High-Temperature Crack Tip Deformation and Crack Propagation in Heat-Resistant Steels

研究了316H奥氏体钢、9Cr-lMo马氏体钢和2(1/4)Cr-lMo铁素体钢在高温应变（T=923K, =-5 × 10^<-6>/s）过程中裂纹扩展和裂纹尖端局部变形的行为，以明确蠕变裂纹扩展的机理。主要结果和结论如下：(1)裂纹扩展速率和裂纹尖端空化频率依次为奥氏体钢、马氏体钢和铁素体钢，但裂纹尖端张开度依次增大。(2)奥氏体钢具有典型的晶间裂纹扩展和晶界空化特征，宏观上表现为脆性，但裂纹尖端的应力集中程度很小（1.2 ~ 1.3），表明裂纹尖端附近的局部塑性变形使裂纹尖端的应力集中得到了广泛的松弛。(3)随着应力双轴拉伸程度的增加，裂纹扩展速率加快，应力集中程度和空穴化频率增加，加工硬化和空穴化集中的裂纹尖端区域尺寸减小。(4)结果(1)~(3)表明，裂纹扩展取决于裂纹尖端加工硬化和空化临界状态的建立。材料在变形过程中表现出工作软化，裂纹扩展速率非常低。

Behavior of crack propagation and local deformation near the crack tip during high-temperature straining (T=923K, =-5x10^<-6>/s) was examined in three types of heat-resistant steel ( 316H austenitic steel, 9Cr-lMo martensitic steel and 2(1/4)Cr-lMo ferritic steel), in order to make clear the mechanism of creep crack propagation. Main results and conclusions are summarized as follows: (1) The rate of crack propagation and frequency of cavitation near the crack tip decreases in the order of austenititic steel, martensitic steel and ferritic steel, though the crack tip opening becomes larger in this order. (2) Even in the austenitic steel which exhibits typical intergranular crack growth with grain-boundary cavitation and is brittle macroscopically, the degree of stress concentration at the crack tip is quite small (1.2-1.3), indicating that a stress concentration at the crack tip is relaxed extensively by local plastic deformation near the crack tip. (3) With increasing degree of biaxial tensile state of stress, the crack propagation rate increases, being accompanied by an increase in the degree of stress concentration and the frequency of cavitation and also by decrease in the size of crack tip area where work-hardening and cavitation are concentrated. (4) The results (1)-(3) indicate that crack propagation depends on the establishment of critical states of both work-hardening and cavitation in crack tip area. Crack propagation rate in a material, which exhibits work-softening during deformation, is very low.