Most heat treatment companies would apply case hardening procedures to a workpiece so that its surface will become harder. Case hardening refers primarily to the process of hardening or coating the surface of a metal workpiece with a thin layer of harder metal while subsequently allowing the base material of the same workpiece to remain soft. The case that surrounds the workpiece can be made from carbon, nitrogen, or both elements. 

To date, there are two popular methods of conducting case hardening: nitrocarburizing and carbonitriding. Some vital factors that can determine the most compatible case hardening method for a specific workpiece include its material, part specifications, and intended uses or application.

Nitrocarburizing

Nitrocarburizing is the dissolution of both carbon and nitrogen into a specific workpiece. However, this case hardening method utilises more nitrogen compared to carbonitriding. For this case hardening method, there are two forms that one can apply: austenitic and ferritic.

Austenitic nitrocarburizing is intended to provide a deeper case depth and longer wear life of a workpiece. It is conducted at temperatures ranging between 675 and 775°C so that there will be no phase change occurrences. This form of nitrocarburizing is controlled to produce a surface layer of epsilon carbonitride and another surface of bainite or martensite, which all provide a great support structure for the hard surface layer. Austenitic nitrocarburizing is great for intermediate stress-point contact-resistance applications.

Ferritic nitrocarburizing, on the other hand, can easily provide case hardening without the need to heat metal parts into a phase change. This form of nitrocarburizing is typically conducted between 525 and 625°C to allow the diffusion of nitrogen atoms to diffuse into the steel. With the same temperature ranges, carbon atoms cannot simply diffuse into the workpiece of steel. This process significantly improves scuffing resistance, fatigue properties, and corrosion resistance of the workpiece.

Carbonitriding

In a carbonitriding process, several parts are heated into a sealed chamber before adding nitrogen and carbon. The heating of the parts is done within the austenitic range, which allows a phase change in the crystal structure of the steel and subsequently permits carbon and nitrogen elements to diffuse into them. Ammonia is introduced into the carburizing process so that nitrogen can be diffused into the parts.

Low-carbon, low-alloys steels are hardened well whenever nitrogen is added into them. Nitrogen, which comes in the form of ammonia gas molecules, can help workpieces maintain their needed hardness during high-temperature operations and applications. Compared to nitrocarburizing, carbonitriding can easily provide greater case depths to metal workpieces. Some limitations with this case hardening process, however, are the long process time and tons of resources involved in achieving great case depths.

Harden surfaces of workpieces, which are processed easily and inexpensively, can benefit from carbonitriding. The application of this case hardening method can significantly boost wear resistance and fatigue strength of the involved workpieces.

While both case hardening methods have their own set of differences, they can still enhance the properties of metal workpieces. If you want to know more about these methods, feel free to contact us at Alpha Detroit Heat Treatment.