In this article, we’ll dive into the modern sand casting process, examining how it works and why it remains one of the most widely used metal casting techniques even thousands of years after its inception.

What Is Sand Casting?

Sand casting, or sand moulded casting, is a metal fabrication process that uses dies made from sand to create metal products. Sand is easily mouldable with common objects. After shaping it as desired, hot molten metal is poured into the mould. As the metal cools, it takes on the shape of the mould as it solidifies. The final metal product is then extracted from the sand mould and undergoes finishing before being put to use.

Using the sand casting technique, products of all sizes and complexity levels can be manufactured. It can be used to cast items as small as pins or as large as railway locomotives. Additionally, it is compatible with a wide range of metals, including cast iron, malleable iron, steel, nickel alloys, copper alloys, aluminium alloys, and magnesium alloys.

Its versatility makes sand casting the most widely-used casting process in the world, with over 60% of all cast metal products being made using this method.

Properties of Moulding Sand

While there are many different types of sands to choose from, not all of them are suitable for the casting process. Each of them exhibits certain traits that may or may not align with the project requirements.

Depending on the casting process and metal, the right combination of properties in the moulding sand is needed. To enhance cohesion, special additives and binders can be added to the basic sand, clay, and water mixture.

These binders, which can be organic or inorganic, include cereals, ground pitch, sea coal, gilsonite, fuel oil, wood flour, silica flour, iron oxide, and pearlite among others. Some of these binders have contradicting effects but trade-offs are often necessary to balance the properties for specific casting applications.

The four most common types of sand used for casting are green sand, dry sand, sodium silicate and resin sand:

1. Green sand is called so because it is essentially raw sand mixed with water. It exhibits a clay-like consistency. Green sand offers a low-cost option for casting sand with good reusability and is compatible with high production rates. However, its low malleability can cause the mould to collapse under certain situations. It also has a propensity to create surface defects.

2. Dry sand, on the other hand, is a moulding sand that uses clay and contains very little water acting as the binding agent. It holds its shape quite well once rammed into place. A benefit of dry sand is that it creates very little steam during the filling stage, which in the case of more moist sands, can lead to mould fracturing if left unchecked.

3. Sodium silicate, aka water glass, is a special sand that uses a sodium silicate binder to integrate the sand. Sodium silicate can create more intricate or complex shapes than green and dry sand.

4. Resin sand, aka Furan sand, is another popular sand used for sand casting It uses a furfuryl alcohol resin to make a coherent mass out of the sand. The robust and highly stable compound has very little tendency to break down and provides excellent dimensional accuracy and surface finish. However, it is the most expensive of all sands. It also has a higher lead time.

Casting engineers choose the type of sand based on the metal, product geometry and tolerance limits. But no matter the specifications, they always screen a sand type for the following properties:

Refractoriness

Refractoriness is a material’s property that allows it to withstand thermal shocks and burning when subjected to high temperatures. The sand serves as a refractory material for molten metal and thus needs to have a high melting point. The selection of sand for casting is largely influenced by the process temperature. If the refractoriness is insufficient, the sand will melt and fuse to the casting.

For many metals, such as aluminium, the sand only needs to withstand a temperature of 700 degrees Celsius. However, sands for steel castings need to withstand at least 1500 degrees Celsius as the melting point is higher.

Permeability

The sand must vent steam and other gases generated during the casting process. Each unit of water in the mould produces 1600 units of steam.

If the gases and steam get trapped, it can cause defects such as blowholes and gas holes in the final product. Their removal is crucial to obtain a good quality finish. Thus, permeability is a crucial property in sand for achieving a high-quality finished product.

The properties of moulding sand are influenced by the grain size and distribution: small grains increase mould strength but reduce permeability, while larger grains enhance permeability. Fine grains yield a smooth surface finish on castings but have low permeability, whereas coarse grains provide higher permeability but a rougher finish.

High Thermal Conductivity

Thermal conductivity is the property of a material that allows heat to flow through it. Efficient heat flow leads to quicker cooling of the molten metal. Sands with high thermal conductivity are required to transfer the heat from the molten metal to the atmosphere.

Low Thermal Expansion

Thermal expansion is the increase or decrease in the volume of a material when heat is added or removed, respectively. For the sand casting process, sand with a very low thermal expansion is needed. This type of sand will maintain its shape during various stages of the casting process, including the pouring of liquid metal and subsequent cooling.

Flowability

The ability of sand to flow into smaller areas, such as cavities, depressions, and deep channels of the pattern, determines whether we can use it for casting complex shapes. Sand with higher flowability will fill all the features of a pattern without requiring external pressure.

A highly flowable sand is not needed for simple shapes, such as a manhole cover, but it may be required for something more complex like a sculpture. The level of flowability needed may therefore be determined on a case-by-case basis.

Cost

The cost can also vary significantly from sand to sand. Silica sand, for instance, has high availability but also issues like dimensional instability and the tendency to burn or even explode to form sub-micron-sized particles under thermal shock. Silicosis is also an extremely concerning issue with workers being exposed to silica for extensive periods. Olivine and chromite sand, on the other hand, are more expensive and less available but do not have the above-mentioned issues.

Sand Casting Process

Sand Casting Process

The casting process is straightforward for simple products. A cavity with the desired shape is created, and molten metal is poured into it through a vertical channel. Once the metal solidifies, it is extracted by removing the mould sand around it.

However, with complex shapes, aspects such as how to fill the mould, remove excess metal, achieve a high-quality surface finish, and ensure the optimum temperature and velocity of the molten metal for uniform filling must be carefully considered.

Let’s explore a typical sand casting process in more detail:

1. Mould Creation

The mould is created using a pattern, which is a replica of the desired product. The pattern may be made from a different material or process, such as machining or additive manufacturing, but it has the same shape and dimensions as the final product. Multiple moulds can be created using a single pattern.

The pattern is separated into two parts along the centreline of the shape. Each half is then placed in separate sandboxes, and the sand mixture is packed around them to create impressions of the patterns in the sand.

Once the patterns are removed from the sand, the two sandboxes are placed on top of each other, securing them to form the complete mould. The top half and bottom half are referred to as the cope and drag, respectively. The mould cavity thus generated has the negative shape of the product. For example, a protrusion on the final product appears as a depression in the mould.

2. Gating System Creation

The gating system provides a pathway for the molten metal to flow into the cavity. A well-designed system can be crucial for the quality of the product. An effective gating system is simple, efficient, and minimises turbulence. It also traps any gas or slag present in the molten metal.

Key components of the gating system requiring careful design include:

• Pouring Basin and Sprue – The pouring basin serves as the funnel-shaped entry point for the molten metal. The metal flows into the sprue from the pouring basin, moving under gravity towards the cavity.

• Runners – Runners are horizontal channels for the molten metal to reach the die cavity from the sprue.

• Gates – Gates are the entry points of the die cavity, ensuring a smooth flow of the molten metal into the die cavity, maintaining pressure, and preventing flashing. They also help restrict any slag from entering the die cavity.

• Risers – To counteract shrinkage as the metal cools, risers serve as reservoirs in the gating system, storing extra molten metal. As the metal begins to shrink, risers provide additional material to the cavity, preventing metal shortages.

• Cores – When a product requires a hollow section or cavity, a solid sand insert called a core is placed inside the die to form the intended cavity. The molten metal fills around the core, creating the cavity in the final casting. Chaplets hold the core in place during casting.

3. Pouring the Molten Metal

The molten metal is poured from the top into the cavity. The volume of metal used slightly exceeds that of the product to ensure that some excess material is stored in the risers.

4. Solidification into Casting

Once the die cavity is filled, the casting is allowed to cool. Natural or forced cooling may be applied depending on the metal and process. Proper cooling is essential for achieving the desired properties and maintaining dimensional accuracy in the final product.

5. Shakeout and Sand Recovery

Shakeout is the process of removing the sand castings from the mould. The sand is shaken and shifted to separate the solidified casting, which is then sent for final finishing operations, while sand recovery begins.

Since not all of the sand can be recovered, some of the old sand is replaced with fresh sand for future sand casting processes. To restore the sand’s properties for reuse, mechanical, chemical, and thermal treatments may be applied, along with adjustments to its composition by compensating for lost moisture and additives.

Advantages of Sand Casting

Despite the development of many advanced technologies in recent decades, sand casting remains one of the most popular metal fabrication processes. Its unique advantages make it indispensable in the modern manufacturing landscape. Today, sand casting is often automated, enhancing its adaptability and efficiency to meet the demands of contemporary production environments. Here are some key advantages that make sand casting such a valuable process.

Suitable for all types of production

Sand casting is compatible with various production setups, allowing for single-piece, batch, or mass production. This method is extensively used in industries like automotive, defence, agriculture, mining, construction, healthcare, and renewable energy.

In the automotive industry, sand casting is used for developing prototypes, engine blocks, cylinder heads, and various IC engine parts. Its application has also expanded to EV manufacturing, where sand casting is employed to produce structural components, motor housings, battery casings, and more.

There is almost no limitation on size as well. Products ranging from a few grams to a few tons in mass can be cast with mathematical precision.

Cost-efficient

One of the primary factors that drive various sectors to adopt sand casting technology is cost. Sand casting is significantly more cost-effective compared to other casting processes. The sand can be reused for future castings with minimal processing.

The automation in high-volume production also reduces the cost per piece while ensuring consistent and reliable quality. Sand casting also incurs low tooling costs compared to other casting methods.

Suitable for all metals

Sand casting can work with almost any ferrous or non-ferrous metal alloy. Examples include gold, iron, lead, and titanium. The only requirement is that it should melt and flow as a liquid metal into the mould. This flexibility allows foundries to work with a wide range of metals without requiring significant process or equipment modifications.

Fast process

Although processes like centrifugal casting can be more efficient for specific shapes, sand casting offers quick turnaround times both for prototyping and large-scale applications.

Limitations of Sand Casting

All manufacturing processes involve trade-offs. In the case of sand casting, however, these trade-offs are minimal compared to the benefits. Let’s examine some of those limitations:

Poor surface finish

Poor sand quality and process deviations can sometimes result in a poor surface finish in sand castings. Any breakdown in the sand mould can lead to a rough surface. For products needing a high-quality surface finish without extensive post-processing, die casting and investment casting are often preferred, but sand casting can also produce refined finishes with the right sand and processes.

Using high-quality, finer sand provides a much better surface finish, but it increases the cost of the process. Proper ramming or packing of sand particles also helps to improve the surface finish. To rectify surface defects such as an overly rough finish, sand castings usually undergo machining operations like turning, grinding and shot blasting.

Loose control of cooling rates

While cooling rates can be controlled more effectively in many metal fabrication processes, the same cannot be said for sand casting. Sand, being an insulator, exacerbates the challenges related to the cooling stage. The cooling stage is considerably prolonged in sand casting, impacting lead times and production rates.

Some variation in microstructure anisotropy can occur due to varying cooling rates. Anisotropy refers to the changes in mechanical properties resulting from alterations in the grain orientation of the material.

Forming internal cavities can be difficult

The sand casting method requires the placement of cores in the mould cavity to form internal cavities. For instance, when manufacturing an engine block, empty cavities for the engine cylinders need to be created during the casting process.

Creating internal chambers, holes, or spaces within the mould cavity is challenging as they must be positioned correctly and adequately supported to withstand the casting process. The more complex these cavities get, the more challenges need to be overcome.

Skilled labour is required if the process is not automated

Automated processes can be costly. However, without automation, the process relies on skilled sand casting experts to craft patterns, moulds, and oversee the process to ensure a high-quality product. Without skilled labour, the outcomes may fall significantly below acceptable standards, resulting in reputational and material losses.

Sand disposal can be a challenge

Foundries recycle moulding sand as much as possible, but after a specific number of cycles, it is no longer fit for reuse. However, waste sand from foundries is a harmful contaminant that can lead to environmental issues.

Construction is one of the sectors where waste foundry sand is now being diverted for disposal. It is being used as a substitute for fine aggregate (sand) in concrete. This practice not only addresses the issue of sand disposal but also lowers the cost and carbon emissions linked to concrete production.

