Microstructural Evolution and Creep-Rupture Behavior of A-USC Alloy Fusion Welds
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DUCTION
DESPITE its use as the largest global source of electricity, coal faces strong regulatory and economic challenges as the world adopts policies for reducing fossil fuel consumption and curtailing carbon emissions. As world-wide demand for electricity continues to grow, a robust portfolio of power generation options is needed to ensure reliable and environmentally responsible electricity. Renewable energy sources are of course the focus of many major research efforts, but in the decades before those power generation methods become industrially viable on a global scale, advancements in coal technology are needed. A key aspect of the roadmap to cleaner coal technology is the deployment of higher efficiency pulverized coal combustion using the advanced ultrasupercritical (A-USC) process cycle. The International Energy Agency has proposed, in its high efficiency low emission (HELE) roadmap for coal technology, that coal generation from inefficient subcritical plants be replaced by higher efficiency DANIEL H. BECHETTI, Research Assistant, and JOHN N. DUPONT, Professor, are with the Department of Materials Science & Engineering, Lehigh University, Bethlehem, PA. Contact e-mail: [email protected]. JOHN A. SIEFERT, Senior Technical Leader, and JOHN P. SHINGLEDECKER, Program Manager, are with the Electric Power Research Institute, Charlotte, NC. Manuscript submitted January 25, 2016. Article published online June 22, 2016 4502—VOLUME 47A, SEPTEMBER 2016
Ultrasupercritical (USC) and A-USC plants as a first step to carbon reduction, prior to commercial deployment of carbon capture and storage (CCS) technologies.[1] A-USC operating conditions are defined as steam temperatures up to 1033 K (760 C) and steam pressures up to 35 MPa. These conditions are anticipated to reduce all emissions, including CO2, by 20 pct or greater compared to today’s U.S. fleet of supercritical boilers.[2–5] However, achieving such significant emission reductions and planned increases in plant efficiency demands the utilization of new materials and new technologies to implement these materials. Current USC boilers operate at temperatures near 873 K (600 C), which is near the limit of 893 K (620 C) for current, widely used creep-strength-enhanced ferritic steels (e.g., Grades 91 and 92). Thus, for planned temperatures approaching or exceeding 973 K (700 C), nickel-based alloys are required. With the aim of operation at 1033 K (760 C), age-hardenable nickel alloys are considered to be leading candidates over solution-strengthened alloys[5] such as 617. INCONEL alloy 740 (ASME Boiler & Pressure Vessel Code, Section I, Code Case 2702)[6] is an age-hardenable nickel-based superalloy which is purposely designed for A-USC steam boiler applications.[2] Developed by modifying NIMONIC 263 for better corrosion resistance, the typical microstructure of wrought alloy 740 is an austenite matrix containing 16 to 20 mol pct c¢ and a
METALLURGICAL AND MATERIALS TRANSACTIONS A
small amount of primary MC carbides at the grain boundaries. During long-term high-temperatur
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