Effect of artificial aging time on the mechanical properties of weldment on API 5L X-52 line pipe steel

The effect of artificial aging time on the microstructure and mechanical properties in the weld and base metals of an API 5L-X52 line pipe steel was studied. Artificial aging was performed for 1000 hours at 250 °C and was monitored every 100 hours. Vickers hardness and tensile tests were used to examine the aging effect on the mechanical properties. Scanning electron microscopy and transmission electron microscopy (TEM) studies were carried out to analyze the microstructure evolution. The Vickers hardness results showed that the weld and base metals displayed a hardening tendency up to a maximum value at 500 hours of aging. The yield strength increased with aging time while the elongation-to-fracture decreased. The maximum yield strength was found at 500 hours, which was attributed to the peak-aged condition. After 500 hours, both the Vickers hardness and yield strength were reduced while the elongation was increased due to the overaging condition. The TEM observations and fracture analysis of specimens showed that the improvement of strength was associated with the nanoparticles’ precipitation, while the degradation of the microstructure and mechanical properties was related to the coarsening process of iron carbide (cementite) and niobium carbide for the weld and base metals, respectively. The largest amount of precipitation in both alloys occurred at 500 hours.