But apart from this, most of the waste foundry sand ends up in a landfill, which is certainly not ideal. The options for recycling casting sand are limited.

Conclusion

Despite traditional sand casting being an ancient process, it has undergone remarkable refinements, making it highly relevant and useful in today’s manufacturing landscape. Its popularity is partly attributed to its versatility across various metal types and product sizes, and partly to the cost efficiency compared to other casting processes.

Ongoing innovations in material science and process automation further expand sand casting’s significance in industries ranging from automotive to construction. As industries evolve, sand casting will persist in shaping the industrial landscape, blending tradition and innovation in metalworking techniques.

The post Sand Casting – Process, Sand Properties, Pros & Cons appeared first on Fractory.

In this article, we’ll look into centrifugal casting process to understand its operation in detail, its types, and the benefits it offers.

What Is Centrifugal Casting?

Centrifugal casting is a metal casting process that uses a spinning die to produce cylindrical products like pipes and tubes. During this process, molten metal is poured into a spinning die. As the metal flows into the spinning die, centrifugal force distributes it uniformly along the inner diameter of the die.

The thickness of the casting can be accurately controlled by regulating the amount of metal poured into the cylinder. The liquid metal starts cooling and solidifying from the outside surface in contact with the cylinder.

While cylinders can be cast through other methods, they would have limitations on size, tolerances, seam strength and corrosion resistance. The formation of internal cavities would require cores and the final part may exhibit issues such as gas porosity, oxides and non-metallic inclusions.

Centrifugal casting, on the other hand, has no limitations on the length of the finished part. It allows the casting of very long parts with remarkable accuracy and consistency. Most machines can produce pipes up to 15 meters (49 ft) in length with a diameter of up to 6 meters (20 ft). The wall thickness can vary from 2.5 mm (0.1 in) to 125 mm (~5 in).

The process works best for thin-walled cylinders but it can also be used for thick-walled applications such as gear blanks, ball bearings, railway carriage wheels, and more. Centrifugal casting can also produce non-circular shapes with a near-constant radius.

Multi-layer casting is another unique feature of centrifugal casting. In this casting technique, two dissimilar materials can be combined to form a multi-layer centrifugal casting. This finds many applications in several industries. For example, steel pipes can be coated on the inside or outside with concrete to enhance their corrosion resistance and compressive strength.

Common applications of the centrifugal casting process are in piping, aerospace, automotive and military sectors. Some common products made using this process include engine cylinder liners, jet compressor cases, metal pipes, railway carriage wheels, bearings and many other tubular parts in high-reliability applications.

Centrifugal Casting Process

Centrifugal casting is a fairly straightforward process. Most often, the process is used to create standard-sized products rather than specific ones. Standardisation allows the process to take advantage of economies of scale and reduce part costs.

Let’s explore a typical centrifugal casting process, its stages and their significance. The four stages are as follows:

1. Mould preparation

2. Pouring of molten metal

3. Directional solidification

4. Removal and finishing operations

Centrifugal Casting Process

1. Mould Preparation

The mould preparation begins with cleaning the die’s inner surface to remove contaminants and debris from previous use. Once it is clean, the motors responsible for spinning the die are powered up.

The amount of force applied to the molten metal is proportional to the die’s diameter and spinning RPM. Typically, a die has a rotational speed between 300 and 3000 RPM. The centrifugal force produced by these high speeds can reach up to 100 times the force of gravity.

Then, it is time for preheating and applying a ceramic slurry to the inner surface of the die. The heat dries and cures the ceramic slurry, ensuring that the refractory lining adheres to the die’s surface.

The ceramic slurry is necessary to protect the die and facilitate the removal of the casting upon solidification. An ideal slurry also provides a smoother surface finish for the part.

2. Pouring of Molten Metal

The metal is melted in an external furnace and poured into the pouring basin of the casting machine using a ladle. The pouring basin then transfers the metal to the centre of the die through a spout. The high centrifugal forces ensure that the final product is free of cavities or gas pockets.

The centrifugal force also separates the low and high-density components in the molten metal. All the impurities float towards the inner diameter of the molten metal due to their lower densities. This results in an increased density of the molten metal at the outer diameter compared to the inner diameter.

3. Directional Solidification

The metal casting rotates continuously during the rapid cooling stage until its temperature is below the transition range. Cooling in centrifugal casting begins from the outside diameter towards the inside diameter. This type of controlled solidification in castings is also known as directional solidification.

Directional solidification offers advantages such as improved grain structure, lower concentration of impurities and the development of beneficial microstructures. Solidification shrinkage always occurs at the inner wall of the finished product in centrifugal casting.

4. Removal and Finishing Operations

Once solidification is complete, the casting is removed from the die using extraction equipment if no heat treatment is needed. In some cases, the casting is subjected to annealing before the steel mould is removed.

It is then sent for finishing operations. The impurities and lighter material that floated inward during the second stage are now machined, leaving a strong, defect-free part with excellent mechanical properties. The outside is shot blasted to remove any traces of the refractory material.

Visual and dimensional inspections are also carried out at this stage.

Types of Centrifugal Casting

Centrifugal casting processes can be classified in different ways. One method is based on the nature of the casting process, where they are divided into true centrifugal casting, semi-centrifugal casting and centrifuging processes. A second, more common method is to classify them based on the die’s orientation, i.e., horizontal and vertical centrifugal casting processes.

The processes between the two classification methods can overlap. For instance, a true centrifugal casting may involve either a horizontal or vertical centrifugal casting process.

Classification Based on the Nature of the Process

Animation of Centrifugal Casting Types

True Centrifugal Casting

The true centrifugal casting process is the most popular type of centrifugal casting process. This is the same process described in the previous section. Unlike other processes, this method does not involve a gating system. The material is poured directly into the cylindrical mold, where it contacts the mould’s internal surface and spreads uniformly.

Semi-Centrifugal Casting Method

The semi-centrifugal casting method is similar to the true centrifugal casting process, except that the former creates solid castings, unlike the latter process which produces only tubular shapes. It handles the production of solid components such as spokes, gear blanks and pulleys.

The material away from the rotational axis is denser compared to that around the axis. Impurities, due to their low density, also accumulate around the axis. After solidification, the material around the axis is machined away to make room for the axis on which the produced component will spin.

Semi-centrifugal casting provides us with a product that has excellent mechanical properties and is free of impurities.

Centrifuge Casting

Centrifuging is a unique form of centrifugal casting method that uses centrifugal forces to fill conventional moulds instead of applied pressure. In this process, we use a rotating mould with a central sprue at the axis. The sprue channels the molten material into the rotating mould as it spins.

The centrifugal force from the rotations pushes the molten metal outward and into the cavities spread along the circumference. This process includes all the components of the gating system, such as sprue, runners, gates and risers.

The spinning motion of the die pushes the material into the die until complete distribution and solidification occur. The centrifuging process is used for small-sized products such as jewellery, sleeves and bushings.

Classification Based on Die Orientation

When designing a die for centrifugal casting, we can choose to align it either vertically or horizontally. Each orientation has its benefits and limitations:

Horizontal Centrifugal Casting

Horizontal centrifugal casting refers to the casting process in which the mould’s spin axis is parallel to the ground. The horizontal axis arrangement enables the production of castings with a very high length-to-diameter ratio. Horizontal centrifugal casting is a cost-effective method that delivers high-quality results.

Some common products made using the horizontal centrifugal process include ductile iron pipes, reformer tubes, furnace rolls, sleeves and roller shells.

Vertical Centrifugal Casting

In vertical centrifugal casting, the mould spins along the vertical axis, perpendicular to the ground. This technique is more suitable for products with a short length-to-diameter ratio. Typically, the length should be less than twice the product’s width for optimal results in vertical centrifugal casting.

The effect of gravity needs to be accounted for in vertical centrifugal castings, as it can lead to distortion in the absence of sufficient centrifugal force.

Some examples of products made with this manufacturing process include ball bearings, gear blanks and pulleys.

Advantages of Centrifugal Casting

Centrifugal casting is a special type of casting process that offers unique advantages compared to other casting processes:

• Unrivalled rotational symmetry in parts

• Fine-grained structure

• Ability to seamlessly integrate two materials

• Can work with a variety of materials besides metals, such as glass and concrete

• Eliminates cores and gating system

• Lower manufacturing costs and reduced machining requirements

Limitations of Centrifugal Casting

Each manufacturing process has its limitations, centrifugal casting is no different:

• Restrictions on complex shapes: Centrifugal casting is limited to producing cylindrical castings and, to some extent, other shapes. Even within cylindrical castings, the complexity achievable through this process is restricted. For applications needing more flexibility in shape or size, other methods, such as sand casting is often chosen, offering adaptability across a broad range of geometries and product sizes.

• Inaccuracies in the inner surface diameter are fairly common. However, it is possible to generate net or near-net shapes with the right controls.

• The process requires a high initial investment. However, similar to die casting, it boasts low per-part costs, making it cost-effective in mass production scenarios.

• Parts with small inside diameters are difficult to cast.

• Operational safety can be a concern with this process because the die operates at high rotational speeds along with other rotating and moving parts.

• The centrifugal process is not particularly effective for small-sized products.

The post Centrifugal Casting – Working Principle, Types & More appeared first on Fractory.

What Is Die Casting?

Die casting is a metal casting process that uses metal dies to create metal parts. Molten metal is poured or injected into the metal dies and allowed to solidify. Once solidified, the parts are removed from the mould and put to use after minimal finishing operations.

The die casting process is primarily used for casting non-ferrous metals, such as aluminium, magnesium, zinc and their alloys, but it can also be used for other metals with comparable melting points.

Die casting is essentially the same as injection moulding, as the primary difference lies in the materials used – molten metal vs molten plastic. Additionally, die casting requires higher temperatures and pressures due to the properties of metals, and the moulds are typically made from high-strength steel to withstand these conditions, whereas injection moulding can use moulds made from a variety of materials due to the lower temperatures and pressures involved in processing plastics.

Applications of Die Casting

Die casting is widely used in various industries to produce parts such as:

• Automotive components (engine blocks, transmission cases and wheel rims)
• Consumer electronics (housing for laptops, smartphones and cameras)
• Appliances (handles, brackets and heat sinks)
• Aerospace components (aircraft parts and fittings)
• Industrial equipment (pumps, valve bodies and enclosures)

Die Casting Process

The die casting process can have different mechanics depending on the metal, application and the process employed. For instance, the molten metal could be poured via gravity or injected into the die under high pressure.

Let’s explore a typical die casting process, which involves six main stages:

1. Mould Creation

2. Mould Preparation

3. Metal Injection

4. Cooling

5. Casting Removal

6. Finishing Operations

1. Mould Creation

The mould in die casting is a metal die with an internal cavity the same shape as the product. Nowadays, CAD/CAM and CFD (Computational Fluid Dynamics) software are used to design moulds. These programs can accurately simulate the smallest details about a mould such as internal friction, temperature distribution, flow rates, heat dissipation rate and lubrication efficiency.

After successful simulations, the engineers share the drawings and specifications, such as tolerance and tonnage, with specialist toolmakers. These specialised toolmakers create the mould through CNC machining and run trials to ensure that the final product meets the specifications. In addition to the cavity, an industrial die also contains channels for metal flow, a cooling system and ejector pins.

This is one of the critical stages in die casting. A high-quality mould can eliminate many issues downstream, such as surface defects, material wastage, shrinkage, warpage and incomplete filling.

2. Mould Preparation

The mould preparation stage involves setting up the mold in a way that is most favourable to producing high-quality castings.

The mould is made up of two halves: the fixed die half, which is connected to the injection system, and the removable die half, which separates to remove the casting.

Engineers clean and heat the mould before pouring molten metal into it. They also spray the inner surfaces of the mould with lubricant or refractory material. Of course, these processes can be automated as well.

The lubricant serves multiple functions, including reducing internal friction, aiding in maintaining the die’s temperature, preventing premature solidification of the molten metal in the die, and facilitating the removal of the cast from the die once the metal has solidified.

Finally. the mould halves are closed and secured together using high pressure before pouring the metal.

