Fracture Analysis on Galvanized Screw

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.