Cryogenic nickel steels show larger elongation as temperature decreases. The presence of retained austenite (γ_R) is suspected to be one of the causes of this behavior. In order to clarify the effect of this phase on the deformation behavior, 5.5 Ni, 9 Ni and 11 Ni steels were studied.
The increase in elongation was mainly due to the increase in uniform elongation and independent of the existence of γ_R. Although γ_R of these steels were considerably stable against sub-zero treatments, they were easily transformed to martensite by plastic deformation at -196°C and to the lesser extent even at room temperature. The mechanical stability of γ_R was quantitatively determined using the parameter, s=-(1/V_γ) (Dv_γ/dε), where V_γ is volume fraction of γ_R and ε is true strain. It is concluded that TRIP effect of γ_R, if any, can hardly contribute to the increase in elongation of these steels. The temperature dependence of thermal activation parameter measured in the 11 Ni steel indicates that the plastic deformation mechanism in the vicinity of room temperature is maintained even at very low temperatures. This fact may relate to the excellent low temperature ductility of the cryogenic nickel steels.