3. Metal Injection

In this stage, molten metal fills the die uniformly. This action can be performed in two ways. One method is gravity die casting, where the force of gravity is utilised to fill all the internal cavities in the mould. This process involves pouring the molten metal from a ladle into a downward sprue that leads to the mould cavity.

The second method, also known as high-pressure die casting, fills the mould by injecting the molten metal into it under high pressure. The pressures can range from 100 bars (1,400 psi) to 1800 bars (26,000 psi). This method offers several advantages over gravity die casting, such as faster and better filling of the mould, the capacity to create intricate structures with thin walls, high dimensional stability, smoother surface finish and the minimisation of finishing operations downstream.

4. Cooling

Once the molten metal has completely filled the mould, the cooling stage begins. It is important to ensure a uniform and controlled cooling rate to obtain the desired microstructure in the final product.

Molten metal must be given enough time to cool to prevent surface or internal defects from forming in the part.

5. Casting Removal

Once the cast is set, the mould halves are unclamped to facilitate the removal of the casting. This may be done manually or automated using ejector pins. The ejector pins are built into the mould to safely and automatically separate the cast when the mould halves are separated.

6. Finishing Operations

Finishing operations are necessary to remove the sprue, runners and other gating system components from the casting. These components are then remelted in the furnace for reuse.

The casting may also need finishing operations, such as surface grinding and machining before it is ready for handover to the customer.

High Pressure Die Casting Process

Types of Die Casting

Die casting processes can be carried out under different conditions depending on the specifications of the process and the final part.

Among the many specialist processes in die casting, two are worth elaborating on: the hot chamber die casting and cold chamber die casting processes. Let’s have a look at both of these processes to gain a better understanding.

Hot Chamber Die Casting

The hot chamber in the hot chamber die casting process refers to an internal furnace within the casting machine. The furnace is connected to the die cavity through a feeding system known as the “gooseneck”. As the metal is melted within the casting machine itself, this process offers faster cycle times of about 20 minutes, making it a worthy contender for mass production.

The hot chamber, however, cannot reach very high temperatures. Therefore, the process is most suitable for low melting point metals such as lead alloys, zinc alloys and magnesium alloys. As the metal melts, it is forced into the die through the gooseneck by a plunger.

The plunger withdraws when the metal in the die solidifies. The mould opens and the casting is removed. Then, the mould closes once again, the plunger transfers the hot metal into the die cavity, and repeats the process.

The hot chamber die casting process is popular for its speed, accuracy and consistency. Additionally, it offers a longer die life and improved corrosion resistance.

Cold Chamber Die Casting

The cold chamber die casting process is similar to the hot chamber process, except that the metal is not melted in the casting machine.

The metal is melted in an external high-temperature furnace and transferred to the cold chamber die casting machine via a ladle. In the cold chamber machine, a hydraulic piston forces the metal into the metal die for casting.

This process is suitable for metals with higher melting points such as copper, magnesium, aluminium and their alloys. Magnesium alloys start melting at about 360°C (680°F) and go up to 650°C (1200°F). Therefore, some magnesium alloys can be cast using hot chamber die casting, while others can be cast using cold chamber die casting.

Metals with higher melting points offer greater strength, durability and hardness compared to low melting point metals. Therefore, cold chamber die casting is a better choice for demanding applications.

Advantages of Die Casting

The die casting process is a modern casting method with numerous unique advantages. Although it has been around for many centuries, it has garnered popularity in recent decades. Let’s explore the advantages of die casting to comprehend why it is a favored choice in the casting industry today:

Complex shapes are a breeze

The die casting industry can create highly complex shapes through this process. It is one of the few processes that can consistently produce high-quality parts at a rapid rate while maintaining narrow tolerances. The high pressures involved are capable of pushing molten metal into the thinnest channels with extreme reliability.

Master at mass production

The quick cycle times, high potential for automation, high material efficiency, and usage of low melting point metals make it highly effective for high-volume manufacturing. The industry uses the process in most serial production applications.

Made for the 21st century

The die casting process can seamlessly incorporate all the latest technology. It is highly compatible with automation and other supporting technologies such as 3D printing and simulation, CAM and CAD software.

First-rate low-cost operation

The cost per piece decreases significantly as the volume increases. Although the initial investment in mould creation and automation integration may be higher than other processes such as sand casting, it more than compensates for it by having one of the lowest operating expenses among all the various types of casting processes.

Buttery smooth finish

Die casting is popular for delivering one of the best surface finishes among casting processes. In many cases, finishing operations may not even be needed, and the product can be put to use after trimming off the excess material from the gating system components.

Additional finishing operations such as grinding, machining and powder coating are only necessary when the application requires a high-quality polish or specific textures.

Limitations of Die Casting

It is equally important to be aware of the limitations and shortcomings of a manufacturing process. Die casting limitations are as follows:

High initial investment

The die casting process involves a significantly higher initial investment compared to other processes. This aspect of die casting can deter interested manufacturers who only have a small number of parts to produce. The economic advantage of die casting lies in mass production. It is not suitable for manufacturing prototypes or small batches of products.

Turbulence can be an issue

The high-pressure injection of molten metal is likely to generate turbulence, which can result in defects like porosity. To address this, most foundries now use flow simulation during mould design. By optimising the mould design and casting process parameters, turbulence can be minimised in the majority of cases.

For example, the metal is initially pushed into the mould with a slow initial velocity to minimise turbulence and the formation of gas bubbles.

Size and weight limitations

Die casting is most suitable for small to medium-sized applications. There are practical limitations on the weight of die-cast parts, as larger castings will naturally require larger dies and related equipment. Process parameters, such as the clamping force and heat dissipation capacity, cannot exceed a certain limit.

As the size of the casting increases, the cost of the process also increases at a greater rate, rendering the process unfeasible beyond the inflection point. In such cases, alternative methods like centrifugal casting may be more appropriate, as they are better suited for larger or cylindrical parts, where uniformity and precision are critical.

To Sum It Up

The die casting process is a modern marvel that can produce the most delicate and complex shapes effortlessly. It surpasses other casting processes, such as sand casting and investment casting, in terms of speed, accuracy, consistency and surface finish. Aluminium die casting, in particular, has been widely utilised in manufacturing to create lighter vehicles while maintaining their original strength.

However, die casting’s disadvantages, such as high initial cost and limitations on size and weight, restrict its use to a small group of applications. Nevertheless, advancements in simulation and automation are enhancing die casting’s capabilities, ultimately expanding its scope within modern manufacturing.

The post Die Casting – What Is It, Process, Types and Benefits appeared first on Fractory.

The founding principles of investment casting have remained the same over thousands of years. However, adding supporting technologies such as CAD/CAM, additive manufacturing, and real-time monitoring capabilities has significantly improved the final part quality, consistency, and affordability of this industrial process.

In this article, we shall explore the workings of modern investment casting processes, along with their advantages, disadvantages and use cases.

What Is Investment Casting?

Investment casting, also sometimes known as precision casting or lost wax casting, is one of the most popular metal forming techniques. The term ‘investment’ originates from the archaic meaning of ‘invest,’ which refers to clothing or surrounding an object with a layer of material.

In this process, a wax pattern is encased in refractory material to form a ceramic shell mould. The pattern is slightly larger in dimensions than the final product to accommodate shrinkage when the material cools.

Since the shell mould has a hard surface, investment casting delivers smoother surface finishes than sand casting. The process can work with most metals. Some popular metals that are highly compatible with investment casting are bronze, magnesium, stainless steel alloys, glass, carbon steel, brass, aluminium and aluminium alloys.

Investment Casting Process

Investment Casting Process Walkthrough

The investment casting process is more intricate than the widely used sand casting process, thus involving more stages. Investment casting involves 8 stages and these are as follows:

1. Creation of the master pattern

2. Creation of the master die

3. Creation of the wax pattern

4. Creation of the shell mould

5. Removal of wax

6. Pouring of molten metal and subsequent cooling

7. Removal of casting from the shell mould

8. Post-processing operations

Let’s look closer at what is happening in each of the stages:

1. Creation of the Master Pattern

The master pattern is an exact duplicate of the product required. It may be made from wood, metal, plastic, clay, wax or other specialist alloys. Over the years, manufacturers have shifted to 3D printing master patterns due to their compatibility with CAD/CAM and improved dimensional accuracy.

2. Creation of the Master Die

The master pattern is used to create a master die. The master die’s creation technique depends on the master pattern’s material. For instance, if the master pattern is from steel, it can be placed in molten aluminium, since aluminium melts at 660°C (1,220°F), whereas steel melts at around 1500°C (2,732°F). Thus, the molten aluminium will not dissolve the steel. Upon solidification, it will create an aluminum-based master die.

3D-printed master dies are increasingly being used because they offer a cost-effective alternative to metal dies when only a few castings are needed.

3. Creation of the Wax Pattern

There are several ways to produce wax patterns from master dies. One method involves filling the die with a small amount of wax and shaking it until it evenly coats the inner surface of the die and solidifies. This process is repeated until the desired thickness is achieved, resulting in a hollow wax pattern.

The second method involves filling the master die completely with wax. The wax may be fed directly or through high-pressure injection into the die. The high pressures enable the wax to fill all of the cavity’s features in the case of complex components. The wax patterns thus produced are solid.

During this stage, cores can be added to create internal cavities in the wax pattern. Common materials for cores include soluble wax or ceramic. The soluble wax is removed in stage 5, while ceramic cores are removed after the final product has hardened.

4. Creation of the Shell Mould

Wax patterns cannot withstand the high temperatures of molten metal. Therefore, ceramic moulds are created from wax patterns to endure the heat of molten metal. The ceramic mould is formed by immersing the wax pattern into a slurry of refractory material and allowing it to solidify.

The process is essentially the same as coating vanilla ice cream with a layer of chocolate. When the cold vanilla ice cream comes into contact with hot melted chocolate, the chocolate sticks to the ice cream and solidifies almost immediately, creating a thin shell of chocolate covering the ice cream.

Similarly, when the ceramic slurry comes into contact with the wax pattern, it adheres to it and solidifies. Several passes of the wax pattern are made, starting with a finer slurry and then a coarser one to achieve the desired thickness of the ceramic mould. The mould is then left to cool down.

5. Wax Removal

Once the slurry has hardened, it is placed in a furnace to remove the wax and for further sintering of the ceramic mould. Sintering is the process through which items are hardened through pressure and heat but without liquefaction. As the temperature rises, sintering hardens the mould while the molten wax is reclaimed for future use. This stage gives us the solid ceramic mould for the actual casting process.

6. Pouring of Molten Metal and Subsequent Cooling

After the wax removal stage, the mould is cooled for testing. If any cracks or other imperfections are detected, they can be rectified using ceramic slurry or special cement.

Upon successful test results, the mould is heated again before pouring liquid metal into it. This preheating ensures that the molten metal remains in the liquid state for a longer period until it uniformly fills the mould.

To pour the metal, the ceramic mould is inverted and placed in a sand-filled container. The molten metal is poured into the mould under gravity or external pressure.

Vacuum-assisted filling may also be used. It comes at a higher price point but offers significantly better resource efficiency compared to gravity and pressure-based filling.

The vacuum pulls the molten metal into the mould, and once all the crucial parts solidify, the vacuum is released, allowing the unused melt to drain out. This process minimises material solidification in the sprue and gates, resulting in up to a 95% material yield, compared to 15-50% in gravity pouring.

The material is then allowed to cool and solidify until it is hard enough to withstand separation from the ceramic shell mould.

7. Removal of Casting From the Mould

The divesting is typically carried out by hammering the mould to release the casting. Other methods to separate the metal casting from the mould include waterjetting, media blasting, vibration and chemical dissolution of the mould. The sprue, runners and other gating system components are then separated and recycled.

8. Post-Processing Operations

The investment casting process generally does not require subsequent machining. However, if the results are not as expected, the casting may need surface finishing operations before use. Typically, surface grinding with machine tools is adequate for polishing and trimming any surface defects. Nevertheless, depending on the situation, other finishing processes such as hand tooling, welding, and hydraulic straightening may be necessary.

