| The background of this study is the application of a kind of novel ball bearing steelwhich called M50NiL with high alloying element. The target of this study is how toprepare modified layer with enough depth and well mechanical property in the steel.Because this steel can not be treated with high temperature, high flow ratio of nitrogenand hydrogen and long time, so we propose two new methods, one of which is calledcyclic nitrocarburizing, the other is called carburize and nitride composite treatment. Inthis study, it discusses the effect of the technological parameter in cyclicnitrocarburizing and different composite treatments on the layer depth, phase structure,microhardness gradient and abrasive resistance. At the same time, it provides themechanism. With the help of OM, SEM, XRD and EDS, it analyzes the phase structureand elements which diffuse into the material in the modified layer and represents themicrohardness and abrasive resistance simultaneously.The outcome shows that when the temperature range is low and the cycle time isshort, the flow ratio of nitrogen and hydrogen has little effect on the layer depth. Thelayer depth mainly depends on the maintain time in high temperature stage, but the flowof nitrogen in low temperature stage could also be an important issue. After increase thecycle time of the experiments, the results indicate that on the condition that the totalamount of nitrogen is same, nitrogen could diffuse into the deepest distance in relativelylow temperature range, but the microhardness of the surface is lowest among all theexperiments. The phase of γ’-Fe4N, ε-Fe2-3N and a little α’-Fe consist the phasestructure of the modified layer. However, we can not found white layer on the surface. Itattributes to the special technological parameter of the cyclic method. The shape of thenitrides which found on the surface of the specimens is not regular, between which therehave a lot of gaps. The surface microhardness of the specimens which treated with thecyclic method can reach1100HV0.1. The wear rate of the specimens is only2.49*10-5mm3/Nm. The main worn mechanism is adhesive wear and accompany with alittle oxidation and abrasive wear.Cyclic nitrocarburizing accompany with laser quenching is not a proper surfacetreatment method for M50NiL steel. It will cause crack both on the surface and in themodified layer, then the material become invalid. However, gas carburizing accompanywith nitrocarburizing obtains well effect when apply on the M50NiL steel. A high microhardness platform which last400μm from surface is formed when the compositemethod is applied on the steel. The average microhardness of the platform is higher than900HV0.1. We could see distinct layers in the metallograph. But this composite methodhas little effect on the layer depth and surface microhardness. At meanwhile, the surfacemicrohardness which obtains from the composite method is lower than the one obtainsfrom the cyclic nitrocarburizing method. The main phase exists in the surface ismartensite that contain some nitrogen and carbon atoms, expect that, there still havesome low nitrogen compounds and alloy carbide. However, there is no high nitrogencompounds exist in the surface phase structure. The abrasive resistance of thespecimens also improves after the composite treatment. Comparing with the untreatedspecimens, the wear rate of the treated specimens decreases a lot. The curve whichreflects the frictional coefficient is stable. The main worn mechanism is adhesive wear. |
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