Effect of cathodic protection on ductility of X65 linepipe steel weldments

Abstract

The combined effects of strain rate, monotonic loading, and cathodic protection and overprotection on the mechanical properties of X65 linepipe steel parent plate and gas metal arc weld metal have been investigated in NaCl solution. The results obtained on longitudinal all parent plate and all weld metal specimens indicate that the latter material, having a higher hardness and strength level, is more susceptible to environmental cracking due to cathodically evolved hydrogen entering the steel. However, with transverse specimens, taken perpendicular to the weld, the gauge length contains parent plate, heat affected zone material, and weld metal. In this case, irrespective of the greater inherent susceptibility of the weld metal, environmental cracking and failure always occur in the parent plate. This result is attributed to the ultimate tensile strength of the lower strength parent plate being exceeded at stress levels significantly lower than the yield strength of the higher strength weld metal. The practical implications of these results are discussed. In addition, the results are presented of a fractographic analysis which relates the morphology of the environmentally induced fracture surfaces to microstructural features in the weld metal and the parent plate.