Local laser hardening of heat treated medium manganese steel initially processed by laser powder bed fusion (LPBF)

Abstract

A method of laser-hardening using an oscillating beam was utilized to locally increase mechanical properties of laser powder bed fusion (LPBF) manufactured thin-walled specimens made of medium manganese steel. Those results showed a martensite microstructure was obtained in the hardened zone over the entire depth of the specimen and indicated its suitability for laser hardening. The average hardness of the fully hardened zone increased from 300 HV to approximately 440 HV. The martensitic transformation caused an increase in the ultimate tensile strength (UTS) and yield strength (YS) of 66 % and 28 %, respectively, and decreased the level of ductility to 5% of elongation. Fracture analysis of the tensile specimens showed shallow dimples in the center and shear lips on the edges and in the laser hardened zones. The crack was initiated by the lack of fusion just below the surface and was characteristic for LPBF material tensile specimens. Based on bending tests, improper localization of the local hardened zones in the component, i.e. in areas of high plastic deformation, led to the formation of a microcrack due to a significant reduction in ductility and microstructural inhomogeneities. The knowledge of the strategy and technological parameters of the hardening process of heat-treated LPBF medium manganese steel will be beneficial to the automotive industry as this material class is gaining increasing attention.