超超临界机组中压汽门阀盖Alloy783合金螺栓断裂失效分析

Fracture failure analysis on Alloy783 bolts in medium-pressure valve bonnets of USC units

超超临界机组汽轮机中压联合汽门(中压汽门)阀盖Alloy783合金螺栓断裂会对人身和设备安全造成极大威胁。为了研究造成Alloy783合金螺栓断裂的主要因素,本文以断裂的Alloy783合金螺栓为研究对象,对螺栓进行了整体数据统计、宏观检查、显微组织分析及力学性能试验。结果发现:Alloy783合金螺栓断裂位置无明显规律,且存在纵向裂纹,因此由于预紧力过大导致断裂的可能性不大;断裂螺栓Alloy783合金中一次?相偏析明显,呈条带分布,晶界二次?相析出较少,分析认为主要由于固溶处理及其后的?时效热处理不充分所致;所有断裂螺栓的疲劳裂纹基本沿晶界扩展,这主要是由于应力促进晶界氧化并导致疲劳裂纹沿着氧化损伤区扩展,裂纹扩展速率由晶界氧化速率控制;断裂螺栓抗应力促进晶界氧化能力差主要是由于晶界缺乏二次?相。

Fracture of Alloy783 bolts in medium-pressure valve bonnets of USC units will cause a significant threat to personal and equipment safety. In order to study the main failure factors, the whole data statistics, macroscopic inspection, microstructure analysis and mechanical performance tests were carried out for a fractured Alloy783 bolt. It was found that, all the fracture positions lacked regularity and longitudinal cracks were observed, so it is impossible that the excessive preload during installation will cause the cracks. The segregation of primary β phases was very serious, showing a clear band distribution. The precipitation of grain-boundary secondary β phases was less, which was mainly due to insufficient solution treatment and subsequent β aging heat treatment. The fatigue cracks of all broken bolts were mainly intergranular, since the stress-induced grain boundary oxidation （short for SAGBO） led to fatigue crack propagation within oxidation damage zone. Thus the crack propagation rate was controlled by the intergranular oxidation rate. The poor resistance against SAGBO was attributed to lack of secondary β phases along grain boundaries.