Effect of aging on high temperature fatigue properties of Ni-23Cr-7W alloy for boiler pipes and tubes

Abstract

This study focuses on the effect of aging on fatigue behavior of nickel-based Ni-23Cr-7W alloy for boiler pipes and tubes to reveal the fatigue properties of the alloy after long-term exposure (1000–5000 h) to high temperature (700 °C). Tensile, Charpy impact, and high temperature fatigue tests were conducted for both aged and non-aged alloys. An increase in the tensile strength and a decrease in the tensile ductility and Charpy absorbed energy caused by the aging were observed. The fatigue life of the aged alloy is slightly shorter than that of the non-aged alloy under PP (fast-fast) type of strain waveform. Under CP (slow-fast) type of strain waveform, the fatigue life of the aged alloy is slightly shorter in the high strain range region and equal to or slightly longer in the low strain range region as compared with the non-aged alloy. The non-aged alloy shows strong cyclic hardening behavior. Therefore, the stress–strain response of the non-aged alloy after fatigue loading locates in the higher stress side than that of the aged alloy, although the stress–strain curve of the non-aged alloy obtained by the tensile test locates in the lower stress side. The observations of the morphology of precipitates and dislocations for the alloy before and after the fatigue test reveal that fine carbides precipitate uniformly in the non-aged alloy during the fatigue test. Furthermore, it is revealed that the cyclic hardening caused by aging has a significant influence on the generation of inelastic strain in the non-aged alloy under low strain range conditions. Although a large influence of aging at high temperature occurs on the cyclic hardening behavior, aging does not significantly affect the fatigue life of Ni-23Cr-7W alloy.