Effect of PWHT Cycles on Impact Toughness of Austenitic Stainless Steel Weldments

Austenitic stainless steels are widely used welding materials in nuclear reactors and power plants because of their high strength, good ductility, excellent corrosion resistance and a reasonable weldability. These properties make austenitic stainless steels attractive candidate materials for use in the fabrication of piping systems, automotive exhaust gas systems and in a variety of equipment associated with the chemical and nuclear power industries. PWHT is a stress relieving process whereby residual stresses are reduced by typically heating to 550–650 °C for a set time depending upon plate thickness. It concerns have emerged about possible effects on the mechanical properties of the base (parent) and weld plates (PM and WM). The 6 mm AISI 304L, 316L, and 347 austenitic stainless steels were used for this work. These welds were produced by SMAW and GTAW techniques using a single vee preparation and multiple weld beads, and welded by various types of consumables. The fracture surfaces of the Charpy V-notch PM and WM (before and after PWHT) samples were examined by SEM. Scanning electron fractographs was critical in this study, in that valuable information regarding the mechanism and nature of failure could be determined. This paper reports work on the impact toughness of the three types of austenitic stainless steels. The parent and weld regions were examined for all types of steels used, and then exposed to temperature in the PWHT range. The effect of exposure to multiple PWHT cycles on these properties is discussed. A decrease in impact energy and fracture toughness with an increase in the number of heat treatments was evident in the parent metal. Similary, the weld metal showed a decrease in impact energy after two PWHT cycles.