Cause analysis of V-shaped crack pairs on drawn welding wire surface of ER70S-6 steel

Welding wires are widely used as electrodes in various welding processes. To produce welding wires, wire rods are drawn gradually to final diameters at welding wire manufacturers. During the wire drawing process, premature failure happens frequently due to various reasons, and a type of wire drawing failure originated from wire surface V-shaped cracks or so-called chevron-like cracks is often found. In this research, V-shaped cracks on drawn welding wire surfaces of ER70S-6 steel were characterised carefully and its underlying mechanism was discussed thoroughly. It was found that the V-shaped cracks on wire surfaces are often present in pairs with opposite directions. Martensite microstructure was found nearby the V-shaped cracks, and its formation can be attributed to deteriorated local lubrication during wire drawing, which causes the austenitization of the wire surface and the subsequent formation of martensite. The large friction on wire surface induces the development of V-shaped cracks along the drawing direction. Meanwhile, as martensite is difficult to deform, strain localization happens in the ferrite/pearlite matrix, and V-shaped strain localization areas were found around the martensite, leading to the development of V-shaped cracks in the opposite direction of the drawing direction.