Research On Heat Treatment Optimization And Microstructure And Properties Of Spring Steel For Urban Rail

With the rapid development of economy and the continuous improvement of the speed of urban rail trains,the spring performance needs to be further improved.Although China's spring steel has made great progress after years of development,compared with foreign spring steel products,there is a huge gap between the spring steel grade and the purity and comprehensive performance.Quality spring steel also needs to be imported to meet demand.One of the ways to solve the above problems is to optimize the heat treatment process system under the existing spring steel grades in China to improve the overall performance of spring steel.Based on the 51CrV4 spring steel,the influence of quenching process and tempering process on the microstructure and properties of spring steel was studied systematically,and the heat treatment process parameters were optimized.The effects of uniform temperature,quenching temperature,temperature equalization time,oil temperature,tempering cooling mode,out-oil temperature,tempering temperature,tempering cooling time shortening,tempering time,and other heat treatment parameters on the microstructure and properties of 51CrV4 spring steel were investigated..The main results are as follows:(1)The increase of the uniform temperature will increase the retained austenite content of the spring steel.The coarse austenite grain is favorable for the formation of lath martensite,and finally the austenite grains will be homogenized,thereby improving the spring steel.Strength and plasticity.With the increase of the uniform temperature holding time,the solid solubility of alloying elements increases,the austenitizing is more complete and the stability is higher.The carbide size becomes coarser and shorter with the increase of the average temperature time,and the number of long-shaped carbides Reduced,increased granular carbide.51CrV4 spring steel is subjected to heat treatment experiments with a temperature equalization temperature of 880?980? and a uniform temperature holding time of 20?40min.The heat treatment system with the best strength and plasticity is:the uniform temperature is 980?,and the uniform temperature is 40 min.(2)The microstructure after quenching is martensite.As the quenching temperature increases,the martensite size of the lath begins to coarsen and then begins to decrease.As the heating process progresses,the austenitizing temperature increases.The rate of grain growth of the austenite becomes very fast,and the rate of grain growth increases,which leads to a larger grain size,which causes the martensite structure to gradually become coarser as the quenching temperature increases..And as the quenching temperature increases,more carbon atoms are dissolved into the austenite,resulting in an increase in the stability of austenite,which increases the difficulty of converting to martensite.However,when the quenching temperature reaches 870?,the lath martensite size has a smaller size and the distribution is more compact and uniform.There are shear lips around the spring tensile fracture of different oil temperature and out-oil temperature.The fracture morphology is mainly dimple,which is a ductile fracture.The 51CrV4 spring steel is subjected to a quenching heat treatment process.The optimum quenching heat treatment process parameters are:quenching temperature 870?,oil temperature 20?,and out-oil temperature 90?.(3)The tempering structure under the tempering heat treatment process is tempered troostite.With the increase of tempering temperature and the tempering time,the martensite begins to decompose,the martensite content decreases,and the carbide begins to grow.This leads to a decrease in strength and hardness and an increase in plasticity.(4)The cooling methods after tempering mainly include air cooling and water cooling.The finer dispersion of ferrite and cementite in the tempered structure under water cooling is conducive to the improvement of mechanical properties.By comparing and analyzing the microstructure and properties of the air-cooled and water-cooled samples of the 51CrV4 spring steel after tempering,the 51CrV4 spring steel adopts water cooling to improve its strength and plasticity.(5)A small amount of martensite appeared in the microstructure of the spring steel when the tempering time was 30 min,indicating that the tempering was insufficient and the martensite was not completely decomposed.When the tempering time rises to 45 min,the martensite content is significantly reduced and the carbides are gradually increased.When the tempering time is 60 min or more,the tempering structure is all tempered troostite with uniform tempering time,and the carbide distribution is uniform.Shortening the tempering time will increase the strength and hardness of the 51 CrV4 spring steel,but this will reduce the plasticity of the 51CrV4 spring steel.The 51CrV4 spring steel is subjected to a tempering heat treatment process.The optimum tempering heat treatment process parameters are:tempering temperature 420?,tempering time 60 min,tempering cooling mode is water cooling.