Comparison of creep behaviour of 2 center dot 25Cr-1Mo/9Cr-1Mo dissimilar weld joint with its base and weld metals

Evaluation of the creep behaviour of 2.25Cr-1Mo and 9Cr-1Mo ferritic steel base metals, 9Cr-1Mo steel weld metal, and 2.25Cr-1Mo/9Cr-1Mo ferritic-ferr itic dissimilar weld joints has been carried out at 823 K in the stress ran ge 100-260 MPa. The weld joint was fabricated by shielded metal are welding using basic coated 9Cr-1Mo electrodes. Investigations of the microstructur e and hardness variations across the joint in the as welded, post-weld heat treated (973 K/1 h), and creep tested conditions were performed. The heat affected zone (HAZ) in both the steels consisted of a coarse prior austenit ic grain region, a fine prior austenitic grain region, and an intercritical structure. In the post-weld heat treated condition, a white etched soft de carburised zone in 2.25Cr-1Mo steel base metal and a black etched hard carb urised zone in 9Cr-1Mo steel weld metal around the weld fusion line develop ed. Hardness troughs also developed in the intercritical HAZ regions of bot h the steels. The width of the carburised and decarburised zones and hardne ss differences of these zones were found to increase with creep exposure. T he 9Cr-1Mo steel weld metal showed higher creep strength compared to both t he base metals. The 9Cr-1Mo steel base metal exhibited better creep resista nce than the 2.25Cr-1Mo steel base metal at lower applied stresses. The dis similar joint revealed lower creep rupture strength than both the base meta ls and weld metal. The creep strain was found to concentrate in the decarbu rised zone of 2.25Cr-1Mo steel and in the intercritical HAZ regions of both the steels. Creep failure in the stress range examined occurred in the int ercritical HAZ of 2.25Cr-1Mo steel even though this region showed higher ha rdness than the decarburised zone. Extensive creep cavitation and cracks we re observed in the decarburised zone. (C) 2001 IoM Communications Ltd.