| The hydraulic piston rod is a key component for the jack-up offshore platform to achieve lifting.During service,hydraulic piston rods are prone to surface scratches,wear and other damage,which seriously affects the stability and safety of jack-up offshore platforms.In order to improve the friction resistance of the piston rod,a plasma spraying method is used to spray a nickel-based coating on its surface.However,the bonding strength between the coating and the substrate is low,and there are many defects such as pores and layered structure inside,which greatly reduces the overall performance of the coating.Plasma flame in-situ remelting is a low-cost,reliable and repeatable remelting method,which can significantly eliminate various defects in the coating.For this reason,this subject has plasma sprayed Ni Cr BSi coating on the surface of 2Cr13 martensitic stainless steel cylinder,and the coating is remelted in-situ by plasma flame,and the performance and structure of the coating are analyzed.On this basis,the mechanism of crack formation in the remelting process was discussed and solutions were proposed.The sprayed coating was remelted with 25 k W,30 k W and 35 k W power respectively.The results showed that when the remelting power was 35 k W,the porosity of the coating was the lowest(0.75%),the layered structure disappeared,and the coating and substrate From mechanical interlocking to metallurgical bonding,the density and cohesion of the coating are greatly improved.XRD analysis shows that the sprayed coating is mainly composed of Ni3 B,Cr B,Cr7C3,γ-Ni,Cr3B4 and Cr3C2 phases,and the remelted coating is composed of Ni3 B,Cr7C3,γ-Ni,Cr B,Cr3C2,among which the sprayed sub The stable phase Cr3B4 disappeared after high temperature remelting.The performance of the coating before and after remelting was analyzed and tested.The results showed that the hardness of the remelted coating was about 829.1HV0.5,which was significantly higher than 619.3HV0.5 of the sprayed coating;the wear quality of the remelted coating was 0.0005 g,which is much lower than 0.017 g of sprayed coating.The average friction coefficient of remelted coating is 0.634,which is slightly lower than 0.688 of sprayed coating.In summary,the hardness and wear resistance of the remelted coating are higher than that of the sprayed state.This is mainly because after remelting,the internal pores of the coating are reduced and the layered structure disappears,thereby improving the compactness and cohesion of the coating.When the surface of 2Cr13 steel cylinder is remelted with plasma flame in situ,cracks are easy to form,mainly cold cracks.Among them,the source of cracks in remelted coatings mainly comes from pore cracks and unhealed boundaries in sprayed coatings.Different compounds have different effects on crack propagation.The crack will bypass the carbides during the propagation process,but directly pass through the borides.At the same time,cracks generally extend along the gap direction and penetrate to the bottom of the coating.Combined with the change curve of the thermal expansion coefficient,the internal stress change of the coating during the heating and cooling process of remelting is analyzed.The internal stress of the remelted coating is about 300℃,and the internal stress is the largest,which is prone to cracking.On this basis,a method of reducing the cooling rate is proposed to reduce the occurrence of cracks.The asbestos thermal insulation device effectively reduces the cooling rate of the remelted coating and the substrate.The number of cracks on the test sample section is reduced from 36 to 16. |
PDF Full Text Request