Influence of Symmetric and Asymmetric Hold Periods on Creep–Fatigue Interaction of Hastelloy C-276

This study examines the creep–fatigue interaction (CFI) behaviour of Hastelloy C-276 under fully reversed strain-controlled loading (R = –1), with emphasis on the effects of symmetric and asymmetric hold periods at elevated temperatures. Tests were carried out at 650 °C and 700 °C using strain amplitudes of 0.4% and 0.6%. Symmetric CFI tests, incorporating a 60-second hold at both peak tension and peak compression, were performed at both temperatures, while asymmetric conditions tensile-hold only and compressive-hold only were applied at 650 °C to assess directional creep effects.

Under symmetric hold conditions, Hastelloy C-276 exhibited pronounced cyclic hardening. Nearly balanced creep relaxation during both dwell periods limited mean-stress development, stabilised hysteresis loop evolution, and delayed crack initiation. Although higher temperatures increased creep deformation and shortened fatigue life, cyclic hardening remained the dominant material response. At 650 °C, asymmetric hold conditions produced distinct hardening characteristics. The compressive-hold condition generated the strongest hardening due to restricted creep strain accumulation during compression. The tensile-hold condition also showed hardening, but to a slightly lesser extent as moderate tensile-dwell creep relaxation reduced peak stress evolution. Fatigue life consistently decreased with increasing strain amplitude.

Fractography revealed predominantly transgranular cracking under lower creep influence, whereas mixed transgranular–intergranular features emerged at higher temperatures or with greater creep exposure. Overall, the results highlight the significant role of directional creep relaxation in governing cyclic stability and life prediction of Hastelloy C-276 under high-temperature CFI conditions.

Keywords: Hastelloy C-276, CFI, Dwell Effect, Creep, Fracture behaviour