Modification And Mechanism Of Nano-Y₂O₃ On Fe-Cr-C-N-Al Wear-Resistant Surfacing Alloy

Fe-Cr-C surfacing alloys are widely used in surface engineering technology for their excellent wear resistance and low cost.The primary M₇C₃ carbide with high hardness in the microstructure is the main factor to ensure the wear resistance of the surfacing alloy.However,during the wear process,the coarse primary M₇C₃ carbide is easily exfoliated from the matrix and can also become a cracking source,leading to cracks initiating in the alloy.This will ultimately deteriorate the wear resistance of the material and accelerate the wear of mechanical components.Therefore,it is necessary to carry out research on the compositions optimization of Fe-Cr-C surfacing alloys,in order to improve the wear-resistant performance of surfacing alloys.This has a practical application value for the development of potential application fields of wear-resistant surfacing alloys,which is of great significance and economic benefits for promoting the development of surface engineering technology in our country.In order to improve the wear resistance of Fe-Cr-C hardfacing alloys,Fe-Cr-C-N-Al and Fe-Cr-C-N-Al-Y self-shielded wear-resistant surfacing flux-cored wires were independently developed in this paper.The welding process was optimized by numerical simulation technology.The surfacing alloy was prepared on the surface of Q235 base metal by open arc surfacing.The effects of different Al and Y contents on the microstructure evolution of surfacing alloy were analyzed by XRD,OM,SEM,EDS,EBSD and TEM.The effects of Al and Y with different contents on the mechanical properties of surfacing alloys were studied by hardness test and abrasive wear test.The bonding interface relationship of Al N/Y₂O₃ was calculated by first-principles method,and the effectiveness of Y₂O₃ as a heterogeneous nucleation sites for Al N was analyzed.The main results are as follows:The plate surfacing welding model and the welding heat source model were established,and a single pass surfacing alloy was welded to verify the accuracy of the finite element model.The comparsion of the simulation results and the experimental results of the single-pass surfacing alloy indicateed that the two surfacing layers possesseed the same melting depth and the similar morphology.The simulation results and the experimental results of thermal cycle curves were aggrement with each other.The numerical calculation results of residual stress were also basically consistent with the measured results.This confirms the accuracy of the simulation results in the temperature stress distribution of the surfacing alloy.The temperature field and stress field of 2-layer 4-pass plate surfacing under 16 sets of process parameters were simulated.After comparative analysis,it was found that the higher the welding heat input,the greater the residual stress,the higher the peak temperature of the temperature field,the longer the cooling time.In the simulation results,the minimum peak temperature of scheme 13 was1683°C,the minimum longitudinal residual stress was 184Mpa,and the minimum transverse residual stress was 101 Mpa..Therefore,the optimized optimal process parameter was Scheme13,corresponding to the following welding parameters:welding voltage U=30V,welding current I=150A,and welding speed v=0.015 m/s..The final solidification microstructure of Fe-Cr-C-N-Al surfacing alloy was composed of M+γ-Fe+M₇C₃+Al N.The Al N particles were small in size,dispersed in the matrix of the surfacing alloy,and partially distributed at the austenite grain boundaries.The two-dimensional lattice mismatch calculation of Al N（120）//Fe₃Cr₄C₃（001）showed that the mismatch was 9.452%,less than 12%,which proves that Al N was moderately effective as a heterogeneous nucleation particle of M₇C₃.When the Al content reaches 3.77wt.%,the Al N precipitation amount was the largest and the microstructure distribution was the most uniform.At this time,the highest hardness value was 56.4HRC;the minimum wear weight loss was0.198g,and the wear resistance was the best.Under this content,the surface furrow depth of the surfacing alloy was the smallest and discontinuous,and there were no spalling pits and cracks.The wear mechanism of the surfacing alloy was mainly plastic deformation mechanism,and the material removal process was micro-cutting and ploughing.This was because with the increase of Al content in the surfacing alloy,the precipitation amount of Al N hard phase increased,the microstructure grains were gradually refined,and the precipitation amount of M₇C₃ eutectic carbides increased.The in-situ self-generated Al N precipitates had higher hardness and could play a role in precipitation strengthening.Under the synergistic effect of these strengthening mechanisms,the mechanical properties of the surfacing alloy were improved and the wear resistance was significantly enhanced.Different contents of nano-Y₂O₃ were added to Fe-Cr-C-N-Al surfacing alloy.It was found that when the content of nano-Y₂O₃ was 0.456wt.%,the surfacing alloy had the best formability and the smallest wetting angle of 50.8°.Due to the addition of nano-Y₂O₃,the electrical conductivity and the thermal conductivity of the metal in the melting pool were reduced,thereby reducing the surface tension of the molten metal,decreasing the wetting angle and improving the weldability.At this time,the precipitation of Al N hard phase and M₇C₃eutectic carbide was the most,and the grain size was the smallest.Compared with no nano-Y₂O₃,when the content of nano-Y₂O₃ is 0.456wt.%,the hardness value of the surfacing alloy was up to 62.3HRC,and the Rockwell hardness was increased by 11.8%.The minimum wear weight loss was 0.125g,and the wear resistance was increased by 41.86%.As the heterogeneous nucleation core of Al N hard phase,nano-Y₂O₃ promoted the precipitation amount of Al N,thereby increased the number of heterogeneous nucleation cores of M₇C₃eutectic carbides.A large number of Al N hard phases and M₇C₃ eutectic carbides constituted the wear-resistant skeleton,which increased the wear resistance of the surfacing alloy.After studying the abrasive wear mechanism,it was found that when the external conditions were constant,the wear resistance of the surfacing layer was not only affected by the hardness,the type of the matrix structure and the size,number and distribution of the secondary phases also play a decisive role in the wear resistance of the surfacing alloy.The effectiveness of Y₂O₃ as the heterogeneous nucleation site for Al N was analyzed by first-principles.The calculation results showed that the mismatch between Y₂O₃（100）crystal plane and Al N（100）crystal plane was only 1.5%,and Y₂O₃ had the basic crystallographic conditions as the heterogeneous nucleation particle of Al N.The O-Al interface had the largest binding work W_ad=12.112J/m² and the smallest interface energyγ_O-Al=-9.665J/m².This indicated that when Y₂O₃ was used as the nucleation substrate of Al N,the Y₂O₃（100）/Al N（100）interface is more inclined to the O-Al interface.The results of differential charge density and electron localization function analysis at the Y₂O₃（100）/Al N（100）interface showed that the O-Al interface was easy to form and had high stability due to the formation of strong ionic bonds between O atoms and Al atoms near the interface.The intense bonding effects between O and Al atoms accelerated Al N nuclei attachment to Y₂O₃,leading to continuous precipitation of Al N in the matrix,which significantly improving the wear resistance of the alloy.