Now, let’s delve deeper into copper corrosion.

Does Copper Rust or Corrode?

Corrosion is a natural process that occurs as metals react with the atmosphere, chemicals, or other specific conditions. This transformation leads to a different appearance along with changes to the metal’s mechanical properties and a weaker structural integrity. Copper forms a reddish-brown cuprous oxide layer during its electrochemical reaction with the environment.

Rust forms when metal alloys containing iron undergo the oxidation process. However, copper is a non-ferrous metal, meaning it doesn’t contain iron. As iron content is a prerequisite for the formation of rust, copper certainly does not rust, it corrodes or oxidises as oxygen molecules land on its surface and combine with copper atoms to form copper oxide.

Unlike iron oxide, copper oxide does not disintegrate over time. It forms a protective film on the surface of the copper which gradually thickens until it becomes copper carbonate. This new layer of material, called patina, serves as a shield that preserves the unspoiled copper inside. What’s more, damaged patina regenerates itself.

Copper corrosion is a slow process, especially in unpolluted environments. Therefore, it takes months or even years for the surfaces to tarnish and gradually turn dark brown or black and finally into a distinctive blue-green colour.

The formation of patina can be forced, as for some applications a specific look is often desired while no one has the time to wait for the copper to achieve this look naturally. This is achieved by treating the copper surfaces with various chemicals or corrosive agents, such as ferric nitrate, sodium thiosulfate, and sulfuretted potash. By using different methods and exposing the copper to different temperatures and moisture levels, various shades and colours can be obtained.

Some applications of copper are most efficient when the patina is removed altogether and copper is in its cleanest form. An example of this would be copper wires, which exhibit their most electrically conductive state without the patina. Wax coating, polishing, and solutions will seal copper from corroding agents, preventing it from oxidising and tarnishing.

Conditions That Contribute to the Corrosion of Copper

There are specific conditions that promote or accelerate copper corrosion. These include:

1. Exposure to environmental conditions that contain saltwater, heat, or acidic compounds deteriorate the copper surface.

2. Induced direct or alternating currents flowing in the soils accelerate the corrosion rate for underground copper pipes.

3. Galvanic corrosion takes place when dissimilar metals are in contact with copper. An example of this is a copper pipe in contact with a steel pipe, where differences in electrical conductivity promote corrosion. The easiest way to prevent galvanic action from taking place is to insulate copper from other metals.

4. Abnormally aggressive soils can facilitate corrosion in copper when it has high concentrations of chloride, sulfate, ammonia compounds, and moisture.

5. Contact with high amounts of organic and inorganic acid deteriorates the metal surface of the copper, removing the protective film.

6. Corrosion fatigue may occur from constant stress applied to the ductile copper metals. High velocity and turbulent water flow inside copper tubes may create localised erosion and corrosion. Periodic contraction and expansion of copper tubes induces stress which promotes fatigue.

7. Elevated levels of oxygen atoms present in the environment corrode the metal’s surface through accelerated oxidation.

Copper Corrosion Example – Statue of Liberty NYC

A great example of copper corrosion can be seen on the Statue of Liberty in New York. Erected in 1886, the statue was originally shiny brown, but it took about 10 years of exposure to the natural environment by the water in New York for its colour to turn to a bluish-green patina. Another 15 years later, the patina was full-blown.

There were even politicians who suggested that the statue be painted shiny brown again, but luckily the wider public didn’t like this plan one bit. Today, the blue-green look is loved by everyone. Some have argued that the statue should be polished every 50 years, so every generation could relive the gradual tarnishing and change in colours. However, this idea isn’t practical. As the statue is only about 2.4 millimetres thick, it would end up being too thin after a few cycles, and soon there would be no statue left.

Corrosion had an effect on the statue’s original design, which had a combination of an iron skeletal structure with copper skin. Galvanic corrosion occurred between the two elements, with rainwater acting as an electrolyte. The iron structure was applied with a zinc coating while the corroded parts were replaced with stainless steel.

Major restoration required 8,000 square feet of copper sheet to replace parts, such as the roofing and torch, while the torch flame was replaced with solid copper and gold leaf. The replacement torch was pre-patinised to match the rest of the statue before being mounted in 1986. The artificial patina wore off within a few years, exposing the dulled copper. It took over twenty years for the dull brown copper to develop its own patina.

The Effects of Corrosion on Copper Alloys

Copper is commonly combined with other metals, as it is an extremely malleable and ductile metal element. Common alloys of copper include bronze (88% copper, 12% tin) and brass (66% copper, 34% zinc with some traces of iron and lead).

Copper alloys are different compared to pure copper and corrode differently from how pure copper corrodes. A copper alloy may turn a different colour than green as it undergoes corrosion. For example, brass takes on a golden brown colour while bronze may turn lime green to dark brown.

The corrosion behaviour between copper alloys varies depending on their physical and chemical properties, environment, stress, and other factors.

Copper alloys demonstrate exceptional corrosion resistance under particular conditions. Here are a few examples:

• Aluminium brass is highly resistant to impingement corrosion from high-velocity saltwater.

• Aluminium bronze is resistant to chemical attacks from sulphite solutions.

• Copper-silicon alloys offer substantial resistance against stress-corrosion cracking compared to brass.

• Nickel silvers offer excellent protection against corrosion from freshwater and saltwater exposure.

The post Copper Corrosion Explained appeared first on Fractory.

Let’s delve into the nature of corrosion, the types of corrosion and how to combat their effects.

What Is Metal Corrosion?

Metal corrosion occurs when the metal surface reacts with a corrosive environment or is subjected to other unfavourable conditions that cause the surface to corrode. Oxidation or rust formation occurs on the exposed surface of the metal and causes the material to weaken over time, leading to structural damage. Corrosion is not only specific to metals, it can occur in other materials, such as polymers and ceramics, for example. However, for these materials, the term degradation is more common.

Corrosion is a natural process that converts a refined metal to a more stable metal oxide. An electron exchange occurs between the metal and the environment, with the metal losing electrons in the process. Metals corrode naturally over time, but the type of environment that the material is subjected to can accelerate the corrosion process. Stable metals, such as gold and platinum, are less likely to corrode due to their stable chemical nature. Alloys that contain certain elements, such as iron, are more prone to rusting when exposed to air and moisture.

Localised corrosion forms cracks and pits, however, corrosion may span over the entire surface of a metal. Corrosion in metal surfaces is influenced by a variety of factors, such as acidity levels in the environment, temperature, mechanical stress, and humidity. These factors may either induce accelerated deterioration of the metal surface or rapidly speed up the corrosion process, ultimately leading to failure.

Protective coatings, galvanisation, and heat treatment are general methods for reducing the risk of corrosion. However, corrosion may still occur under specific conditions due to its complex nature.

Types of Corrosion

There are several types of corrosion that are all influenced by a variety of mechanisms and conditions. Let’s explore each type and understand how they occur.

General Corrosion

General or uniform corrosion is the most common type of corrosion as it occurs across the surface of a metal. This is commonly caused by the absence of a protective coating, leaving the metal exposed to corrosive agents. Electrochemical and chemical reactions occur which cause the metal to dissolve, allowing it to thin out as it forms oxides.

Continuous exposure to these corroding agents will eventually dissolve the whole metal structure. Examples of metals affected by uniform corrosion are aluminium, iron, lead, steel, and zinc. This type of corrosion is predictable and detectable to the naked eye as it is visible in the form of rust over the entire surface.

Pitting Corrosion

Pitting corrosion is an unpredictable type of localised corrosion wherein rust pits form at the metal surface. These pits grow into cavities or holes, penetrating the surface in a downward direction. This type of corrosion is caused by structural defects, poor coating application, non-uniformities, moisture, or damage to the protective oxide layer of the metal. Early stages of pitting corrosion have pit diameters of ≤20 µm.

Pitting corrosion is an insidious type of corrosion since only a small amount of material on the surface is lost while the deep metal structure is damaged. Failure due to pitting can be sudden and is often devastating. Pitting can be observed in aluminium, nickel alloys, and steel. Polished metal surfaces are more resistant to pitting due to their more uniform surface area.

Crevice Corrosion

Crevice corrosion is another type of local corrosion that occurs at the crevices or confined areas, usually between two metals (commonly found in assemblies), a metal and nonmetal, or from debris. These restricted areas usually allow a buildup of corrosive fluids while only having a limited supply of oxygen which prohibits shielding of the material. The imbalance in the pH level turns the fluid within the crevices acidic, which in turn breaks down the passive oxide layer, leaving the surface vulnerable to corrosion attack.

The risk for crevice corrosion can be reduced through a better assembly or a joint design where gaps are eliminated between welds and joints. Protection from moisture or fluids which promote an electrolytic environment may further reduce the risk of this form of corrosion.

Galvanic Corrosion

Galvanic corrosion or bimetallic corrosion is a type of corrosion wherein two dissimilar metals in physical and/or electrical contact are subjected to an electrolytic environment. The active metal (anode) undergoes corrosion at a faster rate than the other metal (cathode) which is more stable. It can also occur in a metal that is exposed to an electrolyte with varying degrees of concentration.

Galvanic corrosion is common in sea vessels where saltwater acts as an electrolyte between the two dissimilar metals. It can also occur in toxic environments, such as in handling molten metal or in chemical laboratories.

A sacrificial anode or galvanic anode that is more active than the metal workpiece is used to prevent/lessen galvanic corrosion. These consumable anodes prevent the main metals from being oxidised by supplying their electrons, corroding the consumable anode instead.

In the case of copper, when connected to a more anodic metal in the presence of an electrolyte, copper corrosion can be significantly reduced, as copper acts as the cathode in the galvanic series.

Fretting Corrosion

Fretting corrosion occurs when two metals in contact undergo small movements caused by slips and vibrations. These oscillations lead to fretting that removes the protective oxide film and allows surface asperities on freshly exposed metals to stick to one another. This connection is subsequently broken again by the vibrations, causing the build-up of wear debris.

This debris and the freshly exposed metal surfaces are prone to oxidation. Since the particles cannot escape the contact, they initiate further abrasive wear and subsequent oxidation, and the process continues with elevated wear volumes.

Reactive soft metals have lower corrosion resistance since the oxide layer is easy to remove under cyclic loading and material transfer takes place.

Intergranular Corrosion

Intergranular corrosion is a form of corrosion that occurs at or adjacent to the metal’s grain boundaries. Grain boundaries act as an interface between grains in the material and are considered imperfections in the material’s crystal structure. These grain boundaries result from uneven growth or impurities present as the metal alloy crystallises. Each grain can vary from 1 µm to 1 mm in size.

Intergranular corrosion or intergranular attack exhibits corrosion when the grain boundaries have a difference in reactivity to the grain. It can occur when metals are subjected to high temperatures (carbide precipitation) or due to defective welding. This lowers the material’s corrosion resistance, making it prone to damage when exposed to a corrosive agent.

Erosion Corrosion

Erosion corrosion is a type of corrosion that is caused by relative movement between the metal surface and a corrosive liquid. This movement results in mechanical abrasion which damages the surface and creates cavities.

The fluid usually flows at high velocities along the metal surface, dissolving the passive oxide layer and removing it as the fluid travels. Erosion corrosion occurs mostly inside metal tubes which are used to transfer corrosive liquids that slowly deteriorate the surface.

High Temperature Corrosion

High-temperature corrosion or hot corrosion occurs when a combination of high temperature (< 400°C) and atmospheric contaminants is present. A chemical attack occurs as the metals operating under these temperatures chemically react with the corrosive contaminants.

High-temperature corrosion is common in industrial environments where furnaces and gas turbines that have atmospheric contaminants and sulfuric gases are used. These corrosive contaminants might also leave ash deposits and molten salt while under operation. Cooling mechanisms and heat-resistant alloys are utilised to prevent high-temperature corrosion.

