Study On Bonding Property Between Fe-C-Cr-Nb Surfacing Alloys And 45 Steel Base Metal For Cement Squeeze Roller

As the foundation of China’s national economy,the cement industry has comprehensive advantages in labor force,resources,and technology in international competition and has a good prospect for development.In the cement production process,the roller press is an important mechanical device.The core component of the roller press is the squeeze roller.The main failure mode of the cement squeeze roller is fatigue wear.When the bonding perperty between the surfacing alloy and the base metal of the squeeze roller is poor,it is easy for the surfacing alloy to peel and shed,thus the fatigue wear occurs.Therefore,it is necessary to study the bonding property of the surfacing alloy to the base metal of the squeeze roller.Three kinds of commercially available flux-cored wires and several kinds of self-made self-shielding Fe-C-Cr-Nb flux-cored wires with different high carbon ferrochrome and graphite additions were used to prepare several kinds of surfacing alloys on the surface of 45steel base metal and the compositions,microstructures,hardness and wear resistance of the surfacing alloys were studied.A new type of lifting-up and pulling-down test method was designed and the bonding strengths between the surfacing alloys and the 45 steel base metal were tested using this method.The fracture location was in-situ observed and the fracture mechanism was analyzed and summarized.The mechanical properties of all areas of tensile pecimens were calculated using the Jmatpro microstructure and property simulation software.Combined with the finite element analysis method,the stress analysis of the specimens were operated.The necessarily theoretical basis and test data were provided for improving the properties of the surfacing alloys.The main results of this article are as follows:（1）The compositions,microstructures and wear resistance of commercially available surfacing alloys were studied,and the wear resistance of commercially available surfacing alloys is related to the microstructures of the alloys.The trend of wear resistance is consistent with the trend of surface hardness.The surfacing alloys both alloyed steel strengthened by niobium carbide and high-chromium hyper-eutectic alloyed cast iron have a small difference in hardness and the wear resistance.The bonding strengths between commercially available surfacing alloys and 45 steel base metal were tested by push-away test.The bonding strengths of austenitic steel,alloyed steel strengthened by niobium carbide,and high-chromium hyper-eutectic alloyed cast iron to 45 steel base metal were 316MPa,241MPa and 60MPa in sequence.The bonding strength is directly related to the composition and microstructure near the fusion zone and is determined by the microstructures of the bottom of the surfacing alloy,the fusion zone and the overheated zone of the HAZ.（2）A lifting-up and pulling-down test method method and the specimen for the test which can be used to test the bonding property between surfacing layers or between surfacing alloy and steel base metal were proposed.In-situ observation and analysis of specimens can be performed to determine the fracture process,fracture properties,and fracture mechanism in this method.And the effect of metallurgical factors and process factors on bonding property of surfacing alloys and base metal were studied.The bonding strengths of three commercially available surfacing alloys and 45 steel base metal were measured to be 549MPa,314MPa,and 170MPa,respectively.The bonding strengths of both alloyed steel surfacing alloy strengthened by niobium carbide and high-chromium hyper-eutectic alloyed cast iron to the transition layer（austenitic steel）were351MPa and 233MPa in order.The bonding strength mechanisms of three different types of commercially available surfacing alloys to 45 steel base metal are different.The cracks initiated in the fusion zone on both sides of the lifting-up and pulling-down test specimen in austenitic steel,and the fracture occurred at the bottom of the surfacing alloy in the middle of the specimen,forming a mixed fracture area containing the fusion zone and the bottom of the surfacing alloy.The cracks initiated at the bottom of the surfacing alloy on both sides of the specimen and propagated inward in alloyed steel strengthened by niobium carbide,with the secondary cracks formed,the fracture occurred in several directions at the same time.The secondary cracks propagated into the fusion zone and the HAZ,finally forming a mixed fracture area containing HAZ,fusion zone and the bottom of the surfacing alloy.The cracks initiated at the fusion zones of the surfacing alloy on both sides of the specimen in high-chromium hyper-eutectic alloyed cast iron and the cracks extended along the austenite layer and hypoeutectic interface.When the cracks finally met,the overall fracture occurred and the fracture area of the fusion zone was formed.（3）The effects of different additions of high-carbon ferrochrome on the compositions,microstructures and wear resistance of Fe-C-Cr-Nb surfacing alloys were studied.With the additions of high-carbon ferrochromium increasing,the contents of C and Cr increased.The amount and size of precipitated NbC which was synthesized in-situ increased.The mixed microstructures of martensite and retained austenite decreased and austenite gradually increased.The additions of high-carbon ferrochromium significantly improved the wear resistance of the surfacing alloy.When the addition of high-carbon ferrochrome was 5%,the wear resistance of the surfacing alloy reached the best,which was up to 5.7 times of 45 steel.The average microhardness of the surfacing alloy was 850HV_0.2.The bonding strengths between the surfacing alloys with different contents of C and Cr to 45 steel base metal were tested by the lifting-up and pulling-down test method.And the influence of the microstructure change in the fusion zone on the bonding strength was analyzed.The bonding strength shows a U-shaped curve distribution.When the contents of C,Cr and other elements at the bottom of surfacing alloy are low,the mixed-structure martensite and retained austenite show good properties,the cracking effect is not significant because of small size of NbC.The propagation to the bottom of the surfacing alloy of the crack was blocked after initiation.Therefore,the crack propagation direction turned to the interface between the fusion zone and the overheating zone.So the bonding strength is high（381MPa）.With the addition of high-carbon ferrochromium increasing,higher contents of C at the bottom of the surfacing alloy resulted in the formation of large size of NbC with a strong fragmentation effect and the crack propagating toward the bottom of the surfacing alloy after initiation.The bonding strength decreased（320MPa-267MPa）.Much higher contents of C and Cr will lead to the existence of independent austenite,which enhanced the plasticity and toughness of the bottom of surfacing alloy,although with large size of NbC.The propagation to the bottom of the surfacing alloy of the crack was blocked after initiation.The crack propagation direction turned to the fusion zone and overheating zone and the bonding strength was improved（418MPa）.On the whole,the bonding strengths showed a U-shaped curve.（4）The effect of graphite addition on the compositions,microstructures and wear resistance of Fe-C-Cr-Nb surfacing alloys was studied.When the contents of C in the surfacing alloys prepared with flux-cored wires with different graphite additions increase,the precipitated NbC particles increased and the size of the particles increased from 0.1-1μm to 0.5-5μm.The wear resistance was related to microstructure and NbC particle size.The wear resistance was consistent with the trend of surface hardness.When the amount of graphite addition is 3%,the matrix was mainly composed of martensite and retained austenite.The hardness of surfacing alloy is 61.8HRC and the wear resistance arrived to the highest level.The lifting-up and pulling-down test method was used to test the bonding strength between the surfacing alloys with different content of C and 45 steel base metal.The bonding strength decreased first and then increased,There was also a U-shaped change overall.（5）According to the phase transformation conditions of the interface area between the surfacing alloy and the 45 steel base metal,the mechanical properties of the tensile pecimens were calculated using the Jmatpro microstructure and property simulation software.Combined with the finite element analysis method,the specimen fracture can be judged to be the brittle fracture with small deformation.The material is in the range of elastic deformation.The failure occured at the bottom of the surfacing alloy near the fusion zone,which was consistent with the actual test results.