Strain Ageing in Welded Joints of API5L X65Q Seamless Pipes

In this work was investigated the influence of the strain ageing on joints obtained by gas-metal arc welding (GMAW) of API 5LX65Q seamless pipes. Specimens were machined from the welded joints, plastically strained to 3% and aged at 250 degrees C for 1h posteriorly, in order to characterize effects of static strain ageing. For the dynamic strain ageing characterization, the specimens were simultaneously strained and aged. Then, the aged specimens were submitted to tensile tests and representative samples were analyzed at a light optical microscope and a scanning electron microscope (SEM). From the results, it was observed that the strain ageing phenomenon changed the volumetric fraction of secondary constituents in the weld metal and in the heat-affected zone, increasing the proportion of ferrite-carbide aggregates. Regarding the mechanical properties, it was noticed that the phenomenon led to higher tensile mechanical properties such as ultimate tensile strength and total elongation and lower elastic ratio, and, therefore, not impaired the mechanical properties under tensile of the welded joints. Comparing the static strain ageing with dynamic strain ageing it was observed that the effect on the plastic straining ability of welds was higher after static strain ageing.