Stress Corrosion Cracking (SCC)

Stress corrosion cracking or corrosion fatigue is caused by applying tensile stress to the material while it is in a corrosive environment. Stress corrosion cracking can be especially prevalent when tensile stress is accompanied by temperature extremes. Metal expansion and contraction result from these temperature changes which weaken the structural integrity of the metal.

Initial signs of stress corrosion cracking are fine cracks on the metal surface. These cracks eventually develop over time, leading to structural failure. Stress corrosion can occur in manufacturing processes, such as machining and welding, but it is accelerated when exposed to a corrosive environment. Examples of this are stainless steels in a chloride environment and copper alloys in ammonia.

Microbial Corrosion

Microbial corrosion, also called microbiologically induced corrosion (MIC), is a type of corrosion caused by the presence and activities of microorganisms on metal surfaces. These microorganisms, which include bacteria, fungi, and algae, can accelerate the corrosion process in metals and alloys in various environments.

MIC is a significant concern in industries like oil and gas, marine, and wastewater management because it can lead to the rapid deterioration of materials. Some of these organisms are capable of consuming oil and excreting acids that can cause corrosion of vessels used for storing.

Microbiologically influenced corrosion can cause various types of damage, including pitting, crevice corrosion and stress corrosion cracking. To prevent this, the oil must be purified as much as possible to remove water content. Draining water at regular intervals from fuel tanks after purification is also necessary. Effective management of MIC also involves regular monitoring and using biocides to control microbial growth. Understanding the specific microbes and environmental conditions involved is key to developing targeted prevention and control strategies.

How to Prevent Corrosion

Preventing different corrosion types is crucial in maintaining the functionality and longevity of assemblies and equipment. Applying surface treatment, protective coatings or material finishing are great proactive measures to reduce the impact of corrosion.

In the realm of wastewater management, corrosion-related risks are minimised by using systems like a moving bed biofilm reactor (MBBR System). These innovative systems utilise biofilm carriers that provide robust wastewater treatment and enhance the durability of submerged system components by reducing the prevalence of microorganisms.

But the simplest measure of them all against most types of corrosion is using corrosion-resistant materials in a non-corrosive environment.

The post What Is Corrosion & the 10 Most Common Types appeared first on Fractory.

At first, this description may sound similar to galling but these two processes have fundamental differences. Galling generally refers to the wear damage associated with gross relative motion on a much larger scale than the small amplitude relative motion associated with fretting contacts.

Let’s discover the reasons behind the fretting process, and how to protect mechanical systems from this type of wear.

What Is Fretting Wear and When Does It Occur?

Fretting is a type of wear that stems from low-amplitude oscillating motion or vibration under high contact pressure. These small cyclic movements and micro-sliding generate stresses on the contact surface, degrading the metals over time.

Fretting is a combination of adhesive and abrasive wear. The oscillatory motion causes fatigue wear which is enhanced by the adhesion of contact surface asperities. This bond is short-lived though, as it will be broken shortly after by the small movements, resulting in wear debris.

If the debris and/or surface subsequently undergo a chemical reaction, mainly oxidation, the mechanism is termed fretting corrosion. Fretting leads to increased surface roughness and micropits, which further reduces the fatigue strength of the components.

Many machine components and systems are exposed to small movements causing fretting. These small oscillations can be from parts intended to move by design (e.g. camshafts and leaf springs) or not intended to move (e.g. bolted flanges, riveted lap joints and keyways) and caused by various external factors.

The relative sliding motion has a small amplitude, generally ranging from micrometeres to millimetres but in some cases, it can also be measured in nanometres. Fretting can cause serious damage even with a small friction path.

Smooth surfaces are more prone to fretting wear since the stress at the contact points between the mating surfaces is amplified.

The term fretting wear was considered to be interchangeable with fretting corrosion. Interestingly, fretting wear can also occur in a vacuum (space mechanisms for example) and in materials that do not oxidise, such as gold and platinum. This is why it’s important to cover the terms fretting corrosion and fretting fatigue separately, although these phenomena most often appear in tandem.

Fretting Corrosion

Fretting corrosion develops due to the removal of protective oxide layers on the metal surfaces, exposing fresh metal to corrosive elements. Wear debris created by the constant sliding motion accelerates the corrosion process since the particles cannot escape contact, causing abrasive wear and subsequent oxidation of the freshly exposed metal, continuing the process and leading to elevated wear volumes.

Corrosion depends on the material’s inertness. In the case of steel, iron oxide is harder than the steel itself and thus it acts as an abrasive and causes a lot of damage to the surfaces.

Fretting corrosion may occur in metals such as steel, aluminium, cast iron, and other non-ferrous metals but also in polymers and ceramics. The color of debris particles for fretting corrosion appears different from normal corrosion. Aluminium corrodes to a white color under normal conditions and turns black with fretting corrosion, whereas steel becomes gray and reddish brown respectively.

Examples of fretting corrosion involve wind turbine pitch bearings (false brinelling) wherein the wear mechanism oscillates and orthopedic implants when the two materials in contact are in relative motion with each other.

Fretting Fatigue

The contact area of the two materials is affected by cyclic friction stress, resulting in fretting fatigue. Fatigue cracks start initiating in the fretting zone, propagating further into the material. The surface of the metal has a high contact load and maximum friction stress whereas the inside of the metal has lower stress values.

Thus, fretting fatigue is different from plain fatigue failure. There is, however a correlation between the two as fretting fatigue life is estimated from plain fatigue life data. Additionally, fretting fatigue strength is usually half or less than its plain fatigue strength.

Surface hardness plays an important role in fretting fatigue. Contact surfaces of hard metals result in cold welding of their asperities, subsequently shearing and generating wear debris (fretting corrosion). Whereas if a soft and hard metal are in fretting contact, fretting fatigue wear is more likely to occur. The asperities of the harder metal will indent the softer metal, resulting in plastic deformation to the softer metal and eventually material loss.

An example of fretting fatigue is seen in mechanical joints from aircraft engine blades wherein catastrophic mechanical failure may occur if left unchecked.

Factors That Affect Fretting

1. Load – The magnitude and position of the load are some of the key factors that promote fretting. Uneven loading on mechanical parts may lead to concentrated local stress at specific areas on the metal surface.

2. Environment – The consequence of subjecting the mechanisms to poor temperature and relative humidity is fretting corrosion from oxidation. It’s highly likely that a high wear rate occurs when metal surfaces are in a corrosive environment or around corrosive agents.

3. Material Properties – Ductility, plasticity, surface roughness, and inertness are some properties that affect how surfaces react under load and contact.

4. Motion – Small movements from the sliding amplitude and number of cycles between two metal surfaces affect the amount of fretting wear.

5. Surface – Fretting behavior is greatly influenced by the material’s surface finish, coatings and lubrication.

How to Reduce Fretting

Torsonial Fretting Wear Test

1. Avoiding fretting altogether – This can include design changes, dampening any vibrations, and making certain that all joints are properly tightened.

2. Fretting systems testing – Trial runs may be used in a controlled environment to address fretting issues in specific mechanisms.

3. Coatings & lubricants – Coating and lubrication add another layer of protection from friction, improving fretting resistance. Steel for example is susceptible to fretting if paired with steel. It can be paired with steel coated in cadmium, indium, lead, tin, or silver to reduce fretting. However, regarding lubricants, they might get squeezed out at the contact area, still resulting in metal-to-metal contact. In some cases, lubricants can even have the opposite of the desired effect as a lower friction coefficient may lead to more movement.

4. Material hardness – Hardness generally influences the friction coefficient of the material. To improve the material’s hardness, surface and heat treatments can be used. Another common method is shot peening which is often performed during repairs, improving strength and reducing stress.

5. Metal selection – It is sometimes optimal to use two materials with different ductility or ‘softness’. A pair of soft and hard metals are proven to have less fretting damage compared to two hard metals since the softer metal ‘flows’ rather than ‘rubs’ when in contact.

6. Environment – Favorable conditions such as a controlled environment during operation and proper storage reduce unnecessary moisture to the material, reducing oxidation.

7. Loose metallic inserts – Some instances use loose metallic inserts to avoid fretting or surface contact such as using thin copper plates for titanium surfaces.

8. Rubber – If the application allows for it, vibrations can be absorbed by rubber while preventing slip zones at the same time.

The post Fretting Explained – Definition, How It Works & Prevention appeared first on Fractory.

This type of adhesive material wear poses serious risks as it may lead to costly maintenance from repairs or equipment failure. Thus, unchecked galling can in turn lead to unwanted production delays, using up time and resources significantly.

Let’s delve into the causes, effects, and preventive measures for galling.

What Is Metal Galling?

Galling occurs when two surfaces in relative motion start sticking to each other through molecular forces, eventually resulting in accidental cold welding. It is quite a common problem in metal forming, hydraulic cylinders, bearings, engine pistons, threads, and other applications where metal surfaces are in sliding contact. Metal galling is especially prevalent when there is insufficient lubrication between the surfaces.

This adhesive wear typically occurs when subjecting the materials to high loads and slow sliding speeds, but it also occurs in high-speed applications with little loads. Add temperature and poor lubrication into the mix and the galling effect only gets stronger.

Some metals are more prone to galling than others, aluminium and austenitic steel for example. Typically, softer materials gall easier and harder materials are more resistant. Properties that affect the material’s ability to gall are plasticity and ductility. Galling can happen regardless if the metals in contact are the same or different. Brass and bronze are preferred for situations where sliding occurs (bearings, bushings) as they are rather resistant to galling.

Stainless Steel Thread Galling

Stainless steel, aluminium, titanium and some other metals possess corrosion resistance from the formation of the passive oxide layer. On the downside, it renders them particularly susceptible to galling when this oxide layer is damaged or swept away under high contact forces and the bare reactive material is exposed.

Thus, galling is quite a big problem for threaded stainless steel and aluminium bolts. Galling leads to freezing of the bolt threads, and applying further tightening force may simply shear off the head of the bolt, or strip the threads.

In metalworking processes, mainly in turning, milling, punching and bending, galling refers to the transfer of the work material onto the cutting tool, resulting in the formation of a lump. This lump alters the interaction between the tool and the workpiece, typically leading to greater adhesion and resistance to further cutting. The vibrations generated by this process in turning and milling operations can often produce a distinctive sound that machinists recognise.

How Does Galling Work?

Metal surfaces are visibly smooth to the naked eye but have imperfections such as impurities or voids on a smaller scale. The microscopic high points (asperities) of the contact area between two materials rub against each other, generating heat and friction. Local stress occurs in the contact area and lumps begin to form, continuing to grow while penetrating the protective oxide layer.

The high energy density in the contact area forms bonds and induces electron transfer between the microscopic fibers of the two metal surfaces. This fuses down the contact zone, undergoing cold welding from the metal’s natural plastic behavior. Galling involves visible transfer of material as it is adhesively pulled from one surface to the other, forming a raised lump known as a gall.

Galling typically doesn’t develop gradually as do other forms of wear (e.g. fatigue and abrasive wear). Instead, it tends to emerge swiftly and spread quickly as the elevated lumps induce more galling. Thus, it is important to regularly check machine parts that are susceptible to adhesive wear. Catching the problem late in the process generally brings high costs with it.

Factors That Trigger Galling

The chance of galling increases when the conditions below are met:

1. Exposed surfaces – Freshly cut metals easily bond when pressed together since there is no protective oxide layer on their surface.

2. Debris – Small particles stuck between the metal surfaces become abrasive as they undergo sliding contact, increasing the risk of galling. The areas of contact will start to deform, creating localised deformation, and will continuously deform as the sliding motion continues.

3. High Stresses – Fast movement of the materials while under high pressure and direct contact contributes to higher levels of stress. These stresses increase the effect of the material’s plasticity which makes it more susceptible to galling.

4. Similar Metals – Microscopic electron transfer works best between two metals with close metallurgical properties. A steel fastener for example will bind more easily with steels having similar properties.

How to Prevent Galling?

