Papers by Keyword: Transformation Plasticity

Abstract: In a previous study, we showed the anisotropy of plastic strain due to the pearlitic transformation and proposed a hydrostatic pressure-dependent constitutive equation to describe this phenomenon. In the present study, we assess the validity of this model using a bending-tensile loading system to experimentally and numerically analyze and characterize the pearlitic transformation plasticity. First, the maximum bending deflections due to the austenite-pearlite transformation were measured under different loadings and then transformation-plasticity coefficients were determined. Furthermore, as was done for bending-tensile loading tests, the pearlitic transformation plasticity was simulated using Abaqus Standard under the same austenitization and loading conditions as in experiments, and the calculated results for pearlitic-transformation plastic deformation are compared with the experimental results. The results show that the transformation plastic deflection due to the pearlitic transformation decreases with increasing applied tensile stress. In addition, this behavior can be described by a hydrostatic pressure-dependent model in large-deformation theory.

Abstract: In this paper, in order to study the transformation plasticity behavior of steel materials in heat treatment and other heating manufacturing process, experiment and simulation of heat treatment for SUS420J2 were carried out. With the conditions of the experiments and simulation, SUS420J2 steel was heating to the austenite transformation temperature and stress is loaded when the steel rapidly cooled to the martensitic transformation start temperature. The transformation plasticity behavior and influence in the load stress of the heat treatment process are revealed.

Abstract: During phase transformation of steels, when stress is applied, significant large strain can be observed even though the applied stress is smaller than their yield stresses. This phenomenon is called Transformation Plasticity or TRansformation Induced Plasticity (TRIP). Transformation plasticity is known to play an important role during steel producing processes. Although its importance, the phenomenon is not fully understood because of complicated coupled effect of metallurgical, thermal and mechanical behaviour during phase transformation. There are several explanations which account for the phenomenon. Among those, Greenwood-Johnson effect appears to be appropriate explanation especially for diffusive phase transformation. According to Greenwood-Johnson effect, volume change during phase transformation causes locally heterogeneous stress variation and it results in the macroscopic strain together with small applied stress. Along with the notion, Leblond et. al. developed an analytical model which describes well the phenomenon of transformation plasticity. On the other hand, the authors have developed a micromechanical model of polycrystalline materials using discrete FFT (Fast Fourier Transform) method with diffusive phase transformation. In this study, volume expansion along with phase transformation (Greenwood-Johnson effect) is taken into account in the model in order to evaluate the transformation plasticity and micromechanical behaviour during phase transformation. The results by FFT confirm linear relation between applied stress and transformation plastic strain, only if the applied stress does not exceed a half the value of yield stress of the parent phase. In contrast, if applied stress is relatively large (more than half of yield stress of weaker phase), the linear relation is never satisfied. The numerical results are compared with those of experimental and of Leblond model. Furthermore, pre-deformation (deformation just before phase transformation) effect on transformation plasticity is investigated. As a first step, uniaxial tensile followed by phase transformation simulation is carried out. Back stress develops in the course of tensile process and thus the material will be macroscopically anisotropic. It is found that the pre-deformation causes anisotropic dilatation during phase transformation. The mechanism of this anisotropic dilatation will be discussed in the micromechanical point of view.

Abstract: A tensile/compressive-torsional biaxial testing system was employed and tensile/ compressive-torsional tests were performed for the hollow specimen, which was loaded and the austenized specimen was cooled so that pealrite transformation accompanied by transformation plasticity occurred and axial and torsional strain were measured. Furthermore, the elastic-plastic constitutive equation due to phase transformation based on the hydrostatic pressure dependent model was proposed, and the validity of this equation was discussed experimentally. The test results showed the transformation plasticity coefficient due to pearlitic transformation of S45C depends on the loading direction, and these behaviour can be appropriately expressed by the hydrostatic pressure dependent model than the isotropic model.

