Mechanical aspects of spotvvelded joints in heat-treated lovvcarbon mild-steel sheets

Abstract

Mechanicalproperties of the spot welds made in martensitic, bainitic, cold-rolled, and subcritically annealed sheets produced from the same basic low-carbon mild steel have been assessed. Although the welds made in the stronger materials were able to withstand greater tensile shear loads than those made in weaker materials, the U-tensile strengths of the welds made in allfour materials were similar. The effects of the speed of tensile shear testing were determined, and it was found that, even at high testing speeds, the welds made in the stronger materials retained their load-bearing advantage over the welds made in the weaker materials. Tensile shear impact strengths of these welds were found to be inversely related to the yield strength of the parent sheet. The behaviour of the spot welds under fatigue stresses was determined. The results showed that the endurance limits of the spot welds in the ‘as-welded’ condition were low and comparable, and it was evident that the metallurgical condition of the parent sheet did not have any significant effect on them. A quantitative estimation of the residual stresses present in the spot welds was established. By plastically compressing the welds, the residual stresses were relieved and the endurance limits of these welds were significantly improved.