Experimental Study And Modeling Of Stress-induced Transformation In Low Alloy Mild Carbon Steel

Heat treatment is a complex solid phase transformation process, in which thetemperature, phase transformation, stress and strain are mutually coupled. The thermalstress in heat treatment is very difficult measured online. With the development ofcomputer technology, numerical simulation supplies a method in heat treatment’scontrol online. In the numerical simulation of heat treatment, temperature andphase transformation, temperature and stress, stress and phase transformation, thesethree coupling mathematical models should be built, also the thermal physicalparameters. Temperature effecting phase transformation is mainly performed ontransformation dynamics. Stress-induced Transformation contains the stress affectedtransformation kinetics and transformation induced plasticity.In this article, the mechanical parameters are measured firstly in low alloy mildcarbon steel42CrMo. Thermal physical parameters like modulus of elasticity, modulusof plasticity, yield stress, expansion coefficient are obtained in martensite, austenite,pearlite and bainite.Make a study on martensitic transformation dynamics and the influence of stresson martensitic transformation start point (Ms). The result implies that uniaxial stressinfluences the Ms point unobvious, but pressure can increase Ms point, tensile stress notso significantly.Establish the transformation dynamics of isothermal bainite transformation, andthe transformation dynamics parameters have a regular change with the temperatureincreasing. Because the bainite transformation is incomplete, there is retained austeniteafter transformation. To get the phase volume fraction, using XRD method, analyze theX-ray diffraction.The experimental criterions are founded for martensitic transformation plasticityexperiment. Using the experimental data, fit the martensitic transformation plasticityand stress into linearity, then the martensitic transformation-induced plasticityparameter is taken. For bainite transformation, continuous cooling and isothermaltransformation, the experimental criterions and data processing technique are founded.The transformation plasticity parameter in fixed quench-rate bainite transformation istaken, also in various temperatures in isothermal bainite transformation. The experiment results show that isothermal bainite transformation plasticity parameters in varioustemperatures levels are in linear distribution with temperatures.Based on Green-Johnson mechanism, the model gives prediction of transformationplasticity parameters in20%error rang compared with experiment results. The selectionof transformation plasticity dynamics function is studied to find the influence onsimulation of transformation plasticity strains. Based on isothermal bainitetransformation dynamics and Scheil’s superposition rule, the continuous cooling bainitetransformation and the radial strain are simulated in little cylinder samples. Somesuggestions are proposed to the application of Scheil’s superposition rule.