Gaseous nitriding behaviour of 33CrMoV12-9 steel: Evolution of the grain boundaries precipitation and influence on residual stress development

The gaseous nitriding behaviour of 33CrMoV12-9 steel was investigated. The co-diffusion of nitrogen and carbon, precipitation at grain boundaries and mechanical properties evolutions (microhardness, residual stresses) are compared for several nitriding potentials, nitriding time and temperatures. While hardening unequivocally depends on the precipitation of alloying element nitrides MN (M = Cr, V…), the development of residual stresses follows phase transformation kinetics during nitriding. The transformation of carbides into nitrides is the limiting rate in the supersaturation of nitrogen in the ferritic matrix and the diffusion of the released carbon. The lower carbides transformation kinetics the smaller decarburization occurs during nitriding and residual stresses are more compressive. The kinetics is controlled by both the temperature and nitriding potential.