The Study Of The Microstructure And Performance Of The Diffusion Layer Of Nitrocarbuized/Carbonitrided08F Steel

Nitrocarburizing (carbonitriding) is an important surface thermo-chemical treatment.Microstructures of the nitrocarburized (carbonitrided) layers are usually affected by the heattreatment parameters of nitrocarburizing (carbonitriding) and the subsequent tempering. Sofar, little research contributed to study the effect of tempering on the coating of thenitrocarburised (carbonitrided) components, especially the diffusion layer.In this study, sheets of08F steel are used to nitrocarburizing at570℃and carbonitridingat700℃respectively. Various subsequent heat treatments are followed the nitrocarburizing(carbonitriding) treatment to study the effect of subsequent heat treatments on the diffusionlayer of the nitrocarburized (carbonitrided) components.Steel sheets were plasma nitrocarburized at570℃for5h followed by furnace cooling.Then the various tempering at200℃~500℃were performed on the nitrocarburized samples.(1)For diffusion layer of samples, as the changes of γ′were not apparent, no influence onthe microhardness of the samples tempered at200℃~250℃. After tempering at500℃for2hfollowed by quenching in water, numerous pre-α″/α″appear in the diffusion layer with thesignificantly increase of microhardness.(2) Four stages exist in the process of the precipitation strength resulted from pre-α″/α″during the natural ageing. Density of quenched-in vacancies and the distribution ofN-C-atoms are two main factors in controlling the strengthening effect of pre-α″/α″.Steel sheets were salt bath carbonitrided at700℃for3h and then quenched in water.Various tempering below550℃areconducted on the carbonitrided samples.(1) Three kinds of morphologies in the as quenched microstructures are mainlycomprised of martensite and retained austenite. The morphology of the complicatedmartensite is similar to the lath martensite in Fe-C system. Many twins appear in themartensite laths with two major morphologies.(2)Several orientations of martensite laths occur in the blocks of retained austenite withthe morphology similar to the plate martensite in Fe-C system. No evidence of twins is found in these martensite laths and the martensite maintains the Kurdjumov-Sachs (K-S) orientationrelationship to the retained austenite.(3)The small precipitations separate out between the martensite laths with nice temperingresistance when tempering proceed at180℃~200℃. At the same time, the blocks of retainedaustenite decomposed. The precipitations become thicker with the lower microhardness whilethe temperature of tempering becomes higher at the range of300℃~400℃. Themicrohardness becomes stable when tempering proceeded above450℃.