Advantages of Investment Casting

The investment casting process offers the following advantages:

• Excellent dimensional accuracy even for complex components

• Smoother surface finish

• Works with a wide range of metals

• Parts are created as a single casting with no parting lines

• Compatible with low- and high-volume manufacturing

• Minimal wastage of resources

• Parts can be combined, eliminating the need for assembly operations downstream

• 90-degree angles can be cast without worrying about shrinkage allowance

• More environmentally friendly than other metal fabrication processes

Limitations of Investment Casting

The investment casting process also has certain limitations:

• A very high number of variables that can affect part quality. A high level of process control is essential for producing good-quality parts

• There is an upper limit on the size of investment castings, which is lower than that of some other casting processes such as sand casting and centrifugal casting

• Casting objects with internal cavities can be more challenging than other casting processes

• Longer production cycles and lead times

• Investment casting techniques are more expensive than other processes, especially when the quantity is low and the manufacturing process involves the use of permanent tools

Applications of Investment Casting

The industry uses investment casting to produce the following products:

• Turbocharger rotors and turbine blades

• Firearm components such as triggers, hammers and receivers

• Power generation and cooling system parts such as pumps, valves and combustor components

• Intricate jewellery with delicate components and tight tolerances

• Automotive parts including engine components, transmission components, brakes, door handles, gears, housings, brackets, rods and more

When to Choose Investment Casting

After learning about the investment casting process, let’s summarise our knowledge by noting the five scenarios where investment casting provides the best ROI:

Medium size range – Investment casting gives better results when the weight of the product ranges from a few grams up to 1,000 kg. Parts weighing more than 1,000 kg are better suited for the sand casting process.

Extremely complex geometry – Investment casting can create super complicated shapes such as turbine blades, medical equipment, dental crowns and inlays, power generation equipment, aerospace components and more.

High number of parts – Investment casting is not cost-effective when a small number of items need to be manufactured, as the process becomes more expensive. However, as the quantity of items increases beyond a certain threshold, the cost per piece can decrease below that of other methods, including sand casting.

Need for a high surface finish – Investment casting uses a hard mould with a very fine surface, allowing the process to achieve an exceptional surface finish.

Need for high dimensional accuracy – It is possible to cast net or near-net shapes through investment casting. The need for post-processing is also minimal with this process.

The post Investment Casting – Working Principle, Process & More appeared first on Fractory.

The process of casting metals dates back over 7000 years. Over the centuries, the casting process has been continuously developed and refined. Today, it enables the repeatable production of highly complex shapes, some of which would be impossible to achieve through any other method.

It is now used so extensively that it is impossible to avoid cast products, no matter what type of environment you are in. Some examples of cast metal products include engine blocks, fire hydrants, motors, tools, traffic lights, manholes, pipelines, valves, and various fittings.

In this article, we’ll explore the metal casting process to understand what it is, explore its types and how they work.

What Is Casting?

The casting process enables the production of metal parts with desired shapes by heating the metals over their melting temperature and pouring the liquid metal into moulds. The molten metal flows into the empty cavities, conforming to the mould’s shape. Once the metal cools and solidifies, it is removed from the mould and sent for post-processing. In post-processing, the surface finish and appearance are enhanced and any necessary protective coatings are applied.

Metal casting is only limited by the manufacturer’s imagination. It can form highly intricate shapes.

Casting is preferred for complex geometries because it is more cost-effective and the process is straightforward compared to, for example, CNC machining in such instances. But it is also widely used for the simplest shapes because of its quick turnaround time and large production capacity.

Modern methods of casting can produce near-net shapes, significantly reducing the necessity for deburring and other post-processing operations.

Metal Casting Process

The reason the metal casting process became so popular was its inherent simplicity. There was no need for modern machinery or complex processes to obtain finished products.

Hence, even ancient armies were able to leverage it for weaponry and tool production. But over the years, the advances in casting have added some steps to the process to ensure that the final products have the best quality possible. Let us look at the typical metal casting process used in industries today.

Step 1: Mould Creation

The mould creation is arguably the most crucial step in the metal casting process. The quality of the mould directly impacts the quality of the finished product. The mould creation method varies depending on the type of metal, part geometry, production capacity, and the desired finish. Once these specifications are decided on, the appropriate materials and the method can be determined.

The mould is created as a negative impression of the desired part. It is made of a material that can not only withstand the high temperatures of molten material but also separate easily from the solid metal part after cooling. Some materials used for mould include sand, plaster, refractory slurry and ceramic liquid. Some moulds can only be used once, while others may be used repeatedly.

Step 2: Melting and pouring of metal

The casting metal is heated above its melting point to create a free-flowing liquid. The liquid must be free of contaminants and dross as much as possible. Arrangements can be added to the mould to prevent the entry of dross into the casting. The molten metal is poured into a pouring basin at the top of the mould. The liquified metal then enters the sprue (vertical channel) and is distributed into the mould cavity through runners (horizontal channels) and gates (entrances for the cavities).

Less viscous metals can flow easily into the casting and form finer patterns. Metals with high viscosity exhibit non-turbulent flow, which helps eliminate defects like porosity. The solidification rate and driving force are also influenced by the viscosity of the molten metal.

Step 3: Removal of the metal casting from the mould

In the case of a single-use mould, the mould is broken to retrieve the metal casting.

For permanent mould casting, there are various opening arrangements on the mould to access and retrieve the casting. The mould can then be reused on the next batches.

Step 4: Finishing and post-processing

The casting retrieved from the mould cannot be used in its current state. When the molten metal is poured into the mould, the liquid metal in the sprue, runners, risers and vents also solidifies. All of these appendages are cut off. Surface finishing may also be necessary.

The part is then sent for cleaning and coating if the application requires it.

Types of Casting Processes

Castings can be created from various metals like aluminium, zinc, steel and iron. Each metal possesses unique characteristics, necessitating adjustments in the casting technique to achieve the best possible product.

The casting technique also depends on parameters such as alloy composition, part geometry, cost, desired final quality, production run size, and performance criteria.

The casting processes are divided in two ways: the first is based on the lifespan of the mould (expendable and non-expendable mould casting) and the second is the method of alloy injection (gravity, pressure or vacuum casting). In this article, we will categorise the casting processes into expendable and non-expendable mould castings.

Expendable Mould Casting Process

Expendable mould casting processes use moulds that can only be used once. These moulds are used in various casting processes such as sand casting, shell mould casting, and investment casting among others. It must be noted, however, that the these moulds may not always be discarded after use. The manufacturers may try to salvage the moulds when mould reform is possible.

Let’s have a closer look at some of the processes using non-reusable mould castings:

Sand Casting

Sand Casting Process

In sand casting, sand is used as the mould material. Sand is cheap, easily available, pliable and can withstand high heat better than most materials. Green sand (moist) and dry sand are the most suitable materials for the casting process.

A sand mould in the negative shape of the desired part is created and then the molten metal is poured into it. Once the metal solidifies, the sand is removed to retrieve the casting.

Sand casting is one of the oldest casting methods. It remains very popular to this day due to its simplicity, versatility and affordability.

Sand casting can produce products ranging in size from a few centimetres to tens of metres. Products can weigh as little as 75g and go up to several tons.

However, sand casting does not provide the best surface finish and is therefore more suitable for larger products that can be refined for a smoother finish later.

Plaster mould casting is a type of sand casting method where plaster is used to build the mould instead of sand.

Shell Moulding

Shell Mould Creation Process

Shell moulding is a type of sand casting process. However, instead of loose sand, metal casting occurs inside a thin-walled, hard sand shell.

To create this shell, a pattern must be created first. A pattern is an object that has the shape of the desired part. The pattern is heated and placed into the sand mixed with a thermosetting resin binder. The sand around the pattern takes on the shape of the part, and the resin hardens it. The shell is further cured in the oven. Once the shell is ready, it is placed in the baking material and molten metal is poured into it. The casting is retrieved upon solidification.

Shell moulding offers advantages such as lower manpower requirements (when automated), increased production capacity, and improved dimensional accuracy.

However, shell moulding is slightly more expensive than sand casting due to the resin used. It may also have lower material strength and high porosity issues. The labour costs can also end up being quite high if the process is not automated.

Investment Casting

Investment Casting Process

Investment casting, also known as lost-wax casting or precision casting, uses wax to create accurate castings consistently.

The process starts with a wax pattern obtained through metal dies. Several wax patterns are assembled with gates, runners and a sprue. This allows to cast several parts simultaneously.

The assembly of wax patterns is dipped or “invested” in a refractory slurry that takes on the shape of the assembly.

The wax pattern, along with the slurry shell, is then heated. The wax flows out of the shell and can be reclaimed for reuse.

The slurry shell then forms the mould for the casting process. Molten metal is poured into the mould, where it solidifies into the required part.

Investment casting is highly precise. It is used for complex parts that require accurate parts, such as turbocharger rotors. It also provides a very good surface finish.

On the other hand, the process is more expensive than sand casting and shell moulding. It has limitations on some features, such as the size and depth of holes. Investment casting can be challenging to execute when cores are involved.

Non-Expendable Mould Casting Process

These casting techniques use permanent moulds that can be reused multiple times. There is no need to prepare a new mould for each casting, as is the case with expendable mould casting. This can be advantageous for castings that require a fine surface finish, high dimensional accuracy, and complex shapes.

The following casting processes can be classified as non-expendable:

Die Casting

High Pressure Die Casting Process

The die-casting process uses a metal die to cast other metals. The die is usually made out of non-ferrous alloys of aluminium, copper and zinc, but ferrous metal dies are also used.

The metal dies are expensive to make but they last a very long time. This makes die casting more suitable for high-volume production.

Die-casting, compared to other metal casting techniques, is more suitable for small to medium-sized parts that require a high-quality surface finish.

Die casting processes are of two main types: gravity die casting and pressure die casting. Gravity die casting uses the force of gravity to fill the mould with molten metals. The channels and the mould cavities are designed in a way that allows the metal to flow naturally into the pockets.

Pressure die casting injects molten metal at high pressure to fill the mould. This process offers greater flexibility in mould design but is more expensive and skill-based.

Pressure die casting is more suitable for complex geometries, while gravity die casting offers greater value for simpler designs.

Centrifugal Casting

Centrifugal Casting Process

Centrifugal casting uses centrifugal force to cast circular parts. The process is generally used for creating standard-sized castings, such as pipes, and not finished products.

As the process leverages gravity and pressure, it creates very fine designs with remarkable accuracy.

The molten metal is fed through the centre and the spinning motion of the mould pushes the metal to the edge, forming an annular shape. By adding various materials successively, it is possible to produce pipes with multiple layers. For instance, centrifugal casting can create cast iron pipes with a cement lining on the interior to prevent corrosion and subsequent tuberculation.

The centrifugal force can also push viscous molten metal into extremely fine cavities. The metal can be moulded into intricate shapes and form complex designs. Thus, this concept is commonly used to create jewellery as well.

The casting machines may be horizontal or vertical. Horizontal machines are suitable for long products like pipes, whereas vertical axis machines are more appropriate for shorter products such as bearings.

Centrifugal casting offers advantages such as not requiring cores, producing excellent grain structure, and having no limitations on dimensions like length, wall thickness and diameter.

Centrifugal casting is the best alternative for specific applications, such as thin-walled cylinders. However, it has disadvantages such as shape constraints, inability to work with smaller diameters and compatibility with limited alloys. Impurities gather at the centre and are thus generally removed by machining.

Continuous Casting

Continuous Casting Process

Continuous casting, also known as strand casting or slab casting, produces a continuous casting of constant cross-section.

Molten metal from a furnace is carefully poured into a tundish, from which the metal is transferred to a water-cooled mould with the desired cross-section. The cooling speeds up the solidification of the molten metal.

While the metal solidifies, it is also pulled out of the mould through rollers, where the casting may be cooled and shaped further to final dimensions. Once the solidification is completed, the casting is cut into standard dimensions.

Continuous casting offers a high production rate, increased accuracy, reduced waste, and enhanced grain structure.

However, the process requires a high initial investment, and the final product may have internal defects and poor surface quality.

