Study On The Heat Treatment And Microstructure And Properties Of 5Cr8MoNi2SiV Hot Work Die Steel

In this paper, we analyze the hot-work die steel 5Cr8MoNi2SiV which is used as the material of high strength steel plate stamping forming. Firstly, we use the thermodynamic software Thermo-calc and Jmatpro to simulate the equilibrium phase diagram and CCT of the steel, respectively. Then, we can derive the parameters of heat treatment process. Secondly,we use hardness test, impulse test, XRD analysis, SEM and energy spectrum analysis to analyze the influence that the heat treatment process has made on the structure and property of the steel. Thirdly, we use AES to derive the compositional variation of the grain boundary in the different tempering temperature and aging time, then, we analyze the reason of the brittle fracture of the steel along the grain boundary. Finally, we analyze the high temperature performance of the steel.Depending on the above test. we derive that 5Cr8MoNi2SiV is the Markov shape hot-work die steel. The steel has excellent hardcnability, and is very stable for supercooled austenite.The phenomenon of carbide precipitation is not obvious. So we keep isothermality in the pearlite transformation zone for a long time to realize the carbide spheroidizing. In the process of spheroidizing annealing, austenitizing temperature, isothermal temperature and isothermal time have great influences on carbide spheroidizing. We can derive the carbide spheroidizing based on the best process of the isothermal spheroidizing annealing. This process has a great improvement of as-cast segregation, and improves the toughness of the steel at the same time.The types of carbide are mainly about the carbide of Cr, Mo and V after annealing. We find that this steel has secondary hardening and high temperature tempering brittleness during the drawing process. The peak value of secondary hardening and high temperature tempering brittleness occurs at 530?. The main reason of intergranular brittle fracture is the increase in hardness. The carbide of Cr, Mo, V and impurity element P segregates along the grain boundary lead to high temperature tempering brittleness and the decrease of grain boundary binding force. The different concentration of Cr and Mo is caused by the alloy carbide precipitation under different tempering temperature and aging time. This phenomenon leads to the variation of P and the difference of fracture characteristics. In the state of short ageing, the increase of the concentration of P is caused by the precipitation of carbide Mo. In the state of over ageing, the increase of the concentration of P is caused by the precipitation of carbide Cr.Finally, through the test of room temperature mechanical properties, high temperature thermal stability and high temperature oxidizing, we derive the best heat treatment process of 5Cr8MoNi2SiV after isothermal spheroidizing: oil quenching 1120? and tempering 450?.Thermostability and high temperature oxidizing are related to the state of ageing. It is still keep the state of short ageing after tempering 450?, and keep the state of over ageing after tempering 620?.Thermostability and high temperature oxidizing in tempering 450? are superior to those in tempering 620? by comparison.The performance of 5Cr8MoNi2SiV can meet the requirements of operating conditions after the process of strong heat treatment. The steel is with high hardness and toughness, and are equipped with good thermal stability and high temperature oxidation resistance.