The Plasma Beam Surface Strengthening Technology For New Roller Steel

The roll is an important vulnerable and wearing parts in production steel products.Quality of them directly affect quality&quantity of rolled products. Due to advantagesof plasma beam surface strengthening technology on precision equipment&functiondevice surface strengthening field, the technique are being used to strengthen workingsurface of various of rollers, improve combination properties of working surface and hasbeen proved as a kind of effective method.In this paper, surface morphology、microstructure、micro-hardness distribution ofhigh Ni-Cr infinite chilled cast iron and9Cr3MoNiV cold roll steel have beeninvestigated. The effect of technological parameters on microstructure andmicro-hardness of hardened layer are measured and analyzed. Discipline oftransformation of microstructure after tempering which following plasma beam surfacealloying scanning on high Ni-Cr infinite chilled cast iron was studied. Then, thermalfatigue resistance of the layer was also investigated. Furthermore, wear behavior andcorrosion resistance of hardened layer fabricated on9Cr3MoNiV cold roll steel byplasma surface alloying method was also investigated.The main conclusions of this paper were given as follows:1) As high Ni-Cr infinite chilled cast iron was referred to, firstly, metallurgicalbonding was established between hardening layer and substrate, and crack-free interfacewas observed. Hardening layer was constituted by melting zone、heat affected zone andmatrix. Eutectic structure containing non-equilibrium austenite and M₇C₃wascharacteristiced in melting zone and also M₂₃C₆which transformed from react betweennon-equilibrium austenite and M₇C₃was observed. Retain austenite and martensite wasformed in heat affected zone as well as cementite which was reserved in interdendritic.Carbides diffuse distributed in hear affected zone. When energy density pointed at 1.62J/mm², better strengthening layer depth was obtained as about0.5mm. The highestmicro-hardness(794HV_0.2) was obtained in subsurface. Affect of tempering on plasmasurface alloying layer was investigated and the results showed that formation of temperedmartensite、precipitation of carbides、formation of supersaturated solid solution asmertensite resulted to excellent tempering resistance stability of alloying layer.2) Surface strengthening by plasma beam scanning, high Ni-Cr infinite chilled castiron sample showed obviously improvement on thermal fatigue resistance. Further,sample treated by plasma surface alloying had better performance on thermal fatigueresistance. Refined crystalline strengthening、solid solution strengthening、dispersionstrengthening improved thermal fatigue resistance of surface layer of samples. Crackleszigzaged in the hardening layer indicated that stress condition in the hardening layer hadhelpful action on inhibitation of crack propagation.3) The hardening layer of9Cr3MoNiV cold roll steel produced by plasma surfacestrengthening could be divided into three regions: Molten zone, heat affected zone andthe matrix. Acicular martensite, retained austenite and dispersive distributed carbids wasfound in alloyed layer. When energy of plasma was at a low level, Micrograph inoverlapping region was tempering martensite, sorbite and carbides. By contrary, whenenergy was at a high level, surface of alloyed layer showed micrograph as dendrite likemartensite which was unstable. Depth of hardening layer increased as energy densityincreased. Reversely, micro-hardness decreased as energy density increased. With plasmasurface alloying, martensitic microstructure sharply refined, distortion of latticesimultaneously happened in martensitic crystals which was caused by supersaturatedsolid solution.4) Plasma surface alloying samples showed abrasive wear behavior in friction-weartest. Alloyed layer showed better wear resistance than as-reserved sample. Increase ofmicrohardness、grain refinement、Inhibitation to microploughing caused by cabrides aremain reasons to improvement of friction and wear behavior. 5) Samples treated by plasma surface alloying showed better corrosion resistance.Compared with as-reserved sample, free corrosion potential of samples treated by plasmasurface alloying moved to the positive and anode process was inhibited. The corrosionpotential was significantly higher than the substrate, the corrosion current decreases, thecorrosion resistance higher than the substrate. With the increase of energy density, thegeneral corrosion resistance improved.