There are numerous methods to avoid wear or add protection from galling:

1. Lubrication/coating – A coating or lubricant adds another protective layer over the metallic surfaces while also reducing friction and contact temperature. It may come in the form of anti-galling lubricants, anti-seize products, grease, additives, or other coatings. Using a coating or lubricant when handling stainless steel is almost always necessary.

2. Lower load, temperature and speed – Decreasing the energy in the surface contact area reduces the transfer of microscopic material in the plastic zone. It is best to stop an operation if the surface starts to bind to another workpiece

3. Material Selection – Dissimilar alloys or even material grades work incredibly well against mechanical abrasion since they have different atomic structures. Brass, bronze, and cobalt which are resistant to galling are often selected for bushings and bearings. High hardness is another way to improve galling resistance by adding material hardening.

4. Use clean and undamaged parts – Contaminants on the metal surface promote cold welding which leads to galling. It is best practice to keep the materials free from debris through proper storage, shipment, and handling as parts can rub against each other while transported.

5. Increasing contact area – A larger contact area is effective in reducing the surface pressure. There is less stress on the parts in contact and less depth of wear.

6. Surface Finish – Rough surfaces (>1.5µm) or very smooth surfaces (<0.25µm) have a high tendency to gall. A rough surface can be deburred to improve smoothness but should be monitored to not be less than <0.25µm.

Other factors to consider in minimising the risk of galling is by leveraging advanced technologies in practice, and in the material. Nanotechnology allows for alloys that offer better galling resistance while advanced coatings containing copper or calcium oxide particles are a great solution in preventing galling.

The post Galling – What Is It, How It Works & Prevention appeared first on Fractory.

Let’s explore the unique characteristics of welding stainless steel to maximise its overall result.

The Challenges in Welding Stainless Steel

Stainless steel welding, in particular, presents a unique set of challenges in welding from its mechanical properties. Its various grades and types (such as austenitic, ferritic, martensitic, precipitation-hardened, Duplex, etc) add an extra layer of complexity to the subject.

Let’s explore a few of the challenges in welding stainless steel and learn how to overcome them.

Warping & Cracking

Stainless steel has low thermal conductivity and high thermal expansion, making it susceptible to warping and cracking. Excessive heat input during welding or a rapid change in the temperature puts the metal under stress, resulting in distortion as it cools down.

There are a few ways to prevent warping and cracking, one of which is through working with lower heat input. While it may seem logical, it also may result in poor weld quality due to incomplete fusion between the workpieces. So it is always important to strike a balance between the two extremes. Another solution is to create a heat sink by clamping copper or brass behind the seam, absorbing unnecessary heat into the copper alloys.

Rust

Stainless steel is known to have high corrosion resistance, yet excess heat may still generate rust in the weld. It may also form as the carbon from the filler material contaminates the heat-affected zone.

Choosing an optimal welding temperature and a stainless steel filler alloy depending on the job greatly reduces the risk of forming rust in the material.

Welding Dissimilar Steels

Welding dissimilar metals or various stainless steel grades together can be challenging, especially if their melting points are far apart. It might cause great trouble in fusing the metals effectively.

Selecting the optimal filler rod is necessary when welding stainless steel and another alloy. Preheating the metals can also alleviate this problem.

Switching Between Jobs

The lower melting temperature of stainless steel (1375 – 1530°C) compared to mild steel (1425-1540°C) might become problematic when switching between the two.

Toxic Fumes

Stainless steel welds produce toxic fumes in the form of hexavalent chromium gas. It develops as the chromium oxide layer is destroyed and may occur either during the heating or cooling process.

An effective breathing mask combined with proper ventilation in the area is key to ensuring the welder’s safety.

Stainless Steel Welding Methods

Depending on the project’s requirements, several welding processes can be performed to join stainless steel parts. We’ll highlight a few of those processes but this is not an exhaustive list, processes such as plasma welding, electron beam welding, submerged arc welding, etc are all perfectly capable of welding stainless steels as well.

TIG Welding (GTAW)

TIG welding is widely used in the fabrication sector for stainless steel since it has a stable arc and the process is automated. In the TIG process, the heat applied to the weld can precisely be controlled through a foot pedal or finger control, minimising the possibility of warping. TIG welding machines can also switch between AC and DC polarities, offering flexibility and convenience simultaneously.

Gas tungsten arc welding stainless steel is rather costly since the process needs consumables such as shielding gas (usually pure argon) and optional filler rods. Selecting the right geometry and size of the electrode depending on the welding variables should definitely be considered beforehand.

MIG Welding (GMAW)

MIG welding is another popular choice for welding stainless steel. It offers faster speeds than TIG welding mainly thanks to its continuously fed electrode. One downside of this technique is that it doesn’t look as pleasing to the eye as properly executed TIG welds.

A Teflon wire liner in the MIG gun allows for a consistent wire feed to the weld pool and added protection from contamination. Backstepping, staggering or allowing the joint to undergo a bit of the cooling process helps avoid warpage since stainless steel retains heat well.

Shielded Metal Arc Welding (SMAW)

Stick-welding stainless steel is often the practical choice regarding cost, portability and simplicity. SMAW can be performed in almost any environment and thus is great for various repair jobs and welding stainless steel outdoors.

Thicker pieces of stainless steel (above 2mm) are most suitable for SMAW since it is harder to control the heat input than with other methods. Selecting the electrode (typical grades: 316, 308, or 312) is an important part of the project. Beware of slag removal after the welding as it might be a bit of a struggle.

Flux Cored Arc Welding (FCAW)

Flux-cored arc welding stainless steel is sometimes preferred over SMAW since it generally creates a more uniform weld bead. FCAW can be performed with a shielding gas when working with varying material thicknesses or in demanding welding conditions.

A 10° drag angle allows the flux to rise at the weld pool and gives enough bead coverage.

Resistance Welding

Resistance welding (spot welding, seam welding) stainless steel creates clean welds compared to arc welding processes since it doesn’t use filler material and has no risk of weld spatter. The high electrical resistance of stainless steel is also favorable since it can complete a weld in a short amount of time.

A good quality welder with sufficient power output is necessary for resistance welding stainless steel to prevent the areas close to the HAZ from deformities. It is recommended to have higher current and voltage values when welding stainless steels compared to, for example, copper and aluminium alloys due to the difference in electrical conductivity.

Friction Welding

Friction Welding Stainless Steel to Carbon Steel

Friction welding stainless steel, similar to resistance welding, doesn’t use consumables, making it an economical choice when the part geometries allow for it. It is most suitable for welding austenitic stainless steel due to its composition of chromium (16-26%) and nickel (8-22%).

Some factors that need to be considered are friction pressure, burn-off length and rotational speed. Some sub-types of friction welding may be suitable for specific applications: friction hydro pillar processing (FHPP), friction stir welding (FSW) and friction plunge welding.

Stainless Steel Welding Best Practices

Safety and Preparation

Preparation of equipment, materials and the work area is essential before engaging in any kind of welding process suitable for stainless steels. A dedicated stainless steel wire brush to prepare the metals will decrease the chances of contamination. Gloves, goggles and other protective gear will minimise the risk of accidents such as exposure to fumes and injury due to spatter.

Filler Metal Selection

Choosing the appropriate filler material depending on the scope of work is really important. Most of the time it should match the grade of the stainless steel, achieving almost the same properties. Other considerations in choosing filler material include weld joint design, aesthetics and overall weld performance.

Parameters

An experienced welding provider understands the finer details of each stainless steel grade, weldable metals and welding method. It leads to a better choice when fine-tuning the parameters such as the power supply, torch angle, travel speed and deposition rate. Effective use of these welding parameters will help to achieve the highest quality welds.

Shielding Gas

The choice of shielding gas in processes such as MIG and TIG welding is crucial in protecting the weld pool from contamination. The correct shielding gas composition will reduce the risk of weld defects and improve the weld’s overall outcome.

Heat Input

Consider the variables of the welding project in determining the ideal heat input to weld stainless steel. Generally, the welding current should be about 20% less than for carbon steel to prevent corrosion.

The post Stainless Steel Welding – Challenges, Methods & Best Practices appeared first on Fractory.

Aluminium welding faces a set of unique challenges though, to achieve a successful and defect-free result.

The Challenges in Welding Aluminium

Welding aluminium is a complex process that requires knowledge, skill, and experience. Understanding the different welding processes suitable for aluminium and the challenges and workarounds regarding those methods is paramount for achieving successful welding joints.

Oxidation

Aluminium rapidly oxidises when exposed to the atmosphere, forming a thin oxide layer with a higher melting temperature (2072 C°) than aluminium (660 C°). Welding through this barrier might burn holes through the workpiece.

The oxide layer can be dissolved using solvents or acids before welding the workpiece. It can be physically removed through mechanical abrasive techniques through an exclusive stainless steel wire brush or sandblasting.

Porousness

Aluminium in its molten state, absorbs high amounts of hydrogen, resulting in tiny voids or bubbles forming in the weld metal. This weakens the structural integrity of the metal, making it vulnerable to failure from pressure and stress.

Porosity is minimised by using clean materials and equipment, removing oil and grease from the metal’s surface, and using a shielding gas to limit contamination.

Impurities

Oil, dirt, air and debris can negatively impact the weld zone, affecting the strength and appearance of the workpiece. These impurities can come from multiple sources, such as the environment, equipment, preparation and storage. Discolouration, graining, corrosion and oxidation present signs of impurities in a weld joint.

Proper storage, equipment maintenance and metal preparation are the steps to take to avoid impurities.

Thickness

Working with aluminium typically involves welding materials with varying thicknesses. Welding through a combination of thin and thick material simultaneously can be tricky. Heat applied to the workpiece might melt through the thin material or not penetrate the thick material enough to create a strong weld joint.

A thorough understanding of the aluminium grades, welding methods, and techniques is the key to success in effectively welding different aluminium sheet thicknesses. Welders can effectively weld these metals by setting the parameters such as amperage and heat input to optimal levels.

Hot Cracking

High levels of thermal stress can lead to hot cracking or solidification cracking in aluminium welds. It is one of the reasons why aluminium was once deemed unsuitable for arc welding.

Combining various alloying elements and treating aluminium alloys can drastically improve its mechanical properties. (Al-Cu) 2xxx and (Al-Zn) 7xxx grade series are aluminium alloys that can heat-treated.

Thermal Conductivity

The high thermal conductivity of aluminium makes it challenging to penetrate the weld joint successfully, as it absorbs and dissipates heat quickly.

Adjusting the settings on the welding equipment to higher heat input improves penetration.

Aluminium Welding Methods

Several welding processes are used for aluminium, each having its own strengths and weaknesses. Welders usually choose TIG and MIG welding, yet other methods can fill in for a unique, desirable result.

Factors to consider when choosing the type of welding method are the supply, cost (equipment, filler material, other consumables), weld performance, joint design, repeatability, distortion tolerance, production speed, and safety.

TIG Welding

Gas tungsten arc welding (GTAW) or TIG welding is one of the most common welding techniques for joining aluminium because of its precision and quality. It can operate on AC (alternating current) polarity, offering stability to the arc and wiping off the oxide layer. A TIG welder can consistently generate enough heat to fuse aluminium alloys, with the ability to weld both thin and thick sections.

Inert shielding gas, most commonly argon, is fed to the TIG torch to keep the weld puddle free from contamination. Thus, protecting the sensitive aluminium. When the project requires the use of aluminium filler rods, then series 4xxx aluminium filler metal is the most common choice.

MIG Welding

Gas metal arc welding (GMAW) or MIG welding is a great welding technique for thicker aluminium sheets as the heat input is higher than in TIG welding. This welding method is quite similar to the TIG process but instead of a non-consumable tungsten electrode, MIG welding utilises an automatically fed electrode wire. The mechanical wire feeding system offers faster travel speeds from its spool gun. Shielding gases are used to protect the welds from contamination.

Laser Welding

Laser welding is a fast welding process that concentrates a high power density beam at the aluminium surface. It offers precision to weld both narrow widths and large weld depths, resulting in clean weld zones with the supplement of shielding gas. 6xxx aluminium alloys are generally safe for laser welding and are most commonly paired with a 4032 or 4047 aluminium-grade filler rod.

