Carbonitride dissolution and austenite grain growth in a high Cr ferritic heat-resistant steel

X.G.Tao, J.F.Gu, L.Z.Han

Abstract: Dissolution of carbonitrides and growth of austenite grain in X12CrMoWVNbN10-1-1 steel under various austenitizing conditions were investigated. The experimental results showed that the carbonitrides existing in after-forging heat treated state were mainly Cr23C6 and small amounts of NbN and Cr2N. However, Cr23C6 and Cr2N precipitates were dissolved completely after austenitization at 1 070°C for 60 min or at 1 200°C for 15 min, only the NbN particles left. The state of dissolution of NbN particles was also studied in detail as a function of austenitization holding time (tA) at different temperatures. Initially, the NbN particles dissolved with increasing tA at 1 070°C. When tA reached over 360 min, the amount of NbN remained approximately constant. On the contrary, during the process of austenitization at 1 010°C, no NbN particles dissolved into matrix. Starting from the fine and uniform grains, abnormal grain growth was observed after austenitizing at 1 010°C for 960 min or at 1 070°C for 180 min due to the heterogeneous distribution of NbN particles. The plot of grain size against tA indicated that the NbN particle with mean diameter of 117 nm and volume fraction of 3.1×10–4 would be sufficient to inhibit the austenite grain size effectively. Finally, the EBSD measurements demonstrated that the high energy grain boundaries whose misorientation range is 20–45° might be responsible for abnormal grain growth.