Welding advanced martensitic creep-resistant steels with boron containing filler metal

Within the last years, the beneficial effect of boron addition in order to increase the creep strength of martensitic 9-12 % Cr steels has been widely investigated and demonstrated. Several boron-strengthened alloys are the outcome of an intensive European research activity within the scientific framework of COST. The most promising alloy for castings with a targeted application temperature of650 °C for use in future ultra-super critical steam power plants is a modified 0.1C-9Cr-1.5Mo steel with the addition of approximately 110 ppm boron. For joining of this advanced steel grade, new "chemically matching" welding consumables have to be developed to provide similar creep strength in the weld. Within this paper, the development of an advanced flux-cored wire with a good "out of position weldability"-based on a rutile slag system is presented. The work discusses the optimization of the chemical composition of the filler, the influence of various post-weld heat treatments on short term mechanical properties, first results of creep tests and results of thick section welds. The boron content of the consumable was optimized to achieve highest creep strength combined with acceptable toughness and lowest hot cracking sensitivity. For a butt weld of a 34 mm-thick casting, the optimum welding parameters and layer sequence were determined. A post-weld heat treatment consisting of stress relieving twice at 730 °C for 12 h seemed to be the best compromise between applicability in the manufacturing of large turbine casings and achievement of acceptable weld toughness.