This chapter presents a review of several important phenomena associated with the influence of hydrogen on the process of the growth of fatigue cracks in steels. In the first part of the chapter, we describe the influence of internal hydrogen for higher-strength low alloy (Cr–Mo) and austenitic stainless steel; in the second part, a corresponding description of the influence of external hydrogen (hydrogen gas) on fatigue-crack propagation in several classes of lower-strength pressure vessel and piping steels is given.
We show that several critical mechanistic phenomena can be enhanced by the presence of internal hydrogen, including the localization of slip bands in fatigue, the in situ transformation to strain-induced martensite in stainless steels (Types 304, 316 and 316L), and the effect of frequency on fatigue-crack growth rates. The nature of the fatigue fracture surface, specifically the morphology of the fatigue striations, and the consequent role of fatigue crack closure, can also be influenced by hydrogen.
With regard to the influence of external hydrogen (gaseous hydrogen), based on measurements over a wide range of growth rates from 10−11 to 10−5
m/cycle, we show that crack-propagation rates can be significantly higher in dehumidified gaseous hydrogen as compared to moist air in two distinct regimes of crack growth, namely, at the intermediate range of growth typically above ~10−8
m/cycle and at the threshold region below ~10−9
m/cycle approaching lattice dimensions/cycle. In lower strength steels, both effects are seen at maximum stress intensities K
max far below the threshold stress intensity for hydrogen-assisted cracking under sustained (non-cyclic) loading.
Comparing the influence of internal and external hydrogen, the acceleration in fatigue-crack growth rates due to hydrogen can be interpreted as a function of hydrogen concentration at crack tip, slip localization and any variation in the active mechanisms of crack closure.
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