Research On Hot Deformation Behaviors Of Modified High Cr Ferritic Heat-resistant Steels

High Cr ferritic heat-resistant steel has many advantages, such as excellenthigh-temperature endurance, good creep resistance properties and heat conductivity,low thermal expansion coefficient. Based on the above advantages, high Cr ferriticheat-resistant steel has been widely used in advanced power plants. In recent years,the problems of energy shortage and environment pollution are getting worse, it isimperative to improve thermal efficiency of the plant. Therefore, it put forward higherrequirements to high Cr ferritic heat-resistant steel.The main purpose of this paper is to confirm a thermo-mechanical process of highCr ferritic heat-resistant steel. The hot deformation behaviors of austenite areadequately researched. After analyzing the microstructural evolution and precipitationunder different conditions, we can obtain a result on determining an effectivethermo-mechanical process of high Cr ferritic heat-resistant steel. The conclusions areas follows:（1） After deformation at the temperature of austenite recrystallization region,dynamic recrystallization occurs need to achieve a certain condition. The increasingof the deformation temperature, the amount of deformation and the rate ofdeformation will promote the process of dynamic recrystallization. The larger theamount of deformation and the rate of deformation, the finer the dynamicrecrystallization grain size. The deformation can also influence the martensitictransformation. The increasing of the amount of deformation can promote themartensitic transformation and refine martensitic laths. The rate of deformation makesno difference to martensite starting point, but the high rate of deformation can alsorefine the martensitic laths.（2） After deformation at the temperature of austenite non-recrystallization region,with the increasing of the amount of deformation, the hardness values are higher. Butthe distribution of microstructure and hardness values are uneven. After a shortrelaxation process, the hardness values have a tendency of decreasing. However,when prolonging relaxation time, we can observe ferrite transformation. This isbecause the M₃C phase may dissolute and decompose during the relaxation process. Catoms with the alloy elements form MX precipitations, result in increasing the content of Fe in the matrix. Meanwhile, the deformation provides a large amount of ferritenucleation points. The above reasons cause the ferrite transformation. The MXprecipitations play the role of precipitation strengthening and improve theperformance of high Cr ferritic heat-resistant steel.（3） The mainly precipitation in high Cr ferritic heat-resistant steel is M₂₃C₆phaseafter normalizing and tempering. During the tempering process, M₂₃C₆phase willgather and grow up. After deformation at the temperature of austenite recrystallizationregion, the mainly precipitation is M₃C phase. Besides M₃C phase, a small amount ofM₂₃C₆phase and MX phase will precipitate after deformation at the temperature ofaustenite non-recrystallization region. As we know, MX phase can improve theperformance of high Cr ferritic heat-resistant steel. Therefore, in order to precipitatemore MX phase, a long time relaxation process after deformation at the temperatureof austenite non-recrystallization region is necessary when we determine thethermo-mechanical treatment process.