Gradient Composition Design Andmicrostructure Of Hardfacing Material For Cold Roll

The failure of the widely used back-up rolls in industrial production has a negative effect on the quality of products.In order to save cost and avoid material waste for the replacement of new rolls,Overlaying welding technique is usually employed to deposit hardfacing.However,the great difference of physical and chemical properties between base metal and hardfacing layer can result in large surface stress and some cracks.Therefore,it is essential to control microstructure and reducing surface stress of hardfacing to improve roll's performance and extend its service life.In this study,gradient composition of hardfacing for cold roll was designed using Fe-based material.The main contents are as follows:1.Considering type,shape and size of hard phase and microstructure of deposited metal,Ti,Nb and other strong carbide forming elements were added into the alloy system of hardfacing to get MC type carbides.In addition,nano TiO₂ particles as a second phase was added into hardfacing to achieve dispersion strengthening and grain refinement strengthening,thus,hardfacing composition with good toughness and wear resistance was designed.The effect of TiO₂ on hardfacing alloys was studied by observing the phase,microstructure,hardness and wear properties of deposited layers.The results showed that with appropriate content of TiO₂,hardness of the hardfacing layer is slightly improved,friction coefficient is reduced,grain is refined and the inclusions become more complex.For 0.2wt.%TiO₂,the hardness of the hardfacing layer is 55HRC and the average friction coefficient is the 0.5615.When the TiO₂ content is 0.8wt.%,oxide-sulfide inclusion of Ti-Mn-Nb-Si-O-S is generated.2.The effect of tempering on the microstructure and properties of the hardfacing layer was investigated.The results showed that the hardness does not change much after a tempering treatment at 500?,560?and 620?.What's more,microstructure of the hardfacing layer is more uniform and the hardness changes little after the second tempering treatment at 560?,which illustrated that the hardfacing has a high resistance to tempering.3.The fatigue properties of the hardfacing and the matrix alloys were simulated and studied at different temperature.The results showed that the fatigue properties of the hardfacing alloy is no better than the matrix below 400?,however,the fatigue properties of the hardfacing alloy is as good as the matrix above 600?,which indicates that hardfacing alloys have good high temperature fatigue properties.4.Transition layers were designed by simulating the thermal physical properties of thermal expansion coefficient,thermal conductivity,resistivity and solidification interval and CCT curves of different alloys,which effectively controls the microstructure and surface stress of hardfacing.Thermophysical properties and microstructure of the transition layer were adjusted by reducing carbon content and adding alloying elements.The effect of dilution on microstructure and properties of transition layer was investigated.The results showed that the alloy obtained by carbon reduction at a dilution of 20%can gain the mixed microstructure of bainite and martensite,and the addition of0.5wt.%Al refine the thermophysical properties of the transition layer.5.Transition layer were adopted between the matrix and hardfacing layer.Metallographic observation showed that the addition of transition layer could avoid coarse microstructure near the fusion line.