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Abstract
Fatigue crack propagation in the threshold region has been investigated in a ½Cr-½Mo-¼V steel heat treated to produce a range of microstructures. Large grain size precipitation-hardened ferritic microstructures were found to give the highest threshold, the threshold being lower for higher strength bainitic or martensitic microstructures. Microstructure-sensitive fracture paths, intergranular failure, and Stage IIa crack growth were observed as the threshold was approached. S/N tests on the ½Cr-½Mo-¼V steel and a simple Fe-1V-0.2C alloy showed that the precipitation-hardened ferritic microstructures gave lower fatigue limits than tempered martensite. This indicates that there are conflicting microstructural requirements for resistance to fatigue crack initiation and to fatigue crack growth in these microstructures.
