柴油发动机进气阀断裂分析

柴油发动机进气阀为一体式锻造、硬质合金堆焊而成，在使用过程中发生断裂。为了分析断裂原因和制定对策，对断裂进气阀的断口宏观特征、微观形貌、金相组织、化学成分、显微硬度和残余应力进行分析。结果表明:进气阀断裂模式为疲劳断裂；零件堆焊层中存在较高的硬度和残余拉应力；进气阀在复杂的工作环境中受到机械冲击负荷和高残余拉应力的叠加作用，加上高的脆性，导致堆焊区局部裂纹产生；在重复的工作负荷下，裂纹以疲劳方式进一步扩展，直至断裂。建议严格控制进气阀堆焊后的热处理工艺，降低硬度和残余拉应力，从而有效避免此类失效的发生。

Failure Analysis of Diesel Engine Inlet Valve

The inlet valve of diesel engine that made of one-piece forging and hard alloy surfacing welding broke during using process. In order to analyze the root failure cause and formulate the countermeasures, the analyses of macroscopic characteristics, microscopic morphology, metallographic structure, chemical composition, microhardness, and residual stress of the fractured intake valve were performed. The results indicate that the failure mode of the inlet valve is fatigue fracture. The hardness and residual tensile stress are high in the hard-facing layer. The intake valve is subjected to the superposition of mechanical shock load and high residual tensile stress in a complex working environment. Coupled with the high brittleness, all of these effects led to the appearance of the localized cracks in the surfacing area. And then, the cracks further expanded in a fatigue manner under repeated working loads until they finally broke. It is recommended to strictly control the heat treatment process after surfacing of the intake valve, in order to reduce the hardness and residual tensile stress, and avoid the recurrence of such failures.