Fatigue life prediction of high-strength low-alloy steel with force-thermal coupling

The fatigue life of high-strength low-alloy (HSLA) steel used in coiled tubing (CT) is influenced by thermal-mechanical coupling effects, and the underlying mechanisms governing its fatigue behavior remain to be further clarified. In this study, low-cycle fatigue tests combined with in situ EBSD observations were conducted to investigate the effect of tempering on the fatigue performance of CT, with a systematic comparison between tempered CT (TCT) and untempered CT (UCT) specimens. The results indicate that both TCT and UCT exhibit cyclic softening during low-cycle fatigue, characterized by a gradual decrease in stress amplitude with increasing number of cycles. However, compared with UCT, TCT exhibits a significantly higher cyclic softening rate, leading to a substantial reduction in fatigue life. Microstructural analysis reveals that tempering increases the average ferrite grain size and causes a dispersion of Schmid factors, which restricts the activation of slip systems and consequently reduces fatigue life. Notably, after fatigue deformation under high strain amplitude conditions, the accumulated dislocation density in TCT is lower than that in UCT. This transition in fatigue deformation mechanism depends on the ratio of elastic strain to plastic strain, which is entirely governed by the tempering process. This study provides a theoretical basis for the fatigue life prediction and safe application of HSLA steel under thermal-mechanical coupling conditions.