Influence of Softened Heat-affected Zone on Stress Oriented Hydrogen Induced Cracking of a High Strength Line Pipe Steel.

Abstract

The susceptibility to hydrogen induced cracking under applied stress was examined for a large diameter high strength sour service line pipe steel produced by employing the thermo-mechanical control process, i.e. controlled rolling followed by accelerated cooling. The low carbon content aimed at high toughness and weldability resulted in softening in the grain-refined and inter-critical heat-affected zone by thermal cycles of longitudinal sub-merged arc welding. The cracking susceptibility of weldment was transversely evaluated by TM0177-90 method A and compared with the parent metal.The parent metal indicated high resistance to cracking. Nonethelss, the weldment indicated the decrease in threshold stress, since the weldment ruptured preferably along the softened heat-affected zone. The cracking morphology in the softened heat-affected zone was recognized as stress oriented hydrogen induced cracking. Also performed was a large scale test to reveal the internal incipient cracks in the softened heat-affected zone. The observation confirmed that the initiation site was a small cluster of non-metallic inclusions which had probably no adverse effect on the resistance of the parent metal. He stress analysis of the weldment proved that the tension through thickness was generated at mid-thickness under the deformation constraint from surrounding harder parts. The enhanced tri-axiality of stress in company with the through thickness tension facilitates the onset of cracking caused by the hydrogen pressure mechanism. The crack morphology was consistently associated with the stress distribution.