Mechanisms of Hydrogen Embrittlement - Crack Growth in a Low-Alloy Ultra-High-Strength Steel Under Cyclic and Sustained Stresses in Gaseous Hydrogen

Rates of crack growth in a low-alloy ultra-high-strength steel D6aC under cyclic and sustained stresses have been measured as functions of stress- intensity, for different cyclic wave-forms and frequencies, in low pressure 13. 3 kPa hydrogen, dry air, and vacuum .001 Pa environments sustained-load cracking in liquid mercury was also studied. Frequency and wave-form had large effects on rates of fatigue-crack growth in hydrogen but little influence on fatigue in air and vacuum. For triangular wave-forms, rates of crack growth in hydrogen were determined mainly by the stress-intensity range and the load-rise time. For square-wave loading, rates of crack growth in hydrogen were proportional to the time at maximum load. Quantitative relationships between rates of cracking under sustained and cyclic loads were not found. Many similarities between hydrogen-embrittlement and liquid-metal embrittlement e.g. surfaces of the intercrystalline fractures induced by hydrogen and mercury were sometimes indistinguishable suggested that the mechanism of embrittlement is basically the same in both cases. It is considered that previous explanations for embrittlement are not consistent with the present fractographic observations e.g. dimpled transcrystalline fractures were sometimes observed after crack growth in mercury and hydrogen. The present results suggest that the effects of hydrogen and liquid-metal environments on crack growth can generally be explained on the basis that chemisorption influences interatomic bondsspacings at surfaces crack tips and thereby facilitates nucleation of dislocations at crack tips.