Effect Of Different Quenching Rates On Microstructure And Mechanical Properties Of 60Si2CrVAT Spring Steel

60Si2CrVAT is an appointed spring steel of bogie spring for high speed train in China. The future development trend is high strength, light weight and high life. It is always the focus of research that how to improve the mechanical properties and service life of the spring steel. In this paper, the test for 60Si2 CrVAT spring steel heat treatment was conducted through the use of quenching medium with different cooling performance and utilizing DIL805A/D thermal simulation machine which could control cooling rate precisely during the quenching process. The change regulation of mechanical properties for 60Si2 CrVAT spring steel was investigated under the condition of different quenching rates. The effect of different quenching rates on the morphology and size of martensite sub structure, the carbide precipitation in microstructure was observed and analyzed by means of optical microscopy(OM),scanning electron microscopy(SEM), electron backscatter diffraction(EBSD), and transmission electron microscopy(TEM) techniques. The results showed:Strength and plasticity showed the trend of first increasing and then decreasing with the decreasing of quenching medium cooling performance after 60Si2 CrVAT spring steel quenching in PAG with different concentration and 15# mechanical oil and tempering at 440℃ and 480℃. When the spring steel quenched by using13%PAG with moderate cooling rate, can the best mechanical properties obtain. After tempering at 440℃, the size of martensite lath width increased with decreasing of cooling rate for quenching medium, which is the consequence of both low quenching rate coarsening martensite sub structure and the emergence of martensite lath merger,recovery. With the decrease of cooling rate of quenching medium, more fine carbides precipitated along martensite lath boundaries. Besides, compared with conventional oil quenching, the spring steel presented a better fatigue performance after quenching by 13%PAG with relatively faster cooling rate, and its fatigue limit was improved by near 70 MPa.In the high temperature region(900℃~300℃) of quenching process, when quenching rates going up from 10℃/s to 800℃/s,the strength and hardness increased first then decreased and increased again at last, and the plasticity rising first and then fall. At the quenching rate of 200℃/s, the strength and plasticity reach to the maximum, which owe to the thinnest nano-twin plate acquired at this quenching rate.The widths of martensite block and lath turn to narrow with quenching rate ascending,which indicated that martensite sub structure mainly affected by quenching rate in the high temperature region. In the low temperature region(300℃~room temperature),with quenching rates improving from 0.5℃/s to 100℃/s,the strength and plasticity increasing monotonically, the changing regulation of plasticity was complex, but the best plasticity appeared at 1℃/s of quenching rate. A large number of fine carbides precipitated along the boundaries of martensite block and lath when the quenching rate was 0.5℃/s, and the phenomenon gradually disappeared with the increasing of quenching rates, which indicated the self-tempering process of martensite mainly affected by quenching rate in low temperature region.