Papers by Keyword: Micro-Structure Evolution

Abstract: The tensile tests of DP590 dual-phase steel tailor-welded blanks were carried out at different temperatures and strain rates. Quantitative analysis of metallographic was utilized to study the microstructure evolution of the base metal area and the weld zone of the tensile fracture. By combining the microstructure evolution model with the secondary development technologies of ABAQUS, the microstructure evolution of dual-phase steel tailor-welded blanks was simulated during warm tensile tests. The results show that, with the temperature increasing and strain rate decreasing, the dynamic recrystallization volume fraction of ferrite increases, and the martensite transform into equiaxed ferrite gradually. The results of microstructure simulation are in good agreement with experimental results.

Abstract: Thermal mechanical experiment of step-cooling of 600MPa hot-rolled DP steel after compressed was carried out on Gleeble-1500 thermal mechanical simulator, using design of butterfly-shaped sample. Microstructure evolution during step-cooling and its effect on mechanical property of tested steel were analyzed. It is shown that, dual phase microstructure which martensite islands disperses in fine grain ferrite matrix is obtained by holding for 8~10s at 670°C followed accelerated cooling to 200°C. With increasing of holding time, grain size of ferrite coarsens, and volume fraction of ferrite has few changes. Meanwhile, tensile strengths of tested steel are all near 600MPa. That is to say, ferrite transformation is adequate during holding for 8~10s at 670°C and tensile strength of dual phase steel is stable accordingly.

Abstract: The microstructure evolution of a medium-carbon Si-Mn steel during deformation of undercooled austenite at different degree of deformation, temperatures and strain rates has been investigated by means of a hot compression simulation test, metallographic microscope, scanning electron microscope and transmission electron microscopy. Also, the mechanism of carbide spheroidized during deformed process has been discussed. The experiment results demonstrate that the process of evolution experienced three stages: that is, strain-induced transformation, austenite eutectoid decomposed to carbides and ferrite matrix, and spheroidization of pearlite at the range of A₃-Ar₃. The austenitic grains would be refined for the extra-product of ferrite above the Ar₃. The eutectoid reaction was induced on the grain boundaries of ferrite and non-transformed austenite and deformation bands with the increasing volume of deformation. An optimum combination of deformation temperature and strain rate is important to obtian the dulplex microstructure consisting of ultrafine ferrites and dispersed carbide particles. The fine spheroidized microstructures are obtained while the deformed temperature reaches 650°C with ≥1.0, meanwhile, The carbides precipate in globular and shot-rod shapes. Keywords: Medium-carbon Si-Mn steel, Undercooled austentite, Microstructure evolution, Deformation induced transformation, Carbide spheroidization

Abstract: In this work microstructure evolution in a columnar polycrystal of pure aluminum is studied using a microstructure sensitive crystal plasticity finite element model (CPFEM). In the model, based upon the kinematics of crystal deformation and dislocation interaction laws, dislocation generation and annihilation are modeled. Dislocation densities evolve in the form of closed loops and are tracked as state variables, leading to spatially inhomogeneous dislocation densities that show patterning in the dislocation structures. The hardening law is based on the strength of junctions between dislocations on specific slip systems. The CPFEM model is able to show the anisotropic hardening behavior of aluminum single crystals. The measures of accumulated plastic strain in the experiment and the simulation are compared with varying degrees of success.

Abstract: The flow stress at the temperature of 250~450°Cand different strain rate of casting AZ91D magnesium alloy was studied through experiment, which adopting the Gleeble 3500 system of DSI company. The mathematical model of flow stress containing the softening factor which is suitable for casting AZ91D magnesium alloy was proposed. The temperature and strain rate conditions during full dynamic recrystallization were found by observing the microstructure.

Abstract: Microstructure evolution in a P911 heat resistant steel was examined under conditions of aging and creep at a temperature of 600°C and an applied stress of 200 MPa. The tempered martensite lath structure (TMLS) evolved after heat treatment consisted of prior austenite grains (PAG), packets, blocks and laths. The mean transverse lath size and the interior dislocation density were about 345 nm and 3.5 × 10¹⁴ m^-2, respectively. Various second phase particles precipitated upon tempering. Fine MX carbonitrides were homogeneously distributed throughout the tempered martensite laths, while relatively coarse M₂₃C₆ carbide particles were located on high-and low-angle boundaries. Upon creep test, precipitation of Laves phases was found. The stability of TMLS during creep is discussed in detail.

Abstract: Microstructure evolution in low-carbon bainitic steel during tempering is investigated by hardness measurements and metallographical examinations. It is found that the microstructure evolution and the hardness variation can be divided into four stages when samples were tempered at 600°C and 700°C, and the evolution of bainte is similar to recovery and recrystallization of deformed metals. It is also found that the newly formed ferrite during recrystallization grows more rapidly along the long axis of bainite laths, and there is evidence of composition changing during recrystallization.

Abstract: In order to get high quality forgings, it is significant to predict the microstructure evolution during hot forging process accurately. In this study, a simulation model is built by combining FEM with the dynamic recrystallization model of 42CrMo, and the finite element model is proved to be reliable by a serial of upsetting deformation experiment. Then the distributions of microstructure evolution are obtained on upsetting process. Upsetting is beneficial to refine the grain size and drawing can make the distribution of grain size homogeneous. By comparing the simulation results with experiments, the distributions of microstructure are a close match in the middle part of steering arm. The forgings formed by this process have a good microstructure and have high comprehensive mechanical properties.

Abstract: In the present study, the evolution of the grain structure of a Mg-Al-Ca-based alloy during hot extrusion was simulated with the cellular automation method. The Laasraoui-Jonas microstructure model was used to describe the dislocation evolution inside crystallites during dynamic recrystallization. The parameters in the Laasraoui-Jonas model, such as the hardening parameter, recovery parameter and material constants, were determined from the flow stress-strain data obtained from hot compression tests using a Gleeble-1500 thermomechanical simulator. The extrusion process was simulated using a DEFORM 3D FEM code. The influence of ram speed on grain structure evolution was analyzed. It was found that the average grain size increases with increasing ram speed. Good agreements between the predicted and observed grain structures were achieved.

Abstract: Prediction and control of the microstructure to improve product performance are very important for the industry practice. In this study, microstructure evolutions of 30Cr2Ni4MoV steel under different conditions were simulated by changing the process parameters using the Deform 3D software. Effects of the forming process parameters on the microstructure were revealed: (1) the higher the temperature and the lower the strain rate, the smaller the strain are needed for the dynamic recrystallization; (2) when strain is enough, the higher the strain rate, the easier the uniform and small grain size can be obtained; (3) under a certain strain rate, the grain size increases as the deformation temperature increases. The microstructure of metal can be predicted and controlled according to the effects of hot forming process parameters on the microstructure evolution.