Application Foundation Research On Double Glow Plasma Surface Metallurgy Ni-Cr Corrosion-resistant Steel Plate

Carbon steel was widely used in many fields such as building, traffic, mechanism and energy,because of its well plasticity and toughness, easy processing and low cost. However, the carbon steelhas a common problem, low corrosion resistance. Scholars developed corrosion-resistant steel andweathering steel to improve the corrosion resistance of steel, but the corrosion-resistant alloyingelements increased the cost and can not be used effectively. The steel plat is very important for thesteel materials. Therefore, the formation of stainless steel on carbon steel plate has been the dream ofworld metallurgical industry. In this paper, to form a new type of plasma surface metallurgycorrosion-resistant carbon steel plate, double glow plasma surface metallurgy technique was used toprepare the nickel-base alloyed layer on carbon steel. This new surface metallurgy material, whichexhibits good plasticity, toughness and excellent corrosion resistance, can economize costly alloyingelements and reduce the cost of materials. Meanwhile, a series of basic scientific issues related to thescale application of surface metallurgy steel plate were studied systematically.In order to solve the problem of insufficient allowance of alloying elements in scale production,grid-like hollow cathode source structure was introduced. Using Ni80Cr20alloyed target as sourcematerial, Ni-Cr alloyed layer was formed on Q235steel. The alloyed layer combined excellently withsubstrate and had gradient distribution of composition. The main phase of alloyed layer was austeniticsteel Ni2.9Cr0.7Fe0.36and a little of awaruite Ni3Fe. Compared with planar source structure, thegrid-like hollow cathode source structure could provide more adequate allowance of alloyingelements, so its Ni-Cr alloyed layer had thicker thickness and higher content of Ni, Cr alloyingelements which result in more excellent corrosion resistance.In the paper, the effect of process parameters on microstructure and properties of Ni-Cr alloyedlayer were studied systematically. The results showed that the thickness of Ni-Cr alloyed layer and thecontent of Ni and Cr alloying elements increased as the treatment temperature increased. As a result,the corrosion-resistant of Ni-Cr alloyed layer was improved, and the best corrosion resistance wasobtained when the treatment temperature reached about1000℃. With the increasing of workingpressure, the thickness of Ni-Cr alloyed layer and the content of Ni and Cr alloying elements inalloyed layer showed trend of first increased and then decreased. Meanwhile, the corrosion resistanceof alloyed layer also increased firstly and reduced afterwards, and the best corrosion resistance wasobtained when the working pressure was about51Pa. Increasing the electrode spacing, the thicknessof Ni-Cr alloyed layer, the content of Ni and Cr alloying elements and the corrosion resistance of alloyed layer exhibited the same results. The optimal electrode spacing was about15mm. Prolongingthe holding time could obtain thicker Ni-Cr alloyed layer, and improve the corrosion resistance ofalloyed layer. But the improvement was not significant anymore after heat for3h.The influence of the carbon content of the substrate on the microstructure and properties of thealloyed layer was investigated. Ni-Cr alloyed layers were obtained on surface of Q235steel,45steeland T8steel. The thickness of diffusion layer of Ni-Cr alloyed layer increased as the carbon contentincreased. The Ni-Cr alloyed layer on Q235exhibited compact fibrous structure, while the alloyedlayer on45steel and T8steel showed porous tapered column structure which showed coarseningphenomenon as the carbon content increased. The fibrous structure is dense, while the tapered columnstructure contains voids. Therefore, the Ni-Cr alloyed layer on Q235steel had the bestcorrosion-resistant. For the tapered column structure Ni-Cr alloyed layer, the corrosion-resistant ofalloyed layer on T8steel was better than the alloyed layer on45steel due to its thicker thickness ofdiffusion layer.The composition of Ni-Cr alloyed target has direct effect on the composition and phase of Ni-Cralloyed layer. When the content of Cr alloying element in alloyed target was20%, the main phase wasNi2.9Cr0.7Fe0.36and a little of Ni3Fe. Then the content increased to40%, the phase of alloyed layerchanged into Cr0.19Fe0.7Ni0.11and a little of Fe-Cr. As the content of Cr alloying element in alloyedtarget increased, the content of Cr0.19Fe0.7Ni0.11decreased and the content of Fe-Cr in alloyed layerincreased. Finally the content reached80%, the main phase of alloyed layer was Fe-Cr and a little ofCr0.19Fe0.7Ni0.11. Under different alloyed targets, the Ni and Cr elements on surface of Ni-Cralloyed layer exist in the form of simple substance Ni, Cr and NiO. The composition segregationeffect of Ni and Cr alloying elements in alloyed layer was investigated, and then the relation formulaof the composition of alloyed target and the composition of Ni-Cr alloyed layer was founded. Thenano-indentation test of Ni-Cr alloyed layer indicated that the plasticity and toughness of alloyedlayer reduced as the content of Cr alloying element in alloyed target increased. As the content of Cralloying element in alloyed target increased, the content of Cr alloying element in Ni-Cr alloyed layerincreased, the continuous dense passive film can be easily formed on surface of alloyed layer whichcould improve the corrosion-resistant of alloyed layer. However, thickness of Ni-Cr alloyed layerunder Ni60Cr40alloyed target was thin, thus its corrosion-resistant was worse than that underNi80Cr20alloyed target.The atom model Ni9Cr2Fe1of Ni2.9Cr0.7Fe0.36, which was the main phase in Ni-Cr alloyed layer,was built. According to the energy minimum principle, equilibrium geometric structure ofNi2.9Cr0.7Fe0.36was obtained. The bonding electron of Ni9Cr2Fe1was contributed by the3s and3pvalence electron of Cr, while the density of states at the Fermi level was contributed by the3d electron of Ni and Cr. The charge interaction of Ni, Cr and Fe atoms in Ni-Cr alloyed layer made theFe atom exhibit positively charged and increased the potential of Fe atom which inreased thedifficulty of the electrochemical corrosion reaction of Fe atom. Therefore, the Ni-Cr alloyed layercould provide effective corrosion resistance for the substrate.In order to meet the needs of industrialization, a new type of double glow plasma surfacemetallurgy device was designed. The large area of carbon steel plate could be processed in theequipment to obtain plasma surface metallurgy Ni-Cr corrosion-resistant steel plate. The results ofstatic immersion corrosion test and salt spray corrosion test demonstrated that the corrosion-resistantof plasma surface metallurgy Ni-Cr corrosion-resistant steel plate was better than that of Q235lowcarbon steel, and its corrosion-resistant was close to that of316L stainless steel.