Study on the softening in overlapping zone by laser-overlapping scanning surface hardening for carbon and alloyed steel

Softening in overlapping passes by laser-overlapped scanning surface hardening is a difficult problem of laser surface modification. Despite the advantage of laser quenching, softening in overlapping zone limits its practical application. In this paper, 45, 9Cr2Mo and W18Cr4V steel were hardened by laser-overlapping quenching. Softening occurred in all overlapping zones. The results of hardness testing indicated the softening width of 45 steel was the broadest and that of W18Cr4V steel was the narrowest. Different mixed microstructures composed the overlapping zone for three steels, i.e., tempered sorbite and a little tempered martensite in 45 steel, tempered martensite, tempered sorbite and a small amount of carbides in 9Cr2Mo steel, tempered martensite and a little carbide in W18Cr4V steel. The effect of activation energy of diffusion for carbon in steel and cooling rate on the decomposition of martensite have been investigated by developing a diffusion model based on the principle of carbon diffusion in martensite. The results indicated that action energy for diffusion of carbon in steel plays a main role in hindering decomposition of carbide and cooling rate has a limited action in reducing temper softening during laser-overlapping scanning.