Summary

Metal casting is a proficient and cost-effective method for producing dependable metal products. It offers a wide range of processes. Each process has pros and cons that make it suitable for specific applications.

For instance, sand casting is perfect for one-off complex pieces, whereas die casting is more appropriate when intricate products are needed at a high production rate.

Moulds used in casting are categorised as expendable moulds and non-expendable moulds. Expendable moulds are destroyed after each use, such as in sand casting, while non-expendable moulds are more permanent and can last from a few days to a few years.

The versatility of metal casting makes it an essential metalworking technique for various industries, from automotive to aerospace, ensuring the high accuracy and repeatable production of complex parts.

The post Metal Casting – Definition, Process & Types appeared first on Fractory.

The process was originally discovered by Alfred Wilm, who aimed to strengthen aluminium alloys. He decided to try to apply the quenching method used for carbon steels to aluminium alloys. The process of age hardening aluminium was patented by him in 1906. His research also led to the discovery of the earliest types of age-hardenable aluminium–copper alloy, Duraluminium.

Since then, age hardening has also been adopted for carbon steels and other metal alloys to enhance their strength, hardness, corrosion resistance and other mechanical properties.

What Is Precipitation Hardening?

Precipitation hardening is a type of heat treatment process used to increase the yield and tensile strength of metal alloys. in which material hardness improves as a function of time. 

The process uses dissolved impurities to improve the mechanical properties of a material. It works by separating specific constituents that bind the mixture together and inhibit relative motion.

The mechanism is somewhat similar to steel-reinforced concrete. Adding steel bars to a concrete mixture enhances the tensile strength of the concrete. When the concrete column encounters a tensile load, the steel beams restrict the expansion of the concrete by absorbing the tensile stress. The impurities in the precipitation hardening process improve the overall strength of the alloy similarly.

Age hardening can lead to a significant improvement in the yield strength and hardness of alloys. For instance, the yield strength of 0.3% carbon steel when annealed is around 300 MPa. The same steel, when work-hardened, has a yield strength of around 600 MPa. When it is precipitation hardened, the yield strength increases to 1500 MPa. Thus, precipitation hardening provides a fivefold increase in the yield strength in this case. 

What Is the Difference Between Tempering and Precipitation Hardening?

Both processes sound really similar in terms of heating the alloy to a certain range, holding it there, followed by rapid cooling, and then heating again below the critical temperature. However, tempering and ageing are thermodynamically really different, performed on different types of metals and result in different properties depending on the time and temperature they’re performed at.

In terms of the effects on the mechanical properties, tempering increases toughness and ductility and slightly decreases hardness of the material. On the other hand, ageing increases the hardness and strength, making the metal somewhat brittle.

Precipitation hardening hardens by forming a fine precipitate phase in the matrix of an alloy. The precipitates increase hardness by blocking dislocations in the crystal lattice. Tempering is actually stretching out the lattice because you have a Body Centered Tetragonal (BCT) lattice that is diffusing out carbon to reduce the internal stresses.

Ageing is generally performed at lower temperatures than tempering and takes longer to achieve the desired effects. Typically, precipitation hardening is a process performed mainly on aluminium, nickel alloys and stainless steels, while tempering is specific to alloy steels because it is dependent on the formation of the martensite phase.

Applications for Age Hardening

Age-hardened alloys are used in a wide range of applications. They are ideal for applications that require rigid materials that do not flex under stress.

Thus, you’ll find that components, such as valves, gears, shafts, engine parts, turbine blades, ball bearings, bushes, dies and fasteners are precipitation-hardened. The age-hardening process improves most structural aluminium alloys (2xxx, 6xxx and 7xxx series), magnesium, nickel and titanium. The process also finds use with mild steel, stainless steel and duplex stainless steel.

Here are a few examples from various industries that rely on precipitation-hardened alloys:

Aerospace and automotive

Both the aerospace and automotive industries use age-hardened aluminium alloys as the parts need to be light and strong. Age-hardening further improves the excellent strength-to-weight ratio of aluminium. Aircraft parts, such as wings and fuselages, as well as automobile parts like engine blocks, cam covers and other critical engine components, utilise precipitation-hardened aluminium alloys.

Telecommunications

Copper beryllium hardens copper in the compound. The copper starts behaving like a spring within the material. Age-hardened copper-based compounds are used in electrical contacts and switchgear, mainly because of their high conductivity.

Mining & power generation

Age-hardened materials with copper-based compounds are used in mining tools since they have very high hardness and do not spark. When underground, preventing sparks is critical for safety.

Apart from the aforementioned sectors, precipitation-hardening alloys are used in many more types of products, such as bicycle frames, rifles, stamping tools and pressure vessels. 

How Does the Precipitation Hardening Process Work?

A typical precipitation hardening process consists of three stages: solution treatment, quenching, and ageing.

1. Solution Treatment

The solution heat treatment phase, or solutionising phase, is where the homogeneity of the solid mixture is improved with the constituent metal. This could be beryllium in copper or copper in aluminium.

The solution is heated above the solvus temperature to ensure maximum dissolution of the precipitate in the alloy. The mixture sits at this temperature for a specific duration. This stage is therefore also known as soaking.

Let’s try to understand what exactly happens in this stage: Imagine you want to dissolve sugar in water. Increasing the temperature of the water enables to dissolve more sugar. On the other hand, if you decrease the water temperature, some of that sugar which was previously dissolved might come out of the solution.

The solvus temperature is the maximum temperature at which you can dissolve the most amount of a solid in a solution. Thus, by holding the alloy at a temperature above the solvus temperature, we can ensure that the maximum amount of solute is uniformly dissolved into the solid solution. This solution is also known as a supersaturated solid solution.

2. Quenching

Once maximum dissolution of the impurity occurs, the alloy is rapidly cooled to room temperature. This step is known as quenching. Quenching enables to trap the supersaturated solution as a metastable phase.

A metastable phase is an intermediate energy state that should not exist at lower temperatures. However, by rapidly cooling the material, we can freeze the phase at lower temperatures.

Quenching achieves this by preventing the diffusion of nucleation sites. Rapid cooling prevents the solute from precipitating out of the solution, which would typically occur during slow cooling processes. Materials that are just quenched, are soft solid solutions with low strength.

3. Ageing

In the ageing stage, the solution is heated again. However, the temperature is lower than in the solution treatment phase and does not reach the solvus temperature. The elevated temperature initiates precipitate formation but within very short ranges. Fine precipitates develop in the solution. These precipitates form massive strain fields and enhance the strength and hardness of the final product.

Ageing can also occur at room temperature for some alloys. This is referred to as natural ageing while ageing above room temperature is referred to as artificial ageing.

We must remember that ageing is a function of time. As time passes, the hardness of the material increases and reaches a peak value. At that stage, the material is optimally aged and exhibits the highest strength. This is because an optimally aged material interferes the most with atomic motion.

If you heat the material further, it passes the peak strength point and the strength starts to decrease again. This is known as overageing. Overaged materials are not as hard as optimally aged materials.

Age Hardening Benefits

Age hardening provides the following benefits:

• Improved strength – For many materials, the strength can be increased up to four-five times. This provides many benefits, such as smaller parts, lower costs, reduced weight and safer operation.

• Improved hardness – Particle hardening makes metals more durable as they won’t wear out as easily.

• Improved corrosion resistance – Depending on the alloy, age hardening can enhance the corrosion resistance. This is particularly desirable in components that are frequently exposed to corrosive compounds, such as chemicals and seawater.

• Improved ductility – Precipitation hardening can also improve a product’s ductility. This enhances resilience and prevents cracking and breakage.

Conclusion

Precipitation hardening is a widely used process to improve the strength rating of many types of metals and alloys. However, there are two main limitations: cost and the potential for distortion. The process is costlier than many other heat treatment methods as it requires additional heating after quenching. This may take a few hours up to several days in some cases. Material distortion is a risk with certain alloys in the quenching phase.

However, the unique benefits precipitation hardening offers make it a compelling choice. The process guarantees a harder and stronger metal when carried out properly which is especially useful for soft metals like aluminium.

The post Precipitation Hardening – How It Works, Benefits & More appeared first on Fractory.

The sandblasting process was first patented by General Benjamin Chew Tilghman in the 1870s after observing the abrasive effects of sand on windows in the desert. Thomas Wesley Pangborn took this concept even further by adding compressed air in 1904.

That’s enough about the history, let’s get into it.

What Is Sandblasting?

Sandblasting, often also called abrasive blasting, is a method used to remove surface contamination, smoothen rough surfaces, and also roughen smooth surfaces. This is a rather low-cost technique thanks to its inexpensive equipment, and it is simple while delivering high-quality results.

Sandblasting is considered a gentler abrasion blasting technique compared to shot blasting. However, intensity can vary depending on the type of sandblasting equipment, the pressure of the compressed air, and the type of abrasive media used.

Sandblasting offers a wide selection of abrasive materials that are effective in different applications, such as removing paint and surface contamination that is lighter in intensity. The process is also ideal for cleaning sensitive electronic components and corroded connectors delicately. Other sandblasting applications that require greater abrasive blasting power may use a high-pressure setting and a more abrasive shot media.

How Does the Sandblasting Process Work?

The sandblasting process works by propelling sandblasting media to a surface through the use of a sandblaster. The sandblaster has two main components: the blast pot and the air intake. The blast pot holds the abrasive blasting media and funnels the particles through a valve. The air intake is powered by an air compressor that applies pressure to media inside the chamber. It exits the nozzle at high speeds, impacting the surface with force.

The sandblast can remove debris, clean surfaces, remove paint, and improve the surface finish of the material. Its results greatly depend on the type of abrasive and its properties.

Modern sandblast equipment has a recovery system that collects the used media and refills the blast pot.

Sandblasting Equipment

1. Compressor – The compressor (90-100 PSI) provides a pressurised air supply that propels the abrasive media to the surface of the material. Pressure, volume, and horsepower are often the key factors to take into account when selecting an appropriate sandblasting compressor.

2. Sandblaster – Sandblasters (18-35 CFM – cubic feet per minute) deliver the abrasive media onto the material using compressed air. Industrial sandblasters require a higher volumetric flow rate (50-100 CFM) as they have a larger area of application. There are three types of sandblasters: gravity-fed, pressure blasters (positive pressure), and siphon sandblasters (negative pressure).

3. Blast cabinet – A blast cabinet is a portable blasting station that is a small and compact enclosed system. It usually has four components: the cabinet, abrasive blasting system, recycling, and dust collection. Blast cabinets are operated by using glove holes for the operator’s hands and a foot pedal for controlling the blast.

4. Blast room – A blast room is a facility that can accommodate a variety of equipment which are typically used for commercial purposes. Aircraft parts, construction equipment, and automotive parts can be comfortably sandblasted in a blast room.

5. Blast recovery system – Modern sandblasting equipment has blast recovery systems that recoup sandblasting media. It also removes impurities that may cause media contamination.

6. Cryogenic deflashing system – Low temperatures from cryogenic deflashing systems allow for safe deflashing of materials, such as diecast, magnesium, plastic, rubber, and zinc.

7. Wet blast equipment – Wet blasting incorporates water into the abrasive blasting media to reduce overheating from friction. It is also a gentler abrasion method compared to dry blasting since it only scrubs the target area in the workpiece.

Sandblasting Media

As the name suggests, earlier forms of sandblasting primarily used sand due to its availability, but it had its drawbacks in the form of moisture content and contaminants. The major concern with sand as an abrasive is its health risks. Inhaling silica dust particles from sand can cause serious respiratory diseases, including silicosis and lung cancer. Thus, nowadays sand is rarely used and a wide range of modern abrasive materials have replaced it.

The blasting media varies depending on the desired surface finish or application. Some common blasting media include:

1. Aluminium oxide grit (8-9 MH – Mohs hardness scale) – This blasting material is extremely sharp which is perfect for preparation and surface treatment. It is cost-effective as it can be reused many times.

2. Aluminium silicate (coal slag) (6-7 MH) – This by-product of coal-fired power plants is a cheap and dispensable media. The oil and shipyard industry uses it in open-blasting operations, but it is toxic if exposed to the environment.

