Project No: FWP-12461
Performer: PNNL - Pacific Northwest National Laboratory
Robert Romanosky Crosscutting Research Technology Manager National Energy Technology Laboratory 3610 Collins Ferry Road P.O. Box 880 Morgantown, WV 26507-0880 304-285-4721 Robert.Romanosky@netl.doe.gov
DOE Share: $8,315,000.00
Performer Share: $0.00
Total Award Value: $8,315,000.00
Performer website: PNNL - Pacific Northwest National Laboratory - http://www.pnl.gov
This project will initially focus on three main tasks: (1) a parametric study to determine process parameters (i.e., spindle rotational and travel speeds, plunge force, and pin diameter/shoulder geometry) that yield defect-free welds in ODS and superalloy materials, (2) mechanical and creep testing of FSW samples welded under optimized conditions, and (3) oxidation and corrosion of FSW material. As part of the latter two tasks, the research team will also perform tests on an equivalent series of specimens fabricated from the unwelded base material. Tensile testing will be conducted over a range of temperatures, from room temperature to 800 °C, initially under inert gas conditions, and creep testing will be conducted at 650, 725, and 800 °C under inert gas conditions. Comparative microstructural analysis will be conducted on a set of witness specimens, as well as select mechanical test specimens, to identify mechanisms responsible for any potential changes in mechanical behavior. If the welding conditions truly have been optimized, there will be little difference in mechanical behavior between the unwelded base material and the FSW region. PNNL investigators will work with researchers at ORNL to devise the appropriate test conditions for oxidation and corrosion testing. The team will experimentally produce highquality welds at various amounts of heat input during the FSW process (controlled via the various tool parameters or secondary induction heating) and the resulting specimens will undergo subsequent oxidation/corrosion testing to determine whether FSW alters the high-temperature corrosion resistance relative to the base material. Select corrosion specimens will be chosen for subsequent tensile, creep, and fracture testing.
Program Background and Project Benefits
To remain economically competitive, the coal-fired power generation industry needs to increase system efficiency, improve component and system reliability, and meet ever tightening environmental standards. In particular, cost-effective improvements in thermal efficiency are particularly attractive because they offer two potential benefits: (1) lower variable operating cost via increased fuel utilization (fuel costs represent over 70 percent of the variable operating cost of a fossil fuel-fired power plant) and (2) an economical means of reducing carbon dioxide (CO2) and other emissions. To achieve meaningful gains, steam pressure and temperature must be increased to advanced ultrasupercritical (A-USC) conditions; that is, operating at temperatures above 760 degrees Celsius (°C) and pressures above 35 megapascals (MPa). The upper bounds of operating pressure and temperature are limited by the properties of the current set of materials employed in the boiler components. Key concerns are creep resistance, corrosion resistance, and cost effectiveness of the materials for critical pressureboundary omponents, such as headers, piping, and superheater/reheater tubes. Materials for boiler components can be divided into three general categories: (1) ferritic steels, (2) austenitic steels, and (3) nickel (Ni)-based superalloys. These materials are listed in order of increasing temperature of effective resistance to both creep and corrosion, as well as increasing cost and difficulty of working. In general, the major performance drivers for heavy section components such as headers and pipes are to minimize thermal fatigue while achieving high creep strength (resistance to deformation at higher temperatures). Historically, materials selection for these components has focused on the ferritic steels. These alloys display greater thermal conductivities and lower coefficients of thermal expansion (CTE) than do the austenitic steels, making them less susceptible to thermal fatigue cracking. However, at temperatures higher than 620 °C, the ferritic steels are prone to corrosion. This can be overcome to some extent by increasing the chromium content of the steel, but at levels greater than 10 percent in ferritic steels, chromium can reduce creep strength. The primary material issues driving the materials selection process for superheater and reheater tubes is the resistance to steamside oxidation and to ireside corrosion, considerations which typically necessitate the use of austenitic steels. Although Ni-based superalloys meet the creep- and oxidation/corrosion-resistance requirements of the various boiler components, they tend to be cost prohibitive in terms of raw material cost and processibility (e.g., casting and welding). Development of effective joining methods to maintain the material performance of high-performance alloys will enable their use in high-temperature, high-pressure, corrosive environments, including A-USC steam turbines and boilers. As USC power plants are being developed to reduce carbon dioxide emissions and increase fuel efficiency, this project will contribute to more efficient use of fossil fuels, which simultaneously leads to lower emissions of greenhouse gases and better management of the subsequent long-term effects of global climate change. Goals and Objectives
The goal of this project is to contribute to the development of cost-effective methods of joining high-performance alloys for use in advanced coal-fired power generation plants. The project will initially focus on ODS steels and on Ni-based superalloys that are susceptible to sensitization upon fusion welding, with the objective of achieving joints that exhibit high-temperature strength, creep resistance, and corrosion resistance properties equivalent to the base material. Specifically, researchers will develop linear and rotary friction-stir welding processes to meet this objective.
Researchers continued creep testing of FSW creep enhanced ferritic steel A387-Gr91-M (V modified) plate. A sample taken along the weld line (longitudinal, weld-only material) ruptured after 9247 hours of testing at constant-load creep conditions of 130 MPa and 625 °C. Based on creep strength vs. time to rupture for T91(mod) from Kimura et al. ("Long-Term Creep Strength Property of Advanced Ferritic Creep Resistant Steels," Advances in Materials Tech. for Fossil Power Plants: Proc. Sixth Intl. Conf., 732-751, 2010) and assuming that A387-Gr91 behaves similarly to T91(mod), the parent plate material should have a rupture life of approximately 800 hours at 625 °C. This weld metal specimen has surpassed the expected parent material life. The microstructure and hardness were evaluated to determine why the weld-only material has superior creep performance compared to the base material. Initial results indicate that the martensite-lath length of the FSW nugget material is smaller compared to the base material. A cross-weld FSW sample ruptured after 198 hours during the first cross-weld FSW Gr91-M creep test at 130 MPa and 625 °C. This is a shorter duration than the team’s base metal results (670 hours) but almost twice that of published data of fusion-welded 9Cr1MoNbV pipe with a post-weld heat treatment (PWHT) of 760 °C for 2 hours that ruptured between approximately 55 and 100 hours (V. Gaffard et al., Nuc. Eng. Design, v235, 92547-2562, 2005). (The project’s FSW cross-weld samples did not receive a PWHT.) Because the cross-weld specimens fail sooner than the base metal, the team did not test cross-weld specimens at 175 MPa and 625 °C, but instead conducted a cross-weld FSW Gr91-M creep test at 100 MPa and 625 °C. This sample ruptured after 789 hours, approximately three times longer than the approximately 250 hours for published values for fusion welds with PWHT (V. Gaffard et al.). Creep tests at the more accelerated condition of 175 MPa and 625 °C are also underway for Gr91 base and FSW weld-only specimens to determine if the trends are the same as those observed as in the 130 MPa tests. The base material ruptured after 34 hours, which fits the extrapolated trend from the work of Kimura et al. The weld-only specimen is currently at over 2600 hours without rupture, indicating that the trend of higher creep resistance for FSW Gr91 weld-only material holds at both 130 MPa and 175 MPa at 625 °C. Kanthal APMT, an ODS alloy, was successfully friction-stir welded up to ¼ inch in thickness with current commercial FSW tools. FSW welds of Kanthal APMT had creep performance at 750 °C, which was virtually identical in the weld nugget as in the parent rolled plate (in the longitudinal or rolling direction).