排气门阀桥开裂失效分析

The Cracking Failure Analysis of an Exhaust Valve Bridge

某商用车管状气门阀桥在车削加工过程中多件出现开裂现象,裂纹分布在侧孔端面、中间孔端面及阀桥管壁。对其中一件开裂阀桥,进行化学成分分析、显微形貌分析、金相分析、SEM分析,同时结合阀桥热锻成型工艺、机械加工工艺、热处理工艺,认为该桥热锻成坯料后未及时去应力退火,在后续热处理(淬火+回火)过程中由于碳析出增加了内部组织应力,最终在车削时受外应力最终导致开裂。

Several cracked parts of exhaust valve bridges for a commercial vehicle were found during turning process.The cracks distributed on the end surfaces of the middle and side holes,and also on the wall of the tube.One of the cracked parts has been studied by chemical composition analysis,microstructure analysis,metallographic analysis and SEM.Considering the processes including hot forging,machining,and heat treatment,it would suggest that the cracked bridge would lack stress annealing immediately after the hot forging.The internal stresses in the part would increase due to the carbide precipitation in the later heat treatment.Consequentlly the bridge were cracked in the turning process.