Temperature Dependence of Fracture Toughness of Large Steam Turbine Forgings Produced by Advanced Steel Melting Processes

Three advanced steel melting processes—low sulfur vacuum silicon deoxidation, electroslag remelting, and vacuum carbon deoxidation—were applied to produce three Cr-Mo-V ASTM A470, Class 8 steel forgings for steam turbine application. Plane strain fracture toughness (K_Ic) values were obtained using 2T and 3T compact specimens between 24 and 93°C (75 and 200°F). The elastic-plastic fracture toughness (J_Ic) was obtained using 1T-CT and round compact specimens at 149, 260, and 427°C (300, 500, and 800°F). Both multiple specimen “heat-tint” techniques and single specimen “unloading compliance” techniques were employed at 149°C (300°F) to generate J-R curves. Both of these techniques yielded results with excellent agreement. Round CT specimen results agree well with standard CT specimen results.

These advanced technology forgings show significant improvement (factors of two to three higher) in fracture toughness over conventionally produced forgings. This increase in toughness is attributed mainly to the ability of these processes in producing cleaner steel, especially with very low sulfur content, and the associated reduction in the amount of nonmetallic inclusions. Minimums in toughness and the tearing modulus at 260°C (500°F) were observed. These minimums correspond to the occurrence of minimum in the tensile ductility at the same temperature which may be due to the dynamic strain-aging phenomenon.