The influence of surface layer microstructure evolution of M2 steel cold-ring rolling mandrel roller on fatigue crack initiation

Failure mode of M2 steel mandrel roller is mainly fatigue fracture. For different mandrel rollers with short or long service life fatigue cracks initiate at surface defects or subsurface defects, respectively. In this paper, the surface layer changes of M2 steel mandrel rollers with different service life were investigated by using Vicker micro-hardness tester, SEM and EDS. The results show that the microstructure changes near surface during rolling have a great influence on the site of fatigue crack initiation. The changes in surface layer include increasing of surface hardness, re-dissolution of carbide, transformation of austenite and martensite, dislocation pile-up around carbide, etc. The result of the surface layer changes is the formation of a hardened layer, which transfers the crack initiation site from surface to subsurface of M2 steel mandrel roller.