This work contains a detailed description of the influence of continuous heating from as-quenched state on microstructure development in the 17-4PH steel. The temperature ranges of the phase transformations occurring during heating from the as-quenched state (tempering) were determined from dilatometric tests. An interpretation of heating dilatometric curves was also presented. The temperature ranges of the occurred transformations during tempering of investigated steel were drawn on CHT (Continuous Heating Transformations) diagram. Moreover, the microstructure development in investigated samples, reflecting the extension of the phase transformations during tempering, was discussed as well. In as-quenched state the microstructure of investigated steel consist of low carbon martensite, ferrite δ banding, and niobium secondary carbides of MC type, which have not been dissolved during austenitizing. Changes in the microstructure of the investigated steel, during the heating from the as-quenched state result in precipitation of transition iron carbides, transformation of small amount of retained austenite into martensite, and precipitation of the Cuε phase. There were no significant changes in the chemical composition as well as in the morphology of the δ- ferrite.Keywords: 17-4PH steel, tempering, kinetics, phase transformations, CHT diagram
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