Carburising Explained – How It Works, Benefits & Types

Carburising is a traditional and reliable thermochemical process 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’s thermodynamics and kinetics.

In this article, we explore this process to understand how it works, the advantages it provides and its most common methods. Let’s start by defining what carburising is.

What Is Carburising?

Carburising is a heat treatment process that improves mechanical properties such as hardness by adding carbon to a metal’s surface.

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. Other examples of case hardening processes are nitriding, carbonitriding, ferritic nitrocarburising, etc.

When Is Carburising Used?

Carburising is generally used to harden the outer surfaces of steel parts after they have been machined into their final shape.

The amount of carbon in a part’s 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.

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.

This makes the process ideal for low-carbon steels that are either plain carbon steels or steels with alloying elements.

Carburising Benefits

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

Simple and versatile process

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, where the maximum carbon content is 0.4%.

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.

Low-cost

Carburising can provide parts with a hardness similar to that of tool steel 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.

Mass production

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

Good dimensional control

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

High wear resistance and ductility

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.

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.

Thus, a part can endure impact loads as well as resist abrasion competently.

High fatigue strength

Carburised parts have high fatigue strength 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.

Moreover, higher surface hardness suppresses fatigue crack initiation and changes the fatigue failure mode from surface failure to internal failure.

Carburising Processes

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

If the carbon only enters the solid solution, we need to carry out further heat treatment to increase the metal’s 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.

To add carbon to a metal’s surface, four different mechanisms can be used. The difference lies in the way the base material is exposed to the carbon-rich substance.

Solid Carburising or Pack Carburising

Pack Carburising Process

One of the oldest and most affordable methods of carburising is pack carburising. In this method, the metal workpiece is placed in the presence of solid carbonaceous material, like crushed charcoal briquettes, for example. This is why it is also known as solid carburising.

Let’s break pack carburising down even more and discuss the process for 1018 carbon steel. It consists of three main steps:

• Carbon addition

• Heat treatment

• Stress relieving or tempering

Carbon addition

The steel is placed in contact with the carbonaceous material on all sides in a tightly sealed steel box. Refractory clays, such as fire clay, can be used for sealing the box.

Once packed, the box is placed into a furnace and heated at around 925 °C (~1700 °F) for about 8–10 hours. In the furnace, the carbon dioxide from the carbonaceous material dissociates into carbon monoxide and elemental carbon. The carbon diffuses into the metal.

Over time, this increases the carbon composition of the surface layers. The absorption rate and the depth to which the carbon reaches depend on time, furnace temperature and the atmosphere.

Once the part has absorbed sufficient carbon, the box is removed from the furnace and the part taken out of the steel box. The metal is still soft at this point but has a higher carbon potential.

Heat treatment

Now, it’s time for the heat treatment process to increase the hardness. The steel is placed directly in the furnace at about 790 °C (~1450 °F) and held there for about 15 minutes. This allows the pearlite microstructure to transform completely into austenite. 

The parts are taken out from the furnace and quenched rapidly in water. The quick cooling does not allow the reverse transformation back to austenite, and instead, the martensite microstructure develops, which is extremely hard.

Stress relieving or tempering

Quick cooling can lead to the formation of internal stresses in the part. It is recommended to relieve these stresses by tempering – heating to a lower temperature of about 205 °C (~400 °F) for about an hour and this time around, the part is not quenched but cooled in air.

After all these steps, we have a case-hardened 1018 steel part that is up to six times harder than the untreated part.

Despite its obvious merits, the pack carburising method is not as common for commercial use these days because it is too slow compared to some more modern methods. But it still finds applications in small workshops where the process is used for case-hardening large gears, knives, shafts, etc.

Vacuum Carburising

In the vacuum carburising method, carbon diffusion takes place in a vacuum or low-pressure furnace. The metal part is exposed to propane, which quickly breaks down into its constituents of carbon and hydrogen. The carbon is absorbed readily and at much lower temperatures than other methods, which saves both time and money.

The process also doesn’t emit any harmful gases. Thus, in vacuum carburising, we have a fast, sustainable, and affordable process that we can control precisely. Vacuum carburising is therefore used in the manufacture of precision tools, gears, and complex parts that require a high degree of accuracy.

Liquid Carburising (Cyaniding)

As the name suggests, liquid carburising is carried out in molten salt baths. The salts are carbon-based salts. Since the most commonly used salt is sodium cyanide, the process is also known as cyaniding.

The process takes place at 850–950 °C and is the fastest of all carburising processes. However, extreme precaution is needed during the process as cyanide is highly toxic to humans.

Gears and nuts and bolts are commonly hardened using the liquid carburising process.

Gas Carburising

When a gas atmosphere is the carbon source for the carburising process, it is known as gas carburising. Commonly used gases for carburising are propane and methane. When they are exposed to hot steel in the presence of a calculated quantity of air and catalysts, they dissociate at the metal surface into elemental carbon, which is then absorbed into the surface.

The typical carburising temperature for the process is around 925 °C (1700 °F). By controlling the process temperature and the atmosphere, we can control this process very well. This makes it suitable for mass production. Engine crankshafts are normally carburised using the gas carburising method.

Carbonitriding

Carbonitriding is a special type of case-hardening process that involves exposure to materials that add not only carbon but also nitrogen to the metal. Similar to how carbon forms carbides, nitrogen can form nitrides in the metal’s lattice. Nitrides show properties similar to carbides with their high hardness and high melting point.

Thus, the formation of nitride can also help us improve the surface hardness of low-carbon steel parts.

In carbonitriding, along with propane/methane, ammonia is added to the gas mixture as the nitrogen source.

Carbonitriding requires lower furnace temperatures compared to the carburising process. The typical temperature for a carbonitriding process is 840 °C (1550 °F).

Unlike carburising, carbonitrided parts can retain their hardness at higher service temperatures.