3. Crushed glass grit (5-6 MH) – Glass grit blasting uses recycled glass beads which are non-toxic and safe. This sand-blasting media is used to remove coatings and contamination from surfaces. Crushed glass grit can also be used effectively with water.

4. Soda (2.5 MH) – Bicarbonate soda blasting is effective in gently removing metal rust and cleaning surfaces without damaging the metal underneath. Sodium bicarbonate (baking soda) is propelled at a low pressure of 20 psi compared to regular sandblasting at 70 to 120 psi.

5. Steel grit & steel shot (40-65 HRC) – Steel abrasives are used for surface preparation processes, such as cleaning and etching, due to their rapid stripping capability.

6. Staurolite (7 MH) – This blast media is a silicate of iron and silica sand which is ideal for removing thin surfaces with rust or coatings. It is generally used for steel fabrication, tower construction, and thin storage vessels.

In addition to the aforementioned media, there are plenty more available. It is possible to use silicon carbide, which is the hardest abrasive media available, and organic shots, such as walnut shells and corn cobs. In some countries, sand is still used to this day, but this practice is questionable as the health risks are not justified.

Shot Media Properties

Each type of shot media has these 4 main properties that operators can consider when selecting what to use:

1. Shape – Angular media has sharp, irregular edges, making it effective in removing paint, for example. Round media is a gentler abrasive than angular media and leaves a polished surface look.

2. Size – Common mesh sizes for sandblasting are 20/40, 40/70, and 60/100. Larger mesh profiles are used for aggressive application while smaller mesh profiles are for cleaning or polishing to produce a finished product.

3. Density – Media with higher density will have more force on the metal surface as it is propelled by a blast hose at a fixed velocity.

4. Hardness – Harder abrasives generate a larger impact on the profile surface compared to softer abrasives. Media hardness for sandblasting purposes is often measured through the Mohs hardness scale (1-10). Mohs measures the hardness of minerals and synthetic materials, characterising the scratch resistance of various minerals through the ability of harder materials to scratch softer materials.

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What Is Shot Blasting?

Shot blasting or grit blasting is one of the abrasive blasting processes that refine metal surfaces through rapidly moving particles. Spherical shot media is propelled onto the metal’s surface, cleaning and polishing the target area using centrifugal force.

The shot-blasting process is an effective method for giving these metals smooth and clean surfaces. Debris and undesirable imperfections on the surface may cause problems further down the line in subsequent manufacturing processes. This can be fixed beforehand through shot blasting.

Shot blasting is an effective method for preparing surfaces, but it has a few downsides if not performed correctly. Possible dangers include damage to the equipment, to the workpiece and its surface, as well as generating dust which creates a mess. Some industries prefer shot blasting over other abrasive surface preparation methods since this process yields high-quality results while being less expensive and safer.

Shot peening works similarly to shot blasting but the former strengthens the material through compressive force from the shot media.

Key Applications

The versatility of shot blasting allows it to have a wide range of applications across different sectors. Some of the surface preparation methods include:

• Deburring – Burrs and other imperfections are removed with concentrated abrasive media, resulting in a more polished and smoother surface.

• Rust removal – The high velocity of the shot blasting media generates enough force to remove rust from the material’s surface.

• Scaling – Shot blasting improves the durability of the workpiece by eliminating mill scale or oxidised flaky material on the surface. Shot media can penetrate hard-to-reach areas and remove mill scale.

• General cleaning – Oils and grease as well as accumulated debris on the surface are removed with this process. Cleaning surfaces before the next step in the production process reduce the risk of damage over time.

• Ceramic shell removal – In the investment casting process, shot blasting is utilised to remove the ceramic shell after the metal has solidified and cooled, ensuring a clean and accurate final product.

• Coatings – An even and clean metal surface results in better paint and coatings adhesion.

Some industries that utilise shot blasting in some of their processes are:

1. Automotive – Body panels or engine parts that come through production are cleaned and the surfaces are smoothened through the shot blasting process.

2. Welding – Surface preparation through high-velocity shot blasting helps to achieve optimal surface quality, helping to avoid welding defects from debris and other imperfections.

3. Aerospace – Shot blasting at pinpointed areas while working on tight tolerances applies to aerospace components that have strict surface requirements. Some applications include cleaning compressor blades and deburring turbine blades after laser drilling the workpiece.

Materials

Shot blasting is typically employed on metals, such as mild steel, cast iron, titanium, copper and engineering steel to name a few, but also on concrete and advanced technical ceramics, such as silicon carbide, alumina, and boron carbide. 

Shot blasting everyday ceramics like pottery or porcelain should definitely be avoided due to their lower strength and higher fragility. Also, soft metals like aluminum or soft copper can deform or get damaged due to the high impact force of shot blasting. This also applies to thin sheets of metal as they’ll easily warp or distort.

How Does the Shot Blasting Process Work?

Conveyor-Type Shot Blast Machine

The shot blasting process works by subjecting the surface of a workpiece to a barrage of shot media at high velocity. The shot material is propelled by a centrifugal wheel in which each individual shot generates enough force to act as an abrasive.

It can clean the surface, remove debris, remove imperfections, and improve the adhesion quality for paints and coatings. Shot blasting is performed inside blast cabinets where the equipment and workpiece are isolated from the environment.

This operation is an economical method for preparing the workpiece before it is processed as a finished product. It would be wise to keep an eye out for issues such as dust collection or excessive shot blasting.

Shot Media Recovery

Modern shot blasting equipment recycles the shot media through a vacuum which collects the used particles. Separators remove broken abrasive media and feeders replace these shots for operation.

Shot Blasting Media

The type of abrasive material propelled by a spinning wheel onto the metal surface in a blast cabinet can vary. Some of the common abrasive media are:

1. Aluminium oxide – An aggressive media that is non-corrosive and versatile, mainly used for hard surfaces requiring fine polishing. Aluminium oxide shots are low-cost and reusable. They break down into smaller particles from usage and are thus categorised as angular shot media.

2. Steel shots – Small steel balls that leave a high polish and shine to the workpiece. Each steel shot is uniform in size which allows equal distribution of impact to the surface.

3. Glass beads – A gentle abrasive suitable for soft metals that allow to avoid contamination of the surface.

4. Cut wire shots – These are obtained with a cutting wire that is divided into equal lengths relative to its wire diameter. They are inexpensive, have high strength, and are highly abrasive.

5. Ceramics – Beads that generate less dust than other media and apply a smaller amount of force.

Shot blasting media used for specific applications are synthetic materials such as sodium bicarbonate and agricultural materials such as crushed kernels. Other shot media include black beauty (coal slag), staurolite, walnut shells, and corn cobs.

Shot Media Properties

Blast media has four main properties that significantly influence the surface finish of the metal.

1. Shape – Most shot blasting media comes in the form of a round shape which is used for almost any application. Angular shot blasting media are more abrasive which is ideal for aggressive cleaning applications. The surface of the workpiece becomes “etched” which gives better adhesion to coatings, paint, epoxy, and rubber.

2. Size – Smaller media sizes or “mesh” are suitable for polishing and achieving clean surfaces. A larger mesh is used for aggressive applications, such as profiling or etching the workpiece in preparation for coating or welding.

3. Hardness – Harder shot media allows for better material penetration compared to softer media. Ideally, suitable blasting media hardness should be selected for the workpiece to avoid damaging the material.

4. Density – Higher-density shot media applies more force to the workpiece when propelled at a fixed velocity.

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Shot Peening Process

Shot peening is a cold working process that subjects the material’s surface to plastic deformation using peening media. The process uses shot media that generally comes in the form of steel, ceramic, or glass beads. This media is bombarded onto the part’s surface at high velocities, generating enough force to compress the surface and create small indentations. It converts the residual tensile stress into beneficial residual compressive stress which improves the material’s strength under load.

The application of the shot media may either be delivered using compressed air or a wheel turbine. The principle of shot peening remains the same with both types of equipment. Factors such as coverage, intensity, angle, and shot quality have a way higher impact on converting tensile stresses to compressive residual stress than the chosen process type.

Each shot particle hits the component’s surface at around 30 to 100 m/s, encasing the layer with residual compressive stresses. Plastic deformation, enabled by shot peening can also correct the surface roughness of a workpiece. Thousands of indentations compress and strengthen the metal, making it more resistant to failure. A correctly shot peened component will last longer and have higher maximum load capacities. Too much peening may induce excessive cold working though, which can lead to fatigue cracks and lower fatigue strength in return.

An Almen Strip Test can be performed throughout the shot peening process to measure the intensity of each shot particle. This test uses a flat strip bombarded by a shot stream which is usually adjusted through a shot flow controller. An Almen Gauge measures the bentness of the strip which determines the intensity.

Separators are used for the recovery of used media and removal of shot fragments. Feeders replace the number of damaged media for operation.

Other uses of this process include shot peening forming, a method in which the component is slightly bent from the force applied. It may be used for thin panels, skins, and wing flaps. Shot peening may also be used for correction of distortion which aims to flatten surfaces (opposite of forming).

Portable shot peening or flap peening is used for extremely difficult areas or repairs (mostly aircraft). It consists of a kevlar matrix with a flap of balls that are propelled pneumatically or electrically to the targeted area.

Shot Peening Media

There are various forms of shot peening media used to generate compressive stress on the material surface. Each has its own HRC (Hardness Rockwell C Scale) value which impacts the amount of force applied to the surface.

1. Cast steel shots (40 ~ 50 HRC) are round metallic in shape and are made from steel scrap and alloys which are melted at ~1650°C (3000°F). Quenching, tempering, and other heat treatments are used to manufacture these steel shots to achieve the desired hardness. Cast steel shots are suitable for most peening intensities of ferrous and non-ferrous metal components. The major downside of a cast shot is the fact that it may shatter into sharp steel fragments at high speeds/forces, damaging the component.

2. Ceramic shots (57 ~ 63 HRC) are pills made from approximately 67% zirconium dioxide, 31% silica, and 2% aluminium oxide. They have an extremely high density and hardness which makes it optimal for increasing the surface hardness of non-ferrous metals such as alloys of aluminium and titanium.

3. Glass beads (>40 HRC) are made from soda lime which can come from scrap glass. The glass is crushed, melted, and formed into spheres. Glass beads are primarily used for aluminium, magnesium, titanium, and other ferrous metals. Peening with glass beads may be used as a secondary process to steel shot peening to remove iron from the surface and improve the part’s smoothness.

4. Cut wire shots (50 ~ 60 HRC) are cylindrical with rounded edges and are homogenous in their composition. A cut wire shot is obtained from a cutting wire that is divided into equal lengths relative to its wire diameter. This type of shot peening media is quite popular due to its low surface contamination, solid and uniform hardness, and no dust generated. It is used for ferrous and nonferrous applications but is ruled out when small-sized shots are needed. It leaves a consistent peening intensity on the surface and is unbreakable, unlike other media. Cut wire shots may come in the form of aluminium, carbon steel, copper, stainless steel, and zinc.

Shot Media Considerations

There are quite a few variables to consider when selecting shot media aside from its type. These considerations are done in comparison to the properties of the workpiece and its desired outcome.

• Shot velocity

• Shot flow

• Shot size

• Shot hardness

• Conditioning

• Exposure time

Some other parameters to consider are generally associated with the peening equipment used. Air blast systems, for example, have to account for the nozzle speed while centrifugal systems should consider the wheel speed.

Shot Peening Methods

There are two main methods to send shot media to the metal surfaces:

1. Air blast systems eject media through a nozzle directed at the targeted surface using high-pressure compressed air. The shot particles travel through the nozzle at high speeds, effectively targeting the area to be peened at great precision.

2. Centrifugal blast wheels use a high-speed paddle wheel to bombard the metal surface. Shot media is introduced into the centre of the spinning wheel and is propelled towards the targeted area on the workpiece by paddle wheels utilising centrifugal force.

There are, however, other methods available such as laser shock peening, ultrasonic peening, and wet peening, that are used for specific applications.

Shot Peening Applications

Gear Shot Peening

Shot peening is widely used to increase resistance to fatigue, including stress corrosion cracking, cavitation erosion, intergranular corrosion and also resistance to repetitive loading cycles. The compressively stressed zone squeezes the grain boundaries of the metal minimising the amount of fretting, galling, spalling, and wear on the workpiece.

