Degradation of impact fracture during accelerated ageing of weld metal on microalloyed steel

The effect of accelerated ageing on the toughness and fracture of the longitudinal weld metal of an API5L-X52 linepipe steel was evaluated by a Charpy V-notch impact test, fracture analysis and transmission electron microscopy. Ageing was performed at 250°C for 100-1000 h. The impact results indicated a significant reduction in the fracture energy and impact toughness as a function of ageing time, which were achieved by the scanning electron microscope fractographs that showed a decrease in the volume fraction of microvoids by Charpy ductile failure with the ageing time, which promoted brittle fracture due to transgranular cleavage. The minimum volume fraction of microvoids was reached at 500 h due to the peak age. The microstructural analysis indicated the precipitation of transgranular iron nanocarbides in the aged specimens, which was related to the deterioration of toughness and change in the ductile to brittle behaviour.