Effect of artificial aging on the microstructure of weldment on API 5L X-52 steel pipe

The effects of artificial aging on the microstructure in the weldment of an API 5L X-52 steel pipe were studied. Aging was performed at 250 °C over a period of 1000 h and values were recorded at every 100 h intervals. Transmission electron microscopy observations showed precipitation strengthening from nearly circular Nb–C containing nanoparticles for the base metal and heat affected zone, and cementite for the weld metal. The largest amount of precipitation in the weldment zone was obtained at 500 h, due to peak-aging, which showed the highest particle density. The weld metal was more susceptible to aging, exhibiting the highest increase in precipitation at 500 h, followed by the heat affected zone. After 500 h, the deterioration in the microstructure was caused by the coarsening of particles due to over-aging. The base metal showed the larger increment in particle size after 900 h of aging accompanied by a bigger decrease in fine particles than in the weld metal.