The mechanical behavior of a cast form of an advanced austenitic stainless steel, CF8C-Plus, is compared with that of its wrought equivalent in terms of both tensile and creep-rupture properties and estimated allowable stress values for pressurized service at temperatures up to about 850°C. A traditional Larson-Miller parametric model is used to analyze the creep-rupture data and to predict long-term lifetimes for comparison of the two alloy types. The cast CF8C-Plus exhibited lower yield and tensile strengths, but higher creep strength compared to its wrought counterpart. Two welding methods, shielded-metal-arc welding (SMAW) and gas-metal-arc welding, met the weld qualification acceptance criteria in ASME BPVC Section IX for the cast CF8C-Plus. However, for the wrought CF8C-Plus, while SMAW and gas-tungsten-arc welding passed the tensile acceptance criteria, they failed the side bend tests due to lack of fusion or weld metal discontinuities.

As conventional coal-fired power plants seek to reduce greenhouse gas emissions by increasing efficiency, the temperature limitations of traditional ferritic/martensitic steels used in high-temperature components present a significant challenge. With Advanced Ultra Supercritical (A-USC) power plants proposing steam temperatures of 760°C, attention has turned to nickel-based superalloys as potential replacements, since ferritic/martensitic steels cannot withstand such extreme conditions. However, the current absence of cast nickel-based superalloys combining high strength, creep-resistance, and weldability has led to the development of cast analogs of wrought nickel-based superalloys, including H263, H282, and N105. This paper examines the alloy design criteria, processing experiences, as-processed and heat-treated microstructures, and selected mechanical properties of these materials while also discussing their potential for full-scale development.

A study is being conducted on turbine materials for use in ultra-supercritical (USC) steam power plants, with the objective of ensuring no material-related impediments regarding maximum temperature capabilities and the ability to manufacture turbine components. A review of the state-of-the-art and material needs for bolting and casing applications in USC steam turbines was performed to define and prioritize requirements for the next-generation USC turbines. For bolting, several potentially viable nickel-base superalloys were identified for service at 760°C, with the major issues being final material selection and characterization. Factors limiting inner casing material capabilities include casting size/shape, ability to inspect for discontinuities, stress rupture strength, and weldability for fabrication and repairs. Given the need for precipitation-strengthened nickel-base alloys for the inner casing at 760°C, the material needs are two-fold: selection/fabrication-related and characterization. The paper provides background on turbine components and reviews the findings for bolting and casing materials.

Current state-of-the-art coal-fired supercritical steam power plants operate with high-pressure turbine inlet steam temperatures close to 600°C. The best of the recently developed and commercialized advanced 9-12Cr martensitic-ferritic steels may allow prolonged use at temperatures to about 620°C, but such steels are probably close to their inherent upper temperature limit. Further increase in the temperature capability of advanced steam turbines will certainly require the use of Ni-based superalloys and system redesign, as seen in the European programs that are pioneering advanced power plants capable of operating with 700°C steam. The U.S. Department of Energy (DOE) has recently undertaken a concerted effort to qualify ultra-supercritical boiler tubing and piping alloys for 720/760°C steam for increased efficiency and reduced emissions. It is, therefore, necessary to develop the corresponding USC steam turbine, also capable of reliable operation at such conditions. This paper summarizes a preliminary assessment made by the Oak Ridge National Laboratory (ORNL) and the National Energy Technology Laboratory (NETL) of materials needed for ultra-supercritical (USC) steam turbines, balancing both technical and business considerations. These efforts have addressed an expanded portfolio of alloys, that includes austenitic stainless steels and alloys, in addition to various Ni-based superalloys for critical turbine components. Preliminary input from utilities indicates that cost-effective improvements in performance and efficiency that do not sacrifice durability and reliability are prime considerations for any advanced steam turbine technology.