Q550D高强板断口分离缺陷的微观分析

A Microscopic Analysis of Q550D High-strength Sheet Segregation Failure

Q550D高强板在进行室温拉伸试验后,断口出现分离缺陷。通过扫描电镜、金相显微镜、透射电镜对其断口及分离面的微观形貌、分离裂纹末端及远离裂纹的断口横截面组织及亚结构等进行了观察分析。结果表明:拉伸变形前,组织为贝氏体+碳化物,部分晶界及碳化物周围有位错塞积,但不存在裂纹。拉伸变形时,贝氏体晶界及亚晶界呈链状分布的大尺寸渗碳体颗粒附近的位错塞积引起应力集中,形成微裂纹及孔洞;同时,拉伸断口的颈缩变形引起垂直于拉伸轴方向产生新的拉应力,导致微裂纹及孔洞在拉伸轴方向优先扩展形成裂纹;这两者成为产生分离缺陷的主要原因。分离裂纹的长度与裂纹扩展时的应力状态及组织变形程度有关。

After the tensile test with room temperature for Q550D high-strength plate was conducted, segregation failure appeared at the fracture. The microscopic appearance of fracture and segregation face, the end of segregation crack, fracture cross section structure away from the crack and sub-structure were observed and analyzed by the scanning electron microscope, metallurgical microscope and transmission e- lectron microprobe. The result shows that segregation failure is caused by stress resulting from dislocation blockade due to the chain distribution of martensite grain boundary and sub-grain boundary and new stretc- hing stress from the neck reduction deformation at the tensile fracture.