Creep-rupture performance of 0.07C–23Cr–45Ni–6W–Ti,Nb austenitic alloy (HR6W) tubes
Introduction
Increased efficiency and decreased emissions of pulverized coal-fired boilers can be simultaneously realized through increasing steam temperatures and pressures [1]. In an earlier study, it was shown that, compared to a standard U.S. subcritical steam boiler, an advanced ultrasupercritical (A-USC) steam boiler operating with steam parameters of 760 °C and 35 MPa will increase the thermal efficiency of the boiler to 46% (HHV), thereby reducing levels of all effluents, including CO2, by 20–25% [2]. This earlier work was sponsored by the U.S. Department of Energy (DOE) and the Ohio Coal Development Office (OCDO), and conducted by a consortium of the U.S. Boiler manufacturers, the Electric Power Research Institute (EPRI), and Energy Industries of Ohio (EIO) with support from Oak Ridge National Laboratory (ORNL) and the National Energy Technology Laboratory (NETL), The main limitation to realizing such a boiler is the lack of materials which can be fabricated and put into service for long-times in such an aggressive environment [3]. Creep-rupture strength is a critical property of interest for A-USC plants, and the advanced austenitic alloy HR6W was developed for improved creep-rupture strength and oxidation/corrosion resistance compared to currently available austenitic stainless steels that are used in steam boiler superheaters and reheaters [4]. In this study, the creep behavior and microstructure of one heat of HR6W tubing, including welds, was evaluated for A-USC application.
Section snippets
Experimental procedure
The material used for this study came from a single heat of tubing received in the solution annealed condition (1190 °C for 3 min followed by water quenching), having a 50.8 mm outside diameter and a wall thickness of 10.2 mm, and a vendor reported composition, weight percent (wt%), of 0.07C–0.26Si–1.0Mn–23.4Cr–44.8Ni–0.12Ti–0.25Nb–6.0W. Some tubing contained gas tungsten arc (GTA) butt welds with Inconel 82 filler or Inconel 617 filler metal. Welding was performed and qualified to ASME Section IX
Results
Creep-rupture experiments on base metal were conducted from 650 to 800 °C for a range of stresses. The minimum creep rate is plotted in Fig. 1 as a function of the applied stress for each temperature. Little change is observed in the slope of the isotherms indicating no change in creep mechanism for the range of conditions tested. These data were normalized to determine the Norton power-law exponent (n) utilizing a standard Arrhenius equation where A and Do are constants, Q is the
Discussion
The high power-law exponent (n = 7.2) for HR6W suggests a significant contribution from precipitate-dislocation interactions (dispersion strengthening). Furthermore, since this n-value stays constant from 85 to 200 MPa at temperatures as high as 800 °C, the precipitates contributing to strengthening are relatively stable for the times investigated. The activation energy for creep, 428 kJ/mol, is much higher than self diffusion coefficients for the most prominent elements in the alloys, but studies
Conclusions
The creep-rupture behavior of a heat of an advanced austenitic alloy, HR6W, was characterized by testing and creep data analysis, microstructural studies, and computational thermodynamics. The findings were as follows:
- •
No change in creep mechanism was observed for the tested conditions however creep failure mode showed a wide range of behavior consistent with most engineering stainless steel alloys
- •
Creep ductility was very good and contributed to good cross-weld creep-rupture performance, even
Acknowledgements
Research at Oak Ridge National Laboratory (Oak Ridge, TN USA) was supported by the U.S. Department of Energy (DOE), Office of Fossil Energy, Advanced Research Materials Program, the DOE/OCDO USC Steam Boiler Consortium, and the ORNLSHaRE User Center, Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. DOE, under Contract DE-AC05-00OR22725 with UT-Battelle, LLC. Special thanks to B. Holbrook and B. Vitalis of Babcock (Riley) Power for providing the material for
References (9)
- et al.
The development of a new 18-8 austenitic steel (0.1C–18Cr–9Ni–3Cu–Nb, N) with high elevated temperature strength for fossil fired boilers
Mechanical Behaviour of Materials – VI
(1992) - et al.
Materials for Ultra-supercritical coal-fired power plant boilers
- et al.
U.S. program on materials technology for ultra-supercritical coal power plants
Journal of Materials Engineering and Performance.
(June 2005) - et al.
Evaluation of the materials technology required for a 760 °C power steam boiler
Cited by (47)
Component test of nickel-based alloys and their welded joints under long-term creep loading
2023, International Journal of Pressure Vessels and PipingMicrostructural evolution of a new Ni-Fe-based superalloy deformed by creep
2023, Materials CharacterizationEffect of long-term aging on microstructural stability and tensile deformation of a Fe–Ni-based superalloy
2022, Materials Science and Engineering: AFatigue-creep behaviors of Ni–Fe based superalloy under various testing conditions
2021, Journal of Materials Research and Technology