Influence of plate thickness on the mechanical properties of welded joints subjected to long-term postweld heat treatments

During their service lives, storage spheres, pressure vessels, and other welded structures are subjected frequently to postweld heat treatments (PWHT). Repeated treatments may result, in some cases, in a reduction in mechanical strength and fracture toughness of the weldments such that their mechanical properties may not meet the code specifications after those treatments. Steel ASTM-A-516 Gr 70 is used frequently to build storage spheres and pressure vessels in the petrochemical industry. In this context, the thickness of the plate is an important variable relative to the mechanical behavior of the welded joint after repeated PWHT. In a previous work (Pimenta and Bastian, JMEPEG, 2001, 10, pp. 192-202), the effect of the duration of a PWHT on the mechanical properties of a 65 mm thick plate was evaluated. In the present work, the effect of the same heat treatment on the properties of a 46.4 mm thick plate was evaluated and compared with those of the 65 mm thick plate. For this purpose, hardness measurements, tensile and Charpy V-notch impact tests, and a metallographic analysis were performed. The results show that both the base metal and heat-affected zone of the thinner plate present higher mechanical strength and impact resistance for all heat-treated conditions. The obtained mechanical properties were then compared with the requirements of the ASME Code, Section VIII.