Influence Of Cr, C On The High Chromium Cast Iron Layer And Its Microstructure By Plasma Cladding

In this article, the high chromium cast iron mixed powders with four levels ofcarbon concentration (2.5%,3.0%,3.5%,4.0%) and three levels of chromiumconcentration (20%,28%,35%) was confected according to the design principle ofplasma cladding materials. High chromium cast iron wear-resistant cladding layer wasprepared on the surface of Q235steel by plasma spray welding facility after adjustingprocess parameters, and a study on these cladding layers was carried out followed.Processing property of high chromium cast iron mixed powders was discussed basedon flow uniformity of powders, homogeneity of microstructure, defects, and dilutionrate. Microstructure morphology and phase distribution in cladding layers withdifferent composition was studied by OM, SEM, XRD, EDS, and Image-Pro Plus,which is a piece of software used to analysis on image professionally. The influencerules of Carbon and chromium concentration on morphology of microstructure wasexplored, as well as hardness distribution by microhardness tester. The mainconclusions are as follows:By measuring, maximum deviation of powder feeding increments was0.142g,the maximum of non-uniform degree was5.6%，which could be consideredapproximately uniformity, while the feeding voltage changed in range of10V～20V.After optimizing the cladding parameters based on the principle of limit value,cladding layers with good shape and compact structure were obtained, the distributionof dilution rate were24～36%. In terms of the analysis on panorama of the verticalsection and microstructure in random sights from surface, obvious micro-defect suchas microcrack and pore was not found obviously, but inhomogeneous distribution ofmicrostructures instead in micro-regions, include of abnormity of microstructuredensity, abnormity of microstructure morphology.Cladding layers with2.5%and3.0%level of carbon concentration wereclassified as hypoeutectic microstructure, constituted by primary phase γ(Fe,Cr,Ni)and eutectic phase MyCx/γ(Fe,Cr,Ni), primary phase closing to welding line grewperpendicularly to the bonding interface depend on the condition of directionalcooling builded by matrix quench cooling, while growing in random direction closingto surface. Small amounts of eutectic austenite transformed into martensite during thesubsequent cooling, most eutectic carbides presented strip morphology, which kept astate of cluster and displayed directionality. In cladding layer of2.5%C-18.53%Cr, eutectic carbides tended to grow along the boundary of primary austenite grain, intoparticle groups.Cladding layers with3.5%and4.0%level of carbon concentration wereclassified as hypereutectic microstructure constituted by primary phase (Fe, Cr)7C3and eutectic phase (Fe, Cr)7C3/γ(Fe,Cr,Ni); Morphology of primary carbide underSEM presented hexagonal prism structure with a hole in centre, microhardness ofcladding layers was promoted significantly, about1500～1700HV in cross section,due to a large number of iron atoms were replaced by chromium. Eutectic carbidepresented solid or hollow fiber rod, whose growth direction tended to be parallel tovertical section. Isolation degree and structural integrity of fiber-rod eutectic carbideisolation were higher than strip carbides.With the increase of carbon and chromium concentration, the area ratio andaverage size of primary austenite in hypoeutectic organization showed up the trend ofdecreasing, which displayed the same trend for the area ratio and the opposite trendfor the average size of primary carbide in hypereutectic organization, andeffectiveness of carbon concentration was more significant than chromium; Carbonplayed a primordial role in morphology and size of eutectic carbide, while chromiumconcentration mainly affected size through affectting the number of primary phase. Ineffective range of chromium carbon ratio, keeping carbon concentration in constant,microhardness of cladding layer was raised significantly along with the increase ofchromium, but this influencing effect weakened while concentration of chromium wastoo high, which resulted in a big surplus of Cr. Keeping chromium concentration inconstant, if the type of microstructure has transformed caused by the increase ofcarbon concentration, the microhardness of cladding layer was raised widely, if not,which changed in a narrow range relatively.