Gas Nitrocarburizing Of Low Carbon Steel And Its Composite Nitriding/carburizing

Nitriding,nitrocarburizing and carburizing are surface chemical heat treatment processes that infiltrate nitrogen and carbon atoms to the surface of the workpiece,which can improve the hardness,wear resistance,corrosion resistance of the workpiece surface.The surface obtained by the traditional nitriding process is characterized by poor compactness,poor toughness,low hardness and thin thickness.In view of these defects,first of all,by adjusting the flow of NH₃ and CO during the process of nitrocarburizing,and then exploring the nitriding and subsequent heat treatment process,the composite processes involving cyclic nitriding and carburizing in turn（CNC processes）are put forward.The microstructure and properties of nitrided/nitrocarburized layers were characterized by using optical microscope（OM）,X-ray diffractometer（XRD）,scanning electron microscope（SEM）,transmission electron microscope（TEM）and multiple performance testers.The main conclusions are as follows:（1）The addition of a small amount of CO in the gas nitriding atmosphere will significantly promote the growth of the compound layer and the rapid formation of theεphase of the surface.The addition of excessive CO will cause the formation ofθphase in the compound layer,and as the flow of NH₃ increases,the critical CO flow value of the compound layer is also increased accordingly.In the atmosphere of gas nitrocarburizing,the compactness and toughness of the compound layer gradually increase with the increase of the flow rate of CO,but the thickness of the compound layer increases first and then decreases when the flow rate of NH ₃ is constant.When the CO flow is constant,the number of holes in the compound layer increases with the increase of NH₃ flow rate.With the increase of NH₃ flow,the CO flow accessible to the maximum thickness of compound layer should be increased correspondingly.Therefore,the CO and NH₃ flow can be increased properly to obtain the thicker and compact layer,which make the sample to have higher hardness,excellent wear resistance and better toughness.The corrosion resistance of the compound layer is mainly determined by the N content and the compactness of the compound layer,while the change of C content has little effect on the corrosion performance.（2）The process involving cyclic nitriding and carburizing in turn（naming CNC process）is put forward,and fabricat the strengthened layer with better mechanical properties.The strengthened layer is composed ofε-Fe_2-3（C,N）,θ-Fe₃C,γ-Fe andα-Fe.When the carbon content and CNC process are variable,the microstructure and properties are changed.The CO flow rate of 50sccm in CNC process is suitable to gain a good strengthened layer with the best microstructure and properties.In this case,the strengthened layer is uniform and dense.The thickness of the strengthened layer is as high as 90μm,and the hardness is up to 900HV_0.05.Although parameters in the CNC process are different from nitrocarburizing followed by quenching process and austenite nitrocarburizing process,the sublayer all consists ofγphase andαphase.There are a lot of holes and cracks in the nitrocarburized for 4h and subsequent quenching sample and austenite nitrocarburized samples,although the strengthened layer are thick.For the nitrocarburized for 1h and subseq uent quenching samples,the thickness of the strengthened layer is only 35μm although the pores are smaller.However,the strengthened layers forming in the CNC process are thick and compact,and more,the strengthened layers have excellent corrosion resistance,higher wear resistance,higher surface hardness and better toughness as compared with that of nitrocarburized layer.（3）The cooling rate and aging have influence on the strengthen layer fabricated by the means of CNC process.As the cooling rate slows down,theαphase in the sublayer increases gradually,theγ′-Fe₄N phase precipitates in the sublayer of the CNC samples cooling in the furnace.After tempering at 180-300℃,there is no phase transition in the surface layer of the CNC samples quenched by oil,but in the sublayer martensite containing N and retained austenite must transform to tempered martensite and bainite.With the increase of tempering temperature,the laths or lamellaes in the sublayer are gradually increasing,and the sizes are gradually decreasing.A similar bainite transformation occurred in the austenite area at the Grain boundary in the diffusion layer.