Cause of bending fracture in high-strength fasteners and heredity of surface recarburization in high carbon steel

Abstract

In this work, the cause of bending fracture in high-strength fasteners after electroplating was investigated and the heredity of surface recarburization from continuously cast high carbon steel blooms to the final fastener products was explored. The fracture surface of the fasteners displayed intergranular cracking and numerous intergranular facets accompanied by ductile tearing. Hydrogen-induced stepwise cracks were identified near fracture zones, indicative of hydrogen embrittlement. The surface decarburized layer on the fasteners featured a significant number of fine partial decarburization grains and a high carbon content, resulting in low electrical conductivity. The observed dense coating, possibly induced by the poor conductivity of the decarburized layer, could elevate the hydrogen content in the substrate, ultimately resulting in hydrogen embrittlement. Additionally, the surface carbon pick-up at the corners of the blooms could be inherited by the rolled billet. The extensive deformation experienced during the hot rolling process resulted in the uniform distribution of recarburization on the surface of both the wire rod and spheroidized wire rod. Consequently, this recarburization contributed to a low degree of decarburization on the surface of the fasteners. Simulation results obtained through DICTRA also indicated that the recarburization on the surfaces of various samples could not be eliminated during the heat treatment process.