{"id":21848,"date":"2023-07-27T11:31:53","date_gmt":"2023-07-27T08:31:53","guid":{"rendered":"https:\/\/fractory.com\/?p=21848"},"modified":"2024-01-26T13:07:07","modified_gmt":"2024-01-26T11:07:07","slug":"carburising-explained","status":"publish","type":"post","link":"https:\/\/fractory.com\/carburising-explained\/","title":{"rendered":"Carburising Explained – How It Works, Benefits & Types"},"content":{"rendered":"

Carburising is a traditional and reliable thermochemical process<\/a> used by small workshops as well as large industries to harden low-carbon steel parts. The method has seen incredible advancement over the years due to automation and a better understanding of the chemical reaction\u2019s thermodynamics and kinetics.<\/p>\n

In this article, we explore this process to understand how it works, the advantages it provides and its most common methods. Let\u2019s start by defining what carburising is.<\/p>\n

What Is Carburising?<\/strong><\/h2>\n

Carburising is a heat treatment process that improves mechanical properties<\/a> such as hardness by adding carbon to a metal\u2019s surface.<\/p>\n

As the carbon addition is limited to the surface, the process forms a hard case around the part along its surface area. Thus, the carburising process is a type of case-hardening process<\/a>. Other examples of case hardening processes are nitriding<\/a>, carbonitriding, ferritic nitrocarburising, etc.<\/p>\n

When Is Carburising Used?<\/strong><\/h3>\n

Carburising is generally used to harden the outer surfaces of steel parts after they have been machined into their final shape<\/a>.<\/p>\n

The amount of carbon in a part\u2019s composition directly determines the final hardness. If the initial carbon content is low, just heating and quenching a part is not enough to increase the hardness.<\/p>\n

For such parts, we must first increase the carbon composition of the material. This is done by carburising. By manipulating the factors in a carburising process, we can increase the carbon content to the desired concentrations and case depths from the surface and then harden the part as needed.<\/p>\n

This makes the process ideal for low-carbon steels<\/a> that are either plain carbon steels or steels with alloying elements.<\/p>\n

Carburising Benefits<\/strong><\/h3>\n

The carburising process has many advantages. As a result, it has become widely accepted across different verticals to provide reliable, case-hardened parts<\/a>. Let us take a look at some of these advantages:<\/p>\n

Simple and versatile process<\/h4>\n

The process is simple and can be performed in rudimentary furnaces by unskilled labour in many cases. It works with a wide range of carbon steels, alloy steels and cast iron<\/a>, where the maximum carbon content is 0.4%.<\/p>\n

It can also work with very complex designs, as long as the uneven cooling rates of different sections are accounted for. If they are not considered, the material will develop excessive stress and break.<\/p>\n

Low-cost<\/h4>\n

Carburising can provide parts with a hardness similar to that of tool steel<\/a> without their high cost. Of course, there are limitations, such as the allowable service temperature and durability, but carburised steel can replace expensive steel in many applications without safety or functionality issues.<\/p>\n

Mass production<\/h4>\n

Carburising is suitable for high-volume production as the process can be automated to churn out large batches of surface-hardened products continuously.<\/p>\n

Good dimensional control<\/h4>\n

The parts will almost always undergo small deformations, but compared to other heat-treating operations, the changes are smaller and more manageable.<\/p>\n

High wear resistance and ductility<\/h4>\n

Wear resistance and ductility are normally mutually exclusive in materials. A wear-resistant surface is obtained by hardening a workpiece. But hardening also makes a material brittle, which means it loses its ductility and, with it, its impact strength.<\/p>\n

Case-hardening gives us parts that are hard at the surface but have a soft and tough core. This enables the parts to have high wear resistance and ductility at the same time.<\/p>\n

Thus, a part can endure impact loads as well as resist abrasion<\/a> competently.<\/p>\n

High fatigue strength<\/h4>\n

Carburised parts have high fatigue strength<\/a> because of their unique construction. The tough interior along with the hard outer layer allows it to withstand higher fatigue loading. As is seen in the case of case-hardened 18CrNiMo7-6, the fatigue limit after carburisation is about 60% higher than before.<\/p>\n

Moreover, higher surface hardness<\/a> suppresses fatigue crack initiation and changes the fatigue failure mode from surface failure to internal failure.<\/p>\n

Carburising Processes<\/h2>\n

The carburising process adds carbon to a metal\u2019s surface. This is done by heating the metal in a carbon-rich atmosphere. At higher temperatures, the carbon will either diffuse into the material\u2019s solid solution or form carbides. For both of these processes, high temperatures are necessary.<\/p>\n

If the carbon only enters the solid solution, we need to carry out further heat treatment to increase the metal\u2019s hardness to its maximum level. The surface must be free of any contaminants, such as oil or an oxide layer, for easy diffusion into the surface. Carbides, on the other hand, are naturally hard materials with a high melting point<\/a>.<\/p>\n

To add carbon to a metal\u2019s surface, four different mechanisms can be used. The difference lies in the way the base material is exposed to the carbon-rich substance.<\/p>\n

Solid Carburising or Pack Carburising<\/h3>\n
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