Electron Beam Welding

Electron beam welding aluminium is an option for thick sections as it produces a small and precise weld pool. This welding method is performed inside a vacuum chamber, protecting the weld from impurities without the need for additional shielding gases.

Resistance Welding

Resistance welding aluminium involves the application of pressure and current to undergo fusion. Spot and seam welding is often used to join aluminium sheets. One downside of this technique is the possible challenges when experiencing high thermal and electrical conductivity.

Friction Welding

Friction welding is a solid-state fusion welding process wherein mechanical friction between aluminium metals generates heat. The nature of this process removes the necessity to add a shielding agent to protect the weld pool. Friction welding is used across all aluminium alloy grades, including the hard-to-weld 2xxx and 7xxx series.

What Processes to Avoid When Welding Aluminium?

Processes that use flux (filler material containing flux, granular flux, or other forms) aren’t ideally suitable to weld aluminium as it faces a greater risk to porosity.

• Flux-cored arc welding

• Shielded metal arc welding

• Submerged arc welding

Aluminium Welding Best Practices

1. Safety and preparation – Wearing protective gear (goggles, helmet, gloves, etc.) is necessary as with any other welding method. Knowing and understanding how the project will be executed beforehand is a great way to prepare for the welding process. Some preparation may include removing the metal’s oxide layer through chemicals or a steel brush and ensuring that the equipment is clean.

2. Understanding the finer details – Experienced welders bring much value in understanding how things should pan out in executing a project. Finer details such as the welding processes, equipment parameters and approach will ensure a successful project. Getting your assemblies welded by experienced welding service providers helps to prevent problems down the line.

3. Patience – It takes time to execute a stellar welding job perfectly, so it is important to be patient and continually test the parameters. Reproducible results with high weld quality are keys when welding multiple pieces in succession.

4. Storage – Aluminium is stored indoors at room temperature and must be kept dry. It is best not to stack these metals if possible (ex. aluminium sheets), but rather vertically since the metal is soft.

5. Bead profile – A stringer bead profile is preferred in welding aluminium most of the time, unlike in welding steel, which uses a weaving technique.

The post Aluminium Welding – Challenges, Methods & Best Practices appeared first on Fractory.

There are many ways to classify testing techniques. One of the most popular classifications is destructive and non-destructive testing.

In this article, we shall take a deep dive into what non-destructive testing (NDT) is, some of its popular types and its applications in some common industries.

What Is Non-Destructive Testing?

Non-destructive testing refers to the use of testing techniques that do not alter any of the properties of the tested product. These properties could be its strength, integrity, appearance, corrosion resistance, conductivity, wear resistance, toughness and so on.

Non-destructive testing is also known as non-destructive evaluation, non-destructive analysis, non-destructive examination and non-destructive inspection.

When the product passes an NDT test, it can still be used. There’s no detrimental effect on the specimen because of the test.

This advantage makes non-destructive testing a very useful method for products that are freshly manufactured as well as for those that are already in service.

When the scope of work is simple, using a single NDT process may be sufficient. But in a lot of cases, a combination of techniques and test methods are used for concrete information about the product characteristics.

Difference Between Non-Destructive and Destructive Testing

Non-destructive and destructive testing have some similarities in their objectives but there are significant variations in the core use cases and application methods. In this section, we shall compare and contrast them based on some important factors:

• Purpose

• Cost efficiency

• Time

• Wastage

• Safety

• Reliability of results

Purpose

The purpose of each type of testing is to ensure that we have a safe product. With destructive testing, however, the intention is to find the operational limits for a product through tests such as fatigue and tensile tests.

On the other hand, with NDT, we check whether a manufactured product or one that is already in service is good enough to function satisfactorily in its service environment. We may also use it to assess the extent of wear and tear such as the use of ultrasonic thickness measurement for steel plating of ships.

Cost efficiency

There are two ways in which non-destructive testing is more cost-efficient compared to destructive testing.

Firstly, it does not damage the test specimen. After evaluation through NDT, it will remain just as effective as before and can be put into service right away.

Secondly, NDT can identify potential issues in machinery that is in service, such as a pressure vessel, and recommend replacement before failure occurs, thereby saving breakdown costs that are far costlier than temporary planned downtime for a single part replacement.

Time

When it comes to time, NDT is more effective again. Destructive methods by nature are far more time-consuming processes. This is mainly because destructive testing processes are mostly manual and we can automate fewer components of it. They also require longer preparation and inspection times.

NDT, on the other hand, does not even always need the removal of parts from service thereby saving valuable time. For destructive testing, work must be halted and machines stopped for testing which increases downtime.

Wastage

A test product that undergoes destructive testing becomes unsalvageable. At times, entire machines have to be discarded. 

Some examples of destructive methods that create wastage are tensile tests, 3-point bend tests, impact tests and drop tests.

This is not the case with non-destructive testing. In some cases, the destruction may be necessary but in a lot of other cases, non-destructive methods will give us similar or better results.

Safety

Destructive testing is carried out before a product is put into service to determine its operational limits. This may be necessary for some products such as PPE where they must be made to certain standards but destructive testing cannot be used for products in service.

In such cases, NDT methods can help us identify worn-out products and parts so we can replace them. By ensuring that critical equipment is well within desired limits, the number of safety incidents can be brought to a minimum.

Reliability of results

Both destructive and non-destructive testing can give very reliable results. Destructive testing can only test a small number of samples (lot sampling). Non-destructive testing, on the other hand, can test entire batches. 

NDT is also a better alternative to find discontinuities and defects in a part.

Non-Destructive Testing Methods

Visual testing

Visual Testing

Visual testing remains the most popular NDT method across all industries. It involves taking a thorough look at the specimen and finding defects that are visible to the naked eye.

It is a quick and feasible method of tracking product quality at every stage of the manufacturing process as well as for those products that are in service.

With visual inspections, we can detect corrosion, cracks, welding defects, deformation, etc. All we need are simple instruments such as rulers, gauges or a camera.

When inspectors are not able to reach hard-to-access places or dangerous environments, drones can often be the solution.

Many industries are in fact using AI and machine learning to improve visual inspection results. For instance, such technology is becoming common in the maintenance of conveyor belts, rollers and pulleys in conveyor systems.

Advantages of visual testing:

• Safe

• Portable

• Effective

• Inexpensive

• Easy to train

• Minimal or no downtime

• Minimum or no part preparation needed

Disadvantages of visual testing

• Only works with surface defects

• Possible misinterpretation of flaws

• Cannot detect minute defects without additional optical instruments

Ultrasonic testing

Ultrasonic Testing

Ultrasonic testing remains the most popular nondestructive testing method after visual testing. 

In this method, a high-frequency sound wave generated by a transmitter travels through the object under test. The frequency of this wave is usually between 1 and 10 MHz.

The wave distorts when encountering a change in the density of the material. This change in the transmitted wave is captured by a receiver.

The equipment then measures and analyses the received wave to understand the nature and depth of the defect. The equipment can also calculate the thickness of the specimen by dividing the wave speed in the material by the time taken for travel.

There are many types of ultrasonic testing available each with its nuances and field of application. These are pulse-echo testing, immersion testing, guided wave testing and phased array ultrasonic testing to name a few.

We can identify defects such as cracks, abrasions, thinning, pitting and corrosion using ultrasonic inspection.

Advantages of ultrasonic testing:

• Quick

• Clean

• Reliable

• Portable

• Safe and easy to use

• Highly accurate and sensitive

• Ability to gauge dense materials

• Detection of surface and subsurface defects

• Identifications of minor defects not visible to the naked eye

Disadvantages of ultrasonic testing:

• Requires training

• Needs a smooth surface

• Difficult to use with thin materials

• Part geometry may create complications

• Wave propagation speed in tested material must be known for accurate results

• Couplants are required for smooth wave transfer from the transmitter to the specimen

Liquid penetrant testing

Liquid Penetrant Testing

Liquid penetrant testing is another popular non-destructive testing method used to identify surface-level defects.

In this method, a low-viscosity liquid (penetrant) enters the surface defects such as cracks, fissures and voids. The excess liquid is then wiped off and the specimen is left alone for some time (penetrant dwell time).

The inspector then applies a developer that allows the penetrant to move towards the surface. The specimen is again left alone for a prescribed amount of time (developer dwell time).

Now, the inspector performs the surface inspection. If the dye is visible, it can be inspected with the naked eye. In the case of fluorescent dyes, black light is needed for inspection.

We can detect surface discontinuities such as cracks, porosity, seams, laps and leaks using this method.

Advantages of liquid penetrant tests:

• Works with many materials. Material properties such as magnetism, conductivity and metallic/non-metallic do not matter

• Can spot tiny defects such as hairline cracks

• Suitable for complex part geometries

• Low cost

• Can test large areas

• Portable

• Easy to use

Disadvantages of liquid penetrant tests:

• The depth of defects is not known

• Risk of exposure to toxic fumes

• Cannot identify subsurface defects

• Does not work with porous materials

• Time-consuming, generally needs more than 30 minutes

• Messy operation, pre- and post-cleaning are necessary

• Involves handling of chemicals and therefore not it’s not as safe as other methods. Chemical disposal may also become an issue

Radiographic testing

Radiographic Testing

Radiographic testing uses radiation to spot internal defects in parts. X-rays work well with thinner materials whereas gamma rays are better for thicker materials.

The specimen is placed between the radiation source and a recording media. When the radiation falls on the part, the amount of radiation that exits the part in different locations is captured. A physical radiography film or a digital detector is used as the recording media.

The test allows us to obtain the shape and size of internal defects by changing the angle of radiation exposure.

We can use radiographic testing to pinpoint defects such as cracks, thinning, corrosion, voids, insufficient fusion, porosity, excess root penetration and laps.

Advantages of radiographic testing:

• Can test complex structures

• Documentation is permanent

• Works with a range of materials

• Needs minimum surface preparation 

• Can record surface and subsurface defects

• Portability is possible for gamma ray testing

• Less misinterpretation of results compared to other methods

Disadvantages of radiographic testing:

• More expensive

• Needs two-sided access to specimen

• Not as effective for planar and surface defects

• High voltage and radiation can be harmful to personnel

• Skilled personnel needed for execution and accurate interpretation of results

Magnetic particle testing

Magnetic Particle Inspection

Magnetic particle testing is also a fairly popular NDT technique because of its fast execution where no surface preparation is needed.

In magnetic particle testing, the part is placed between permanent magnets or electromagnets. The strength of the field is an important factor since a stronger field gives better results.

When the part under inspection is placed into the field, a magnetic current starts flowing through the specimen. If there’s no defect, an uninterrupted magnetic flux field is obtained.

But if it comes across a defect, the magnetic field bends and a part of it leaks out. This leakage is also known as the flux leakage field.

In order to identify the defects via these leakage points, magnetic particles are used. These particles are applied to the test specimen and they are pulled into these leakage points because of the uneven magnetic flux density.

We may either use magnetic particles that can’t be seen with the naked eye or fluorescent ones for better visibility. 

The width of the magnetic particle strips is wider than the defect’s width. As a result, it can reveal minute defects with an opening width of up to 0.001 mm and depth of 0.01 mm.

With this technique, we can detect defects such as cracks, pores, laps, inclusions, seams, laminations, shrinks, flakes, welding defects, machining tears and also service-related or fatigue cracks.

Advantages of magnetic particle testing:

• Easy to use

• Portable setup

• High sensitivity

• Immediate results

• Usually inexpensive

• Can work through thin surface coatings

• Parts with complex geometries are also suitable

• Visual indication of the shape and size of the defect

• Can detect surface defects well. Also works for subsurface defects to an extent

Disadvantages of magnetic particle testing:

• Can only test small areas at a time

• Does not work with non-magnetic materials

• Testing may burn the particle if the field is too strong

• Coatings thicker than 0.1 mm need removal for testing

• Demagnetisation of test specimens is necessary but may be tricky

• Can only work for subsurface defects that have a depth of up to 3 mm

Eddy current

Eddy Current Testing

Like magnetic particle testing, eddy current testing is another electromagnetic testing technique. It works on the principle of electromagnetic induction.

