Research And Numerical Simulation Of Heat Treatment Process Of 5CrNiMoV Hot Work Die Steel

Hot work dies are widely used in various industries and are high value-added technology-intensive products.5CrNiMoV is a commonly used steel for the manufacturing of hot work dies.Hot work die forgings are required to be heat treated to warrant their properties.Numerical simulations of heat treatment process es provides a basis of the intelligent manufacturing of steel products.It is imperative to quantify the microstructure transformation characteristics of the steel and to develop kinetic models of phase transformations and the interactive relations between phase transformations and stresses at various temperatures.Thermal physical properties of the steel corresponding to its microstructure are also required for the numerical simulation of the heat transfer in the heat treating processes.This thesis presents the research of the microstructure transformation characteristics of 5CrNiMoV steel during heating and cooling utilizing DIL 805 ADT dynamic transformation thermal dilatometer,optical microscope,scanning electro n microscope,TTRAX?X-ray diffractometer,and other experimental instruments.Kinetic parameters of different mi crostructure transformation models are obtained from the experimental data.Effects of martensite transformation on plasticity and stress on martensite transformation are quantified.Experimental measurements using differential scanning calorimetry and laser flash diffusivity apparatus are combined with calculations using JMat Pro to acquire thermal physical properties of the5CrNiMoV steel.The end-quench test is carried out using a self-made apparatus and the process is numerically simulated utilizing a finite element heat transfer model with commercial FEA code ABAQUS which integrates phase transformation models in the form of user subroutintes coded in FORTRAN laguange.Microstructural examinations and hardness measurements are conducted on the end-quenched bar.Predicted microstructure and hardness in the end-quenched bar from the numerical simulation are found in good agreement with the exper imcal observations.Consequently the numerical model for the heat treatment of 5CrNiMoV steel is validated and can be extended to simulation the actual heat treatment process es of5CrNiMoV steel hot work dies.For the austenitizing process,the expansion data of 5CrNiMoV steel heated to austenitizing temperature at different heating rates of 0.05K/s to 50K/s are analyzed and are fitted in to the.Johnson-Mehl-Avrami(J-M-A)model for non-isothermal diffusion transformation using the Kissinger method to obtain the kinetic parameters of austenitization process.It is found that the activation energyis 1.29?10⁶ J/mol,exponent9)is 1.054,and ln6)₀ is 7.8×10^{6 3} for the austenitization of 5CrNiMoV steel.Calculated amounts of transformed austenite utilizing the J-M-A model with the given kenetic parameters are in good agreement with the experimentally observed.The model can be used to predict non-isothermal as well as isothermal as austenization of 5CrNiMoV steel dies and to design austenitization process in engineering practice.For the isothermal transformation of bainite,the experimentally measured dilation data between 245?and 500?are analyzed and fit ted into the J-M-A isothermal transformation model to determine the kinetic parameters.TTT curves are plotted.Shortest incubation time of bainite transformation in 5CrNiMoV steel is found to be around 320?.Maximum amount of bainite transformed at different temperatures are determined from the expansion data.For the continuous cooling transformations,dilations of 5CrNiMoV steel cooled to room temperature at the cooling rates between 0.01?/s to 10?/s are experimentally measured.The critical cooling rate of martensite transformation is found between 0.1?/s and 0.2?/s and the average value of the martensite transformation temperature is 238?.Bainite transformation occurs between 238?and 450?.The transformation parametersandin K-M equation for the martensite transformation are 0.0167 K^-1 and 231.15?respectively.and the5CrNiMoV steel martensite transformation kinetic equation was established.Then the Li model is modified from the CCT curve to establish the kinetic equation of bainite transformation under continuous cooling conditions.It accounts for the variations of steel chemistry on continuous cooling phase transformations.For the interaction between stress and martensite transformation,kinetic constantsandin the Koistinen-Marburger equation are simultaneously determined along with the transformation plasticity coefficient6)in the Greenwood-Johnson mechanism from the dilation data of martensite transformation in 5CrNiMoV steel under stresses from-80MPa to+80MPa.The value of transformation plasticity coefficient6)fluctuates with the stresses,but approaches a fixed value at 8.092?10^-5MPa^-1.So determined value of t ransformation plasticit y coefficient is compared with the calculated values using the Greenwood-Johnson model and the Leblond model.The Leblond model is mo re in line with the test result.Thetemperature increases slightly with the applied stress,but the effect of applied stress with magnitude less t han 80 MPa is not significant.The value ofunder tensile stresses is slightly higher thanunder no stress,andunder compressive stresses is slightly lower thanunder no stress,indicating that tensile stress promotes transformation and compressive stress hinders transformation.Specific heat and thermal conductivity of different micros tructures of 5CrNiMoV steel are determined using STA449 F3 synchronous thermal analyzer and Netzsch LFA-457 laser flash diffusivity apparatus and the corresponding data are fitted to a quadratic function that changes with temperature.The density and heat transfer coefficient.The JMat Pro software was used to calculate the densi ty at different temperature.Heat transfer coefficient of the end-quenching is from published data in the open literature.The end-quenching test results show that 5CrNiMoV steel has good hardenability.The prelimina ry exploration of the simulation of the austenitization process basically conforms to the objective situation.