Process And Microstructure Studies Of Nitridation Combined With Trivalent Chromium Electroplating On Pure Iron

Recently, advanced modern structural materials of high corrosion resistance and excellentwear properties are significantly required. Chromium nitrides have been considered the mostpromising substitute material for TiN coating in the frication application. Although the currentprocess of duplex chromizing can achieve low temperature chromizing in a salt-bath below700°C and gain the nanostructural grains of chromium nitrides meanwhile improve themicrostructure and preformance of substrate, there exists deficiency: complex process etc. Inthe present study, trivalent electroplating combined with nitriding process is explored toobtain chromium nitride in a relative low temperature.This method is simple andenvironmentally friendly, improving the performance of the surface matrix as well as breakingno squares to deep zone substrate.In this work, dimethyl formamide(DMF) trivalent electroplating bath was chosen topre-treat the steel before nitriding. In order to compare the effect of pre-nitriding andpost-nitriding on the chromium deposition, two different processes were explored: one istrivalent chromium electroplating on pre-plasma nitrided pure iron followed by an Arprotection atmosphere annealing heat treatment at 200, 400, 600°C for 2 hours, the other isthat developing chromium nitride layer through a two-stage hybrid process comprisingelectrodeposition of different thick chromium layer thought trivalent chromium bath followedby gas nitriding in pure NH3 atmosphere at the temperature range of 550,640,700°C for 4hours. The surface and across-sectional morphology ,microstructure, phase structure andconcentration distribution were investigated using optical microscopy(OM), scanning electronmicroscopy(SEM) attached energy dispersive spectrometer(EDS), X-ray diffractometer(XRD)and electron probe micro-analyzer(EPMA), mainly studying the evolution of microstructureand the phase identification of chromium nitrides. The microhardness of differentacross-sectional layer was obtained by microhardness tester.The pure iron is nitrided before electroplating, developing duplex microstructure consistsof a electrodeposition on the top,ε-Fe2-3N layer inner, nitride diffusion layer, and matrix. Anew transition layer is formed between the electrodeposition andε-Fe2-3N layer, whichcontains Cr,Fe,C,N,O elements and so on. The microhardness of the trivalent chromiumdeposition increases from 600HV25g (unannealing) to 1200HV25g (600°C).The microhardnessof transition layer is lower than the chromium deposition on the top andε-Fe2-3N/γ'-Fe4Nlayer on the bottom. The XRD analyses of the as-deposited and as-annealed coatings indicatethat the phase structure of the trivalent chromium deposition evolves from amorphous/microcrystalline to crystal when annealed. Moreover, some new phases such asCrC appeares annealed at 400°C and Cr2O3, Cr7C3 and Cr23C6 present in the coatings at600°C.The pure iron is nitrided after electroplating, developing duplex microstructure composedof three distinct regions:a mixed-nitride (~2-5μm thick) at the top,ε-Fe2-3N/γ'-Fe4N phase inthe middle (10-20μm) andγ'-Fe4N phase distrubited on theα-Fe matrix layer at the bottom ofthe coating in order of vertical distance from the surface.At 550°C,only little Cr converts intoCr2N phase.As the temperature increases to 640°C, mass of CrN phase appears in thecompound nitride layer, combined with most CrN and little Cr2N. At 700°C, peak intensity ofCr2N (111) exceeds CrN (110), a mixture phases of CrN and Cr2N are obtained.To learn more about the formation of CrN/ Cr2N in the process of nitriding afterelectroplating, different nitriding time of 1, 4, 9 hours at 640°C were adoped, the data revealthat during nitriding of Cr, Cr2N is formed preferentially. With an increase in nitriding time,more nitrogen diffuse from surface to center and the Cr2N tends to partially dissolve andallow formation of higher nitride CrN.Comparing the different process, results indicate that a relative thick and better adhesionalloy layer is obtained by electroplating after nitriding while the alloy layer formed bynitriding after electroplating showed not high interface adhesive.Compare the difference of duplex layers on different matrixs: pure iron, 45 steel and H13steel through the process of nitriding after electroplating. It manifests that the compound layercomprises of chromium nitride layer on the top, inner nitride diffusion layer, and interiorsubstrate. A parallel microstructure of chromium nitrides on the surface presents in the threedifferent matrixs. Due to the high carbon content of 45 steel, relatively strongcarbon-chromium phase tendency is showed.