Effect of prolonged isothermal heat treatment on the mechanical behavior of advanced NANOBAIN steel

The microstructural evolution and consequent changes in strength and ductility of advanced NANOBAIN steel during prolonged isothermal heat-treatment stages were investigated.The microstructure and mechanical properties of nanostructured bainite were not expected to be influenced by extending the heat-treatment time beyond the optimum value because of the autotempering phenomenon and high tempering resistance.However,experimental results indicated that the microstructure was thermodynamically unstable and that prolonged austempering resulted in carbon depletion from high-carbon retained austenite and carbide precipitations.Therefore,austenite became thermally less stable and partially transformed into martensite during cooling to room temperature.Prolonged austempering did not lead to the typical tempering sequence of bainite,and the sizes of the microstructural constituents were independent of the extended heat-treatment times.This independence,in turn,resulted in almost constant ultimate tensile strength values.However,microstructural variations enhanced the yield strength and the hardness of the material at extended isothermal heat-treatment stages.Finally,although microstructural changes decreased the total elongation and impact toughness,considerable combinations of mechanical properties could still be achieved.