The torsion fatigue limit of 4340 steel in the quenched and tempered condition is compared with the quenched, tempered and strain-aged condition. Previous experimental work by the authors indicated that strain-aging of tempered martensite of 4340 improved the tensile strength, yield strength and tensile elastic limit. Other workers have shown that the tensile elastic limit and rotating beam fatigue limit of 4340 steel were related at high hardness. The results of this work show that strainaging tempered martensite of 4340 at a hardness of 54-55 Rockwell C does not improve the high cycle torsion fatigue limit.

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