Effect of heat treatment temperature on microstructure and mechanical properties of Cr-Ni-Mo-V steel

Effect of heat treatment temperatures( normalizing temperature,quenching temperature and tempering temperature) on the microstructure and mechanical properties of a Cr-Ni-Mo-V steel was investigated by means of OM,SEM and TEM. The results show that the prior austenite grain size of the tested steel changes little( 8-17 μm) when the normalizing and quenching temperatures are in the range of840-920 ℃,which has little effect on the mechanical properties of the final tempered steel. When the tempering temperature increases in the range of 460-660 ℃,the lath width of α-Fe increases from 50 nm at 460 ℃ to 500 nm at 610 ℃,and the laths become obscure at 660 ℃.Meanwhile,the matrix α-Fe decomposes gradually and precipitates out relatively bulky strip-like carbides,which makes the strength of CrNi-Mo-V steel to reduce gradually. When tempered in the range of 510-560 ℃,however,the strength of the steel increases slightly due to secondary hardening caused by precipitation of fine dispersive acicular carbides( 50-500 nm) and spherical carbides( < 50 nm). In addition,the elongation increases,and the impact absorbed energy at-50 ℃ improves obviously since 560 ℃. Therefore,the tested Cr-NiMo-V steel can obtain preferred comprehensive mechanical properties by adopting a heat treatment process of 840-920 ℃ normalizing+840-920 ℃ quenching+510-610 ℃ tempering.