EH36钢焊接接头海水腐蚀疲劳性能及断裂机制

以EH36钢焊接接头为研究对象，研究了其在南海实际海水环境下不同应力范围的腐蚀疲劳寿命及腐蚀疲劳断裂机制。根据DNV-RP-C203标准中规定修正实际海水环境腐蚀疲劳试验结果，修正后的S-N曲线方程为lg N=15.414-4.005lg Δσ。从S-N曲线上可看出，EH36钢焊接接头在实际海水中寿命随着疲劳应力范围的增大而显著降低，其腐蚀疲劳性能优于DNV-RP-C203标准模拟海水环境下的焊接接头的腐蚀疲劳性能。采用扫描电镜观察焊接接头腐蚀疲劳断裂面发现，腐蚀疲劳裂纹源区主要发生在焊接接头表面，分析原因为焊接接头表面产生的腐蚀坑周围具有显著的应力集中，导致裂纹萌生。腐蚀疲劳裂纹扩展区宏观上较为平坦，焊接接头表面多处裂纹源区产生的撕裂棱在此区域汇合成一条撕裂棱；微观上裂纹扩展区出现了明显的疲劳台阶和二次裂纹，裂纹扩展方式主要为穿晶；腐蚀疲劳瞬间断裂区主要形貌为韧窝，断裂形式主要为塑性断裂。创新点： 基于成组法疲劳寿命预测理论，得出EH36钢在南海海水环境下的焊接接头的S-N曲线。同时结合微观分析方法研究焊接接头腐蚀疲劳断裂面微观特征，揭示实际海水自由腐蚀环境下EH36钢焊接接头腐蚀疲劳机制。

Seawater corrosion fatigue properties and fracture characteristics of EH36 steel welded joints

Taking welded joints of EH36 steel as the research object, corrosion fatigue life and fracture mechanism of welded joints under different stress ranges in seawater environment of South China Sea were studied. According to DNV-RP-C203 standard, actual seawater environment corrosion fatigue test results were corrected, and the corrected equation of S-N curve was lg N=15.414-4.005lg Δσ. The S-N curve showed that fatigue life of welded joints of EH36 steel in actual seawater decreased significantly with the increase of fatigue stress range, whose corrosion fatigue performance was superior to that of welded joints in simulated seawater environment under DNV-RP-C203 standard. Scanning electron microscope was used to observe corrosion fatigue fracture surface of welded joints, and source area of corrosion fatigue cracks mainly occurred near the surface of welded joints. The reason was that there was a significant stress concentration around the corrosion pit on the surface of welded joints, which resulted in crack initiation. The crack propagation area was relatively flat macroscopically, and a number of tearing edges generated from crack source area on the surface of welded joints converged into a tearing edge in this area. Obvious fatigue steps and secondary cracks appeared in the microcosmic crack growth zone, and crack growth mode was mainly transgranular. The main morpholgy of instantaneous fracture area of corrosion fatigue was dimple, and fracture form was plastic fracture.Highlights: Based on fatigue life prediction theory of group method, S-N curve of welded joints of EH36 steel in seawater environment of South China Sea was drawn. Combined with microscopic analysis method, microscopic characteristics of corrosion fatigue fracture surface of welded joints were studied, and corrosion fatigue mechanism of EH36 steel welded joints in actual seawater free corrosion environment was revealed.