Abstract
The fatigue properties of ultra-fine grain austenitic steel (UFG16-10), which has a 1 μm average grain size, were studied as part of the project aimed at the development of high-strength low-cost stainless steels for hydrogen service. The fatigue properties of the UFG16-10 were compared with that of a coarse grain material with the same chemical composition (CG16-10) and two kinds of commercial steels, JIS SUS316 and JIS SUH660. The fatigue strength of the UFG16-10 was 2.8 times higher than that of the CG16-10. The effect of hydrogen on the fatigue limit of the UFG16-10 was not significant. However, the fatigue life of the UFG16-10 was reduced by hydrogen in the short life regime. In the fatigue crack growth test, the UFG16-10 showed a good crack growth resistance that was equivalent to that of the SUH660 and significantly higher than that of the SUS316. However, the crack growth rate was significantly accelerated by hydrogen. The cause of the hydrogen-assisted fatigue crack growth of the UFG16-10 was transformation of the microstructure at the crack tip from austenite to strain-induced martensite. This was also the cause of the reduced fatigue life of the hydrogen-charged UFG16-10.
Highlights
The studies of ultra-fine grained materials produced by ECAP [1], HPT [2], ARB [3], Warm tempforming [4], HCR [5], etc., are actively being advanced
Regarding the research on the fatigue properties of the ultra-fine grain materials, a unique fatigue mechanism was found such that the recrystallization at the crack tip during crack propagation occurs in low-melting-point materials such as copper [1, 8] and a magnesium alloy [7]
This resulted in no improvement in the fatigue strength due to the grain growth, whereas the static strength was significantly improved by the grain refinement
Summary
The studies of ultra-fine grained materials produced by ECAP [1], HPT [2], ARB [3], Warm tempforming [4], HCR [5], etc., are actively being advanced. Regarding the research on the fatigue properties of the ultra-fine grain materials, a unique fatigue mechanism was found such that the recrystallization at the crack tip during crack propagation occurs in low-melting-point materials such as copper [1, 8] and a magnesium alloy [7]. This resulted in no improvement in the fatigue strength due to the grain growth, whereas the static strength was significantly improved by the grain refinement. In this context that grain refinement is a promising method to develop high-strength material and austenitic stainless steels are hydrogen compatible but low strength, the fatigue properties of an austenitic stainless steel having a 1 μm average grain size and the effect of hydrogen were studied
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