新型Cr-Mo-V热作模具钢的高温断裂机制

High temperature fracture mechanism of a new type Cr-Mo-V hot-working die steel

通过高温拉伸试验研究了新型Cr-Mo-V钢的高温断裂行为,探究不同拉伸温度(25~700℃)下试验钢的热力耦合区(断裂区)和热应力区(夹持区)的组织演变和碳化物分布特征,揭示钢中碳化物的种类、分布及尺寸与高温断裂行为之间的关系,阐明新型Cr-Mo-V热作模具钢在高温变形过程中裂纹附近显微组织的演化机制。结果表明:随着拉伸温度的升高,试验钢的抗拉强度和屈服强度降低,伸长率和断面收缩率升高,断裂方式由脆性断裂向韧性断裂转变,脆性断裂向韧性断裂的转变温度约为400℃;在高温变形过程中,试验钢的位错密度降低,回火马氏体发生分解、回复、再结晶及晶粒长大,M_(7)C_(3)和M23C6碳化物沿晶界析出长大,少量的细小颗粒状MC碳化物在晶内弥散分布;由于外力作用对变形区做功将导致变形区温度高于拉伸温度,这为断口附近细小碳化物回溶、大尺寸碳化物的长大提供热力学条件,导致热力耦合区小尺寸碳化物减少,大尺寸碳化物增多。

The high-temperature fracture behavior of a new type Cr-Mo-V steel was studied through high-temperature tensile tests,and the microstructure evolution and carbide distribution characteristics in the thermal-mechanical coupling zone(fracture zone)and thermal stress zone(clamping zone)of the experimental steel at different tensile temperatures(25-700℃)were studied.The relationship between the types,distribution and size of the carbides in the steel and the high-temperature fracture behavior was analyzed.The evolution mechanism of microstructure near cracks in the new type Cr-Mo-V hot-working die steel during high-temperature deformation was clarified.The results show that as the tensile temperature increases,the tensile strength and yield strength of the experimental steel decrease,while the elongation and reduction of area increase.The fracture mode changes from brittle fracture to ductile fracture,and the transition temperature from brittle fracture to ductile fracture is about 400℃.During the high-temperature deformation,the dislocation density of the experimental steel decreases,and the tempered martensite undergoes decomposition,recovery,recrystallization and grain growth.M_(7)C_(3)and M23C6 carbides precipitate and grow along the grain boundaries,and a small amount of fine granular MC carbides are dispersed in the grain.Due to the external force acting on the deformation zone,the temperature of the deformation zone will be higher than the tensile temperature,which provides thermodynamic conditions for the dissolution of small carbides near the fracture and the growth of large-sized carbides,leading to a decrease in small-sized carbides and an increase in large-sized carbides in the thermal-mechanical coupling zone.