The compression stresses that resist metal fatigue help prevent the propagation of cracks on the material’s surface. Tensile stresses inside the material are not as problematic as the cracks are far less likely to start propagating from the inside of the material.

Many industries such as automotive, aerospace, machinery manufacturing, and medical utilise the shot peening process as it is an economical and practical way to improve the fatigue life of a component. Springs, gears, camshafts, drills, and turbine blades undergo this cold working process to achieve resistance to cyclic loading. Shot peening removes debris and smoothens the surfaces of cylinder heads and engine blocks. Blasting shot media through a liquid or powder may also be used to embed surface coatings onto the material.

Shot Peening vs Shot Blasting

Shot peening and shot blasting are two different methods that have different purposes. These two might seem similar as they both use shot media at high velocities, but the techniques differ in several aspects.

Shot peening is used to improve the mechanical properties of the material by increasing its strength, minimising the risk of failure. The purpose of the shot material is to increase fatigue life, much less than improving its aesthetics. Shot peening may be used in embedding coatings and lubricants to metals to achieve its desired surface coating.

Shot blasting and sandblasting are used in preparation for other surface treatments such as heat treatment and painting. It improves the texture of the material’s surface, removing dents or imperfections that can sometimes arise during production. The media used in a shot blast stream is generally more abrasive than in shot peening. Shot blasting may also use other abrasive shot media such as silicon carbide, copper slag, and aluminium oxide to achieve the desired surface finish.

A part can undergo both processes in the manufacturing procedure if required by the project.

Wrapping It Up

Shot peening effectively improves the fatigue life, strength, and durability of metal components. The induced compressive residual stresses on the material’s surface make it significantly more resistant to corrosion, crack initiation and wear. It is an essential method employed by various industries due to it being precise, fast, reliable, and rather inexpensive.

The post Shot Peening Explained – Definiton, Process & More appeared first on Fractory.

In this article, we will explore this surface finishing technique and its ability to enhance a range of beneficial properties in steel.

What Is Galvanising?

Galvanisation or galvanising, as it is most commonly called, is a metal plating process in which an iron or steel component is coated with a thin layer of zinc. The zinc acts as a protective layer that shields the underlying surface from corrosion and extends its lifespan. The process may sometimes be driven by electricity until a sufficient thickness of the zinc layer is developed. This layer is not discrete, i.e. the zinc merges into the steel at the surface and forms a strong metallurgical bond.

Galvanising is commonly used in places where the metal needs to endure harsh environments or in structural applications where the metal needs to maintain its strength over many decades. The protective coating ensures that the base metal withstands challenging conditions without compromising its critical properties, such as tensile strength, elasticity and durability. The extensive utilisation of galvanised steel can be observed in the construction of bridges, guardrails and steel equipment in chemical plants.

Why Galvanise Steel?

The primary reason for galvanising steel is to increase its lifespan. Regular steel components, if not galvanised, can succumb to corrosion attacks and deteriorate quickly.

Even when embedded in concrete, steel products can corrode due to carbonation (exposure to carbon dioxide), moisture and high chloride levels. Applying a zinc coating improves the corrosion protection of the base metal and thereby enhances its durability.

Galvanising processes also provide the toughest coatings that can resist mechanical damage during transportation, storage and installation. The cost of installation is also low because galvanised components are ready to use upon arrival. Galvanised steel does not require any surface preparation, painting or inspection prior to installation.

Once installed, it provides a lifespan of over 50 years in rural areas and a minimum of 20 to 25 years in urban and coastal areas. The protective coating offers unparalleled protection by also covering sharp corners, recesses and other inaccessible areas. This feature is hard to find in some other coating processes.

Even when the coating is lost at a certain area, it still does not lead to corrosion because zinc has a higher negative reduction potential. This causes the zinc to oxidise instead of the iron, ensuring the safety of the application. To inspect the coating, only the physical condition of the layer is assessed. A close visual inspection is sufficient. If the coating appears intact, we can be confident that no damage has occurred to the steel underneath.

Galvanised steel is generally a cost-effective alternative compared to other methods, except for some large-scale applications. In the construction sector, galvanised steel may become so expensive that builders may abandon the idea of using it altogether when the budget is restrictive. A second disadvantage is that, although the zinc layer is resistant to red rust, it can still develop white rust when continuously exposed to moisture.

Galvanising Methods

There are several galvanising processes available, each with its own advantages and limitations. Let’s take a look at some of the most common galvanisation methods in use today.

Hot-dip galvanising

Hot-dip galvanising is one of the most straightforward and economical galvanising methods. It is capable of coating a variety of complex shapes with relative ease. The surface of the material is cleaned of contaminants and any oxides before being dipped in a molten zinc bath. The bath of molten zinc applies a layer of zinc coating onto the steel component. The quality of the metallurgical bond is determined by how efficiently the surfaces were cleaned beforehand.

Electrolytic galvanising

Electrolytic galvanising, also known as electrogalvanising, is the second most widespread galvanising method. It uses electrolysis to apply a layer of zinc onto a base metal, making it a form of electroplating. In this process, the steel workpiece is placed in a zinc solution with a second electrode and then direct current is passed through it. The current ionises the zinc solution, depositing a pure zinc coating on the base metal. This process provides greater control over the rate of deposition and the layer thickness.

Sherardising

Sherardising is a type of galvanising process named after its inventor, Sherard Osborn Cowper-Coles. It uses thermal diffusion to create a zinc coating on ferrous metals. In this process, the metal to be coated is placed in a container with zinc metal dust. The temperature of the workpiece is increased to 400-450 °C in the presence of this dust. The high temperature causes the dust to diffuse into the metal, forming a zinc-iron alloy.

Pre-galvanising

Pre-galvanising can be understood as a type of hot-dip process adapted specifically for long steel sheets. In this process, the sheet is passed through a cleaning agent that prepares the material for galvanising. Once the metal sheet has been cleaned, it is quickly passed through a molten zinc bath. The rapid movement enables the formation of a thin protective zinc coating that is more uniform than the one achieved in the conventional hot-dip galvanising method. This method provides the fastest turnaround times for sheet metal.

Hot-Dip Galvanising Process

As mentioned previously, the hot-dip process is the most widely used method for galvanising steel components. It provides a reliable method for coating various shapes and sizes, ranging from nuts and bolts to large structural beams. The complexities can range from simple steel rods to artistic pieces with intricate shapes.

In this section, we’ll explain the general hot-dip galvanising process to understand its stages and their purpose. The process can be divided into four major stages:

1. Removal of hard contaminants

2. Chemical cleaning

3. Flux application

4. Immersion in a zinc bath

Removal of Hard Contaminants

When the surface of a workpiece contains hard contaminants, such as welding slag, heavy grease and paint, chemical cleaning is not going to cut it. These impurities must be removed beforehand by using a paint stripper or by mechanical means such as blasting, grinding, etc, as they’ll affect the final quality of the galvanised products. Once the surface is free of any hard contaminants, chemical cleaning will be sufficient to further prepare the part for immersion in a zinc bath.

Chemical Cleaning

In the chemical cleaning stage, all contaminants, such as dirt, grease, scales and oxides, will be removed from the surface of the steel.

To achieve this, several operations take place in sequence. The first step is degreasing, where the component is dipped into a caustic solution, held for a predetermined amount of time, and then rinsed.

After removing the surface grease, the part is immersed in a hydrochloric acid solution to eliminate mill scale and oxides. It is then rinsed once more. This step is also known as the acid pickling process.

Flux Application

Once out of pickling, the part’s exposure to air can lead to the formation of a very thin oxide layer on its surface. This layer can interfere with the adhesion of the zinc coating to the steel component and, therefore, needs to be removed.

To accomplish this, the component is exposed to a chemical flux (typically zinc ammonium chloride) that facilitates the dissolution of the surface oxide into it. The concentration varies from case to case. The flux usually also contains wetting agents, anti-foaming agents and viscosity adjusters. The flux is allowed to dry before being dipped into liquid zinc.

Immersion in Zinc Bath

Immersing steel parts in a tank of molten zinc

After applying flux, the openings for venting and draining on the component are checked before the actual dipping process. The air will exit through the vent opening, while the molten zinc will enter through the drain opening.

Once they are found to be satisfactory, the part is cleared to go through the hot-dip galvanising process. The component is immersed in a bath of molten zinc at 450°C. The bath cannot be heated too much, as excessive temperature can cause the chemical flux on the component to burn away. The component is held in place until its temperature becomes equal to that of the bath.

The immersion creates layers of zinc-iron alloys with different compositions. There is no clear demarcation between the layers, and the steel component gradually transitions into a pure zinc coating on the surface.

Upon successful coating, the component is cooled down in a quench tank to prevent exposure to air.

Wrapping It Up

Galvanisation is an excellent method for providing low-cost and effective protection to steel and other ferrous metals. The formed coating is strong, corrosion-resistant and durable. As a result, the process has found widespread usage across various industries, ranging from the production of fasteners to the construction of entire stadiums. Thus it is important to ensure that all the necessary steps of galvanisation are carried out properly to achieve a reliable finish and desired performance.

The post Galvanising Explained – How It Works, Types & More appeared first on Fractory.

Instead, black oxide finish converts the surface layer of a part into one that boasts a great appearance, exhibits greater corrosion resistance and is non-reflective. The process is also often referred to as blackening, black passivating, oxidising and gun bluing.

What Is Black Oxide?

Generally, black oxide or black iron oxide refers to the oxide of iron known as magnetite (Fe3O4) that is formed when a ferrous metal is exposed to special oxidising chemicals. It is a type of conversion coating, thus the surface layer of a metal is formed into its oxide through conversion. The layer thickness on a finished part is between one and two micrometres.

The thin layer or coating along the part’s surface provides advantages such as greater corrosion protection, an attractive look and a smooth finish. As a result, many applications such as automotive parts use the black oxide coating process.

The blacking process works for some non-ferrous metals, such as zinc and copper, as well.

When Is Black Oxide Coating Used?

Black oxide coatings are used to increase corrosion and wear resistance while complying with high tolerance requirements. One of the advantages this process offers is the minimal buildup which means that the overall dimensions of the part only increase by a miniscule amount.

It is used on parts that require a certain degree of abrasion resistance as the black oxide does not wear away as easily as some other finishes. Black oxide parts are often used in the automotive and aerospace sectors in addition to being used in producing hand tools and other hardware. It is used to coat fasteners as black oxide finish improves their grip and corrosion protection while making them aesthetically pleasing.

How Does the Black Oxide Coating Process Work

The black oxide coating or blackening process is a chemical reaction in which the outer surface of a metal reacts to chemical solutions and therefore undergoes transformation.

The process is divided into three main types:

• Hot black oxide process

• Mid-temperature black oxide process

• Cold black oxide process

Hot black oxide process

The hot black oxide coating process takes place at an elevated temperature of 141 °C (286 °F) and converts the surface of the ferrous material into magnetite (Fe3O4). Magnetite by nature is a black, opaque and magnetic material and thus, the surface turns black.

The entire process consists of seven steps::

1. Surface cleaning

2. Rinsing

3. Acid pickling

4. Rinsing

5. Black oxide chemical bath

6. Rinsing

7. Sealing

Surface cleaning

Cleaning the surface is the first step. A suitable alkaline detergent ensures the removal of contaminants such as grease, dirt, dust, rust, scales, fats, oils and even microorganisms. The alkaline solutions also do not stick to the surface and rinse off with ease.

Rinsing

A simple water bath is used to rinse the detergent. With the detergent, the emulsion of all the contaminants such as soil deposits also leaves the surface.

Acid pickling

Acid pickling is used to remove oxide films and rust stains. Technicians may skip this step if the part does not have the above-mentioned issues.

Rinsing

The part is rinsed again to remove the residues of acid from the previous step.

Black oxide chemical bath

The parts are then dipped into a hot bath that contains specific salts of sodium hydroxide (caustic soda), nitrates and nitrites. These blackening compounds convert the top layer of the part into magnetite Fe3O4.

