Carburizing, like other chemical heat treatment, also includes three basic processes.
1 decomposition: the decomposition of carburizing medium produces active carbon atoms.
2 adsorption: the activated carbon atom is absorbed by the surface of the steel and then dissolved in the surface austenite, which makes the carbon content in the austenite increase.
3 diffusion: when the surface carbon content increases, there is a concentration difference between the surface carbon content and the center carbon content, and then the surface carbon diffuses to the interior. The diffusion rate of carbon in steel mainly depends on the temperature, and is related to the concentration difference between the inside and outside of the infiltrated elements in the workpiece and the content of alloy elements in the steel.
The material of carburized parts is generally low carbon steel or low carbon alloy steel (carbon content is less than 0.25%). After carburizing, quenching is necessary to give full play to the beneficial effect of carburizing. After carburizing and quenching, the surface microstructure of workpiece is mainly martensite with high hardness plus residual austenite and a small amount of carbide. The core microstructure is low carbon martensite with good toughness or non martensite, but ferrite should be avoided. Generally, the depth of carburized layer ranges from 0.8 mm to 1.2 mm, and the depth of carburized layer can reach 2 mm or deeper. The surface hardness is hrc58-63, and the core hardness is hrc30-42. After carburizing and quenching, the surface of the workpiece produces compressive internal stress, which is beneficial to improve the fatigue strength of the workpiece. Therefore, carburizing is widely used to improve the strength, impact toughness and wear resistance of parts, so as to extend the service life of parts.