A new approach to hardening mechanisms in the diffusion layer of gas nitrided ?-alloyed steels. Effects of chromium and aluminium: experimental and simulation studies

Abstract

Hardening mechanisms in the diffusion layer of gas nitrided ?-iron and -steels have been investigated
through the study about effects of chromium (binary alloys and industrial steels) and aluminium
(industrial steel). After nitriding (520°C 48h), nitrogen mass balance between total nitrogen
concentration located in the diffusion zone, experimentally determined, and the expected theoretical
nitrogen concentration, reveals for each alloy a “nitrogen excess”. Jack and Mittemeijer [1-3] suggested
that the volume misfit between semi-coherent nitrides and matrix induces local matrix lattice distorsion,
leading to a local increase of nitrogen solubility in the matrix.
We propose a new approach, based on thermodynamical calculations (Thermo-Calc software), confirmed
by different characterization methods (HRTEM, EDX and X-Ray). Indeed no significant solid solution “N
excess” occurs, but the total nitrogen concentration is explained by complex MN nitrides precipitation,
isomorph of CrN FCC, containing chromium, iron (up to 30at.% at 50?m from the surface), molybdenum
and vanadium. During annealing (520°C 48h), atomic iron fraction in MN nitrides decreases and the
corresponding nitrogen atomic fraction diffuses to the core.
Addition of aluminium in industrial steel strongly increases nitrogen concentration and hardening
(?=HVx-HVinitial). Aluminium induces in the diffusion layer precipitation of Fe4N and Fe2-3N and
precipitates in complex MN FCC nitrides, containing chromium, iron and molybdenum.