| 75Cr1 as a high-performance diamond circular saw blade substrate steel,with high hardenability,high wear resistance and good mechanical properties are very widely used in stone processing,construction and mechanical engineering,mineral processing and energy and transportation industries.Due to the harsh service conditions,especially with the demands for increasing saw blade size and cutting speed,saw blades have to bear greater impact and frictions,therefore their vibration and stresses increase during working to result in failures such as fracture and tooth chipping which seriously affects its service life.Therefore,it is urgent to solve the key heat treatment process of domestic high-performance saw blades and improve the quality and market competitiveness of domestic saw blade products to meet the growing market demand.This subject aims to set up the relationship among heat treatment process,microstructures and mechanical properties of saw blade base material 75Cr1,and making comprehensive research on the mechanism of strength and toughness,so as to lay the foundation for the quality improvement and upgrading of domestic high performance saw blades.The CCT curve of continuous cooling transformation was simulated using JMatPro thermodynamic software,the phase transformation temperatures of the experimental steel were determined.The influence of chemical compositions on the transformation curve was analyzed.Relationship between the structure and mechanical properties of the experimental steel under different quenching and tempering processes was studied,which the grain size,carbide particle size and distribution under different heat treatment processes were characterized by means of OM and SEM.The microstructure,particularly grain sizes,carbide number and distribution on the effect of strength,toughness and wear resistance of the materials and their mechanisms were analyzed.Metallurgical qualities,microstructure and properties of a foreign saw blade sample were also analyzed,and try to explore its heat treatment processes.Effects of different original structures on the final treatment structures and properties were compared,and better heat treatment processes and properties were obtained for the experimental materials.The main conclusions are as follows:JMatPro simulations show that phase transformation temperatures of 75Cr1experimental steel are Ac1=735.1°C,Ac3=773.1°C,Ms=213.8°C.Full martensite can be obtained when the cooling rate is above 20°C/s.Cr,Mn,Si elements make the C curve moving to the right,Cr content increases Ac1 and Ac3 temperature which increasing hardenability;Si element increases Ac1 and Ac3 temperature and decreases MS point which meaning inhibit the transformation of bainite.C curve left shift with the carbon content increasing which also increasing Ac1 and Ac3 temperature,decreasing the temperature of Ms.Results of heat treatment process experiments show that structures of fine needle martensite and a small amount of residual austenite were obtained between 780?and840?quenching.With the increase of quenching temperature,the hardness increased slightly,all at the level of 63 HRC,the grain size decreased from grade 10 to grade 8,and the unevenness of grain size increased with the increase in quenching temperature.With the increase in tempering temperature,the structures changed from tempered troostite to tempered sorbite,and the fine granular carbides increased.The analysis of microstructure and mechanical properties show that austenite grain size gradually grows with the increase of quenching temperature which causes a decrease on effects of fine grain strengthening,and the strength,toughness and wear resistance decrease accordingly.Grains are very fine after quenching at 800?,so the effect of fine grain strengthening is obvious.After tempering from 480?to 540?,yield strengths are relatively high which being in the range of 900 MPa to 1300 MPa,and the impact toughness is better from 16 J to 20 J.The Fracture topographical features of tensile and impact specimens show a mixture of quasi-cleavage and small dimples.As rising the tempering temperature,the number of carbide particles increases,and the size increases slightly,and effects of precipitation strengthening increase first then decrease.The wear resistance is related to the hardness of the matrix and the number and size of the carbides dispersed on it.The 75Cr1sample has good properties of strength and toughness after quenching at 800°C,tempering at 540°C.Good strength and toughness and wear resistance are the result of the combined effect of fine-grained strengthening and toughening and dispersed carbides.Analysis of foreign saw blade samples and heat treatment process investigation show that its inclusions species class with class D(epoxide class)fine system i=1.5 grades,metallurgical quality is better than similar domestic materials.The tissue is fine tempered sowthite,the average grain size is 11.25?m,grain size 11,the structures is more uniform and fine,and the hardness is 36.4 HRC.The amount of carbide in the matrix of the sample is approximately 23.3%,with fine rounded particles diffused distribution on the matrix.Comparing the heat treatment process and the mechanical properties of the experimental material,grain size,carbide quantity and distribution and hardness of the 75Cr1 sample quenched at 800°C then tempered at 540°C are basically comparable to those of the foreign saw blade samples.The optimized final heat treatment process is quenching at 800?,then tempering at540?which fine sorbite microstructure was obtained with small carbides distribution on the matrix of hardness 36.5 HRC?The optimized final heat treatment process of saw blade substrate after quenching and tempering pretreatment,quenching and tempering hardness are all increased by about 1HRC,the grain is smaller,the grain size is increased by one level,and the amount of carbide on the substrate is slightly increase to 26.1%,and the impact toughness and wear resistance of the sample have been improved. |
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