Soft nitriding generally refers to nitrocarburizing. The process of infiltrating nitrogen and carbon elements on the surface of the workpiece at the same time is called nitrocarburizing. The process can be carried out in various media such as gas, liquid and solid.

Nitrogen and carbon co-infiltration and single nitriding have the following characteristics:

1. Due to the existence of activated carbon atoms, the nitriding speed is accelerated. Once the surface ε phase is formed, it will create conditions for carburizing at the co-carburizing temperature. This indicates that carbon and nitrogen promote each other in co-infiltration. Therefore, the nitrocarburizing rate is much higher than the single nitriding rate.

2. In the ε phase of the nitrocarburized compound layer: in addition to nitrogen, it also contains a certain amount of carbon (about 2%). Since the ε phase contains carbon, the brittleness of the ε phase is reduced. This indicates that the white bright compound layer after nitrocarburizing does not exhibit brittleness.

Screenshot of Fe-N-C phase diagram at 565 degrees

Screenshot of Fe-N-C phase diagram at 565 degrees

3. It can be seen from the Fe-N-C phase diagram that when the nitrocarburizing temperature is 565 ℃, there is a γ phase, which can dissolve 0.35% C and 1.8% N. During rapid cooling, this γ (nitrogen-containing carbon austenite) is quenched and transformed into α' phase (nitrogen-containing carbon martensite). During slow cooling, eutectoid reaction of γ phase occurs to form (α+γ') eutectoid, and γ' phase is continuously precipitated from α phase, which reduces the hardness of the infiltrated layer. Production practice has proved that nitrocarburizing temperature is preferably 560-570 ℃, and the hardness of rapid cooling is greatly improved (about 10HRC higher than that of slow cooling). Therefore, when nitrocarburizing, especially carbon steel, rapid cooling, such as oil cooling, must be used, and this is also the reason.