A Mn–C steel with an as quenched martensite microstructure expected to be used as a high strength steel for automobile parts. However, the fatigue strength of the steel is relatively low in spite of its high tensile strength. Such low fatigue strength is attributed to a low elastic limit caused by mobile dislocations in the as quenched martensite structure. Shot peening improves the fatigue life markedly below 107 cycles but has little effect at 107 cycles. These results are explained by the fact that the initiation site of the fatigue crack changes from the surface of the as quenched specimen to the interior of the shot peened specimen.
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