Multi-scale hot working tool damage (X40CrMoV5-1) analysis in relation to the forging process

•A failure analysis of used hot forming die is presented.•Different damage phenomena characterizations, occurred in failed die are discussed.•As the complexity damage modes, analysis of residual stresses is also performed.•Local stress is determined by numerical approach based on multilinear hardening model.•A thermal superficial softening of failed die is investigated during forging process.

During hot forming process, tools are exposed to severe and complex loads. As result, damages by oxidation and thermal and mechanical fatigue are generated which cause tool failure. The aim of this work is a multi-scale analysis of damaged hot forging tools. An expertise of main failures on industrial hot-forging die, made of hot-working die steel X40CrMoV5-1 is presented. Die was previously sectioned and examined after its service life. The measurements of micro-hardness depth profiles, SEM and optical observations of oxidized surfaces were achieved on the most loaded sections of the die. An analysis of residual stresses in damaged die was also performed in order to estimate and then predict the most solicited zones on the die, a simplified numerical approach was investigated. The cracks on the observed areas are caused by local elasto-plastic stress due to mechanical load. A good correlation between expertise and numerical results was established.