| 23CrNi3Mo steel is a typical carburized brazing tool steel with good tempering stability and hardenability.However,23CrNi3Mo brazing tool steel is subjected to complex loads such as impact,high-speed rotation,friction and wear during service.The complex stress environment requires that the brazing tool materials have reasonable hardness distribution,high fatigue strength and wear resistance.Low carbon brazing tool steel needs carburizing and low temperature tempering treatment to obtain excellent strength and toughness.Carburizing treatment can effectively improve the surface hardness and wear resistance of the drill steel,while tempering heat treatment can improve the toughness of the drill steel.Therefore,carburizing and low temperature tempering treatment can make the brazing tool steel have good strength and toughness,thus extending the service life of the brazing tool.In this paper,for 23CrNi3Mo carburizing steel,the changes of structure and hardness during gas carburizing,quenching and tempering are studied by means of thermal/dynamic calculation method and in-depth learning,and a"process structure property"relationship model is established,which provides a basis for the organization control and hardness prediction.According to the prediction results of the microstructure and hardness of the model,the carburizing process can be optimized,and the wear performance of the brazing tool in service can be improved.Therefore,according to the prediction results of microstructure and hardness,four groups of carburized layers with different carbon concentration gradients are selected to conduct friction experiments to study the role of residual austenite in the wear process under different carbon concentrations and the corresponding wear mechanism.In four groups of carburized layers with different carbon concentrations,one group of carburized layers with the most retained austenite was selected,and friction tests were carried out after different quenching and cooling rates to obtain the optimized quenching and cooling process and explore its wear performance and wear mechanism.Then,the minimum energy path of retained austenite induced transformation martensite is explored through the first principle,which lays the foundation for clarifying the mechanism of friction induced metastable retained austenite transformation.Finally,the influence of the size,morphology,volume fraction and surrounding phase distribution of metastable austenite in the carburized layer on the wear resistance was investigated.The mechanical stability of metastable austenite,the improvement of wear resistance by transformation martensite in the wear layer and the mechanism of crack nucleation and spalling were systematically expounded,which laid the foundation for designing an excellent carburizing process to improve the wear resistance.The main research contents of this paper are as follows:(1)Firstly,by adjusting the vacuum gas carburizing time(including the total carburizing time and the ratio of strong carburizing time to diffusion time),carburizing layers with different depths and different carbon concentrations can be obtained.After quenching at 860~oC,carburizing layers can obtain different microstructure gradients and hardness gradients.(2)Secondly,the deep learning algorithm is used to process the metallographic images to identify and calculate the residual austenite of the carburized layer so as to predict the hardness.The surface of23CrNi3Mo steel was treated by different carburizing processes.A large number of metallographic images of carburizing layers were collected based on laser confocal microscopy technology.The carburizing layer microstructure database(MCLD)was constructed for training and testing.The contents of residual austenite and martensite in the carburized layer were calculated based on the UNet-6 model,and then the mapping relationship between C content,martensite and residual austenite content and hardness was constructed by using the support vector machine(SVM),so as to predict the hardness.(3)Friction and wear experiments were conducted on P1,P2,P5 and P7of different residual austenite contents in the carburized layer to explore the influence of different residual austenite contents on the wear performance and the wear law.(4)Based on the first principles,the minimum energy path of tribation-induced martensitic transformation was explored.The K-S and N-W transformation paths of pure iron austenite and iron-carbon alloy austenitic transformation martensitic were calculated,and then the influence of martensitic transformation path on the formation of wear cracks and wear properties was explored.(5)Friction and wear experiments were carried out on the carburized surface layer with the same carbon concentration gradient and different structure gradient.The high carbon martensite and residual austenite in the carburized layers of P10 and P11 were refined by carburizing and quenching processes.During the friction and wear process,the fine high carbon martensite imposed several restrictions on the residual austenite,which played a shielding role,leading to its increased stability;Secondly,the size of retained austenite decreases,and the volume fraction of thin-film austenite increases,which further enhances the mechanical stability.The formation of nano twin martensite in the severely deformed layer of wear can effectively prevent dislocation movement,and store high-density dislocations near the twin boundary,thus improving the strength of the wear layer and reducing the wear rate.The carburizing and quenching processes P12 and P13 have high content of residual austenite in the carburized layer,and the austenite is mainly coarse and massive.During the friction and wear process,the hardness of the worn surface is improved due to the increase of induced transformation martensite,which can improve its wear performance in a short time.However,the newly formed irregular coarsened martensite has become the crack nucleation location.Because the stability of the coarsened residual austenite is low,the induced transformation will change rapidly,The content of retained austenite in the subsurface layer is reduced,and the role of retained austenite in preventing crack growth is significantly reduced during crack growth,resulting in an increase in flake wear debris and a decrease in wear resistance. |
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