Failure Analysis of Brittle Cracking in Spark Plug Expansion Tank
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TECHNICAL ARTICLE—PEER-REVIEWED
Failure Analysis of Brittle Cracking in Spark Plug Expansion Tank Jun-yi Wang . Jie Chen . Xin-kuan Liu
Submitted: 22 October 2018 / in revised form: 10 September 2019 Ó ASM International 2019
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 lm, 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
J. Wang Baosteel Research Institute, Shanghai 201900, China J. Chen X. Liu (&) School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China e-mail: [email protected]
electroplating hydrogen, resulting in delayed cracking at the weakest expansion groove in the iron shell. Keywords Spark plug Expansion tank Cracking along the crystal Hydrogen embrittlement
Introduction A spark plug is a device that mixes gasoline and air entering the engine and ignites, and it plays a very important role in the operation of the engine. It is mainly composed of metal body and ceramic components, and its metal part is usually called ‘‘iron shell’’, which can be processed by the combination of bar cold extrusion technology and cutting process [1]. At present, the common raw material for spark plugs is 10 A cold-rolled steel bar. The relevant components of this steel meet the Chinese standard GB/T 6478-2015. The model of the spark plug we study is K7RF-11 and has a total length of about 75 mm, with the iron shell part being about 34 mm long. The u22 mm wire rod is pickled, drawn into u18.20 mm. After being cut, it is preformed and then composite cold-extruded, thread-rolled, cut, nickel-electroplated . And then it is assembled with the ceramic body by riveting on the condition of induction heating. The iron shell is composed of a connecting screw, a rod portion, an expansion groove, an outer hexagon, and a riveting port.
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