Formation of Nodular Bainite in a Fe-9.10ni-0.06c (Wt. %) Alloy: A New Microstructure for Cryogenic Steels

Quenched and tempered Fe-9Ni (wt. %) steels have been widely used in cryogenic applications. For the first time, the formation of nodular bainite in an Fe-9Ni-0.06C (wt. %) steel during isothermal transformation at 550 and 500 °C for 24 h is reported. Electron microscopy revealed that proeutectoid ferrite forms predominantly by a massive transformation at prior austenite grain boundaries or at intragranular inclusions as the first stage of austenite decomposition. Then, nodular bainite composed of a ferritic matrix and rows of fine cementite precipitates forms by interphase boundary precipitation during decomposition of adjacent C-enriched austenite. Both the Bagarayatskii and Isaichev orientation relationships between ferrite and cementite were observed. Also, the microhardness of nodular bainite in the sample transformed at 500 °C (214 ± 4 VHN) was higher than that in the sample transformed at 550 °C (195 ± 5 VHN), due to smaller sizes of cementite precipitates in the former.