Abstract
The factors influencing the occurrence of intergranular fracture during fatigue crack growth tests of quenched and tempered steels have been examined. It is found that failure along prior austenite grain boundaries may be divided into two types. One type is found at high proof strength levels in steels in which impurities have segregated to the grain boundaries and is responsible for increased fatigue crack growth rates. The other is confined to environments containing water vapour and does not appear to significantly influence crack growth rates. The occurrence of the latter mode is thought to be related to hydrogen derived from the water vapour. A model describing hydrogen diffusion to regions of hydrostatic stress ahead of the crack tip can account for the observed variation of intergranular failure with crack velocity, mean load, and steel purity levels.
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