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SynopsisCryogenic nickel steels show larger elongation as temperature decreases. The presence of retained austenite (IR) 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 YR. Although IR 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 IR was quantitatively determined using the parameter, s --(1/ Vr ) (dV1/de), where Vr is volume fraction of rR and e is true strain. It is concluded that TRIP effect of IR, 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. I. IntroductionNickel steels show larger elongation with a decrease in temperature so long as they are ductile.'-3) These steels sometimes contain a certain amount of retained austentte (TB) ; therefore, this phase might contribute to the increase in elongation through transformationinduced-plasticity, i.e., TRIP effect. This effect of YR has been also proposed as one of the possible mechanisms to improve toughness of ferritic steels for cryogenic service such as 5.5 Ni steel,4) 9 Ni steel5) and Fe-12Ni-0.25Ti alloy.6'7) Although it has been reported that TRIP effect is largely dependent on the stability of the metastable austenitic phase against plastic deformation,8-"> the mechanical stability of IR in the cryogenic steels has not been investigated quantitatively with reference to low temperature mechanical properties.Hence, in the present work the stability of IR