What is carburizing?

What is carburizing process?

Carburizing is one of the most widely used surface hardening processes for a long time. 

Carburising is a method to harden low-carbon steel. Carbon atoms are diffused to steel surface after carburisation, quenching is done and steel gets hardened.

Carburizing heat treatment is the introduction of carbon into the surface of the steel and produces a surface which is resistant to wear while maintaining toughness and strength of the core. It occurs when the steel is heated above the critical temperature in a carburizing furnace which contains more carbon than steel contains.

The process involves diffusing carbon into low carbon steel to form a high carbon steel surface.

It is essentially the addition of carbon at the surface of low carbon steels at appropriate temperatures.

Hardening agents :

There are different types of elements or materials that can be used to perform this process as hardening agents. Mostly used hardening agents are following below :

• Carbon monoxide gas ( CO )
• Sodium cyanide and barium carbonate 
• Hardwood charcoal 

Effects of carburizing in material properties :

Mechanical :

• Increased surface hardness.
• Increased wear resistance.
• Increased fatigue or tensile strength.

Physical :

• Grain growth may occur.
• Change in volume may occur.

Chemical :

• Increased surface carbon content.

Now we can discuss about the various carburizing processes commonly used by the industry.

• Pack carburizing 

It is a process in which carbon monoxide derived from a solid compound decomposes at the metal surface into nascent carbon and carbon dioxide.

Nowadays led to reduced use of pack carburizing because of environmental concerns. In this process low carbon steel components are packed in an environment with high carbon content such as carbon powder. 

The components are heated with the production of carbon monoxide, which is called a reducing agent. The steel components are become hardened because of the carbon absorption inside the component. 

Control of this process is difficult because it is difficult to maintain uniform temperatures. It is an effective process in introducing carbon but this method is exceedingly slow. In this process carbon monoxide is given off by coke or hardwood charcoal.

• Gas carburizing 

Gas carburizing is a surface hardening process which is carried out at a very high temperature. 

The parts are surrounded by a carbon-bearing atmosphere that can be continuously replaced and because of that, a high carbon potential can be maintained. 

The gas carburizing process is mostly similar to pack carburizing process aside from the supply of carbon monoxide (CO) gas to the heated furnace and the carbon decomposition. CO gas needs to be contained safely. 

Gas carburizing is the most effective and widely used method for carburizing steel parts in large quantities in many industries.

• Liquid carburizing 

Liquid carburizing is a process used for case hardening of steel or iron made components. 

In this process, the components made from steel are submerged in a liquefied carbon-rich environment. The main component in such baths is cyanide. 

Cases produced by liquid carburizing are lower in nitrogen and higher in carbon than cases produced by cyaniding. However, safety issues have led to baths that are non-toxic that produced similar results. 

Liquid carburizing involves faster heat-up so that cycle times for liquid carburizing are shorter than those for gas carburizing. The components are held in a molten salt that converts carbon into the steel. 

Carbon is diffused inwards producing a hardened case by rapid quenching process. The case produced by carbon diffusion is similar to that produced by the gas carburizing process. Cases that formed by liquid carburizing have low nitrogen and high carbon content.

• Vacuum carburizing :

The vacuum carburizing process involves carburizing under an absolute pressure environment which is free from oxygen. Since the heating environment is oxygen-free, the carburizing temperatures can be improved significantly without border oxidation of the surface or grain. The atmosphere is significantly simplified although the furnace enclosure is more complex. A single component environment containing hydrocarbons like methane and that can be used. 

The higher temperatures increase the solubility of carbon and the diffusion rate that is why the time necessary for case depth is reduced. The carbon potential of the gas in deep recesses and blind holes is rapidly depleted as vacuum carburizing is carried out under very low pressures and the furnace flow rate of the carburizing gas is very low.

Unless this gas is replaced, a great non-uniformity in case of depth over the surface of the part is likely to occur. The gas pressure is increased significantly, another problem arises, that of free-carbon formation, or sooting is an effort to overcome this problem.

Therefore, in order to obtain uniform depth over complex shape, the gas pressure must be increased periodically to refill the down atmosphere in recesses and then reduced again to the operating pressure. 

• Plasma carburizing 

Plasma carburizing is environment-friendly. 

With the use of plasma carburizing improve characteristics such as wear and corrosion resistance, hardness and load-bearing capacity with an even treatment of components with complex geometry.

In plasma carburization method, plasma impinges positive carbon ions on the surface of a steel part that used as a cathode. 

The primary difference between standard and plasma carburization is that plasma techniques achieve reduced carburizing times. 

Insignificant industrial plants, plasma carburization used rises because it improved the surface features of different steels and particularly stainless steels.