显微组织对SA508 Gr.3钢断裂方式的影响

Effect of microstructure on fracture mode of SA508 Gr.3 steel

采用光学显微镜、扫描电镜和电子背散射衍射等手段研究了具有铁素体+贝氏体、粒状贝氏体、板条贝氏体+马氏体和板条马氏体4种显微组织的核电压力容器用SA508 Gr.3钢的低温(-196℃)冲击吸收能量和二次裂纹扩展行为。结果表明:低温冲击吸收能量随实验钢中硬相增多而升高。铁素体+贝氏体混合组织的实验钢中,裂纹大多在晶界形核,二次裂纹数量较少,但易于扩展;粒状贝氏体组织的实验钢因含有大量的马氏体/奥氏体岛,能提供大量的形核位置,致使二次裂纹呈现多而短的特征;裂纹在板条贝氏体组织中比在马氏体中更容易扩展,这是因为高密度的大角度界面能有效阻止裂纹扩展,故板条马氏体组织实验钢的冲击性能最好。

The impact absorbed energy at-196℃and secondary crack propagation behavior of SA508 Gr.3 steel for nuclear pressure vessels with four different microstructures(ferrite-bainite,granular bainite,lath bainite-martensite and lath martensite)were investigated by means of optical microscope,scanning electron microscopy and electron backscattered diffraction technique.The results show that the low temperature impact absorbed energy increases with the increase of hard phase in the experimental steel.In the experimental steel with ferrite-bainite mixed microstructure,the cracks are mostly nucleated at grain boundaries,the number of secondary cracks is relatively small,but they are easy to propagate.The experimental steel with granular bainite structure contains a large number of martensite/austenite islands,which can provide a large number of nucleation sites,resulting in many short secondary cracks.The crack propagates more easily in the lath bainite than in the martensite,because the high-density large-angle interface can effectively prevent crack propagation,and thus the experimental steel with lath martensite has the best impact property.