Hold-Time Effects on Low-Cycle-Fatigue Behavior of Hastelloy X Superalloy at High Temperatures

Total strain-controlled low-cycle-fatigue tests with and without hold times were performed at temperatures ranging from 816°C to 982°C in a laboratory air on a nickel-based superalloy, HASTELLOY X. The influence of hold times on the cyclicstress response and fatigue life was studied. At the temperatures considered, the alloy exhibited initial cyclic hardening, followed by a saturated cyclic-stress response or cyclic softening under low-cycle fatigue without hold times. For low-cycle-fatigue tests with hold times, however, the alloy showed cyclic hardening, cyclic stability, or cyclic softening, which is closely related to the test temperature and the duration of the hold time. It was also observed that the low-cycle-fatigue life of the alloy considerably decreased due to the introduction of strain hold times. Generally, a longer hold time would result in a greater reduction in the fatigue life. For the tests without hold times, the test temperature seems to have little influence on the fatigue life of the alloy at the test temperatures from 816oC to 927oC. However, when the test temperature increased to 982oC, the fatigue life clearly shortened. In addition, the fracture surfaces of the fatigued specimens were observed, using scanningelectron microscopy, to determine the crack initiation and propagation modes. The fatigue life was predicted by the frequency-modified tensile-hysteresis-energy method. The predicted lives were found to be in good agreement with the experiment results. Introduction A solid-solution-strengthened nickel-based superalloy, HASTELLOY X, is currently used in gas-turbine engines for combustion zone components, such as transition ducts, combustor cans, spray bars, and flame holders, as well as for afterburners, tailpipes, and cabin heaters. Moreover, the HASTELLOY X is also used in the chemical-process industry for retorts muffles, catalyst-support grids, furnace baffles, tubing for pyrolysis operations, and flash-drier components. The wide usage of this material in gas-turbine and chemical-process industries is based on an exceptional combination of its hightemperature strength, excellent forming and welding characteristics, and good resistance to oxidation and stresscorrosion cracking. It is expected that the damage from lowcycle fatigue, creep, environment and even their interaction will be the main limiting factors of their service lifetimes at elevated temperatures. Thus, it is very important to investigate the hightemperature, low-cycle fatigue behavior of the HASTELLOY X alloy for safely designing the high-temperature components and finding the potential usage of the alloy. Klarstrom and Lai [1] had examined the effects of thermal aging on the low-cycle fatigue behavior of HASTELLOY X and found that the fatigue life was degraded by the aging treatment at 760°C for 1,000 Hours. They suggested that the cause of the fatigue-life degradation was the precipitation of the sigma-phase and M23C6 carbides after long-term aging. Miner and Castelli [2] studied the cyclic-hardening mechanism of HASTELLOY X during isothermal and thermomechanical cyclic deformation, and observed that the alloy exhibited a broad peak in cyclic * L.J. Chen is currently with School of Materials Science and Engineering, Shenyang University of Technology, Shenyang 110023, P.R. China. 241 Superalloys 2004 Edited by K.A. Green, T.M. Pollock, H. Harada, TMS (The Minerals, Metals & Materials Society), 2004 T.E. Howson, R.C. Reed, J.J. Schirra, and S, Walston hardening between about 200°C and 700°C, with a maximum increase in the cyclic stress amplitude around 500°C. They proposed that the precipitation of M23C6 on the dislocations and solute drag could be attributed to cyclic hardening. In his research concerning the hold-time effect on low-cycle fatigue at 704°C in air, Seaver [3] pointed out that both tension and compression hold times resulted in a substantial life reduction, but the compression hold time was potentially more damaging in the low strain regime because of the development of high tensile mean stresses. This author thought that a contributing cause of the hold-time effect should be an aging reaction that occurred in HASTELLOY X. Tsuji and Kondo [4] studied the effects of cyclic frequency, strain waveform, and hold time on low-cycle fatigue behavior of HASTELLOY X and its modified version, HASTELLOY XR, at 900°C in a helium environment. These authors found that decreasing the cyclic frequency would lead to a notable reduction in the fatigue life. In their tests with different holding types of trapezoidal strain waveforms, the reduction in the fatigue life was found to be the most significant in the tests with tensile hold times, and more effective than in the tests with symmetric hold times, while no appreciable fatigue life reduction was recognized in the tests with compressive hold times. They suggested that creep damage could be attributed to a considerable reduction of the fatigue life in the tests at lower cyclic frequencies and with hold times. In their study on fatiguecrack-growth behavior of HASTELLOY X at 650°C, Hour and Stubbins [5] observed that the alloy remained a transgranular crack-propagation mode at all frequencies. 100 m Figure 1. The microstructure of the HASTELLOY X alloy in the as-received condition.