Ion chloride solution (FeCl3) can also be used as the blackening compound. A 50% black oxide solution is generally sufficient.

The extent of blackening depends on the dipping duration. The blackening is quick at first but deeper shades require progressively longer times.

Rinsing

After achieving the desired shade, the parts are removed from the bath and rinsed once again to remove any compound residues. 

Sealing

This is an additional step that ensures greater corrosion resistance. The part is exposed to a sealant such as oil that fills the pores in the black oxide surface and seals it by ‘sinking’ into it. The oil film also gives a glossy appearance to the part. A matte finish is achieved by replacing oil with clear wax. The sealant takes a while to dry completely.

Upon completion, the metal part will have an attractive black finish. Bearings are frequently blackened using this process.

The hot black oxide coating process is highly suitable for large batches. Automated part carriers can immerse parts for pre-determined durations in different baths.

It is the most popular type of blackening process. But it carries the risk of steam explosion as the process takes place above the boiling point of water.

Mid-temperature black oxide process

The blackening process can also be carried out at temperatures below the boiling point of water. When it is carried out between 90 °C and 120 °C (194 °F – 248 °F), it is known as the mid-temperature black oxide coating process. 

This process does not produce caustic fumes which are highly toxic to humans and, in extreme cases, can even lead to fatalities. 

Cold black oxide process

The cold black oxide process takes place at room temperatures between 20 °C and 30 °C (68 °F – 86 °F). Unlike the previous two processes, this one does not include surface conversion to an oxide.

The cold process deposits copper selenide on the surface. This compound also has the characteristic black colour and produces a finish similar in appearance to the previous two processes.

However, the film is softer and can come off easily. The addition of a sealant is required to improve the corrosion resistance property just as the other two processes.

Other materials

The above processes work well with iron-based metals such as cast iron and carbon steel. However, some other materials can be blackened also. Let’s quickly dive into a few of those methods.

Copper

The black oxide process for copper converts the surface into cupric oxide and is called Ebonol C. A high-quality layer is created that can handle temperatures up to 200 °C (392 °F) with ease.

As with carbon steels, this layer requires sealing through oil, lacquer or wax for increased corrosion resistance.

Zinc

The blackening process for zinc goes under the trade name Ebonol Z. In this process, the zinc part is immersed in an alkaline solution at temperatures between 72 °C – 82 °C (160 °F – 180 °F).

The process imparts a dark black finish with mild corrosion resistance properties.

Stainless Steel

Black oxide is often used to eliminate the reflective nature of stainless steel in surgical instruments, for example. This reduces the mistakes that a surgeon could possibly make as the light does not reflect back to them during a procedure.

For stainless steel, the mid-temperature process works best. It takes place at 93 °C to 98 °C (200 – 210 °F) and provides a surface finish at par with hot black oxide. It works for 200, 300 and 400 series stainless steel.

Benefits

Black oxide coating is an intermediate process between protective coatings and plating processes. It provides many of the advantages of plating processes without being as expensive or difficult:

An attractive look

One of the most popular reasons why people opt for a black oxide finish is because of its aesthetic appearance. Blackening provides a clean and uniform surface without any spots. Either a glossy or matte appearance can be achieved by choosing the appropriate post-treatment sealant.

Affordable

Blackening is more affordable than electroplating and even paint. Cold black oxide process can even be carried out at home with regular equipment such as plastic containers.

Low dimensional changes

The part size grows due to oxide formation. However, this growth is minimal compared to other processes such as painting and electroplating.

The black oxide finish typically has a thickness of one to two micrometres which is almost negligible and does not affect functionality in most cases. Thus, the process is suitable for parts with tight tolerances.

Excellent surface finish

The black oxide process imparts a superior finish to the part. In addition to being smooth, the finish also takes on a protective role.

Minimises light reflection

Minimising light reflection can be a necessary property in many applications such as surgical tools and parts in radiation applications (UV and IR sensors and detectors).

By making the part less reflective and easy on the eyes, control and precision are improved in laparoscopic surgeries.

Improved corrosion resistance

The black oxide coating improves corrosion resistance to some extent provided a sealant is used to close the pores. However, the parts might still corrode in harsh environments.

In the case of stainless steel, the corrosion protection is enhanced further as the steel retains its corrosion-resistant property besides the protection offered by blackening.

Anti-galling properties

The black oxide coatings also impart anti-galling properties. This means that it prevents adhesive wear between parts when subjected to sliding contact. 

This allows for a smoother break-in of mating parts. The black oxide coating is sacrificed but the surface becomes work-hardened, preventing further wear.

High lubricity

The black oxide layer has a low friction coefficient, especially if treated with wax/oil at the end. 

Abrasion resistance

The black oxide layer also provides a certain degree of wear resistance. It does not peel off easily and is harder than the substrate in some applications. The enhanced hardness helps it deal more effectively with abrasive materials.

Limitations

The following limitations may prevent the use of black oxide coating for some purposes:

Mild corrosion resistance

The black oxide coating does not provide exceptional corrosion resistance. There are other cheaper alternatives if corrosion resistance is the main objective.

Layer erosion

Black oxide coatings have lower durability than other finishes and may get eroded, exposing the substrate to harsh conditions, especially hot environments. Moreover, the discolouration of the oil post-treatment to a dirty brown appearance can make the part look like it is rusting prematurely. The coating is also difficult to repair once damaged.

This is the reason why it’s not recommended to use black oxide coating for fasteners that will be securing an engine, for example.

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Processes such as grinding, abrasive blasting, or wire brushing are often used to achieve these objectives. However, these processes can be less accurate and may either remove excessive or insufficient amounts of metal at times.

In this article, we shall explore an alternative, a more reliable surface finishing process known as pickling, which is commonly used to remove scale and rust from metal surfaces. Let’s begin. 

What Is Metal Pickling?

Metal pickling is the process of cleaning contaminated metal surfaces with specific chemicals before further processing or coating. It is most commonly performed after hot working processes since they tend to leave a discolored oxide layer on the surface. To remove this scale, the workpiece is immersed in acid baths. Thus, prior to cold rolling operations, hot rolled steel usually goes through a pickling line to remove surface scale.

Different metals and applications use various acids but the most common ones are hydrochloric acid and sulphuric acid. Since the chemicals used are commonly known as pickle liquor, the process came to be known as pickling. It is sometimes referred to as acid washing if descaling is not needed.

The metal parts are exposed to the pickling liquids through immersion bath, spray, or application with a brush. In the case of pipes that need to be pickled internally, the pickling liquid is pumped through them.  

Pickling is most often performed on ferrous metals, noble metals, copper and aluminium alloys. Pickled metal has various benefits such as an attractive surface, increased durability, increased corrosion resistance, and removal of heat-affected zones.

Pickling Process – How Is Steel Pickled?

The pickling process is pretty straightforward and it can be divided into six simple stages:

1. Pre-cleaning

2. Pickling solution preparation

3. Pickling

4. Rinsing

5. Neutralisation and rust inhibitor application

6. Drying

Pickling of Hot Rolled Steel Coil

Pre-cleaning

The precleaning step refers to cleaning the material using degreasing or caustic solutions. Surface impurities such as loose oil, dirt and any other contaminants must be removed during this step.

Pickling solution preparation

We typically use diluted hydrochloric or sulfuric acid for the pickling process. The concentration and temperature vary for different metals and the level of pickling required. You can find the ideal concentrations by referring to the project requirements or manufacturer’s guidelines.

The solution must be prepared with extreme caution. Acid should always be added to the water, and not the other way around, to prevent splattering of concentrated acid.

Pickling

In this stage, we immerse the carbon steel in a bath of hydrochloric or sulphuric acid if the carbon content is less than 6%. If the carbon content is higher, the steel must first be pickled with hydrofluoric, nitric or phosphoric acid.

The material is submerged fully in the solution for a predetermined amount of time. The duration depends on the level of contamination and the type of metal. The range can be from a few minutes to a few hours. The solution must be stirred from time to time.

A small amount of material (1 to 3% of steel mass) is removed from the surface by chemical reactions, freeing the material of iron oxides. The steel is removed from the solution once sufficient cleaning levels are achieved.

The left-over pickling liquor, now contaminated with rust flakes and iron oxide dust, is called pickling sludge. It is possible to recover some of the acid and ferric oxide through regeneration but the rest must be disposed of properly as it is classified as hazardous waste.

Rinsing

The steel surface must be cleaned of all the acid by rinsing it with clean water. All traces of the pickling solution must be removed before proceeding with further processing.

Neutralisation and rust inhibitor application

In some cases, the part is neutralised using an alkali solution to thoroughly cleanse the surface and remove any traces of the pickling agent. A commercial neutraliser or baking soda, is suitable for this purpose. Alkaline solutions are also supplemented with rust inhibitors to enhance the corrosion-resistant properties of the part.

Drying

After rinsing and neutralising, the part is dried before considering the process complete. Once this step is completed, the part is ready for additional processing or coating, as required.

Acids Used for Pickling

The most common acids used for steel pickling are hydrochloric acid and sulfuric acid. These chemicals also work for a variety of metals other than steel.

Hydrochloric acid is more common now, but previously, it used to be sulphuric acid. Sulphuric acid is more affordable than hydrochloric acid, but it is unsuitable for fast steelmaking lines due to its requirements for longer contact durations. Automatic steel mills can produce hot rolled steel at rates of up to 243 m/min, which necessitates fast pickling times.

Hydrochloric acid is also more efficient at removing scale formation and has reduced heating costs since the process can occur at room temperature. It also has less hydrogen penetration through diffusion and reduced depositions of iron salt on a pickled part. However, hydrochloric acid can be difficult to recover and it also starts to fume at slightly elevated temperatures. It is also more corrosive and harder to dispose of than sulphuric acid.

Sulphuric acid, besides being more affordable, has the advantage of being able to manipulate the pickling rate by adjusting the process temperature. Increased temperatures allow for lower acid concentrations to pickle more effectively. Iron sulfate formed during pickling with sulphuric acid is easier to recover, and the acid can be renewed more frequently.

But sulphuric acid attacks the base metal more aggressively. It also promotes deeper diffusion of hydrogen. The pickling residues are also more adhesive and the process almost always requires heating.

Carbon steels with a carbon content greater than 6% (cast iron) are initially pickled using phosphoric acid, nitric acid, and hydrofluoric acid before being treated with a hydrochloric or sulphuric acid solution. Nitric and hydrofluoric acids are also suitable for pickling corrosion-resistant and acid-resistant chromium-nickel steels.

Phosphoric acid is the best option for removing a thin film of iron oxide or iron scale. The process is also relatively expensive. However, the use of phosphoric acid leads to the formation of a thin layer of iron phosphate, which immediately passivates the metal.

Copper alloys are typically pickled using dilute sulphuric acid, while brass is best pickled with a mixture of concentrated nitric and sulphuric acid, along with soot and sodium chloride.

Benefits of Pickling

Pickling of materials is done for the following advantages:

1. The materials are cleaned of all surface impurities, such as rust and scale. The end result is a smooth and clean surface finish.

2. The process removes heat tint from previous operations. The final product has a uniform colour that is achieved through very few finishing processes.

3. It improves the corrosion resistance of the material, as is the case with stainless steel.

4. Less abrasion of the metal surface compared to the mechanical method. Chances of particle embedment are also eliminated.

5. Enhances the surface’s appearance. Jewellery is often pickled to remove the unattractive copper oxide layer from the surface.

6. Pickling is a simple, affordable and relatively easy process.

7. We can recover valuable products from the pickling sludge by sending the waste mill scale to a sintering plant, where it can be transformed into a solid steel mass.

Limitations of Pickling

1. It is difficult to create consistent results as the acid becomes weaker over time. The amount of material removed varies over time as the acid bath degrades.

2. The pickling sludge is a hazardous waste. Strict EPA regulations around this waste material limit its use to some extent.

3. The handling of hazardous pickling chemicals is a significant safety concern in the pickling process.

4. In some cases, hydrogen diffusion can result in hydrogen embrittlement, causing steel to become brittle and weak, and negatively impacting the material’s physical properties.

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