Abstract: Molecular dynamics simulations were carried out to clarify the atomistic mechanism of transformation plasticity. As the first step for the purpose, a simple thin-film model consisting of 8640 atoms was prepared. Phase transformation was assumed to be expressed by switching the material parameters of Lennard-Jones potential function. As a preliminary calculation, phase transformation was forced to occur homogeneously in the whole region of the model, resulting in no extra strain except volumetric transformation dilatation. In that case, perfect single crystal structure was maintained in the new phase. Simulations were carried out under external load, but specific strain was not generated. On the contrary, when the transformation region was set partially in the model and the region was expanded with time, a large deformation was observed. In the middle process of the phase transformation, slip-like deformation behavior and the change in crystal orientation occurred, indicating that extra plastic strain was induced during phase transformation. The strain was observed even when external load is not applied, and hence it was concluded that not only external load but also local stress distribution may cause the transformation plasticity.

Abstract: Three-point bending system with one end simple support and the other end fix support has been proposed to analyze the transformation plasticity (TP) behavior and obtain transformation plasticity coefficient. In this investigation two types of materials SCM440 steel and S45C steel have been studied. The specimens were heated to austenite temperature and the temperature kept constant for several minutes, then cooling and loading processes were performed. Austenite to martensite phase transformation with forced cooling for SCM440 steel and austenite to pearlite phase transformation with natural cooling for S45C steel due to bending stresses have been occurred. The deflections of specimen were measured during loading process. By obtaining the maximum deflection due to transformation plasticity, the transformation plasticity coefficient was determined.

Abstract: A numerical model of the tailored hot stamping process was developed in the framework of the commercial FE code ForgeTM and accurately calibrated in order to take into account the influence of applied stress and strain on the phase transformation kinetics. The calibration was carried out by introducing in the numerical model data on the shift of the TTT curves due to applied stress and the transformation plasticity coefficients, which were obtained through an extensive dilatometric analysis. The numerical model was validated through a laboratory-scale hot-formed U-channel produced using a segmented die with local heating and cooling zones. The predicted distribution of Vickers hardness and evolution of microstructure given by the numerical model was compared with the experimental results to show the significant predictive improvements introduced by considering the influence of the transformation plasticity and deformation history on the phase transformation kinetics.

Abstract: The paper presents the results of longitudinal investigations of transformation plasticity, kinetic plasticity, elastic-plastic strain and auto deformation of different grades of steel during quenching. A special device was used to evaluate transformation plasticity and determine the maximum normal bending stress, modulus of transformation plasticity for high alloyed and high corrosion resistance steel. Other targets of experiments were to determine relations between magnitude of normal bending stresses and plastic deformation of test pieces, to examine influence of hardening temperature on the transformation plasticity, to calculate modulus of transformation plasticity Etp. As a complimentary survey we study the mechanical behaviour of high chromium and medium chromium steels during quenching. Understanding phenomenon of steel behaviour during quenching it is possible to renovate and restore the details and components deformed during exploitation, for production of steels with high formability, and to predict properties of steel.

Abstract: In this paper, an approach for modeling transformation plasticity using a phase field model is presented. A conventional formula is utilized to represent the strain due to transformation plasticity as well as thermal expansion and transformation dilatation. A phase-field variable is introduced to express the state of phase in material instead of volume fraction, and numerical simulations under simplified conditions are demonstrated. As a result, the strain induced by phase transformation is suitably regenerated, and qualitatively appropriate temperature-strain curves are obtained. In addition, the effect of each parameter is investigated, and various dependencies, such as transformation temperature and stress, on the induced strain are demonstrated. It is then concluded that the results indicate the applicability of the presented model for practical use by adjusting the parameters.

Abstract: A phenomenological mechanism of transformation plasticity is discussed, in the first part of the paper, why the transformation plastic deformation takes place under a stress level even lower than the characteristic yield stress of the material: This is principally based on the difference in thermal expansion coefficient of mother and new phases. Some calculated data of induced stress and strain depending on applied stress are represented. Bearing in mind that it is also a kind of plastic strain, a unified plastic flow theory is derived by introducing the effect of progressing new phase into the yield function of stress, temperature and plasticity related parameters. Thus obtained strain rate reveals to include the transformation plastic part in addition to thermo-mechanical plastic components. Application of the theory is carried out to simulate some complicated cases of varying stress and temperature, and the results are compared with experimental data.