Thermal fatigue evaluation of AISI H13 steels surface modified by gas nitriding with pre- and post-shot peening

In this work, the H13 hot work die steels were surface modified and experimentally evaluated in a homemade thermal fatigue simulation tester. Two types of nitrided compound layers, and compound-free layers with and without surface deformation were prepared by a selection of surface modifications including traditional gas nitriding, controlled gas nitriding, gas nitriding with pre-shot peening, and controlled gas nitriding with post-shot peening. Thermal cracks formed in the nitrided compound layer propagated faster on surface but slower along the depth direction as compared to those in the compound-free layer. Thermal crack advancing along the depth direction in the sample nitrided with pre-shot peening had a tendency to be suppressed by the higher cross-sectional hardness distribution. The deformed compound-free nitrided layer showed the excellent thermal fatigue cracking resistance in both surface and cross-section, which was attributed to the good combination of hardness and toughness, as well as the underlying enhanced compressive residual stress.