14Cr17Ni2不锈钢过滤阀门失效分析

Failure Analysis of 14Cr17Ni2 Stainless Steel Filter Valve

为研究14Cr17Ni2不锈钢过滤阀门在海洋大气环境下工作18个月后的断裂失效原因,对断裂阀门进行断口形貌、金相组织和化学成分等检测分析。结果表明:阀门开裂位置为壳体与滤芯重合部位,即承受气体压强最大部位,点蚀源和裂纹源起源于内螺纹根部,向阀门壳体内表面向外表面扩展;金相组织显示过滤阀门基体材料为铁素体+马氏体双相组织,测得铁素体组织的体积分数为15%~25%,成分分析显示基体中的铬含量大于20%,未检测出镍元素存在;扫描电镜下观察到阀体断口面平齐,晶粒之间遍布裂纹,呈现沿晶脆性断裂的特征。过滤阀体基体组织中Cr,Ni含量存在较大偏差,且铁素体的含量大于10%,降低了材料的塑韧性,这是材料发生失效的内在因素;由于海洋大气中Cl-的加速作用,点蚀在材料回火析出的富碳、铬第二相夹杂处形核,并在应力集中的区域产生裂纹,在交变应力的作用下,裂纹进一步扩展,最终使材料发生脆性断裂。

The reason for fracture failure of 14Cr17Ni2 stainless steel filter valve after operation in marine atmosphere for 18 months was studied.The fracture morphology,metallographic structure and chemical composition of the broken valve were analyzed.Results showed that the valve cracking position was where the casing and the filter element overlapped,namely the position that bore the greatest gas pressure.The source of pitting and cracks was originated from the root of the internal thread and extended from the inner surface of the valve casing to the outer surface.The metallographic structure showed that the matrix material of the valve was two-way-structure ferrite+martensite,and the volume fraction of ferrite structure was 15%-25%.Composition analysis showed that the chromium content in the matrix was greater than 20%,and nickel was not detected.Under the scanning electron microscope,it was observed that the fracture surface of the valve body was flush,and there were cracks between the crystal grains,showing the characteristics of brittle fracture along the crystal.There was a large deviation in the content of Cr and Ni in the matrix structure of the filter valve body and the content of ferrite was greater than 10%which reduced the plastic toughness of the material,which was the internal factor of the material failure.Moreover,because of the accelerating effect of chloride ions,the pitting corrosion nucleated in the second phase of rich carbon and chromium,and the cracks appeared in the stress concentration area.Finally,under the action of alternating stress the cracks further expanded and finally caused brittle fracture of the material.