When current passes through any current-carrying conductor (primary conductor), it generates a magnetic field (primary field). 

When we place a second conductor (test specimen) in this magnetic field, the primary magnetic field induces an opposing electric current in this conductor.

This current is known as eddy current and it is proportional to the change in the magnetic field as the alternating current in the primary coil rises and falls during every cycle.

The fluctuating eddy current creates its own magnetic field (secondary field) that opposes the primary field and affects the voltage and current flowing through the primary conductor.

As the test specimen’s magnetic permeability and electrical conductivity changes because of the defects, the magnitude of the eddy current changes. These changes can be recorded by using the primary or the secondary coil and analysing them to get more information about the defect.

There are also other testing methods similar to eddy current available. Remote field testing, for example, finds use in detecting defects in steel pipes and tubes. The main difference between the methods lies in the coil-to-coil spacing.

With eddy current testing, we can detect flaws such as cracks, corrosion, laps, lack of fusion, magnetic inclusions, porosity and wear.

Advantages of eddy current testing:

• Quick

• Portable

• Immediate results

• Minimal part preparation

• Can be a non-contact process

• Works with complex part geometries

• Capable of automation for uniform parts

• Can spot surface and subsurface defects up to 0.5 mm in size

• Multipurpose. It can also measure coating and material thickness, identify materials and their heat treatment conditions

Disadvantages of eddy current testing:

• The penetration depth depends on many factors

• Cannot detect flaws parallel to the part surface

• Only works with electrically conductive materials

• Requires high-skill personnel for accurate interpretation of results

These are some of the most popular NDT techniques used across the industry today. Apart from them, there are many other NDT methods for very specific applications. Some of these are acoustic emission testing, thermal/infrared testing, vibration analysis, leak testing (e.g. mass spectrometer testing), rebound hammer testing, laser testing and so on. 

NDT Applications

NDT has a wide range of practical applications. But we can divide all of them into one of the following two categories: quality control and condition monitoring.

We can use nondestructive testing to carry out a quality assessment of the manufactured parts to determine that they meet the desired specifications. We may also use it to assess the condition of parts that are already in service and whether they are safe for further use.

Some common industrial applications of NDT are as follows:

Structural mechanics

NDT can verify the structural mechanics of a wide range of products and structures. It requires minimum intervention and is capable of carrying out routine as well as special inspections without aggravating the issues present in the structure. We may also use it to inspect parts that are not as accessible.

When it comes to civil engineering applications, nondestructive testing can inspect structural foundations, cultural heritage monuments, bridges, buildings, etc.

In mechanical engineering, we use NDT for the inspection of loaded machinery such as shafts, turbomachinery, batteries, etc.

Techniques used in this field include ultrasonic testing, radiography testing, visual testing, acoustic emission testing, terrestrial laser scanning, photogrammetry, tacheometry, infrared thermography, etc.

Welding

NDT techniques can identify the various defects that occur during the welding process. We have enumerated many welding defects in the previous sections that we can reliably test through NDT.

As a quick recap, we can use NDT inspections to identify both external and internal welding defects on both metals and plastics.

External defects include cracks, porosity, undercut, underfill, spatters, overlaps, arc strike and excessive penetration. Internal defects include internal cracks, slag inclusion, tungsten inclusion, internal porosity, internal blowholes, lack of penetration and lack of fusion.

Medical

NDT methods have been revolutionising the medical industry for several years now. They help to accurately diagnose and treatment at the skin as well as the internal level.

Some notable NDT technologies that have become common terms in the medical industry are ultrasound imaging, radiography testing, and echocardiography.

Manufacturers also use NDT methods to test medical implants for fine defects. Such testing prevents subsequent failures when the components are already in use.

Summing it up

Nondestructive testing is quickly becoming the go-to test method for spotting defects in almost every discipline. Today, NDT finds application in sectors such as aerospace, military, medical, nuclear, marine, power generation, manufacturing, etc.

The increasing popularity has prompted further research and the existing processes are becoming better and more capable with every passing year.

The post Non-Destructive Testing (NDT) – Process, Types & Applications Explained appeared first on Fractory.

However, untreated aluminium has low wear resistance. On exposure to the environment, it forms a thin aluminium oxide layer naturally that provides aluminium with its characteristic corrosion protection. But this naturally formed oxide film can erode upon reaction with other environmental elements.

The answer to providing better protection lies with anodising. This procedure has other benefits as we will learn further into the article. But let’s start from the beginning.

What Is Anodising?

It is an electrochemical process that develops an aluminium oxide coat on the surface of the part or product. This protects the product from wear and tear while improving the aesthetics. In this process, the product to be coated acts as an anode in an electrolytic cell, hence the name. 

On an industrial scale, anodising made its first appearance in 1923.

Soon, many variations of this process came into use for different materials using various electrolytic chemicals. It was around this time that Gowen and O’Brien used sulfuric acid to anodise aluminium.

This process from almost a century ago still remains the most common and effective method today.

Anodised Aluminium Benefits

The aim of the process is to increase the thickness of aluminium oxide on the surface of the product.

Aluminium oxide layer is extremely hard. On the Mohr’s scale, it has a score of 9 and is second in hardness only to diamond. It is so hard that it is commonly used as an abrasive in sandpapers. Depositing a layer of this material on the product ensures that the product will have high wear resistance.

The thickness of this layer depends on the purpose of anodising. For decorative purposes, a thin layer is enough. A thicker layer protects the surface besides improving the appearance.

Having a thick layer of aluminium oxide also makes the metal surface more receptive to dying as pores are created on the surface when it is anodised. Then, desired pigments are introduced that fill the pores from the surface to its very depth. This makes the pigment quite durable as it cannot be scratched away.

Anodising can also act as an excellent primer for a regular coat of paint on the surface instead of accommodating it into the actual oxide layer.

Anodising aluminium improves the insulation properties of aluminium as aluminium oxide is not a good conductor of electricity.

Working Principle

Anodising works on the principle of an electrolytic cell. In this procedure, the anodising tank is filled with a suitable electrolyte. In this tank, the part is usually suspended to expose most of the surface to the electrolyte.

We then place plates of suitable elements (usually lead or aluminium) in the tank. The next step is completing the circuit between the cathode and anode through a power source. 

The aluminium product is connected to the positive terminal, and the plates are connected to the negative terminal of the battery. As the circuit is now complete, the current passes through it.

The value and duration of the electrical current passed will determine final features such as the thickness of the aluminium oxide layer on the anodised aluminium product.

Aluminium Anodising Process

Most anodising setups today still use the original sulfuric acid bath for the process. However, many new features have been added to significantly improve the final result in terms of aesthetics and functionality of the product.

The modern aluminium anodising process is very technical. Generally, aluminium anodising consists of the following steps.

Aluminium anodising process

Cleaning

The surface of the aluminium product needs cleaning prior to anodising. Exposing the surface uses acidic or alkaline cleaning agents to clean grease/dirt from the surface.

Pre-treatment

This step eliminates any surface imperfections. The goal is to provide a visible finish with a clean and smooth surface. This is done by using two main processes – brightening and etching.

Brightening

Brightening or bright finishing cleans any heavy metal residues left over from the cleaning process. Using a concentrated mixture of nitric and phosphoric acids to chemically smoothen the surface provides a metallic finish ready for anodising.

Etching

Etching removes a layer of aluminium from the product surface to provide a matte finish (see more about gloss levels). A hot solution of sodium hydroxide is used to remove surface imperfections.

Anodising

After pretreatment, the product is ready for anodising. As mentioned above, sulfuric acid is the go-to electrolyte for aluminium anodising. Alternatives that are sometimes used are organic acid, borate, tartrate, phosphoric acid, and chromic acid.

Colouring

There are several methods to add colour to anodised aluminium. Different colours need different methods. Let’s look at two of the most popular methods of colouring anodised aluminium.

Electrocolouring

One of these methods is the electrocolouring method. This method is used for darker shades. In electrocolouring, the anodised aluminium product is introduced to inorganic metallic salts through an electrolyte.

The anodised aluminium product becomes one electrolyte, and graphite (or aluminium) becomes the other. The oxide or hydroxide precipitates in the pores adding colours such as black, brown, blue, yellowish grey, and bronze to the film.

Dyeing

Dyeing is the other popular method of adding colour to an anodised aluminium product. The pores that are formed during the electrochemical process readily absorb dies or pigments.

They fill the pores through the entire thickness of the aluminium oxide layer. Since the thickness of this layer can be up to 50 microns in some cases, this method is quite durable. Scratching or rough usage of the part doesn’t affect the colour due to the layer’s thickness. Also, the range of available colours is wide.

Sealing

Sealing is the final step in the aluminium anodising process. This prevents water leakage and improves corrosion resistance of the anodised aluminium product. There are three methods of doing this – hot method, cold or a combination of the two.

Sealing reduces the chances of staining, scratching, colour degradation and crazing of the surface.

Types of Anodising

Based on the thickness of the aluminium hydroxide layer, there are 2 types of anodising.

Decorative anodising

Decorative anodising, as the name implies, has its focus on providing a nice aesthetic finish first and providing protection as more of a nice extra.

For decorative anodising, the recommended layer is between 5µm to 25µm. To get the best result when dyeing the parts, its best to keep the thickness between 15µm to 25 µm. ISO 7599:2018 specifies the method for decorative anodising for aluminium and its alloys.

Hard anodising

In cases where we need superior protection of aluminium alloys (marine applications or exposure to corrosive chemicals), we recommend opting for hard anodising.

The thickness of the oxidation coating must be between 25µm and 50µm. ISO 10074:2017 provides the specifications for hard anodic oxidation coatings.

Final Finish

Anodising gives the aluminium surface a superior appearance. As we know, the surface consists of the pores with pigments as well as the uncoloured portions where the surface reacts with oxygen to prevent further oxidation. As the light strikes both these surface features at the same time, it interferes on reflection, giving the metal an attractive metallic shine.

The surface also has very few imperfections as it reacts uniformly with the electrolyte giving it a smooth finish.

Anodising vs Powder Coating

Powder coating is a type of surface treatment that is most common for coating steels but also available for aluminium. The surface of aluminium is coated with polyester powder for decoration as well as protection. Manufacturers have a choice between these two methods when they are looking for surface treatment options for aluminium.

Anodising is better than traditional powder coating in many ways, some of which are as follows:

• It is an inorganic finish and provides a superior surface finish compared to organic counterparts such as powder coating.
• When it comes to appearance, anodising has a metallic sheen and is extremely well integrated with the surface compared to powder coating. An anodised surface reacts differently to both natural and artificial light.
• Anodising is also better in the long run. Powder coating sometimes suffers adhesion failure and even if it doesn’t, the colour will fade over time.

What Materials Can Be Anodised?

Besides aluminium, many other metals and even plastics are suitable for anodising.

Metals such as magnesium, titanium, zirconium, niobium, zinc, hafnium, and tantalum are anodised, albeit for different purposes.

In the same way, we can anodise conductive plastics. All anodised products develop superior surface finish, attractive appearance, and generally last longer than their untreated counterparts.

FAQ

What is meant by the barrier layer in anodising?

When aluminium is anodised in an acidic solution, its surface starts to lose aluminium ions. This causes erosion of the aluminium surface and to counter this, the surface reacts with negatively charged oxygen ions in the electrolyte.

While the points where the initial erosion takes place continues to be eroded, the rest of the surface forms an aluminium oxide layer that acts as a barrier against further erosion. This layer is known as the barrier layer. It is quite thin compared to the porous layer formed due to anodising.

Can the entire product surface be anodised?

It is not possible to anodise the entire surface of a product. An electric terminal must be connected to the part throughout the duration of the process, so wherever it is connected, that portion will not be anodised.

