材料期刊网

目的：针对某炼油厂波纹管截止阀中双层不锈钢304波纹管组件发生开裂,造成截止阀失效的现况,通过失效分析,寻找腐蚀开裂的原因。方法对失效开裂的不锈钢截止阀双层波纹管组件进行外观检查,采用金相显微镜和直读光谱仪分别对失效组件的金相组织和化学成分进行分析,用电子显微镜观察组件断口形貌与特征。结果波纹管组件外层管壁断口上可以观察到解理面和解理台阶,并且能看到腐蚀产物的存在,这是奥氏体不锈钢发生应力腐蚀断裂的典型特征；内层管壁断口上有韧窝存在,属于机械断裂。金相组织和化学成分分析表明,波纹管组件使用的不锈钢材质合乎设计与使用要求。导热油介质检测结果显示,导热油中含氯55 mg/kg,总硫含量350 mg/kg,有害离子含量较高。结论双层波纹管组件的内层管壁和外层管壁的失效机制不同：外层管壁是由Cl-导致的应力腐蚀开裂；内层管壁是由于外层管壁失效引起波纹管组件失稳,造成抗压强度和寿命急剧降低,在应力的作用下出现韧性断裂。建议降低导热油中有害离子含量,使用耐蚀性更好的材质。

Objective To find out the causes for corrosion cracks by failure analysis in the condition that the bellows subassem-bly made of 06Cr19Ni10 stainless steel in the sealed valves cracked which resulted in the failure of sealed valves. Methods The appearance of the failed bellows subassembly was examined by naked eyes, the metallographic structure and the chemical compo-nents were analyzed respectively by the metallographic microscope and direct reading spectrometer;the morphology and fracture of the failed subassembly were observed by SEM. Results Cleavage plane, cleavage steps and corrosion products were seen on the fracture of the outer wall, which was the typical feature for stress corrosion cracking of austenitic stainless steel. Some dimples were observed on the inner wall fracture, which indicated that the inner wall had ductile fracture. The analysis showed that the quality of microstructure and chemical composition of the bellows subassembly met the requirements on design and usage. The medium test results showed that the content of harmful ions was relatively high in the heat transfer fluid ( HTF) which contained 55 mg/kg chlo-ride ion and over 350 mg/kg sulfur. Conclusion The main fracture causes of outer and inner wall of the double-layer bellows subas-sembly were different;the failure of outer wall was due to the stress corrosion related to the high content of chloride ion, while the fracture of inner wall was ductile fracture, which was due to the significant decrease in compressive strength in the result of the plane instability cause by the failure of outer wall. It is proposed that the content of harmful ions should be reduced in HTF and the stainless steel with better corrosion resistance should be used.