Research On The Working Stress Of Large-scale Backup Roll As Well As The Microstructure And Properties Of Surfacing Layer

Roll is a large and important consumption tool for steel and nonferrous rolling. The selling price of large-scale backup roller for hot rolling is over one million RMB. Because the backup rollers are applied to severe conditions for long time, the decrease of roller diameter caused by abrasion and the spalling caused by high stress cycle are the main reason for the roller failure. How to repair the failure roller with the new material, new technology and new method is a big problem that ferrous metallurgy enterprises pay much attention.The stress state of backup roller during working was simulated by ANSYS. The rolling force (P), the maximal contact stress (σmax) and the maximal shear stress (τ45°) were also calculated with formula. The calculated value of the rolling force is5.799MN while the simulation value is5.33MN. According to the Hertz formula, the values of amax and τ45°respectively are1031.7MPa and313MPa, and the distance between the position of the maximal145°and the surface is6.26mm. According to the result calculated by ANSYS, the value of σmax and τ45°respectively are1158MPa and348MPa, and the distance between the position of the maximal145°and the surface is5mm, which agrees with the practice. These works provide purpose and direction for the preparation of hardfacing materials.The temperature field and stress field of roller during welding repairing process were simulated by ANSYS, and the welding thermal cycle curve and the pool size is quite consistent with the experiment data. The experiment data of residual tensile stress is bigger than the simulation value while the residual compressive stress is smaller than the simulation value. This is because the simulation process didn’t consider the influence of transformation stress. The simulation results show that the influence of circumferential stress on the weld is greater than the transverse stress. The circumferential residual stress after welding appears as compressive stress in initial stage, and then changes to tensile stress because of plastic deformation, the maximal tensile stress appears at the middle of the weld and the weld toe, the value of simulation maximal tensile stress is1086MPa while the experiment data is805MPa. The transverse residual stress appears as compressive stress, the value of simulation maximal tensile stress is531MPa while the experiment data is354MPa, which means the main residual stress after welding appears as circumferential tensile stress. The residual stress field of roller after welding under different preheating temperature was simulated, and the results show that the maximal residual stress is1190MPa without preheating, the maximal residual stress is801MPa while preheating temperature is300℃, the maximal residual stress is642MPa while preheating temperature is400℃, the maximal residual stress is491MPa while preheating temperature is500℃. It is obvious that preheating processing before welding is an effective method to decrease the residual stress.Two kinds of hardfacing flux-cored wire were prepared through optimization experiment. The tempering stability of hardness for the hadfacing alloy under different tempering temperature was studied, and the result shows that the best heat treatment temperature is480℃. The hardness value of hadfacing alloy measured from the high Cr alloy steel series is better than the martensitic stainless series. The influence of Ni and Mo on the tempering stability of hardness alloy was studied. According to the result and economic evaluation, the optimum additive content of Ni and Mo are both4%. Nitrogen alloying make the retained austenite and the Cr dissolves in the base stock more stable, thus the precipitate time of M23C6and M6C is put off, so the secondary hardening temperature of Nitrogen alloying hardfacing alloy is520℃, which is higher than the480℃of carbon alloying hardfacing alloy. The strong carbonitride forming elements, such as Nb, V, and Ti, which added to the flux-cored wire can obviously improve the hardness of hardfacing alloy. Precipitation characteristic of strong carbonitride forming elements was studied, and it is shows that there are two kinds of carbonitride particle. First, these particles are Ti-rich carbonitride with large size, and these particles are formed in the initial stage of weld solidification, thus particles have enough time to grow. The second kind is small and round carbonitride, these particles are precipitated in the low temperature solidification process and the tempering process. Because the growth time of the second kind particles after precipitation is very shot, the new precipitated phase has not enough time to grow on its surface, thus the particle size is very small and the precipitation temperature is above520℃. These small size carbonitride particles are the main reason for the secondary hardening of hardfacing alloy during tempering process.The result of high temperature wear experiment shows that the nitrogen can improve the high temperature wear resistance. The main failure modes of high temperature wear include the spalling of high temperature oxide skin and abrasive wear. There are two main phases in the hardfacing alloys, lath martensite and the residual austenite. Nitrogen alloying make the retained austenite more stable, and the increase of retained austenite proportion is benefit to the toughness of Nitrogen alloying hardfacing alloy. The base of carbon alloying hardfacing alloy with poorer toughness is easier to generate endurance crack and the crack is easier to grow. The crack would cause the base and the oxidation film which is on the base flake off together, and finally cause more wear. The small size carbonitride particles precipitated on the surface of hadfacing alloy under high temperature could reduce the amount of wear caused by abrasive wear.Effect of rare earth Ce on the fracture toughness and abrasive wear behaviour of hardfacing alloy was studied. Nb and RE oxide could refine the microstructure, thus the hardness of hardfacing alloy could also be improved. The hardfacing alloys microstructure in this research were composed of lath martensite, residual austenite and dotted carbides. With the addition of RE oxide, the size of columnar crystal was refined and carbides were distributed more homogeneously in the matrix. The addition of Nb has a certain effect on improving the toughness of hardfacing alloy, but not as effective as RE oxide. The addition of RE oxide not only could refine the microstructure, but also could purify the grain boundary and spheroidize the carbide, and all of these could contribute to improving the toughness of deposit. The wear resistance of hardfacing alloys with RE oxide was improved. Micro-cutting and micro-ploughing are the main abrasive micro-mechanisms.The improvement of hardness can decrease the depth of groove caused by micro-cutting, meanwhile, the improvement of fracture toughness can increase the resistance to plastic deformation and scratch, so that the amount of spalling lips and prows caused by microplugging is decreased. Thus the wear resistance of hardfacing alloys is improved.