In order to minimise the effect of this limitation, the connection is placed at non-critical points. The best place is usually a hidden face on the part.

Hard anodising requires a higher voltage and electrical current. In such cases, the connection is made through a threaded hole in the product for good electrical contact.

The post Aluminium Anodising appeared first on Fractory.

To utilise sheet metal to its full extent, engineers should first know a thing or two about it. This includes standard measurements, materials, differences in manufacturing and possible forming methods.

What Is Sheet Metal?

Sheet metal is one of the shapes and forms metal can be bought in. Sheet metal is any metal that has a thickness in between 0.5…6 millimetres.

There are other measurement units used to categorise metals by thickness, though.

Millimetres, Mils & Gauge

Foils, sheets and plates are pretty much the same, with the only difference being in thickness. Thickness is measured in 3 ways – millimetres, mils and gauges. While millimetre is a pretty straightforward unit, mils and gauges are only common in engineering and manufacturing.

One mil equals to a thousandth of an inch. As the British often use “mils” as a plural for millimetres, it can be a little confusing but the difference is important.

Gauge is another unit for measuring sheet metal thickness. Although official standards discourage the use of gauges, they are not all that rare. Gauge represents the thickness of a metal in relation to its weight per square foot. A higher gauge number means a smaller thickness.

As it is related to the weight of a metal, the actual thickness (mm) for the same gauge, is different for various metals (e.g. 12 mm steel vs 12 mm aluminium).

Foil, Sheet and Plate

Having gone over the primary measurement systems, we can now look at the categorisation of foils, sheets and plates. For our purpose, we are going to stick with millimetres as the unit of measurement.

Metal foil is especially common with aluminium, often referred to as tinfoil. The thickness of foil is usually up to 0.2 mm. 

Sheet metal is the next in line. The thickness of sheet metal starts from 0.5 mm and goes up to 6 mm. Anything above that is a metal plate.

The thin sheet metal is easy to form, while still providing great strength. At a relatively cheap price, it makes a great fit for most engineering purposes. That is why we see it everywhere around us.

At the end of the article, we will be discussing different forming methods used with sheet metal.

Common Materials

Pretty much all the common engineering metals are also used in the form of sheets. Some of them are carbon steel, stainless steel, copper and aluminium. Choosing the right one for you comes down to the application and requirements of your product.

Metal sheets have the same mechanical properties as the base metal. Thus, steel sheets have high tensile strength and durability suitable for use in constructions and machinery.

At the same time, copper sheets often find use as a decorative layer on modern buildings.

Standard Sizes

Standard measurements apply to both sheet sizes and thicknesses. While achieving large sheet sizes have a few workarounds through welding, you cannot really look past the thickness tables.

Standard Sheet Metal Measurements

Knowing standard sheet sizes helps you with optimising your part layouts. Many times have we received parts for production that are just a bit larger than the standard size. That means more scrap and higher overall costs for the customer.

Also, another thing to bear in mind here is the availability of different sheet sizes. While small, medium and large sheets are pretty much always in stock at every sheet metal fabricator’s warehouse, oversized sheets may not be.

Many of them may also not have the machinery to cut such large sheets. So knowing your manufacturers’ capabilities comes in handy here.

Sometimes you can just weld together two smaller sheets but it is not the optimal solution. Especially when putting high emphasis on the aesthetics of your project.

Standard Sheet Metal Thickness

The table above shows standard thicknesses for both sheet metal and metal plates. As you can see, each type of metal has its own standards.

Also, it is good to not only pay attention to the starting and ending points but the actual values. Someone asking for non-standard thicknesses is quite frequent.

Exceptions to this table definitely exist. We would just advise you to follow these thicknesses because of the difficulty of finding the materials. All of the ones above are readily available with many sheet metal fabricators.

Hot-Rolled vs Cold-Rolled Sheet Metal

There are two ways of producing sheet metal – hot rolling and cold rolling. Looking at the standard thickness table above, you can see how those two are used. In the case of construction steels, cold rolling only goes up to 3 mm. From there on, the sheets and plates are hot rolled for cost-efficiency purposes.

Although the hot rolling process again starts from 3 mm for stainless steels, cold rolling covers all the sheets, i.e. up to 6 mm.

For engineering purposes, it is important to differentiate the two. For precision applications, cold rolled steels are preferred as we have more control over the final dimensions. This is because in hot rolled steels, after cooling, the steel shrinks slightly and the shape might change a little.

Forming Processes for Sheet Metal

So we finally made it to the forming methods for sheet metal. There is a plethora of options on the table and the decision rests on you.

The choice mostly depends on the required outcome. But many of those methods get you a seemingly similar or matching result. Then it comes down to cost (often depending on batch sizes), availability and desired accuracy.

Sheet Metal Bending

Bending is a forming process in which sheet metal is bent into the desired shape by applying bending stress. The sheet metal is bent so plastic deformation is reached. That prevents the metal from regaining its former shape.

Parts such as flanges and corrugations are created by bending. The most common form of bending is V-bending. There, a V-shaped die and a punch press together to give the sheet the desired form.

Edge bending is another common method to bend flanges using a wiping die and a punch.

Sheet Metal Curling

Curling is the process of forming a circular ring at the edge of the metal sheet to make it safer for handling.

There is a difference between a curled edge and a tear-shaped hem. In curling, the initial edge is rolled into the formed circle whereas, in the tear-shaped hem, the initial edge is still exposed.

Curling can also be classified into an off-centre and an on-centre roll. Off-centre rolls have the centre above the level of the sheet, whereas on-centre rolls have the centre at the same level as that of the metal sheet.

Sheet Metal Decambering

Decambering is the process of removing camber from a sheet of metal. In a sheet of metal, especially in strip-shaped parts, a horizontal bend is produced when it is flattened into sheets.

In decambering, we remove this horizontal bend by flattening the edges to remove the camber. The force is applied on a deformed edge, and not on the face, to push it into a straight form. It is usually carried out on limited length sections.

Deep Drawing

Deep-drawn manufacturing process for MDI cans

Deep drawing is a sheet metal forming process in which a sheet’s shape is changed to the desired shape in multiple stages using a series of dies. Only if the depth of the shape formed exceeds the original diameter of the sheet, it is considered deep drawing.

Punches and dies are used to create changes at every stage. Using this process, a sheet is converted into many different shapes like a fuel tank, sink, and automobile parts. Deep drawing is mostly used for large-batch production.

Sheet Metal Expanding

Bender SP-1250 expanded metal production line

In this metal-forming process, a sheet of metal is passed through perforating scissors where it is cut and stretched into a pattern. Usually, a diamond-shaped mesh is preferred as it has a structural advantage over other shapes.

This forming process is commonly used for manufacturing fences, catwalks, platforms, grating, etc. The process gives a self-draining, strong product that can support the weight it is designed for.

These products can be used where a passage of air or a liquid is needed but there is a need to prevent larger particles from passing, e.g. sifting. The strength of the original metal is retained.

Hydroforming

Hydroforming Animation

Hydroforming is an innovative way of shaping metal sheets into the desired shape. In this process, the metal is placed on a die, but instead of a punch, high-pressure fluid is used to shape the sheet.

With this process, more complex parts can be created in a shorter time frame. It is also comparatively cheaper and requires less work. Hydroforming is also compatible with almost all materials such as stainless steel, aluminium, carbon steel, brass, and precious metals.

Since matching dies are generally not needed, hydroforming can be used to form unconventional shapes.

Incremental Sheet Forming

Single Point Incremental Forming at University of Aveiro - SPIF-A Project

Incremental sheet forming is a metalworking process that gives sheet metal a certain, desired shape. This forming method is only viable with low volumes.

Changes can be made to the product quickly and without much hassle. In this process, the product is formed in incremental steps.
The sheet material is shaped using a single-point sphere.

While the process gives similar results to deep drawing, it does not need a separate set of punches and dies. So the largest advantage of this method is its flexibility.

Ironing

Ironing process is used when the thickness needs to be reduced in a certain area of the sheet metal. This enables the manufacturer to get a uniform wall thickness when deep drawing products.

A good example of ironing is the process of manufacturing soda cans. The walls of the can need to be thinned to a predetermined thickness. Usually, 2 or 3 ironing processes with different dies are needed to create the desired thickness in aluminium soda cans. This ironing process is carried out in combination with deep drawing.

Laser Cutting

Laser cutting is being used more and more for sheet metal cutting applications. It is precise and the finish is extremely smooth. CNC machines are generally used to cut specific shapes.

After feeding the program, the specific cut is carried out by laser. Laser cutting has the advantage of flexibility. When needed, changes in the required shape can be carried out easily. It is also possible to cut metal sheets into extremely complex pieces using laser cutting services.

Photochemical Machining

Chemical Etching: A Tour Through The Process (3D Animation)

Photochemical machining is a process in which controlled corrosion is carried out to create sheet metal parts as per requirements.

In this process, a photoresist and an etchant are used to give the metal sheet its shape. The method was developed in the 60s and even today is a comparatively inexpensive method.

Photochemical machining can be used to cut any metal. The process is ideal for mass production. If changes are required, they can be easily brought about.

Just like laser cutting, the process is extremely precise and can offer a smooth finish. This process is used to make extremely fine meshes, apertures, flexible heating elements, metal gaskets, electrical contacts and jewellery.

Punching

Punching is a very common technique for cutting holes in sheet metal. The setup consists of a punch and a punching die. There is a very small clearance between the two.

When the punch and the die meet, the material is pressed between the two and with subsequent pushing the shearing force cuts a hole in the sheet. The process is relatively inexpensive in large quantities and capable of punching a hole in strip as well as sheet metal.

Holes of varying sizes can be punched but it is usually recommended that the diameter of the hole required is greater than the thickness of the material being punched. For especially sturdy alloys, the difference between the hole diameter and the thickness needs to be greater.

Rolling

Rolling may be carried out on sheet metals to decrease the overall thickness of the metal sheet or to make it more uniform. In this process, the sheet metal is passed through a set of rolls.

Depending on the temperature the process is classified as hot rolling or cold rolling. In hot rolling, the temperature is around 1400 degrees Fahrenheit for steel. This can help achieve a thickness from 1/16th of an inch to 5/16th of an inch.

In cold rolling, the process is carried out at room temperature. The material is washed with acid and heat treated to achieve a good finish.

Plate roll / Sheet metal roller / Plate roll bending / Rundbiegemaschine Motor / Hengerítőgép

Rolling is also another way to achieve large-radii bends. Instead of using press brakes for step-bending, this gives a large uniform radius without the need for continuous manual work in the process.

Press Brake Forming

In this process, a long sheet of metal is bent around a straight axis going through the material. A ‘V’, ‘U’, or a channel-shaped material may be formed depending on the punch and the die.

Although it looks simple, precision bending is a difficult task to achieve in press brake forming. Material properties need to be considered along with the press and the tooling to perfect the amount of springback.

Press brake forming may also be used for smaller parts making it suitable for smaller pieces along with large pieces. Thickness up to 25 mm can be easily bent and the length of the piece can go up to 6 m.

Wheeling

This forming process uses an English wheel to stretch and curve flat metal sheets into required shapes. The method needs highly skilled labour and is therefore expensive. It is not possible to use this method for mass production. Wheeling is mainly used in producing low-volume customised parts for old vehicles.

It is also used for creating sheet metal parts for car prototypes and aircraft.

The shape is produced in stages and at each stage, the operator must compare the formed piece with the reference shape. Different wheels and passing the piece in different directions may be required.

While there is a great variety of metal production services on offer, the importance lies in finding the right fit for your needs while staying cost-effective. All the info above aims to help you do just that.

The post All About Sheet Metal – Materials, Standard Sizes & Forming Processes appeared first on Fractory.

While limits and fits apply to all sorts of mating parts, their main use is for regulating the sizes of mating shafts and holes for best performance.

