E690高强低合金钢焊接热影响区典型组织在含SO_2海洋环境中的应力腐蚀行为对比研究

Comparative Study of Stress Corrosion Cracking Behaviors of Typical Microstructures of Weld Heat-Affected Zones of E690 High-Strength Low-Alloy Steel in SO_2-Containing Marine Environment

采用U形弯试样干湿交替腐蚀的实验方法,结合电化学测试和裂纹扩展行为分析,对比研究了E690钢焊接热影响区热模拟组织在模拟含SO_2海洋薄液环境中的应力腐蚀行为及机理。结果表明,E690钢焊接热影响区组织在含SO_2海洋薄液环境中具有较高的应力腐蚀敏感性,其中细晶热影响区组织与母材的应力腐蚀敏感性相对较低,粗晶和临界热影响区组织在该环境中应力腐蚀敏感性很高,裂纹扩展速率较快,且呈加速扩展之势。不同组织U形弯试样经过5 d干湿交替腐蚀实验后均形成应力腐蚀微裂纹,裂纹萌生于马氏体-奥氏体(M-A)组元与铁素体基体之间微电偶腐蚀形成的点蚀坑处。

With the extensive exploitation of ocean resources, the steels used in ocean engineering have been developed towards the trend of high strength-toughness and thick plates, which consequently causes welding problem and high risk of stress corrosion cracking(SCC). The heat-affected zone(HAZ)of high-strength low-alloy steel undergoes phase transformation during welding thermal cycle and it’s generally considered to be most vulnerable to SCC. E690 steel, as a newly-developed high strength steel, is currently the leading kind of steel used in ocean platform for its excellent performance. However, there is few research about its SCC behavior in marine atmosphere, especially in SO2-polluted atmosphere.Therefore, it’s of great importance to investigate the SCC behavior and mechanism of simulated HAZ of E690 steel in this environment. However, the HAZ is a narrow zone including various microstructures;thus, the individual performance of different microstructures is inconvenient to study. In this work, various microstructures in HAZ, including coarse grained heat-affected zone(CGHAZ), fine grained heat-affected zone(FGHAZ) and intercritical heat-affected zone(ICHAZ), were simulated by heat treatment according to real HAZ microstructures of E690 steel. A comparative study of SCC behaviors of various HAZ microstructures in simulated SO2-containing marine atmosphere was conducted by using U-bend specimen corrosion test under dry/wet cyclic condition. The results indicated that various HAZ microstructures have high susceptibility to SCC in this environment. The SCC susceptibility of CGHAZ and ICHAZ is very high with a high crack growth rate while that of FGHAZ and parent metal is relatively modest. SCC cracks were initiated after 5 d of cyclic corrosion test for U-bend specimen of various microstructures. The microcracks were initiated from the corrosion pits, which were induced by the galvanic corrosion between martensite-austenite(M-A) constituents and ferritic matrix.