Effects of Full Annealing Heat Treatment on Long-term Creep Strength of 2.25Cr-1Mo Steel Welded Joint

Long-term creep strength of Cr-Mo steel with fully annealed ferrite and pearlite microstructure is higher than those of the steels with martensite, tempered martensite and bainite microstructures. In this study, influence of post weld heat treatment at the temperature of austenite single phase region(full annealing PWHT) on long-term creep strength at 550°C has been investigated on 2.25Cr-1Mo steel, in comparison with that of normal post weld heat treatment at the temperature below A C1 (standard PWHT). Creep rupture strength of the welded joint subjected to standard PWHT is lower than those of base metal and weld metal in the short-term. However, such differences in creep rupture strength disappear after long-term creep exposure for about 50,000h at 550°C. On the other hand, creep strength of the welded joint subjected to full annealing PWHT is obviously higher than those of the welded joint subjected to standard PWHT, base metal and weld metal in the long-term condition. It may be also expected to reduce a probability of mechanical damage such as Type IV cracking by the full annealing PWHT, since inhomogeneous microstructures in weld metal and heat affected zone have been essentially extinguished. Full annealing at the temperature of austenite single phase region has been proposed as a new post weld heat treatment condition available to obtain high creep strength and to improve a reliability of welded joint for low alloy Cr-Mo steels.