Abstract
The strong influence of the fine-scale microstructural features on mechanical properties has become increasingly evident during the past decade. This is particularly true for fracture toughness of quenched and tempered alloy steels. Large differences in microstructure can be produced by isothermal, rather than athermal, treatments in the bainite and upper martensite temperature ranges. The kinetics of transformation, as well as the kinds and volume fractions of transformation products can be varied o ver wide ranges by relatively small changes in chemical composition. The effects of the common alloying elements on transformation kinetics, both separately and in various combinations were determined experimentally. The synergistic effects of individual elements added as pairs of elements were not predictable from a knowledge of the effects of the individual elements. Isothermal treatments, coupled with variations in the kinds and amounts of alloying elements, produced different morphologies, compositions, and volume fractions of the transformation products. The effects of such microstructural differences on tensile properties, fracture toughness, and fatigue characteristics were evaluated. Beneficial effects were found, such as substantial increases in fracture toughness, with small changes in alloy content or with heat treatments that differed from those conventionally used.
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