汽轮机Nimonic 80A材质螺栓频繁断裂分析及“负蠕变”行为研究

Study on Frequent Fracture and Negative Creep Behavior of Superalloy Bolts in Steam Turbines of Power Stations

以失效断裂的Nimonic 80A材质螺栓为研究对象,进行了细致地案例分析、力学性能试验、三维有限元分析以及理论机理研究等,来综合分析造成Nimonic 80A材质螺栓失效断裂的原因。主要结论如下:Nimonic 80A作为紧固件材质使用时,服役温度对于螺栓内应力存在不寻常的影响。525℃以下偏低温服役导致的负蠕变是造成Nimonic 80A材质螺栓断裂的关键原因,而525℃以上高温服役时运行正常。Nimonic 80A螺栓长期低于525℃服役时会引起螺栓内部大范围Ni2Cr有序化转变,产生负蠕变,引起螺栓应力上升,从而在螺纹应力集中最大的局部区域产生过载开裂。针对负蠕变问题对螺栓进行工程改造,525℃以上螺栓维持不变,525℃以下螺栓进行材料替换;改造工程项目最长已经3年以上,至今服役良好。

A broken Nimonic 80A bolt was taken as the focus in this paper.Detailed case analysis,mechanical performance tests,three-dimensional finite element analysis and theoretical mechanism research were carried out to analyze the bolt failure comprehensively.It was found out that the service temperature had unusual effects on internal stress of the Nimonic 80A when as a bolt material.Negative creep caused by low temperature service below 525℃was the key reason for the fracture,while normal operation remained above 525℃.Long-term service below 525℃would cause a large-scale Ni2Cr ordering transformation within the bolts,resulting in negative creep.Negative creep will cause the bolt stress to rise,resulting in overload cracking at the maximum stress location.Regarding the problem of negative creep,the bolts were reengineered.The bolts above 525℃remained unchanged,while the bolts below 525℃were replaced with another matetial.The longest renovation project had been over 3 years and in good service so far.