Hold time effects on low cycle fatigue behavior of HAYNES 230 ® superalloy at high temperatures

Abstract

Total strain controlled low cycle fatigue (LCF) tests with and without hold times were performed at temperatures ranging from 816 to 982 °C in laboratory air on a nickel-based superalloy, HAYNES 230 ®. The influence of hold time on the cyclic stress response and fatigue life was studied. Fracture surfaces and metallographic sections were evaluated in terms of the crack initiation and propagation modes, i.e., transgranular or intergranular. In general, the fatigue life decreased as the temperature increased. However, at total strain ranges higher than 1.0% and without a hold time, the LCF life was longer at 927 °C than at 816 °C. This “abnormal” behavior was found to result from the smaller plastic strain amplitude at half-life at 927 °C than that at 816 °C. The introduction of a hold time led to a decrease in fatigue life. At both 816 and 927 °C, the material exhibited a cyclic hardening/softening behavior at higher total strain ranges and a cyclic hardening/saturation behavior at lower total strain ranges. An increase in temperature and/or the introduction of a hold time decreased the hardening rate and increased the softening rate. The introduction of a hold time and/or the increase of the test temperature progressively changed the fracture mode from transgranular to mixed trans/inter-granular, then to intergranular. Within the two phases of the fatigue process, crack initiation was more severely influenced by the change of hold time and/or temperature.