Study On Microstructure And Properties Of Plasma Remelted Fe-based Coatings Deposited By Supersonic Arc Spraying

In this paper,302A (FeCrNiMnMoTi) and chromium carbide (FeCrMnNiTi/Cr3C2),two Fe-based coatings were deposited onto20steel substrates by supersonic arc spraying.Plasma was used to remelt the arc sprayed coatings. The orthogonal experiments weredesigned to determine the optimum parameters of plasma remelting process.Microstructure, morphologies, phase composition and microhardness of the coatingsbefore and after remelting process, microstructure evolution of the remelted coatingsunder different linear energy, and the effect of tempering on microstructure of the remeltedcoatings were investigated. The effect of plasma remelting on corrosion, oxidation andthermal shock resistance of302A coating was studied.The as-sprayed coatings consisted of Fe-based solid solution phase mixed the pores,oxide phases, un-melted and partially melted particles. The coatings were bothmechanically boned to onto the substrates. The main phase in302A coating was α-Fe.α-Fe and γ-Fe both existed in chromium carbide coating and α-Fe was the main phase. TheCr3C2hard phases gathered in chromium carbide coating. The as-sprayed coating had highporosity and oxides content.The cross section of the coatings was divided into remelted area, HAZ (heat affectedzone) and substrates after plasma remelting. According to the results of orthogonalexperiments, when the surface quality of the remelted coatings was priority, the optimumparameters of remelting process were determined as follows: for302A coating, the currentwas50A, and the scanning rate was120mm/min. for chromium carbide coating, optimumparameters that put emphasis on the remelted depth were: current was60A, and thescanning rate was120mm/min; optimum parameters that put emphasis on themicrohardness were: the current was70A and the scanning rate was120mm/min. With theaddition of Si and B,302A coating showed a better remelting result. The typical lamellar structure of as-sprayed coatings disappeared and defects such aspores and oxides were almost eliminated after plasma remelting. The remelted coatingsappeared to be homogeneous and compact and exhibited good metallurgical bonding tothe substrates. The remelted coatings had rapid solidification microstructure whichconsisted of columnar grain, equiaxed grain and dendrite. Microstructure of the remelted302A coating consisted of α-Fe primary dendrites and interdendritic α-Fe/(Fe, Cr)7C3eutectics. Microstructure of the remelted chromium carbide coating was composed of (Fe,Cr)7C3dendrite and interdendritic Fe-Cr solid solution matrix. With the increase of linearenergy, the carbide in the remelted chromium carbide coating gradually dissolved andfinally bainite structure with high microhardness was formed in the remelted coating.Microhardness and dispersity of it of the coatings decreased after remeltingThe cross section microstructure of multi-way plasma remelted302A coating showedgood lap. After the remelted302A coating being tempered at200℃, the carbideprecipitated in primary α-Fe, and the eutectics coarsened, all these leaded to a decrease ofmicrohardness. At the same time, lamella interval distance of pearlite in HAZ increased,cementite coarsened and spheroidized, which lead to softening of HAZ.Analysis of the polarization curves and impedance spectroscopy all indicated thatplasma remelting improved the corrosion resistance of302A coating in NaOH solution butreduced that of the coating in NaCl solution due to the different corrosion mechanism ofthe coating in each solution. The corrosion of the coatings in NaOH solution was mainlypassivation, with homogeneous and compact structure, the remelted coating couldpromote the formation of steady passive film and inhabit the initiation of corrosion cracks.When being corrupted in NaCl solution, a interphase corrosion of the remelted coatingoccurred due to the potential difference between α-Fe and carbides, which leaded to thereduction of the corrosion resistance.Plasma remelting improved the oxidation resistance of302A coating, which wasattributed to the formation of dense oxide film on the remelted coaing surface and the compact structure of the remelted coating which could hinder O from passing inward orinto the interface between the coating and the substrate. The remelted302A coatingshowed better thermal shock resistance than the as-sprayed coating, which was attributedto the metallurgical bond between the remelted coating and the substrate.