Effect of Nickel Content on the Tensile Deformation Characteristics of Cryogenic Steels

Tensile behaviors of cryogenic steels, which contain 0 to 12wt% nickel, were examined to obtain the fundamental knowledge related to the role of retained austenite in the improvement of fracture toughness. Although retained austenite of 8% and 12% nickel steel, which were manufactured by the special heat treatment composed of quenching, intermediate heat treatment, and tempering, was stable under the deep cooling at –196 ºC, it transformed to martensite by straining around 10% at –196 ºC. The strain hardening behavior was divided into two categories. One is that the maximum strain hardening appears just after yielding, then the strain hardening gradually decreases with increasing strain. It was related to 0% to 2% nickel steels, which include less retained austenite. Another is that the strain hardening increases just after yielding with the increase of strain, then it gradually decreases. In this case, higher strain hardening was derived. It was related to 5% to 12% nickel steels which included a certain amount of retained austenite. The increase of strain hardening under low temperature was attributed to the solute nickel and the formation of martensite, which act as a hard second phase, by the work induced martensitic transformation.