Microstructure,reversed austenite content and tensile properties of 5.5Ni steel after quenching+intercritical quenching+tempering(QLT)heat treatment were studied by means of scanning electron microscope(SEM),electron backscatter diffraction(EBSD),transmission electron microscope(TEM)and X-ray diffraction(XRD).The effect of quenching temperature in two-phase region on the microstructure and tensile properties of the 5.5Ni steel was analyzed.The results show that the microstructure of the tested steel after QLT heat treatment is tempered martensite+critical ferrite+reversed austenite.The quenching temperature in the two-phase region has great influence on the content of reversed austenite,yield strength and yield ratio,but the quenching and tempering temperature have a little influence.With the increase of quenching temperature in the two-phase region,the content of reversed austenite increases,the tensile strength is basically unchanged,and the yield strength and yield ratio decrease at the same time.The stress-induced transformation of bulk reversed austenite at the triple junction into martensite during the tensile process is the main reason for the difference in yield ratio of the 5.5Ni steel quenched in the two-phase region at different temperatures.
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