| C-Si-Mn-Cr steels have a multiphase microstructure dominated by bainite,martensite,and retained austenite.It can control the microstructure and wear resistance through the heat treatment process,so it is widely used in mining and machinery fields.At present,the microstructure of C-Si-Mn-Cr steel can be controlled by quenching-tempering,quenching-partitioning,quenching-partitioning-tempering and other heat treatment processes.However,quenching medium such as oil is usually used in the heat treatment process.There are some problems include large energy consumption,solid waste,and high cost.The water-air alternating cycle control cooling heat treatment process uses water and air as the cooling medium,which can be controlled by the quenching process to achieve controllable microstructure in an economical,pollution-free,and solid waste-free manner.Besides,it has the advantage of preventing cracking caused by uneven heating.The related research on the regulation of microstructure and evolution mechanism through processes is still relatively small and not in-depth.This work takes the 0.4C-1.6Si-2Mn-0.6Cr steel as the experimental materials,and uses the water-air alternate cycle to control the cooling process.The microstructure is studied in steel under the control of isothermal temperatures(240℃,285℃,330℃),isothermal times(5 min～145 min),water quenching times(15 s,20 s,25 s)and circulation times(0,1,2,3)during the different cooling processes.The form,phase composition and evolution of the microstructure are analyzed,and the influence of the microstructure on the performance is clarified,and the wear mechanism of steel is revealed.This work has an important theoretical guidance and practical meaning for the formulation of heat treatment processes and production applications of related wear-resistant materials.Heat treatment was used in 0.4C-1.6Si-2Mn-0.6Cr steel by the water-air alternating cycle control cooling processes,the following conclusions are obtained:the multiphase microstructure include bainite,martensite and retained austenite can be obtained in the water-air alternating cycle control cooling processes,but the composition of the microstructure is various in different processes conditions and positions.As the isothermal temperature increases,the content of bainite and retained austenite increases with the martensite content decreases gradually.The content of bainite decreases slightly from the core to the surface,while the martensite gradually increases.With the increase of isothermal time,the content of bainite gradually increases to the maximum of 46.5%,and the bainite transformation rate gradually increases from the core to the surface.With the increase of water quenching time,the change of bainite content is less than 3.1%,and the martensite increases from 39.7%to 47%.With the increase of circulation times,bainite,martensite and retained austenite of steel all increases,and gradually increases along the core to the surface under air cooling after water-air alternating.However,the content of bainite and retained austenite are increased,and martensite is the opposite under water cooling after water-air alternating.The content of bainite in steel decreases gradually,and martensite and retained austenite increase from the core to the surface.With the help of XRD,OM,SEM,EPMA,EBSD,TEM and the other technologies,the microstructure evolution mechanisms of 0.4C-1.6Si-2Mn-0.6Cr steel under water-air alternating cycle control cooling processes were studied.The results shows that the driving force of phase change gradually increases when the temperature decreases by controlling cooling due to the different cooling rates of the steel in water and air,and the bainite nucleation rates increases.As the water-air alternating progresses,it is beneficial to improve the uniformity of the core and surface temperatures distribution of the steel.The bainite begins to grow and the content gradually increases due to the temperature of the steel reachs the bainite transformation zone.When the temperature continues to decreases,the bainite and martensite of multiphase microstructure are obtained.The difference of cooling rate at different positions leads to different morphology and phase composition of the microstructure under the same process conditions.Combined with numerical simulation calculations and experiments,it is realized that the transformation laws of the microstructure can be controlled by adjusting the process parameters.After the alternating water-air circulation control and cooling,the relationship between the content of bainite transformation and the isothermal time and position during the isothermal treatment of the steel is obtained.The performance is tested under different cooling conditions,the performance results of 0.4C-1.6Si-2Mn-0.6Cr steel are obtained as following:(1)As the isothermal temperature increases,the hardness value of steel decreases,while the impact toughness value gradually increases.The hardness and toughness of the steel first increase and then decrease with the increasing of the isothermal time.The maximum tensile strength of the steel reached 1184 MPa when the water quenching time is increased to 25 s.(2)With the increase in the number of water-air alternating cycles,the strength of steel increased from 831 MPa in air quenching to 1281 MPa after three cycles.Hardness is increased from 25 HRC to 46 HRC,and the impact toughness reached 50 J/cm~2 under the alternate cycle and then air cooling conditions.The strength and toughness of steel gradually is increase,but the hardness decreases under the alternating cycle and then water cooling conditions.The difference in mechanical properties is caused by the different phases composition and content of the bainite,martensite and retained austenite formed during the water-air alternating process.(3)The surface morphology of 0.4C-1.6Si-2Mn-0.6Cr steel under impact abrasive wear is mainly cracks,spalling,micro-cutting and ploughing.As the number of cycles increases,the bainite and retained austenite act as tough phases to slow down the stress concentration and hinder the initiation and propagation of cracks.After repeated cooling by alternating water and air cycles,a fine bainite and martensite multiphase microstructure in steel was formed,which has the effect of fine grain strengthening.The retained austenite undergoes plastic deformation during the impact abrasive wear process,resulting in work-hardening and transforming into martensite,so that the steel has high wear resistance. |
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