Effect of deep cryogenic treatment on martensitic lath refinement and nano-twins formation of low carbon bearing steel

The effect of heat treatment and deep cryogenic treatment on microstructural evolution of low carbon martensitic bearing steel was investigated. The experimental results showed that the lath martensite was obtained by quenching and a few twins as substructures formed in some martensitic laths. The rudiment of sub-interfaces of martensitic lath was formed in the high-density dislocation regions after deep cryogenic treatment; meanwhile, the number of twins increased, especially in the high-density dislocation regions. This phenomenon is due to the increase in internal stress caused by cryogenic treatment. After tempering, the rudiment of sub-interface further evolved into the martensitic lath boundary, and thus the original martensitic laths were refined. The twins formed by cryogenic treatment did not disappear after tempering. In addition, small quantities of annealing twins formed in tempering process. Martensitic laths morphology and substructures in different stages of the heat and deep cryogenic treatment were observed by transmission electron microscopy.