Microstructure And Mechanism Of Interface Layer Of Composite Casting Wear-resisting Thin Plate Based On Liquid-semisolid Bimetal Process

Wear-resisting plates are specially designed for bearing the wear in large area,such as electric power,petroleum,metallurgy,building,mining and other industries.The traditional processes are limited by the poor metallurgical bonding and oxidized inclusions at the interface layer during the fabrication of wear-resisting thin plate.In order to produce wear-resisting thin plate with thickness less than 40 mm,a new technology called liquid-semisolid casting composite is proposed.In this paper,a low carbon steel(LCS)-high chromium cast iron(HCCI)composite wear-resisting thin plate was prepared by this method.The microstructure and formation mechanism of interface layer of wear-resisting thin plate were studied,and installment experiment of the composite thin plates was tested.It is a method that firstly placing the cooling medium at the bottom of the sand mold,then pouring the LCS and then casting HCCI.The advantages of the method is the previous pouring LCS is in a semisolid state under the influence of cold iron,then pouring HCCI.It makes the liquid HCCI contact with the semisolid of LCS wholly,achieving the extensive metallurgical bonding of LCS/HCCI.The relationship between the thickness of the solidified layer and the time of the solidification of LCS from the bottom to the top was established,and the distribution of the temperature field during the solidification process of the liquid-semisolid casting composite was analyzed.The simulation results show that the semisolid region can be formed on the upper surface of the LCS during the final stage of the solidification of the LCS,under the conditions of water-cooled copper plate and external cooling iron in a certain thickness of LCS.The chilling ability of water-cooled copper plate is stronger than external cooling iron,the thickness of the semi-solid area is narrow,which can be avoided the mixture with the HCCI.The relationship between pouring temperature T1,solidification coefficient K1,solidified time ?1 and solidified layer thickness ?1 were built.(1)Under the cooling condition of water-cooled copper plate,the empiric relationship are as follows:K1=-0.0078T1+15.558;?1=-0.0173T1+34.573;?1=3.79?10.48.(2)Under the cooling condition of external cooling iron,the experimental relationship are as follows:K1=-0.01037T1+18.913 and?1=-0.0229T1+42.089.When the water flow rate QW=5.0×10-3m3·s-1,the maximum thickness of LCS is about 26?29 mm and the maximum of pouring time of HCCI is 48?70s.The maximum thickness of LCS is about 25?28 mm and the maximum of pouring time of HCCI is 56?91s under the condition of cold iron,and the width of semisolid area is less than 0.6 mm.The casting composite bimetal thin plates with the minimum thickness of 10mm of LCS were both fabricated under the condition of water-cooled copper plate and cold iron.The microstructure of the casting composite thin plate can be divided into LCS,interface layer and HCCI.The microstructure of the LCS is ?-Fe and P and the microstructure of the HCCI is(Fe,Cr)7C3 and ?-Fe.In addition,the interface layer can be subdivided into pearlite(P)transition layer,composite layer and high chromium cast iron transition region.The concentration distribution of C and Cr in the interface layer increases gradually,and the concentration of Fe decreases gradually from LCS to HCCI.The interface layer consists of y-Fe and granular Cr7C3,the pearlite(P)transition layer contains pearlite and granular Cr7C3 adjacent to the edge of the composite layer and the high chromium cast iron transition region is mainly consisted of dendritic eutectic y-Fe and eutectic structure made of ?-Fe +(Fe,Cr)7C3.The height of the austenite dendrites in the high chromium cast iron transition layer is between 750-1200?m under the condition of water cooling copper plate when increasing the pouring temperature of LCS.Whereas,it decreases to 305-520 ?m under the external cooling iron.The non-uniform nucleation of the liquid HCCI take place on the surface of the LCS and the growth of austenite dendrites perpendicular to the composite layer with the form of planar,cellular and dendritic with the expansion of the supercooled composition area.The formation mechanism of interface layer of LCS/HCCI is plane growth?cell growth?dendritic growth.The mechanical properties results show that the composite interface layer exhibits good metallurgical bonding.The tensile strength of the interface layer is higher than that of the HCCI.The shearing strength of composite plate increases with the height increase of the austenite dendrites in the HCCI transition region.The cracks can be further prevented at the interface layer and LCS.The installment test results show that the service life is 12 months under the conditions of wet grinding and 18 months under the dry grinding conditions,which is superior to the wear-resisting plate used currently.Furthermore,the production cost of this liquid-semisolid method is low,providing a new way to prepare the composite wear-resisting thin plate.