Both ISO and ANSI have standardised fits in three classes – clearance, transition and interference. Each class has a variety of options available for choosing the correct one for a specific application.

Tolerance Grade

With engineering fits, the tolerance will always be shown in an alpha-numeric code. For example, a hole tolerance may be H7. The capital letter signifies that we are dealing with a hole. When indicating tolerance for a shaft, the letter will be lowercase.

The number shows the international tolerance grade (ISO 286). A tolerance class determines a range of values the final measurement can vary from the base measurement.

From the table, we can see that the tolerance grade applies to a range of basic sizes. So if we have a hole with a nominal size of 25 mm and a tolerance class of H7, we will fit into the 18…30 mm basic size group. Looking at the IT7 tolerance grade, the chart gives an allowed variance of 0.021 mm.

The letter signifies the start of the tolerance zone. For H7, the starting point is at exactly 25.000 mm. The maximum hole size is then 25.021 mm. For F7, the tolerance range is the same but the starting point is 25.020 mm, taking the last acceptable measurement to 25.041 mm.

A great way to find all the corresponding engineering tolerances to specific measurements is by using a limits & fits calculator.

Hole and Shaft Basis System

When choosing a system for a fit, you have 2 options – hole and shaft system. The system tells which part has a controlled measurement and which part is made based on the other.

In short, the hole-basis system uses a constant measurement for the hole and the diameter of the shaft is made accordingly to achieve the required fit.

And the shaft-based system works vice-versa.

Engineers tend to follow the hole system because of simplicity. As the hole size stays constant, the shaft’s upper and lower deviation values determine the type of fit. Drilling does not allow for much precision, as the tooling comes in certain measurements.

At the same time, CNC turning services are able to create shafts with exact measurements, so achieving the desired fit is just easier this way.

Limits & Fits

In engineering, we have to define the tolerances of parts to ensure a long lifespan and proper working of a machine. We can choose the fits according to the necessities and working conditions. The three main categories are:

• Clearance fit
• Transition fit
• Interference fit

All these come with another subset of categories, each designed for different circumstances. Of course, we have to keep in mind that closer tolerances and more snug fits will result in higher costs because of higher demands on machining accuracy and the difficulty of assembly.

A clearance fit always leaves room between the two parts. A transition fit is somewhere in between clearance fits and interference fits and can end up either way but without leaving much room nor being too tight. An interference fit is tight and creating the fit requires considerable force and other techniques for easing the process.

Clearance Fits

With a clearance fit, the shaft is always smaller than the hole. This enables easy assembly and leaves room for sliding and rotational movement.

When the shaft diameter is at its minimum and hole diameter at its maximum, we have a situation of maximum clearance. When the shaft diameter is at its max and hole diameter at its minimum, we have a situation of minimum clearance.

Clearance fits come in 6 sub-categories. Starting from the loosest:

• Loose running
• Free running
• Close running
• Sliding
• Close clearance
• Locational clearance

Loose Running Fit

Fit with the largest clearance. Suitable for applications where accuracy is not of the utmost importance and contamination may be a problem.

Example uses in engineering: Fits exposed to dust contamination, corrosion, thermal and mechanical deformations. Pivots, latches, etc.

Example fits: H11/c11, H11/a11, H11/d11 (all hole-basis), C11/h11, A11/h11, D11/h11 (all shaft-basis)

Using a 25 mm diameter, a H11/c11 fit gives a minimum clearance of 0.11 mm and a maximum clearance of 0.37 mm. In this case, the shaft diameter can fall in between 24.76 and 24.89 mm while the minimum hole size is 25 mm and the max 25.13 mm.

Free Running Fit

Suitable where no special requirements apply to the accuracy of matching parts. Leaves room for movement in environments with heavy temperature fluctuations, high running speeds and heavy plain bearing pressures.

Example uses in engineering: Applications where maintaining a film of oil lubrication is important. For example, shaft and plain bearing fits with little rotational movement.

Example fits: H9/d9, H9/c9, H9/d10 (all hole-basis), D9/h9, D9/h8, D10/h9 (all shaft-basis)

Using a 25 mm diameter, a H9/d9 fit gives a minimum clearance of 0.065 mm and a max clearance of 0.169 mm.

Close Running Fit

Close-running fits are a good choice for applications that require smaller clearances and moderate accuracy. Good for withstanding medium speeds and pressures.

Example uses in engineering: Machine tools, sliding rods, machine tool spindles, etc.

Example fits: H8/f8, H9/f8, H7/f7 (all hole-basis), F8/h6, F8/h7 (all shaft-basis)

Using a 25 mm diameter, a H8/f7 fit gives a minimum clearance of 0.020 mm and a max clearance of 0.074 mm.

Sliding Fit

Leaves a small clearance for high accuracy while maintaining ease of assembly. Parts will turn and slide quite freely.

Example uses in engineering: Guiding of shafts, sliding gears, slide valves, automobile assemblies, clutch discs, parts of machine tools, etc.

Example fits: H7/g6, H8/g7 (all hole-basis), G7/h6 (shaft-basis)

Using a 25 mm diameter, a H7/g6 fit gives a minimum clearance of 0.007 mm and a max clearance of 0.041 mm.

Locational Clearance Fit

Location clearance fits provide minimal clearance for high accuracy requirements. The assembly does not need any force and the mating parts can turn and slide freely with lubrication, helping with assembly by hand. Provides a snug fit for stationary parts.

Example uses in engineering: Roller guides, guiding of shafts, etc.

Example fits: H7/h6, H8/h7, H8/h9, H8/h8 (all hole-basis)

Using a 25 mm diameter, a H7/h6 fit gives a minimum clearance of 0.000 mm and a max clearance of 0.034 mm.

Transition Fits

A transition fit encompasses two possibilities. The shaft may be a little bigger than the hole, requiring some force to create the fit. At the other end of the spectrum is a clearance fit with a little bit of room for movement.

Specifying a transition fit means that both outcomes are possible even inside a single batch.

Transition fits come in 2 forms – similar fit and fixed fit.

Similar Fit

Leaves a small clearance or creates a small interference. Assembly is possible using a rubber mallet.

Example uses: Hubs, gears, pulleys, bearings, etc.

Example fits: H7/k6 for hole-basis and K7/h6 for shaft-basis

Using a 25 mm diameter, a H7/k6 fit gives a max clearance of 0.019 mm and a max interference of 0.015 mm.

Fixed Fit

Leaves a small clearance or creates a small interference. Assembly is possible using light force.

Example uses in engineering: Driven bushes, armatures on shafts, etc.

Example fits: H7/n6 for hole-basis and N7/h6 for shaft-basis

Using a 25 mm diameter, a H7/n6 fit gives a max clearance of 0.006 mm and a max interference of 0.028 mm.

Interference Fits

Interference fits are also known as press fits or friction fits. These types of fits always have the same principle of having a larger shaft compared to the hole size.

The assembly stage requires force, sometimes lubrication, heating of the hole and freezing of the shaft. These help to increase/decrease the hole and shaft sizes respectively to make for an easier process.

The interference helps to secure the relative positioning of the shaft and hub even during rotation, making this type of fit good for transmitting rotational speed and power.

Press Fit

Minimal interference. Assembly can be performed with cold pressing.

Example uses in engineering: Hubs, bushings, bearings, etc.

Example fits: H7/p6 for hole-basis, P7/h6 for shaft-basis

Using a 25 mm diameter, a H7/p6 fit gives a min interference of 0.001 mm and a max interference of 0.035 mm.

Driving Fit

Needs higher assembly forces for cold pressing. Another way is by using hot pressing. This interference fit is more prominent than with a press fit.

Example uses in engineering: Permanent mounting of gears, shafts, bushes, etc.

Example fits: H7/s6 for hole-basis, S7/h6 for shaft-basis

Using a 25 mm diameter, a H7/s6 fit gives a min interference of 0.014 mm and a max interference of 0.048 mm.

Forced Fit

High interference fit. Assembly requires heating the part with a hole and freezing of the shaft to force the mating parts together. Disassembly can result in broken parts.

Example uses in engineering: Shafts, gears, etc.

Example fits: H7/u6 for hole-basis, U7/h6 for shaft-basis

Using a 25 mm diameter, a H7/u6 fit gives a min interference of 0.027 mm and a max interference of 0.061 mm.

The post Limits & Fits appeared first on Fractory.

What Is Tensile Strength?

Imagine a strip of paper being pulled at its two ends with your fingers. You are applying a tensile force on the strip. When this tensile force crosses a certain threshold, the paper tears. The tensile stress at which this takes place is the tensile strength of that material, in this case paper.

When excessive tension is applied, both ductile, as well as brittle materials will approach a point of failure. Initially there will be a uniform deformation observed. All throughout the body of the material, its length will increase while its width reduces at the same rate.

Ultimate tensile strength is the amount of stress that pushes materials from the state of uniform plastic deformation to local concentrated deformation. The necking phenomenon begins at this point.

Ultimate tensile strength is an intensive property. In other words, it does not depend on the size of the sample. The same material with varying cross-sectional area will have the same value of tensile strength.

As this type of fracture in a system can cause failure and possibly endanger life, it is imperative that this parameter is considered while selecting appropriate materials for an application.

Ultimate Tensile Strength on a Stress-Strain Curve

There are 4 major regions that a stress-strain curve can be divided into:

• Proportional limit
• Yield limit
• Strain hardening
• Necking

Proportional Limit

In the proportional limit, the specimen material acts like a spring and any strain caused is completely reversible. On the stress-strain curve, this area is called the Hooke’s region. The reason lies with the applicability of Hooke’s Law for forces that fall into the area.

Yield Limit

As soon as the specimen passes the proportional limit, it enters the yield limit region. At this point, permanent deformation sets in. From this point on, it doesn’t matter if you release the tensile force or apply a force in the opposite direction, the specimen will not return to its original dimensions.

Strain Hardening Region

On further increasing the tensile stress, the specimen enters the strain hardening region. This is a very unique section because you are changing the crystal structure of the material. The material is under enough stress that its very microstructure is modified.

As the name suggests, the material becomes harder and tougher. This hardening can be very useful and so it is not necessarily a bad thing (cold hardening, cold forming processes actually use this region to impart strength to the workpiece).

Necking Region

Right before entering the necking phase, the material is the strongest it will ever be. We have strain hardened it to its maximum limit. When we enter the necking phase, the material starts to get weaker. It is characterized by a local reduction in cross-section.

Beyond this point, the material is only moving towards failure. It can handle less stress with increasing strain.

We can sort of go back to the original equation that says stress is equal to force per unit area and infer that the smaller the area, the higher the stress. The material moves beyond this point until rupturing.

Ultimate Tensile Strength on the Curve

The point that separates the strain hardening region and the necking region is the ultimate strength for that material. At this point, the maximum amount of strain hardening has taken place. The material is handling the highest amount of load it can handle safely.

Ultimate strength is, therefore, a crucial point to be considered on the stress-strain curve. It shows the maximum amount of stress a material can bear before failure.

Why Is Tensile Strength Important?

It is imperative to know the tensile strength of a particular metal or any material to ensure it is the right choice for an application. This ensures an incident-free service life.

The results of choosing materials with lower tensile strength than what the application demands can be disastrous.

Engineers turn to yield strength in the design phase to make sure the stress never reaches any higher than that. Otherwise, the structure suffers permanent deformations. But ultimate tensile strength tells us the value that is necessary for complete failure and breaking.

Thus, a roof construction that comes under more stress because of a higher than normal snow load may bend the structure. At the same time, surpassing the tensile strength value means that the roof may fall in.

Tensile Strength vs Yield Strength

Engineers use yield strength when designing products. Keeping the load within this area ensures the product is safe from failure. This means that the maximum load has to stay below the yield strength limit at all times.

A common way of doing so is by determining the maximum load first. Taking the specifics of the chosen material into account, calculations give the answer for the necessary cross-sectional area. Geometry plays an important role in how high loads a part can withstand. 

