Alloy receives ASME boiler code approval

The capability of plants to operate at higher temperature and pressure is dependent on the materials of construction,
and the key limiting factor of these materials is their inherent creep strength. Creep is the time-dependent deformation
of materials, which generally takes place in a material subjected to sufficient stress at a temperature above half its
melting temperature. In turn, the main enabling technology in designing USC plants has been the development of high-temperature materials with greater creep strength, including creep-strength-enhanced-ferritic steels, advanced austenitic alloys, and nickel-based alloys.

A significant milestone has been reached in the national R&D program to develop materials technology for construction of boilers capable of operating at advanced ultra-supercritical (A-USC) steam conditions in pulverized coal plants. Approval of Inconel alloy 740 by the ASME Boiler & Pressure Vessel Code will help pave the way for use of this alloy in future construction of tubing and heavy-wall piping of A-USC boilers.

Announcement of the code approval and a status report on the ultrasupercritical boiler project are contained in a recent EPRI report, Boiler Materials for Ultrasupercritical Coal Power Plants (EPRI document 1022037).

The U.S. Department of Energy (DOE)/Ohio Coal Development Office (OCDO) Advanced Ultrasupercritical Project is a large-scale, multiyear, joint government/industry consortium aimed at identifying, evaluating, and qualifying the materials needed for the construction of the critical components of coal-fired boilers capable of operating at much higher efficiencies than the current generation of supercritical plants. The goal of this project is to assess/develop materials technology that will enable achieving turbine throttle steam conditions of 760°C (1400°F)/35 MPa (5000 psi).

For the steam boiler portion of the U.S. A-USC Project, the components of concern are the boiler headers and piping, superheater/reheater tubes, and waterwall panels. The initial general requirement for materials in an A-USC plant is that the 100,000-hour creep-rupture strength is approximately 14,500 psi (100 MPa) or higher. Austenitic steels possess the requisite strengths at intermediate temperatures, but their physical properties (low thermal conductivity and high thermal expansion) limit their use in thick-section applications. Nickel-based superalloys must be used for components in the hottest sections and where high stresses are encountered. Nickel-based superalloys are, however, quite expensive relative to steel, and any power plant incorporating these alloys must limit their use to the most critical components.

Six alloys were initially selected for testing, based on creep strength. Inconel 740 was selected for tubing as well as heavy-wall piping. The relative strength of the various candidate alloys was measured based on creep-rupture tests.
Tests showed that Inconel 740 is the strongest alloy capable of operating at temperatures approaching 1400°F (760°C). Because Inconel 740 was not code approved, extensive creep-rupture studies were conducted to evaluate heat-to-heat variations in strength, product form variation, stability with time, and heat treatment. Testing of three heats of Inconel 740 were undertaken for code acceptance.

Now that ASME code approval of Inconel 740 has been achieved, a separate data package will be submitted for Inconel alloy 740 welded with Haynes 282 at a future date, because ORNL is still developing the required stress-rupture data to support this material combination.

The DOE, through the National Energy Technology Laboratory (NETL), is the majority funder for the project, with significant co-funding coming from the OCDO. Energy Industries of Ohio is managing the program, and EPRI is providing overall technical direction and coordination. The consortium also has included the best talents of all the major U.S. boiler manufacturers (Alstom Power, Riley Power, Babcock and Wilcox, and Foster Wheeler), steam turbine manufacturers (GE, Alstom, and Siemens), and Oak Ridge National Laboratory (ORNL).

For more information, contact Howard Hendrix (hhendrix@epri.com, 205-668-6922) or John Shingledecker (jshingledecker@epri.com, 704-595-2619).