Creep-Rupture Behavior of SUS 304H – IN 617 Dissimilar Metal Welds for AUSC Boiler Applications

Abstract

Advanced Ultra Super-Critical (AUSC) power plants are envisaged for achieving higher thermodynamic efficiency by operating at temperatures and pressures of 710/720 °C and 310 ata, respectively which are significantly higher than sub-critical (conventional) and supercritical power plants. This has led to tremendous research in selection of new and advanced materials to meet high temperature requirements. Ni-base superalloys having known to have excellent creep-rupture behavior were selected for high temperature sections of boiler while austenitic stainless steels were selected for moderately high temperature sections considering the economical feasibility. Since both these materials have to be fabricated for application in boiler tubes, dissimilar metal welding is inevitable. This work discusses creep-rupture evaluation of one such case of dissimilar welds, i.e., between Inconel 617 (IN 617) and SUS 304H. IN 617 is a Ni-base superalloy while SUS 304H is a Cu-precipitated austenitic stainless steel. Welding was carried out using IN 617 filler material for producing defect free welds. Creep-rupture samples were prepared incorporating the complete cross-weldment for understanding mechanical behavior as a result of prolonged exposure to elevated temperature and stresses. Creep-rupture testing was carried out at 600, 650 and 700°C at suitable stresses to obtain rupture times in the excess of 3000 hours in a few test conditions. Detailed characterization studies when carried out on ruptured samples revealed the weakest zone to be heat affected zone towards SUS 304H which led to failure. This work also provides insight into the possible creep-mechanisms that operate in various zones of weldments. Phases formed as a result of exposure to stress and temperature for a prolonged duration are also discussed. Comparison of the data obtained with data available in literature was carried out and performance of weldments was analyzed and reported.