The microstructure evolution and hardness variation of FB2 steel influenced by tempering are investigated. The results show that water‐cooling from 1100 °C produces lathy martensite microstructure. After tempering at 500 °C, the steel exhibits a martensite structure with 4.2% needle‐like Fe3C particles. The (Cr, Mo)2C and Cr‐rich M7C3 particles are detected in the sample tempered at 570 °CAs the tempering temperature enhances from 620 to 700 °C the (Cr, Mo)2C and Cr‐rich M7C3 in the matrix are replaced by Cr‐rich M23C6. Besides, the (V, Nb)C particles are identified in samples tempered above 620 °C. Thus, the evolution of carbides in FB2 steel during tempering can be summarized as: Fe3C→(Cr, Mo)2C + Cr‐rich M7C3→(Cr, Mo)2C + Cr‐rich M7C3 + Cr‐rich M23C6→Cr‐rich M23C6. Furthermore, the results suggest that M3C, M2C + M7C3, M23C6, and MX have high reaction rates at 500, 570, 620, and 700 °C respectively. In addition, the dislocation density reduces from 6.8 × 1014 m−2 to 2.1 × 1014 m−2, and the volume fraction of carbides increases from 4.2% to 12.6% with increasing temperatures from 500 to 700 °C leading to hardness decrease from 485 to 284 Hv. Finally, the quantitative relationship between the hardness and microstructure evolution during tempering is discussed.
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.
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