A comparative study on microstructure evolution of metamorphic layers induced by grinding and laser quenching in hardened AISI 52100 steel

Abstract

This study presents a comparative analysis of the similarities and differences in the microstructure characteristics between ground and laser-quenched metamorphic layers on the AISI 52100 steel, revealing the respective impacts of thermal and mechanical effects on the microstructure evolution of metamorphic layers (i.e. white layer (WL) and dark layer (DL)). The results show that both thermal and mechanical effects result in grain refinement within WL. However, the increase in the proportions of sub-grain boundaries (SGBs) and low-angle grain boundaries (LAGBs) is caused by the mechanical effect, irrespective of the thermal effect. Furthermore, in addition to the mechanical effect, the thermal effect is also responsible for increased dislocation density within WL. The thermal effect causes the partial dissolution of carbides, while the mechanical effect results in the deformation and refinement of carbides in WL. In the DL, the thermal effect coarsens grains through grain boundary migration, accompanied by forming carbides due to the precipitation of C atoms. The dislocation density and the proportions of SGBs are reduced. However, the influence of the mechanical effect on the microstructure alterations of the DL is negligible. The hardness of the ground WL is increased by approximately 49.1 %, and such an increase is attributed to three factors: solid solution strengthening due to thermal effect, dislocation strengthening due to mechanical effect, and grain boundary strengthening due to thermal and mechanical effects. The thermal effect results in the formation of over-tempered martensite in the subsurface layer, reducing the hardness of ground DL by approximately 35.4 %. The results provide theoretical guidance for controlling the generation or utilization of metamorphic layers on AISI 52100 steel.