Effect of long-time postweld heat treatments on the mechanical properties of a carbon-manganese pressure vessel steel

Postweld heat treatment is a common practice among building codes for welde d steel structures and equipment to reduce the high Its-welded residual str ess level, improve the fracture toughness, and increase the critical size o f acceptable defects. There are many discrepancies among international buil ding codes for storage spheres, pressure vessels, and welded structure comp onents about parameters such as duration and temperature for postweld heat treatments, Furthermore, the codes frequently omit the top number of therma l cycles, which the structure may support to maintain the mechanical proper ties and toughness in an acceptable level, keeping the physical integrity o f the equipment. The present work analyzes the effect of duration of the po stweld heat treatments on the mechanical properties and fracture toughness of a carbon-manganese steel of specification ASTM A-516 G 70, which frequen tly is used to build spheres and pressure vessels in the petrochemical indu stry. The regions corresponding to the base metal (BM) and heat-affected zo ne (HAZ) were studied. Through the results obtained from the tensile tests, hardness measurements, Charpy V-notch impact and crack-tip opening displac ement (CTOD) testing, and microstructural characterization, it is concluded that the mechanical properties and fracture toughness were reduced by incr easing the time of the postweld heat treatment. The degradation of the orig inal properties of the steel is attributed to the changes that occurred in the microstructure. With the welding procedure used, it was verified that t he fracture resistance of the HAZ was higher than that of the BM.