| Medium-high temperature furnace roller is one of the key components in heat treatment furnace.The quality of its surface directly determines the quality and production efficiency of the steel plate.However,the medium-high temperature furnace roller is prone to oxidation,corrosion,nodulation and friction-wear during the long-term service process,which will lead to premature failure of the furnace roller.In this paper,aiming at the thermal corrosion problems existing in the furnace rolls,Cr3C2-25wt%NiCr/NiCrAlY cermet protective coating and Al2O3/NiCrAlY protective coating were prepared through using high velocity oxygen fuel?HVOF?and atmospheric plasma spraying?APS?,respectively.The effects of HVOF process parameters and APS process parameters on the microstructure and mechanical properties of the coating were investigated by orthogonal experiment.The process parameters were further optimized by means of range and variance method,and combining the microstructure of the coating.The hot corrosion behavior of the coating at high temperature was studied and the corrosion mechanism was discussed.The results are as follows:1.HVOF Cr3C2-25wt%NiCr coating surface didn't appear obvious holes and cracks,the working layer and the bonding layer interface to formed an irregular staggered occlusion,bonding mode was mechanical bond.The coating phase was mainly composed of NiCr,Cr3C2 and a small amount of Cr7C3.The order of influence of process parameters on the mechanical properties of Cr3C2-25wt%NiCr coating was:oxygen flow>powder feeding>spraying distance.Low spraying distance,moderate oxygen flow rate and powder feeding rate were beneficial to the preparation of compact coating.The optimum parameters of HVOF Cr3C2-25wt%NiCr coating were as follows:spraying distance 180mm,oxygen flow 250 L/min,powder feeding 50 g/min.In this case,the properties of the coating were as follows:bonding strength 65MPa,microhardness 945.0HV0.3.3 and porosity1.27%.2.The surface of APS Al2O3 coating was not fully spread and there were some pores with different sizes.The interface between the working layer and the bonding layer was continuous and complete without pores or cracks.The bonding mode was mechanical bonding.The phase of the coating was composed of two phases:the main phase?-Al2O3and a small amount of unmelted?-Al2O3.The influence of process parameters on the mechanical properties of Al2O3 coating was in turn:spraying power>main gas flow>powder feeding.With the increase of power and main gas flow rate,the bonding strength and hardness of the coating increased first and then decreased.The optimum process parameters of APS Al2O3 coating were:spraying power 44 KW,main gas flow 40 L/min,powder feeding 40 g/min.In this case,the properties of the coating were as follows:bonding strength 52 MPa,microhardness 1219.4 HV0.3.3 and porosity 4.00%.3.Cr3C2-25wt%NiCr coating showed good thermal corrosion performance at 750?,and the dense Cr2O3 film generated at the interface between the corrosion layer and the coating effectively retarding the corrosion process.Meanwhile,the corrosion mechanism of the coating in Na2SO4 molten salt was that the protective oxide film on the surface dissolves at the interface between the coating and molten salt,and then formed the unprotected oxide at the interface between molten salt and air.4.Al2O3 coating exhibited good hot corrosion resistance performance without obvious spalling and cracking under the 750°C Na2SO4 molten salt corrosion.The transformation from?-Al2O3 to?-Al2O3 occured on the surface of the coating.?-Al2O3 has excellent thermal corrosion resistance and prevents further corrosion.However,as the temperature increased,cracks appeared in the working layer of the coating at 800?and900?,which was attributed to the large difference in thermal expansion coefficient of the metal matrix,bonding layer and Al2O3 working layer. |
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