Failure Analysis of Welding Nut on the Cover Plate of Automobile Hydraulic Torque Converter

Abstract The welding nut on the cover plate failed during the torque converter assembly process, resulting from surface defects and missing corners after spot welding, ring welding, and surface turning. Quantitative analysis of the composition, observation of the microstructure, hardness test, and fracture analysis was carried out, and the results indicated several heat-affected zones after spot welding caused speckle defects. The martensite and carbides produced in the heat-affected zone were inconsistent with the steel matrix deformation during the surface turning. The stress caused by inconsistency induced swelling and spalling. The crack source was pinpointed at the region where large agglomerative inclusions were observed, which was caused by stress concentration. The crack propagated along with the inclusions finally developed into the missing corner. The research results could improve the production process and provide data support for further researches into nut failure analysis.