Failure Analysis of Brittle Cracking in Spark Plug Expansion Tank

Abstract

A cold-extruded spark plug iron shell is assembled by hot riveting and then laterally brittle cracking at the expansion groove. In order to investigate the cause of failure, a comprehensive microscopic analysis was performed by using an optical microscope, a scanning electron microscope, a microhardness tester, and a hydrothermal analyzer. The results show that the machining of the inner wall of the iron shell is obvious and there is a small amount of rust. Manually opening the failed sample, it reveals that the originating source is located on the subsurface close to the inner wall, and the entire cross section has an intergranular fracture, and there are a large number of secondary cracks. The thickness of the electroplated nickel layer of the sample is relatively thin, the local thickness of the inner wall is less than 1 μm, and a small amount of iron oxide remains in the plating layer. The grain size is 7.4, and the hardness of the longitudinal section of the expansion tank fluctuates greatly. The hardness of the coarse grain area is less than 150 HV, and the front of the hot rivet is about 230 HV. The diffusion hydrogen content of the failed sample is 0.042 mg/m³. It is considered that the cause of the failure sample is the coupling of the phase transition stress and the residual stress generated by the increasing electroplating hydrogen, resulting in delayed cracking at the weakest expansion groove in the iron shell.