超低温裂纹尖端张开位移(CTOD)实验装置改进及焊接接头断裂韧性分析

Improvement of ultra-low temperature CTOD experimental device and fracture toughness analysis of welded joint

针对超低温裂纹尖端张开位移(CTOD)实验中存在的试样对中不准确和温度控制不精确等问题,该文对现有设备进行了改进并展开测试实验。首先,选用9Ni钢焊接液化天然气(LNG)储罐并使用扫描电镜(SEM)和电子背散射衍射(EBSD)观察焊接接头的组织形貌;然后,用改进的CTOD实验装置测定接头的断裂韧性;最后,使用光学显微镜(OM)、SEM和EBSD分析CTOD断口形貌和疲劳裂纹的走向。实验观察到在焊接接头的熔合线处存在宽度约17μm的过渡层,元素Mo和Nb发生了偏聚,焊缝疲劳裂纹穿晶扩展;热影响区CTOD值低于焊缝且离散性较大,在主体实验过程中,出现了pop-in现象,产生的微裂纹沿熔合线粗晶区一侧扩展;粗晶区晶粒宽度达50~100μm,组织为粗大的板条马氏体,且大角度晶界比例较低。由实验可知粗晶区是整个接头断裂韧性最薄弱的区域。

Aiming at the problems of inaccurate sample alignment and inaccurate temperature control in the crack tip opening displacement(CTOD)experiment at ultra-low temperature,the existing equipment is improved and the test experiment is carried out.Firstly,9Ni steel is used to weld liquefied natural gas(LNG)tanks,and the microstructure of welded joints is observed by scanning electron microscopy(SEM)and electron backscatter diffraction(EBSD).Then,the fracture toughness of the joint is measured by the improved CTOD experimental device.Finally,the fracture morphology and fatigue crack direction of CTOD are analyzed by optical microscope(OM),SEM and EBSD.It is observed that there is a transition layer with a width of about 17μm at the fusion line of the welded joint,the elements Mo and Nb are segregated,and the fatigue crack of the weld is transgranularly expanded.The CTOD value of the heat affected zone is lower than that of the weld,and the discreteness is large.During the main experiment,pop-in phenomenon occurs,and the generated microcracks expand along the side of the coarse grain zone of the fusion line.The grain width of the coarse grain zone is 50~100μm,the microstructure is coarse lath martensite,and the proportion of high angle grain boundaries is low.It can be seen from the experiment that the coarse grain zone is the weakest area of the fracture toughness of the whole joint.