Effect of gas nitriding on the thermal fatigue behavior of martensitic chromium hot-work tool steel

The influence of heat treatment and gas nitriding on the thermal fatigue behavior of martensitic chromium hot-work tool steel was investigated. Thermal fatigue tests were carried out using a special induction heating apparatus, which consisted of induction heating and water spray cooling unit. The process of thermal cycling was simulated using a coupled heat-transfer solid-mechanics finite element model. It was seen that the thermal fatigue resistance was higher in the gas nitrided samples after austenizing at 1020 °C than for the gas nitrided samples after austenizing at 1100 °C. The thermal fatigue endurance limit was found to be maximum for the samples having a compound layer comprising of the higher phase fraction of γ' (Fe4 N1−x). It was also found that, lower the ratio of compound layer thickness to the total diffusion depth, the higher is the fatigue life. These results were influenced by two major effects of nitrided (diffusion) layer. First is the high compressive residual stresses imparted on the surface which tend to mitigate the effect of thermal tensile stress and secondly very high surface hardness, due to the diffusion of nitrogen, which increase the threshold for crack initiation at the surface.