Failure Analysis Of The Turbine Valve Rod After Plasma Nitriding

With the power development and environmental requirement, the developmentof steam turbines is in the direction of high-capacity unit and supernal parameters.Some steam turbine parts working in high temperature environment, such as valve rodmade of38CrMoAlA steel were substituted by the martensitic stainless steel of1Cr12WMoV to ensure the reliable operation at a high temperature. Up to now theplasma nitriding of1Cr12WMoV martensitic stainless steel has rarely been studied.The existing plasma nitriding parameters of the steel are on the basis of plasmanitriding parameters of38CrMoAlA. During the running of the steam turbine, thepeeling of the nitrided layer of1Cr12WMoV martensitic stainless steel directly affectthe unit’s safe operation.The nitrided layer microstructures, physical and chemical properties of the1Cr12WMoV steel were analyzed by optical microscopy (OM), scanning electronmicroscopy (SEM), energy dispersive spectrometer (EDS), micro-hardness tester,pin-on-disk tribometer, electrochemical testing technique. The paper also studied theeffect of the nitriding temperature and nitriding time on the microhardness, hardnessprofiles, wear resistance and corrosion resistance of the nitrided layer. According tothis study, the main conclusions are as follows:(1) The main reason of the nitrided layer peeling of valve rods made of1Cr12WMoV martensitic stainless steel was decarburization and brittle CrN phase inthe nitrided layer during the plasma nitriding.(2) The raising of nitriding temperature will reduce the surface carbon contentof the nitrided layer to form the decarburizing layer. And the raising of the nitridingtemperature will also lead to the chromium diffusion into the nitrided layer from thesubstrate. CrN brittle phase formed in the nitrided layer also increased the brittleflaking tendency of the nitrided layer of the valve rod. (3) The thickness of1Cr12WMoV nitrided layer increased with the raising ofnitriding temperature. The maximum hardness of the nitrided layer can be reachedwhen the nitriding temperature is550℃.(4) Prolonging the nitriding tempture can reduce the generation of CrN brittlephases. But the chromium depletion in the nitrided layer can reduce its corrosionresistance.(5) The optimized nitriding process parameters of1Cr12WMoV martensiticstainless steels with fine comprehensive mechanical properties is the nitridingtemperature of500℃and nitriding time for7h.