Effects of Nitrogen Content and Hot Forming Temperature on Prior Austenite Grain Size and Mechanical Properties for Normalizing Cryogenic Pressure-Vessel Steels

The effects of nitrogen content and hot forming temperature on prior austenite grain size for normalizing cryogenic pressure-vessel steels were described. The evolution of V(C,N) precipitates during thermal cycle of hot forming was analyzed. The optical microscopy displays that as increasing N content from 0.0094 to 0.0198 wt.% the refinement of prior austenite grain was seen at each hot forming temperature of 870, 910 and 950 °C. The transmission electron microscopy presents that much finer V(C,N) precipitates were found in sample with 0.0198 wt.% N content than in sample with 0.0094 wt.% N content. Thermo-Calc calculation reveals the precipitation temperature of V(C,N) was increased by increasing N content. While the Ostwald ripening rate of V(C,N) was decreased by increasing N content. TEM result and Thermo-Calc calculation indicate that increasing N content enhanced the thermal stability of V(C,N) precipitates. The stable finer V(C,N) precipitates contribute to the finer prior austenite grain and good mechanical properties for sample with 0.0198 wt.% N content even at higher temperature of 950 °C during hot forming process.