长期服役P91钢蒸汽管道焊接接头显微组织及力学性能研究

Study on Microstructure and Mechanical Properties of Long-Term Service P91 Steel Steam Pipe Welded Joint

摘要:

P91钢被广泛应用于大型电站的发电机组和蒸汽管道等关键部位。长期服役会导致P91钢蒸汽管道性能退化，目前研究多关注长期服役对母材性能的影响，而对焊接接头各部位的显微组织和力学性能缺乏系统研究。因此，分别从未服役的P91钢蒸汽管道的母材（NBM）和已服役74 000 h的P91钢蒸汽管道焊接接头的母材（BM）、热影响区（HAZ）焊缝（WZ）取材，利用金相显微镜、扫描电镜、透射电镜、硬度计和拉伸试验机，研究长期服役对P91钢蒸汽管道焊接接头的显微组织和力学性能的影响。结果表明：相比于NBM，BM和WZ的硬度和拉伸性能均未出现退化迹象，而HAZ的拉伸性能在硬度基本未变的情况下显著退化，表明硬度不能用于判断服役时间低于74 000 h的P91钢蒸汽管道焊接接头的寿命。焊接时高温热循环回火使HAZ的马氏体板条和碳化物发生粗化，且碳化物沿马氏体板条界面连续析出分布，对HAZ的拉伸性能已造成一定程度的损害。长期服役使本已老化的HAZ显微组织明显退化，致使其拉伸性能显著下降，断裂模式由韧窝断裂转变为韧窝和准解理复合断裂。

Abstract:

P91 steel is widely used in the key parts such as generator sets and steam pipelines in large power stations. Long-term service will lead to performance degradation of P91 steel steam pipelines. However， the current research mainly focuses on the effect of long-term service on the base metal， and there is a lack of systematic research on the microstructure and mechanical properties of P91 steel steam pipeline welded joint. In this paper， samples were obtained from the base metal of non-service P91 steel steam pipelines （NBM） as well as the base material （BM）， heat affected zone （HAZ）， and welded zone （WZ） of P91 steel steam pipeline welded joint in service for 74000 h. The microstructure and mechanical properties of long-term service P91 steel steam pipeline welded joint were studied by metallographic microscope， scanning electron microscope， transmission electron microscope， hardness tester， and tensile testing machine. The results show that compared with NBM， the hardness and tensile properties of BM and WZ show no signs of degradation， while the tensile properties of HAZ are significantly degraded when the hardness is basically unchanged. This indicates that hardness cannot be used to determine the service life of P91 steel steam pipeline welded joint with service time lower than 74000 h. The high temperature thermal cycle tempering during welding roughens the martensitic laths and carbides of HAZ， and the carbides are continuously precipitated and distributed along the interface of martensitic laths， which has caused a certain degree of damage to the tensile properties of HAZ. Long-term service will significantly degrade the already-aged microstructure of HAZ， resulting in a significant decrease in its tensile properties， and the fracture mode changes from dimple fracture to mixed fracture of dimple and quasi-cleavage.