Effect of Cleanliness of Steel on its Weldability

Abstract

It has been reported that clean steels with very low sulphur and oxygen contents may be more susceptible to hydrogen cracking than traditional grades. However, some investigations showed little effect of sulphur on weldability. In order to clarify the reason for this discrepancy, many steels with widely varied sulphur contents (0.001%-0.0037%) were laboratory-melted and their HAZ hardnesses were examined.
Hydrogen diffusion rates were also examined for the base metals with varying contents of sulphur, and their HAZs to investigate the significance of inclusions in reducing the risk of hydrogen cracking through their roles on hydrogen trapping.
The findings in the experiments are as follows:
1) In C-Si-Mn steels, MnS and MnS-Oxide inclusions act as ferrite nucleation sites. HAZ hardenability of these steels thus increases as the sulphur content decreases.
2) In C-Si-Mn-Nb steels, Nb suppresses the function of MnS as ferrite nucleation. Therefore, HAZ hardenability in this type of steel is not influenced by sulphur content.
3) Hydrogen is found to be temporarily trapped in inclusions. As steel contains less inclusions, diffusible hydrogen tends not to be trapped in base and thus hydrogen accumulation in HAZ is more likely.
4) Inclusions influence the susceptibility of steel to hydrogen HAZ cracking mostly through enhancement of hardenability and partly through hydrogen trapping.