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Three advanced steel melting process—low-sulfur vacuum silicon deoxidation, electroslag remelting, and vacuum carbon deoxidation—were applied to produce three 1Cr-Mo-V [ASTM Specification for Vacuum-Treated Carbon and Alloy Steel Forgings for Turbine Rotors and Shafts A 470-82, Class 8] steel forgings for steam turbine application. A detailed evaluation of the tensile properties up to 565°C (1050°F), fracture toughness (KIc and JIc), high-cycle and low-cycle fatigue properties, and stress rupture (creep) properties were determined for the three forgings.
Compared to conventionally produced forgings, the advanced technology forgings show better tensile ductility and uniformity along the radial and longitudinal directions. Charpy upper-shelf energy shows about 40% improvement, and no temper embrittlement was found using a step-cooled and isothermal aging treatment. Significant improvement in fracture toughness (factors of two to three higher) is realized for these forgings. Low-cycle fatigue life is better at high temperatures due to the absence of nonmetallic inclusions. Creep strength shows slight improvement. However, creep ductility probably is improved due to low residual elements. Especially, the VCD forging shows excellent creep ductility even at long lives. Both the toughness and creep properties are better than those of oil-quenched rotors produced by European practice. These improvements are attributed to cleaner steel, low residual elements (especially very low sulfur content), and the associated reduction of nonmetallic inclusions.
advance melting, Cr-Mo-V steel, steam turbines, mechanical properties, tension, Charpy impact, fatigue (metals), fracture toughness, creep, stress rupture
Senior engineer, Westinghouse Electric Corp., Orlando, FL
Senior technical advisor, Electric Power Research Institute, Palo Alto, CA
Consultant, Engineering Materials & Processes, Inc., Pittsburgh, PA