Abstract
Fatigue crack growth studies have been conducted in a humid air environment for various martensite containing microstructures under low and high R-ratio testing conditions. Tempering reduced the yield strength properties; the greater the amount of martensite the greater the reduction. Tempering generally (i) increased the threshold for fatigue crack growth ΔK th, and (ii) decreased near threshold fatigue crack growth rates for both low and high R-ratio testing conditions. Also tempering inhibited the occurrence of subcritical transgranular cleavage in the high R-ratio tests of the ferrite-martensite microstructures and consequently affected Stage II fatigue crack growth behaviour. Finally the fully martensitic microstructure did not exhibit cracking and the present ΔK th data exhibited good agreement with other data for steels in the tempered condition.
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