Chemical Heat-Treatment (Casehardening) of Steel

 

Chemical heat-treatment, or casehardening as it is called in most cases, consists in adding some elements (for instance, carbon, nitrogen, aluminum, chromium and other) to the surface of steel by diffusion of this element from surrounding media at a high temperature.

Carburizing is the process of adding carbon to the surface layer of steel. Distinction is made between two main procedures: (l) pack carburizing and (2) gas carburizing.

In pack carburizing the saturating medium is a solid carburizer, usually activated charcoal, semicoke or peat coke. A widely used carburizer consists of 20 to 25 % BaCO₃,
3.5 % CaCO₃ and charcoal. Work pieces and carburizer are placed in metallic boxes, closed and heated to temperature from 910 to 930°C. Under such conditions:

 

,                                            (1.23)

                                                        (1.24)

The carbon C_at evolved in this reaction is in the atomic state. It diffuses into the austenite.

Gas carburizing is accomplished by heating the work in a medium of gases containing carbon:

,                                                          (1.25)

 

Gas carburizing time ranges from 6 to 12 hours to obtain a case from 0.7 to 1.5 mm in thickness.

The final properties of carburized works are obtained as a result of hardening and low-temperature tempering. Idea of carburizing is to obtain surface layer, which has high strength, hardness and wear resistance.

Nitriding is the process of adding nitrogen to the surface of steel by heating it in ammonia gas at 480…700°C:

,                                              (1.26)

 

The atomic nitrogen thus formed diffuses into iron and forms solid solution with a-phase and chemical compounds (Fe₄N, Fe₂N, CrN, MoN, A1N) and therefore increases hardness, strength, wear resistance and resistance to corrosion.

Cyaniding and Carbonitriding of steel are processes in which both carbon and nitrogen are added to the surface layer of steel. Cyaniding consists in heating the steel in a liquid medium; if the process is performed in gaseous atmosphere, it is called carbonitriding.

Diffusion coating. The impregnation of the surface of steel with Al, Cr, Si, B and other elements is called diffusion coating, or metallic cementation. Components whose surface is coated in this manner acquire various valuable properties, such as high heat resistance, corrosion resistance, increased wear resistance and hardness.

The diffusion coating of steel with various metals and silicon can be done at 900…1050°C by:

- embedding the components in the corresponding mixtures (usually ferroalloys and 0.5 to 5 % NH₄Cl);

- immersing them in the molten metal if the diffusing element has a low melting point (zinc, aluminum);

- impregnation from a gaseous medium (A1C1₃, CrCl₂, SiCl₄, etc.).