大厚度NiCrFe-7熔化焊焊缝微裂纹

Study on micro-cracks of the NiCrFe-7 weld metal with large thickness

通过金相显微镜、扫描显微镜和电子背散射衍射方法对一种大厚度NiCrFe-7熔化焊焊缝中的微裂纹进行了表征和机制分析,焊接方法分别为焊条电弧焊(SMAW)和钨极氩弧焊(GTAW).结果表明,该种热裂纹为沿晶的再热裂纹,其形成是由于焊接热影响区在高温下发生应力松弛现象,导致晶界大尺寸MC型碳化物和氧化物处出现应力集中,造成裂纹沿这些颗粒内部或界面开裂.相比于GTAW工艺,SMAW工艺的焊缝中存在大量氧化物,造成SMAW焊缝再热裂纹倾向高于GTAW焊缝,进而导致前者焊缝弯曲性能不符合国家标准要求.通过此研究,建议采用保护气效果更佳的GTAW工艺.

Micro-cracks in a NiCrFe-7 weld metal with large thickness were characterized and analyzed by metallographic microscopy,scanning microscopy,and electron backscatter diffraction.Two kinds of welding methods,shielded metal arc welding(SMAW)and gas tungsten arc welding(GTAW),were applied in our study.The results showed that the crack was a kind of reheating crack along the grain boundaries in the heat affected zone(HAZ).These cracks were induced by the stress relaxation in the heat affected zone during welding process,which resulted in local stress concentration around the largesize MC carbides and oxides.The cracks nucleated and propagated within these particles or along the interfaces of the particles/matrix.Compared with the GTAW process,there were a large number of oxides in the weld metal of the SMAW process.Therefore,the tendency of reheating cracks in the SMAW was much higher than that of the GTAW,which resulted in the bending properties of the former weld failed to meet the requirements of the national standard.The GTAW process with better effect of protective gas was recommended for the Ni-base weld metal with large thickness.