9Ni钢断裂过程中的裂纹扩展行为

Propagation behavior of crack in 9Ni steel during fracture process

采用SEM和TEM对9Ni钢断裂过程中裂纹的扩展行为进行了研究,讨论了逆转变奥氏体在断裂过程中的作用。SEM观察表明,基体的塑性对裂纹尖端形状有重要影响,基体塑性较差时裂纹前端尖锐,加剧了应力集中,基体塑性较高时裂纹前端呈"钝角",使应力集中松弛。TEM观察显示,9Ni钢裂纹的扩展有两种形式:一种是在带状薄区中呈"Z"形扩展,另一种是遇到逆转变奥氏体后偏离原来的扩展方向。研究结果也显示在两种条件下获得的逆转变奥氏体,经变形后又发生相变。这表明稳定性高的逆转变奥氏体能够在变形过程中保留下来,阻碍裂纹扩展提高9Ni钢低温韧性,而稳定性较低的逆转变奥氏体则在变形过程中很快发生相变。

Propagation behavior of crack in 9Ni steel during fracture process was studied with SEM and TEM,and the role of reversed austenite in fracture process was discussed.The results of SEM show that the plasticity of matrix has important influence on the shape of the crack tip,which is a acute ＂angle＂ when the matrix plasticity is low and leads to increase of the stress concentration.For the high matrix plasticity,the crack tip shape is an obtuse ＂angle＂,which makes the significant stress relaxation.The TEM results show that the crack propagation in 9Ni steel have two ways.One is ＂Z-shape＂ expansion in strip-shaped thin area,and another is that the crack deviates from its original direction when it meets reversed austenite.The results also show that the reversed austenite obtained under the above mentioned conditions will transform to martensite due to deformation.These also indicate that the reversed austenite with high stability is retained and blocks the crack propagation lead to increase of low temperature toughness for 9Ni steel.The unstable reversed austenite may quickly transform to martensite due to deformation.