Microstructural heredity of Ni-containing cryogenic steel and its effect on the toughness at 77 K

The microstructural heredity of as-rolled lath martensite with different prior austenite grain morphologies during inter-critical quenching has been studied by using a quasi in-situ EBSD technique in a 6.5%Ni-containing cryogenic steel, to reveal the effect of as-rolled microstructure on final microstructure and toughness at 77 K. It is found that the refinement of final microstructure can be achieved by refining the as-rolled lath martensite through both refinement and elongation of prior austenite grains, which is attributed to two aspects of heredity effect. On the one hand, the fine as-rolled martensitic structure in the interiors of prior austenite grains is inherited during inter-critical quenching. On the other hand, the refinement of as-rolled lath martensite promotes the formation of newly oriented grains along the prior austenite grain boundaries to further refine the microstructure. The cryogenic toughness of Ni-containing cryogenic steels is significantly improved through the heredity effect of as-rolled lath martensitic refinement.