Microstructure And Properties Of Surface Layers Of Quenched30CrMnSiA And42CrMo Steels Plasma Nitrocarburized With Rare Earths

In this paper, quenched30CrMnSiA and42CrMo steel were plasma nitridedand nitrocarburized in different atmosphere at different temperature for differenttime with and without rare earths （RE） addition. The effects of processingparameters on the microstructure, mechanical properties, wear and corrosionresistances of the surface layers of the treated steels were characterized by opticalmicroscopy （OM）, X-ray diffraction （XRD）, scanning electron microscopy （SEM）equipped with an energy dispersive X-ray analyzer （EDS） and transmission electronmicroscopy （TEM）.The microstructure of quenched30CrMnSiA and42CrMo steels issupersaturated carbon-bearing ’-Fe. The modified layers of both plasma nitridedand nitrocarburized samples consist of a compound layer and a diffusion layer.When the nitrocarburizing temperature is higher than460oC, the RE addition couldimprove the microstructure of surface layer. The surface layer nitrocarburized at560C with0.5L/min RE can be divided into three sub-layers （i.e. out compoundlayer of5μm, middle “white” layer of40μm, and inner diffusion zone）.The phase composition of the plasma nitrided or nitrocarburized layer mainlycontains ε-Fe_2-3N and γ′-Fe4N (or ε-Fe_2-3（N,C） and γ′-Fe4N). The relative content ofε-Fe_2-3N (ε-Fe_2-3（N,C）) decreases with increasing the treating temperature. Thereexists the diffraction peaks of ’-FeNwhen the nitriding is conducted at400oC and560oC. The relative content of ε-Fe_2-3N in surface layer nitrided at460oC increasesfirst and then decreases with prolonging the treating time, and the γ′-Fe4N possesses（200） preferred orientation when the treating time is16h. However, the （200）preferred orientation of γ′-Fe4N becomes weaker as the nitrocarburizing temperatureincreases and treating time prolongs. The RE flux could control the relative contentsof the phases produced in surface layer. The proper RE addition favours theformation of ε-Fe_2-3（N,C） and the （200） preferred orientation of γ′-Fe4N. Thecarbide Fe3C phase occurs at the surface employing a flux of0.5L/min RE during560oC nitrocarburizing.Nitriding atmosphere has little effect on the surface morphologies of nitridedsample. The nitrided granules assemble and grow up at the surface with increasingthe treating temperature, resulting in an increasing of the surface roughness. For thenitrocarburized samples, the incorporation of RE increases the density of surfacenitrides and helps to the assembling and growing of nitrides. The morphology ofFe3C formed at the surface is changed from petal-like type to fine stick-like one with a mean size of100nm³00nm when the sample is nitrocarburized at560oC for8hwith0.5L/min RE flux. EDS results show that the RE elements can diffuse into thesurface layer to a certain depth, which helps a diffusion of nitrogen element.TEM results show that the microstructure in the surface layer of nitrided andnitrocarburized30CrMnSiA steel are mainly comprised of nano-sized nitrides and asmall quantity of nano-sized martensite ’-FeNwith a scale of50nm¹00nm.Significantly, the involvement of RE is beneficial to the formation of nano-sized’-FeN, and under this experimental conditions, the more the RE addition intoatmosphere, the more degree of nanocrystallization phenomenon. The localnanocrystallization in surface layer is based on the following fact: firstly, thenano-sized nitrides precipitate in the surface layer of30CrMnSiA steel with highsupersaturated microstructure, and secondly, the internal stress in the modified layerincreases resulting from the synergistic effect of the penetration of nitrogen, carbonand big sized La atom and the solid soluted alloy elements. And the nano-crystallineof ’-FeNis formed by the moving, amalgamating and rearranging of the dislocationsubstructures.The hardness of the quenched30CrMnSiA steel is HV500, and the surfacehardness can be improved evidently, up to HV1122.6by nitriding with NH3. Bothsurface and core hardness decreases as the nitriding temperature increases. From theresults of EDS and effective hardening layer, the involvement of RE can help thediffusion of carbon and nitrogen in surface layer and there is an optimum amount ofRE addition into the plasma nitrocarburizing atmosphere. The samplenitrocarburized at500oC with0.05L/min RE has the thickest effective hardeninglayer. When the nitrocarburizing is carried out at560oC for8h, the thickest effectivehardening layer can be obtained with the optimum RE flux of0.3L/min. The corehardness value changed apparently when the quenched30CrMnSiA and42CrMosteels were plasma nitrided and nitrocarburized, and the value is almost equivalentto that of quenched and tempered steels. That is to say, the composite processes ofquenching+high temperature tempering and surface modification can be substitutedsuccessfully by quenching and plasma nitriding/nitrocarburizing for this type ofsteel.The tensile test results show that the strength of the nitrocarburized sample forthe quenched30CrMnSiA is higher than that of the nitrocarburized sample for thequenched and tempered one, but the toughness is lower. However, the quenched30CrMnSiA nitrocarburized with RE has the similar strength and higher toughnesscompared with the quenched and tempered one. It indicates that the quenching andnitrocarburizing treatment with RE could replace the conventional quenching+ tempering and nitrocarburizing one for the nitriding steels.Wear test results show that the wear resistance of the experimental steels can beincreased evidently by plasma nitriding and nitrocarburizing. The frictioncoefficients can be decreased markedly and the volume wear rates of the nitrided-,nitrocarburized-and RE nitrocarburized-layer of30CrMnSiA steel have beenreduced74%,78%and82%, respectively. The wear mechanism of the untreatedsample is mainly adhesive wear with local plastic deformation, as well as fatigueand oxidation on the worn surface. The changes of the load in wear test do notchange the wear mechanism of the samples, but change the wear degree. The mainwear mechanism of nitrided and nitrocarburized layers is adhesive wear withoutplastic deformation. The RE addition further improves the wear resistance of thesurface layer, with only a mild adhesive wear.The corrosion resistance of surface layer can be increased evidently by plasmanitriding and nitrocarburizing, by changing the corrosion mechanism from severelygeneral and uniform corrosion to pitting corrosion. The corrosion resistance is bestfor the surface layer of30CrMnSiA steel nitrided at460C and42CrMo steel treatedat560C, respectively. However, the nitrocarburized layer has higher corrosionresistance than the nitrided layer. The corrosion mechanism of the surface layer for42CrMo steel changes from severely crevice corrosion along the grains in thenitrided layer by nitriding to shallow surface corrosion in nitrocarburizing layer.Moreover, the involvement of RE could further improve the corrosion resistance byextending the passive regions and promoting the pitting corrosion potentials.