EFFECT OF MICROSTRUCTURE, INCLUSION MORPHOLOGY AND COMPOSITION ON THE HYDROGEN-INDUCED HAZ CRACKING SUSCEPTIBILITY OF CAST AND WROUGHT CARBON-MANGANESE-SILICON STEELS (WELDABILITY, COLD, DELAYED)

The research program was undertaken to provide both basic and pragmatic information on the hydrogen-induced cracking (HIC) susceptibilities of cast steels and to compare their behavior with that of equivalent wrought steels. The work was extended into determinations of the effect of inclusion morphology, deoxidation practice, residual elements and the effect of very low sulfur contents (0.001%S) in wrought steels.;The results of the baseline underbead cracking tests indicate that the susceptibility to HIC of cast and normalized C-Mn-Si and 8630 steels can be related to composition using a carbon equivalent formula of the type, C.E. = %C + %Mn/6 + %Si/24 + %Cr/12 + %Mo/10. Wrought C-Mn-Si steels when test welded in the rolling direction were found to be more susceptible to HIC when compared to cast steels of equivalent composition (C.E.). The greater susceptibility of wrought steels when tested in the rolling direction is related to the presence of elongated sulfide inclusions which act as more potent sites for the initiation of HIC compared to the globular inclusions in cast steels. In addition, banding (ingotism) in wrought steels presents a continuous path for crack propagation. When the wrought steels were tested with the weld normal to the rolling direction, the susceptibility to HIC of cast and equivalent wrought steels was similar.;For cast steels, Type II inclusions (eutectic sulfides) located on primary solidification boundaries were found to exacerbate susceptibility to HIC. Increasing the extent of deoxidation which leads to the formation of Type III inclusions (angular sulfides) in preference to Type II's, was found to decrease HIC susceptibility in cast steels.;Trapping of hydrogen at inclusion/matrix interfaces leading to the accumulation of hydrogen sufficient to initiate and propagate a crack is suggested as the primary mechanism of HIC. Fracture morphologies observed in hydrogen-embrittled HAZ's are explained using the mechanism in concert with Beachem's model of hydrogen-assisted cracking. The observance of a predominantly intergranular mode of HIC in low sulfur steels is explained using the above model.;The tests included the self restraint rapidly cooled HAZ; Battelle Underbead Cracking Test, the applied restraint; Implant Test and a composition/hydrogen method; the UT-Modified Hydrogen Sensitivity Test.