Fatigue crack growth behavior of 4340 steels

Fatigue crack growth behavior of 4340 steels was investigated in four gaseous environments; laboratory air, wet hydrogen, dry hydrogen and dry helium. Specimen orientation does not affect crack propagation rate results. The effects of R-ratio (load ratio) and environment on crack growth rate properties are interrelated. Increasing R -ratio increases the rates of near-threshold crack propagation. Nevertheless, the effect of R-ratio on crack growth rates in air is much more significant than that in the two dry environments. Interestingly, the R-ratio effect in wet hydrogen is comparable to that in dry environments. At an R-ratio of 0.1, the rates of crack propagation in air are slower than those in dry environments while crack growth rates are essentially identical in wet hydrogen and dry environments. Increasing R -ratio was found to decrease the environmental effect. Furthermore, increasing yield strength from 700 to 1040 MPa does not affect crack propagation behavior. While surface roughness-induced crack closure is thought to be minimal in affecting gaseous-environment near-threshold crack growth behavior of 4340 steels, oxide-induced crack closure governs crack propagation kinetics. It is suggested that in moisture-containing environments, thick oxide deposits measured on fracture surfaces may not result in high crack closure levels. Nevertheless, oxide-induced crack closure rationalized the effects of R-ratio and environment on near-threshold crack growth rate properties. Furthermore, hydrogen embrittlement is believed not to play an important role in influencing wet-hydrogen environment near-threshold crack propagation behavior. At higher ΔK levels (⩾ 12 MPa √m), an “intrinsic” dry hydrogen effect seems to be present, and crack closure, however, cannot account for the environmental effect.