火花塞膨胀槽脆性开裂失效分析

Failure analysis of brittle crack in the swollen slot of a spark plug

某冷挤压成形的火花塞铁壳热铆组装之后于膨胀槽处横向脆性开裂。为了探讨失效原因,采用光学显微镜、扫描电镜和显微硬度计以及氢热分析仪等对其进行了全面地显微分析。结果表明:铁壳内壁机加工痕迹明显,并有少量点状锈蚀。人工打开失效样品发现起裂源位于靠近内壁的次表面,整个横截面上均为沿晶开裂形貌,并存有大量二次裂纹。失效样品的热铆变形处显微组织为等轴铁素体+马氏体及碳化物颗粒,铁素体为双重晶粒。失效样品的电镀镍层厚度相对较薄,内壁局部厚度不足1μm,镀层内还残留少量氧化铁。膨胀槽纵截面的显微硬度波动较大,粗晶区域的显微硬度低于150 HV,而热铆前约为230 HV。失效样品的氢扩散质量浓度为0.042 mg/m3。由此认为样品失效原因系电镀增氢与热铆产生的残余应力耦合导致铁壳中最薄弱的膨胀槽处发生脆性延迟开裂。

A spark plug was failed with brittle crack from the swollen slot. It was analyzed in detail by optical microscope,scanning microscope and microhardness tester as well as thermal desorption spectroscopy. There were red rust spots on the inner surface with rough tool marks. The origin of crack was at the sub-surface of inner layer and intergranular fractures were all over the cross section. It was equi-axial ferrite and martensite with carbide particles at deformation place,and coarse grain in the interior inwall with duplex grain size. There were galvanized nickel layer on the surface of the samples,and it was thin in the inner layer. It could be found that retained rust under the layer along the inner wall.The micro-hardness on the cross section of swollen slot was different. It was lower than 150 HV in the coarse grain zone while it was 230 HV before heated. The mass concentration of diffusible hydrogen was 0.042 mg/m3 in the failure sample. The conclusion could be drawn that the failure sample with delayed brittle crack caused by hydrogen added by galvanization coupled with retained stress result from heat-riveting.