Influence of multiple nitriding on the case hardening of H13 tool steel: experimental and numerical investigation

Controlled gas nitriding represents one of the most important factors in enhancing the service life of AISI H13 steel dies used for the hot extrusion of aluminum alloys. Such surface-hardening treatment is used repeatedly to re-harden the die surface, which has been exposed to high temperature and abrasion by extruding the aluminum alloy, resulting in the wearing away of the existing nitride layer. Therefore, after certain extrusion cycles, dies require re-nitriding. In the present work, the influence of repeated nitriding on AISI H13 steel is studied. Single-, double-, and triple-nitrided samples, treated under controlled two-stage gas nitriding process, have been included in the study to evaluate their nitride layer morphology, hardness, case depth, and quality. Both experimental and numerical results are presented and compared. In the experimental part, the nitride layers are characterized using different techniques including optical microscopy, scanning electron microscopy, X-ray diffraction analysis, microhardness analysis, and energy-dispersive spectrometry technique. A sequentially coupled heat diffusion analysis of re-nitriding treatments are also conducted numerically using finite element code, ABAQUS. The numerically predicted results are in close agreement with experimental results in terms of nitride layer growth and nitrogen concentration distribution in the diffusion zone. The experimental results reveal that multiple-nitriding treatment on H13 steel has a significant effect on compound layer thickness and its phases, diffusion zone depth and its microstructure, hardness–depth profile, and nitride case depth. It was found that excessive cumulative nitriding time during multiple-nitriding treatment results in greater nitride depth and a significant increase in hardness with deeper effect due to the dense and deeper precipitation of nitrides in the diffusion zone. Multiple-nitrided samples show oxidation and porosity in the near-surface part of the nitrided layer due to the interaction of iron with oxygen of the air upon decomposition of iron nitrides in the compound layer during re-nitriding. This results in reduced toughness and hardness in the near-surface part of the nitride layers.