Study On Microstructures And Properties Of Arc Spraying Coatings And Their Remelted Layers By Argon Arc

Thermal spraying technology is used for preparing wear and corrosion resistancecoatings. It has many advantages such as simple equipment，convenient operation andvariety coating materials. Thus it is widely applied for aerospace, machinery, shipsand petroleum chemical industries. But it also has some drawbacks. First, under thesituation of impact and heavy-loading, it would have some limitations due to themechanical coalescence of spraying coatings with substrates, which has low adhesivestrength. Second, because the spraying coatings are full of porosities, voids and oxideinclusions, these defects could decrease the hardness and deteriorate abrasiveproperties of the coatings. By means of remelting, the metallurgical bondingmechanism is realized between coatings and substrates. The stack layermicrostructures are changed to uniform and dense ones with less or no voids. As aresult, the bonding strength between the coating and the base metal is increased andthe abrasive property is improved as well.In this paper, the arc spraying-melting layers were prepared by self-madeflux-cored spraying wires, in which alloy powders of C-Cr-Fe, B-Fe, Si-Fe, Mn-Fe,Ti-Fe and so on were added. The process of arc spraying plus arc remelting wasapplied to produce the arc spraying coatings on the substrates of65Mn and Q235respectively. And then they were remelted by argon arc under the different processingparameters to achieve the spraying-melting layers. The microstructures，interfacecharacteristics and element distribution were analyzed by means of opticalmicroscope, SEM, XRD and other analysis equipments for both the spraying coatingsand the remelted layers. Their microhardness and abrasion resistance wereinvestigated by means of respective equipments.The spraying coatings have the similar lamellar structures and contain largeamount of oxide inclusions, pores and voids. They consist of some phase componentslike ferrite, austenite and M7C3. In addition, the nonuniform composition phenomenonwere found that lamellar high chromium and silicon particles exist in the coatings.The interface between the spraying coatings and steel substrates was analyzed and theresults showed that their coalescence is mechanical. After arc remelting treatment,defects in coatings like oxide inclusions, pores and voids were eliminated and surface layers became denser. Moreover, the interface between remelted layers and substrateshad such characteristic that the alloy elements were changing gradually. That is to say,the bonding mechanism was changed to metallurgical coalescence. The remeltedlayers on65Mn substrate consisted of austenite, martensite, M7C3, M3C and TiC.Similar results were achieved on Q235substrate. When the arc current is low, themirostructure type of the remelted layers was alloyed cast iron. While high arc currentbrought about alloyed steel remelted layers for the high dilution from the base metal.The elemental micro-distribution such as Cr, Si, Mn and so forth of arc sprayingcoatings and remelted layers on65Mn substrate were analyzed. The main differencewas that fluctuations in element contents for the arc spraying coating were large, butthey were small for the remelted layers. For example, the difference of maximum andminimum values of chromium contents in arc spraying coatings is4.23wt.%, whilethat number is3.88wt.%for the remelted layer at arc current of160A. For furtheranalysis of the element chromium distribution, average values and standard deviationof chromium contents in surface layers were calculated. The average value andstandard deviation of chromium contents for the spraying coating are15.93wt.%and1.71respectively. The average values of chromium contents of remelted layers at160A and220A are14.62wt.%and11.60wt.%. The corresponding standard deviationsare1.30and1.22respectively. According to these data, it was concluded that theelement chromium was distributed more uniform in the remelted layer than in the arcspraying coating. And higher arc currents made chromium distribution much moreuniform. Also, the average chromium content in the remelted layer was lower thanthat in the arc spraying coating. when the arc current increased, the average chromiumcontents of the remelted layers were further decreased because of higher dilutuion.The characteristics of elemental distribution of silicon and manganese were basicallythe same as that of element chromium.The experimental results showed that the microhardness and abrasion resistanceof spraying coatings could be improved by remelting. The average microhardnessvalue of the arc spraying coating on65Mn is517.9HV and its weight loss is42.1mg.But after remelted at arc current of160A, the average microhardness value is as highas756.0HV, which is45.8%higher than that of the spraying coating. At the same time,corresponding weight loss is21.2mg and the abrasion resistance was improved by50%. In the case of Q235substrate, the average microhardness value of the arcspraying coating is551.2HV and its weight loss is11.3mg. After remelted at arc current of120A, the average microhardness value is as high as597.9HV, which is8.47%higher than that of the spraying coating. At the same time, correspondingweight loss is8.3mg and the abrasion resistance was improved by17.7%. The averagemicrohardness and the abrasion resistance was decreased with the increase of the arccurrents. The higher, the arc current, the greater, the dilution effect from the basemetal. And contents of alloyed elements more declined. Then the hard phase contentsof abrasion resistance were reduced.Arc spraying coatings contains large amounts of ferrite which has goodtoughness and low hardness. But it could be pressed by the abrasive particles andmade plastic deformation repeatedly. Finally, it was pelt off. Moreover, the adhensionstrength among lamellar structure particles in the spraying coating is low, it was easyto peel them off during the abrasive particle pushing. Hence, the abrasive mechanismof arc spraying coatings is a synthetical combined function of numerous plasticdeformation and micro-fracture. Carbides like M7C3, M3C and TiC in remelted layersare very hard and abrasive resistant. And austenite structures are tough and couldprovide the favourable support for the hard phases of M7C3, M3C and TiC, whichhelps improve their abrasive performance. So the abrasive resistance of remeltedlayers was increased obviously than that of the spraying coatings. Furthermore, thewear mechanism of remelted layers is a synthetical combined function ofmicro-cutting and numerous plastic deformation.