PSI - Issue 52

Long Jin et al. / Procedia Structural Integrity 52 (2024) 12–19

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Author name / Structural Integrity Procedia 00 (2019) 000–000

strengthening phase with the softer ferrite phase, causing stress concentration around the carbide and leading to crack initiation at the nearby ferrite phase. And the coarser carbides after aging are more vulnerable to stress concentration initiating micro or secondary crack, inducing faster propagation rate and lower fatigue life after aging.

450 ° C 1000h

As-received

BF

GB

PF

Carbides

Carbides

Secondary Crack

Secondary Crack

BF

M/A

Loading direction

Fig. 10. SEM micrographs illustrates the secondary crack propagation of as-received and aged specimens at 0.5% strain amplitude, respectively

4. Conclusions In this study, Axial push-pull fatigue tests were carried out to investigate the low cycle fatigue behaviours of aged low alloy steel at 450 ° C for 1000h. The main conclusions can be summarized as follows: (1) Aging of 16MND5 at 450 ° C for 1000h was a non-hardening process, in which the tensile strength remained almost unchanged while the elongation and section shrinkage decreased. (2) The maximum cyclic hardening ratio H max was proposed to describe the extent of cyclic hardening. Both aged and unaged specimens exhibited cyclic hardening followed by cyclic softening, while the aged samples had a higher value of H max . (3) Fatigue life reduced remarkably after aging at all strain amplitudes, and fatigue strength appeared to be more sensitive to aging compared to tensile strength. (4) Coarser carbides were prone to initiate and propagate cracks leading to a shorter fatigue life after aging. Acknowledgements This work was supported by Innovation Program of Shanghai Municipal Education Commission (2023-05-49), Natural Science Foundation of Sichuan Province (2023NSFSC0912) and the National Natural Science Foundation of China (No.52205173). References Mehmanparast, T., Nikbin, K., 2022. Local creep damage effects on subsequent low temperature fatigue crack growth behaviour of thick-walled pressure vessels. Engineering Fracture Mechanics 272, 108720. Guo, C., Yu, D. J., Sun, X. Y., Yu, W. W., Chen, X., 2021. Fatigue failure mechanism and life prediction of a cast duplex stainless steel after thermal aging. International Journal of Fatigue 146, 106161. Sarkar, A., Kumawat, B. K., Chakravartty, J. K., 2015. Low cycle fatigue behavior of a ferritic reactor pressure vessel steel. Journal of Nuclear Materials 462, 273-279. Fekete, B., Kasl, J., Jandova, D., Joni, B., Misjak, F., Trampus, P., 2015. Low cycle thermomechanical fatigue of reactor steels: Microstructural and fractographic investigations. Materials Science & Engineering A 640, 357-374. Smith, R. W., Hirschberg, M. H., Manson, S.S., 1963. Fatigue Behavior of Materials under Strain Cycling in Low and Intermediate Life Range. Silva, R. A., Pinto, A. L., Kuznetsov, A., Bott, I. S., 2018. Precipitation and Grain Size Effects on the Tensile Strain-Hardening Exponents of an API X80 Steel Pipe after High-Frequency Hot-Induction Bending. Metals 8, 168.