The Effect of Surface Carbon on the Spot Weldability of a Microalloyed 60 ksi Cold-Rolled Steel 830127
The surface condition of sheet steel has been noted in the past to affect the occurrence of interfacial nugget failures of spot welds during qualification testing. A microalloyed 60 ksi minimum yield strength cold-rolled steel was observed to have an unusually high incidence of interfacial nugget tears when welded in the as-received condition, but virtually no interfacial tears when wiped before welding. Samples of welds from both the as-received and wiped materials were examined to determine the surface characteristic causing the interfacial failures.
Using Auger surface analysis techniques, the presence of a high concentration of carbon relative to iron was observed on the surface. This carbonacious surface was found to be detrimental to the formation of successful spot welds.
Scanning electron microscopy (SEM) revealed that the diffusion bonded region of the weld heat-affected zone of the spot welds in the as-received material was significantly smaller than that of the similar region in the cleaned material. This was evidenced by the extent of the ductile dimple structure found within the heat-affected zone of a tested sample. Additionally, a significantly higher amount of secondary cracking was noted in the heat-affected zone of the as-received material. Good spot weldability, as evidenced by ductile nugget pull-out, was found to correlate with significant heat-affected zone solid-state bonding. When little heat-affected zone bonding was observed, interfacial nugget fracture resulted.
A spot weld model describing the plastic zone development and propagation during a standard peel test has been proposed on the basis of the observations noted in the heat-affected zone.