Practical Model to Predict Diffusion Layer's Hardness Profile in Gas Nitrided Low Alloy Steel Containing Chromium

A model to predict the diffusion layer's hardness profile in gas-nitrided low alloy steels such as JIS-SCM435, DIN-31CrMoV9 and JIS-SACM645 was successfully developed taking into account the following considerations. The diffusion of dissolved nitrogen in the surface boundary was given as the nitrogen flux, which can be determined by either the nitriding potential or thermo-equilibrium between α-Fe and γ′-nitride after the formation of the compound layer. The kinetics of CrN and AlN precipitations was characterized as the KP concerned with nucleation and diffusion of Cr or Al and the parabolic rate constant χ concerned with the growth of disc-shaped CrN and AlN. KH was defined as a proportional constant between the increase in hardness and the nitrogen concentration. KP and the solubility product KCrN for CrN were fitted well using the numerical model and the hardness profiles. KH was experimentally determined by using the hardness and nitrogen concentration in the diffusion layer. Then, the numerical results showed good agreement with the experimental ones. In SACM645especially, a more accurate calculation was demonstrated taking into account immobile excess nitrogen in the CrN and AlN.