Hardening Behavior in Diffusion Zone of Fe–Mn and Fe–Cr Binary Alloys Nitrocarburized after Cold Working

Abstract

To clarify the effect of dislocation on the precipitation behavior of nitrides, Fe–Mn and Fe–Cr binary alloys were deformed, nitrocarburized, and examined their hardening behavior. We measured the content of N in a diffusion layer and estimated the amount of alloy nitrides based on the measured N content. With the Fe–Cr alloy, the deforming have little effect on the hardness and CrN precipitation behavior. With the Fe–Mn alloy, the deforming contributes to increase in hardness with denser and finer Mn₃N₂ precipitation. Transmission electron microscopy observation shows CrN precipitates mainly by homogeneous nucleation but Mn₃N₂ precipitates mainly on dislocation. The thermodynamic calculation shows that substitutional alloy element (M) and N can form M–N clusters in Fe–Mn–N and Fe–Cr–N systems through phase separation of bcc lattice. Because Cr–N clusters have a higher nucleation driving force than Mn–N clusters, they can be formed by homogeneous nucleation. We evaluated the degree of the precipitation hardening and revealed that CrN has a larger hardening ability per unit precipitation amount than Mn₃N₂.