液氨储存罐304不锈钢法兰连接螺栓断裂失效分析

Failure Analysis on Fracture of 304 Stainless Steel Flange Connecting Bolts of a Liquid Ammonia Storage Tank

某火电厂液氨储存罐上方气氨出口气动阀上的304不锈钢法兰连接螺栓在服役过程中发生断裂,采用金相显微镜、扫描电镜、显微硬度计、拉伸试验机等设备,从显微组织、断口、硬度、拉伸性能等方面分析了该304不锈钢螺栓断裂失效的原因。结果表明:螺栓失效模式为应力腐蚀开裂;螺栓材料成分不合格(高碳、低铬),导致合金的耐蚀性能大幅降低,晶间应力腐蚀倾向增加;螺栓服役环境为紧邻海岸的海洋大气,空气中氯离子含量较高,螺栓在服役过程中表面易于发生腐蚀,在预紧力、气氨出口气动阀工作过程中产生的拉应力和氯离子的共同作用下裂纹快速沿晶扩展,直至断裂失效;此外,螺栓内部存在较多铸造缺陷,会显著降低合金的力学性能,在发生腐蚀破坏的情况下,使螺栓出现过早断裂失效。

The 304 stainless steel flange connecting bolts of the pneumatic valve of the gas ammonia outlet above the liquid ammonia storage tank in a power plant fractured during service,the fracture failure reasons of the 304 stainless steel bolts were analyzed through the aspects of microstructure, fracture, hardness and tensile property by using metallographic microscope,scanning electron microscope,micro-hardness tester,tensile testing machine,etc. The results show that the failure mode of the bolts was stress corrosion cracking. The unqualified chemical compositions（high carbon content and low chromium content） of the bolts resulted in the decrease of corrosion resistance of the alloy and the increase of the tendency of intergranular stress corrosion. Because the bolts were exposed to the marine atmosphere near the coast which exhibited a higher concentration of Cl-,the surface of the bolts were easy to be corroded during the long time exposure,and the cracks rapidly expanded along the grain boundaries under the combined effect of the pre-tightening force,the tensile stress produced during the service of the pneumatic valve of the gas ammonia outlet and the Cl-until the occurrence of fracture. In addition,there were many casting defects in the bolts,which could significantly reduce the mechanical properties of the alloy and make the bolts fracture prematurely in the case of corrosion.