As an extra precautionary measure, a safety factor is added. The safety factor usually falls somewhere between 1.5 and 2. The simplest way of using it is just multiplying the maximum load value by the factor. Adding the safety factor ensures that unexpected loads and material imperfections will not result in broken parts.

Designing for ultimate tensile strength means your part will permanently deform once subjected to the load it was designed for. The material’s crystal structure may change and it will probably lose an important property. This means that the product no longer has the same characteristics that may have been the very reason for its selection.

An important point to note here is that some tools like knives and spanners are strain hardened so that they can be stronger and closer to their ultimate tensile strength value before they can potentially fracture.

Tensile Testing

Tensile Test

Tensile strength is measured by elongating a specimen in a Universal Testing Machine (UTM). A UTM is a tensile testing machine.

The specimen is held on opposite ends using clamps. One of the ends is stationary while pulling the other with real-time monitoring of the forces. A steady increase of force takes place until reaching a point where the specimen breaks. The recording of tensile test data is constant all through the process.

This tensile tester consists of features such as servo automation control (electro-hydraulic), data acquisition, automatic measurement, screen display and test result calculation.

The maximum force that was applied is then divided by the cross-sectional area to obtain the maximum stress it was subjected to. This maximum stress is the value of ultimate tensile strength.

The SI unit of ultimate tensile strength is N/m² or Pascal with large numbers being expressed in megapascals.

Examples for Material Tensile Strength

The tensile strength of materials varies significantly. Mechanical engineers use mostly metals because they offer a good return for value and other great properties besides relatively high tensile strength. But it is evident that the range within different types of metals alone is huge.

At the same time, we can see that non-metals like carbon fibers far exceed metals in terms of ultimate tensile strength values. Even human hair can take half the load of structural steels before eventually breaking.

So whether it is a 5th-grade boy who wants to know how hard he can pull a girl’s ponytail before getting attention turns into making her bald, or an engineer who needs to know how much the elevator cable can actually take before breaking, the answer lies here – with ultimate tensile strength.

The post Ultimate Tensile Strength appeared first on Fractory.

A property that is considered more than any other is the ultimate tensile strength of the material. This strength refers to the maximum load that the material will be able to withstand before failure takes place. The factor of safety is then selected to further reduce the risk of breakdown.

However, in many applications, the load is not constant. The load’s magnitude and direction may vary in a regular or irregular manner. Such loads are known as fluctuating loads and can result in a breakdown that can happen at load values far below the ultimate tensile strength.

In this article, we shall learn about the phenomenon of material fatigue and how we can avoid failure caused by it.

What is Fatigue?

Fatigue can be explained as the weakening of a material due to the application of fluctuating loads that result in damage to the material’s structure and eventual failure. The damage starts locally and builds up over time and can end in a catastrophe.

There are many fluctuating loads that parts are subjected to during service. A few examples are as follows:

• A point on the wheel of a railway coach as it comes into contact with the railway track. On coming in contact, this point on the wheel is subjected to a compressive load by the metal track. When it moves away from the bottom, the compressive force on that point subsides. This happens once every rotation and will happen millions of times in a single journey.
• Traffic passing over a bridge applies a fluctuating load on the bridge. The bridge is subjected to the highest sagging load when the traffic is closest to the middle of the bridge. This force is removed when there is no traffic.
• The hull of a boat when it passes over waves is subjected to constant tensile and compressive forces as it makes way. This is especially pronounced during rough weather as the vessel pitches.

Understanding Fatigue Failure

Fatigue failure is something everyone has encountered while trying to break a metal wire.

The process includes bending the wire back and forth numerous times. Each back-and-forth bending is one cycle. When the wire finally breaks, you can count the number of cycles it took to lead to the initial crack and final break.

Knowing the number of cycles and the loading stresses gives the fatigue strength of that material.

If you tried to break the wire just by pulling from both ends, the force requirements would have been pretty high. At the same time, bending only requires minuscule forces and arrives at the same result.

The wire was subjected to fluctuating loads at the bending point as the top and bottom part of the wire cross-section were alternately stretched and compressed. With enough number of load cycles, the wire breaks.

This is an example of fatigue failure. Thus, it can happen quite easily even at small loads and a small number of load cycles, depending on the material.

It is also worth mentioning that fatigue failure does not happen gradually. It is instantaneous, like a brittle material just breaking into smaller pieces.

Fatigue Failure

It is also far less predictable than regular failure as there are few prior indications. These indications are difficult to perceive with the naked eye. In regular failure, due to excessive loading, necking will be observed but there is no necking before fatigue breakdown occurs. This makes it impossible to predict exactly at what point in time a part will fail due to fatigue.

To prevent such a situation, the parts must be closely inspected and changed after a recommended number of load cycles. The number of cycles depends upon the material characteristics and the magnitude of the load. A higher load means a shorter lifecycle. This can be better understood using an S-N curve which we will get to shortly.

Failure Process

Fatigue failure happens without warning but on careful observation, some indications are evident.

Formation of the initial microcrack

On continuous exposure of the material to cyclic loading, minuscule cracks start to develop at high-stress points. These cracks can be observed by non-destructive testing methods.

Dye Penetrant Inspection

Dye penetrant testing for inspection of diesel generator engine connecting rods during routine overhauls is one such example of crack detection. If cracks are found, the part is replaced with a new or reconditioned spare part.

Crack propagation

Once a fatigue crack has occurred, it propagates through the part with every load cycle. While spreading through the material, it will usually produce striations on the surface.

Striations are marks on the surface that show the position of the crack tip. These striations are a tell-tale of the development of a fatigue crack. The crack propagation is extremely slow when the crack is first initiated and is referred to as Stage I crack growth.

Fracture

When the crack reaches a critical size and the stress intensity at the point exceeds the fracture toughness of the material, the crack spreads at a high speed.

This type of fast propagation is known as Stage II crack growth and it happens in a direction perpendicular to the applied force.

Over time, the material is unable to handle any further loading and complete fracture occurs. This failure is immediate and can result in serious consequences for the workers using the machine and the machinery itself.

Machine Components Prone to Fatigue

Some parts of a machine are more likely to succumb to breaking due to fluctuating stresses. One must be aware of such parts so that a breakdown can be avoided.

Shafts

Fatigue is on the top of the list when looking at shaft failure causes. Crankshafts, for instance, have to face serious cyclic loading. They form an integral part of many prime movers such as diesel generators, marine engines, vehicle engines, and reciprocating compressors.

Substandard design is the primary cause of damaged shafts. A crankshaft fillet rolling process may be used to improve its fatigue life.

Other shafts such as those in reciprocating and centrifugal pumps, motors, etc. are also subjected to cyclic loads resulting in a danger of breakdown.

The stress comes from different points of load. For example, at one end of the shaft is the motor which rests some of its weight on the shaft. There may be 2 bearings acting as points of support. At the same time, the rotational speed needs transmitting, maybe in the form of a sprocket-wheel and chain, both of which load the shaft further.

During rotation, the direction of the load is constantly changing subjective to the axis of the shaft. Thus, a break can occur similarly to our previous example of a wire.

Bolts

Bolts are known as a source of possible failure. They have to resist the tensile and compressive stresses due to machinery movement/vibration.

Insufficient bolt tightness gives room for more movement. Thus, the bolts must be tightened to a load exceeding the maximum working load.

Gears

Gears transmit power through their teeth. These teeth deal with high loads when they are in contact with the other mating gear’s tooth. When they move away from this position, the stresses are removed. Cyclic loading occurs as the magnitude of forces on a tooth changes continuously.

Furthermore, the load is not evenly distributed on the entire length of the teeth. This localisation of load can further exacerbate fatigue issues.

Fatigue Strength

Fatigue strength is the ability of a material to resist fatigue failure.

ASTM defines it as the limiting value of stress (denoted by S_Nf) at which failure occurs after N_f number of load cycles. This number of cycles can be from a few cycles up to a large number depending upon the load and material.

After a certain number of load cycles have passed, the material can fail at any point and it is prudent to change it as soon as possible.

Fatigue Limit

Fatigue limit, denoted by S_f, is defined as the value of stress at which N_f (number of cycles at which failure occurs) becomes very large. N_f can be increased by reducing the stress value of the cyclic load.

When keeping the load below the fatigue limit, a part can withstand a huge number of cycles, usually more than 10 million but up to 500 million.

The difference between fatigue strength and fatigue limit is in the number of cycles. It is considerably higher with fatigue limit. Therefore, engineers try to design their parts so that they are kept under the fatigue limit during work.

Calculating the Fatigue Limit

There are various fatigue tests available that allow us to determine the fatigue limit of different materials. These are as follows:

1. The stress-life method. In this method, the fatigue life of materials is calculated by subjecting it to different stress amplitudes. Then the curve is plotted. This curve is also known as the S-N curve.
2. The strain-life method. When the strain produced due to cyclic loading is no longer elastic, it can be used instead of stress to plot a graph of strain vs. life.
3. The crack growth method. In this method, the amount of crack growth per stress cycle is calculated. This gives us the rate of crack growth an helps us estimate the number of cycles required to reach critical size for a crack.
4. Probabilistic methods. This is based on stress/strain-life or crack growth methods and improves the accuracy of predicted service life. The above methods do not account for probability distribution by themselves. However, we know that not all parts will fail at the same time once N_f stress cycles have passed. Using probability distribution helps us narrow down the number of cycles without failure.

Endurance Limit

It is evident from the graph that the S-N curve becomes progressively flatter as the stress values are reduced. This is the case for most materials. 

There comes a point where the curve for steel becomes horizontal. This means that below this stress value, the material will never fail because of fatigue. This stress limit is known as the endurance limit.

At the same time, aluminium does not have such properties and while it can last more cycles with a lower load, there is no endurance limit.

The difference between fatigue limit and endurance limit is that the endurance limit does not define a limited number of cycles after which the breakdown will occur. The part basically has infinite life and will not exhibit fatigue failure. It has an indefinite fatigue life.

S-N Curve

Fatigue Failure & S-N Curve

Fatigue life prediction can be done by plotting the S-N curve, where S stands for Stress applied and N stands for the number of load cycles. Most S-N curves are plotted in laboratories where different specimens are tested at varying stress values using a metal coupon testing machine. The specimen’s N_f is noted. N_f is the number of cycles at which failure occurs.

Then these points are plotted on a logarithmic S-N curve to accommodate large values of N_f. Linear regression is then used to form a continuous curve instead of discrete test data points on the graph.

The finished curve resembles a hyperbola to some extent. The equation for the curve is a derivative of SN=constant.

This means that with decreasing stress amplitude, number of stress cycles increases exponentially. A single equation cannot be used for the entire curve and different equations are defined for data points within a selected range. These equations can be calculated manually or through software like Solidworks.

Once an S-N curve is available, it can be used to determine the fatigue life of a part if the cyclic load value is known.

Accounting for Fatigue in Design

Owing to excellent research on the metal fatigue phenomenon in the last few decades, a lot of information is available to design parts more efficiently.

When designed within the endurance limit, the material will practically last forever provided corrosion and other factors are not present. Let’s take a look at how proper design can improve the life expectancy of materials.

1. Designed within the endurance limit. The maximum cyclic load that will be applied during service must be within the endurance limit to prevent fatigue failure.

2. Fault-tolerant design. The design must account for the failure of parts. As far as possible, it must be ensured that a single part’s failure does not cause a complete stoppage of the system or irreparable damage to the machinery.

3. Regular renewal of vulnerable parts. The design must take into account the fatigue limits of various parts and must include recommended service duration for these parts. These service periods can then be added to the preventive maintenance system for easy review and part renewal.

4. Damage tolerance. While designing a part, the likelihood of the existence of defects in a new part must be accounted for. It must be assumed that cracks may already be present.

5. Risk assessment. The risk of part failure must be regularly assessed to ensure it remains within acceptable levels.

Other than design, testing and repair must be carried out to prevent fatigue failure.

The post Material Fatigue Strength appeared first on Fractory.