Abstract
The influence of phosphorus and sulphur on the microstructures and cross-tension strength of spotwelds in sheet steels has been studied. In steels containing 0·06%C and 0·33%Mn (wt-%), the addition of phosphorus had two effects on the microstructure. An addition of 0·10%P increased the hardenability, resulting in a martensitic weld and heat affected zone. A further increase to 0.15%P resulted in the stabilisation of ferrite in the weld and fusion zones. The ferrite along grain boundaries in the fusion zone aided the initiation of cleavage and decreased the cross-tension strength. The addition of sulphur to a steel containing 0·15%P decreased the cross-tension strength. This effect was associated with a change from ductile to brittle behaviour at the interface between the sheets in the heat affected zone, caused by increased wetting of the interface and grain boundaries by oxysulphides as the sulphur content was increased.
MST/920
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