一批M6 mm内六角镀锌螺钉在安装后运行基本正常,但经一段时间试车后头部发生断裂.通过化学成分分析、硬度测试、宏观和微观检验对断裂原因进行了分析.结果表明:螺钉断裂为氢脆引起的延迟断裂;主要原因是烘箱中的一条电阻丝断裂,使烘箱内温度不均匀,导致电镀过程中渗入的氢未彻底去除;次要原因是螺钉硬度偏高,增大了氢脆敏感性,最终导致了螺钉的断裂.

A batch of M6 mm inside-hexagonal screw was ruptured on the head area after trial running for a period of time,which was in normal operation state after installation.The fractured screw was detected from the aspects of chemical composition analysis,hardness testing,macrograph and microscopic observation.The results showed that:the fracture mode of screw was the delayed fracture caused by hydrogen embrittlement;the main reason was that one resistance wire of the oven was broken,causing the temperature in the oven distributed unevenly,and therefore the hydrogen brought in electroplating period was not removed completely;the secondary reason was that too high hardness of the strew increased the susceptibility to the hydrogen embrittlement,and finally led to the fracture of the screw.