The Influence of Stress and Strain on the Tempering of SAE 4340 Martensitic Steel
Rochim Suratman
Abstract. An investigation into the influence of stress and/or strain on the tempering of SAE 4340 martensitic steel is being done. The as-quenched steel is tempered at several temperatures (100o - 600oC) with and without the influence of stress. The resulting changes in hardness and structure were then followed by microhardness measurements and electron microscopy. The primary variables of interest in this work are hardening conditions, tempering time, tempering temperature, stress, and strain during tempering. From the experimental result it appears that the hardness of steel SAE 4340 tempered under stress between 100oC and 300oC is higher than the hardness of the same steel tempered without stress, while the hardness of steel tempered under stress al a temperature higher than 300"C is lower than the hardness of steel tempered without stress. In all cases, at higher stresses the specimens undergo plastic deformation during tempering and it is seen that the plastic strain has more influence on the tempering than the stress. The electron microscopic observations show that the increase in hardness, due to the strain occuring during tempering, is caused mainly by a more abundant precipitation of carbides of about the same shape and size as in specimens tempered without stress. When the strain occuring during tempering causes a lower hardness, the carbide precipitations are coarser, more equlaxed and fewer than after tempering without stress.
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