Papers by Keyword: Bainite

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Authors: Tansel T. Arif, Rong Shan Qin
Abstract: The phase field method is rapidly becoming the method of choice for simulating the evolution of solid state phase transformations in materials science. Within this area there are transformations primarily concerned with diffusion and those that have a displacive nature. There has been extensive work focussed upon applying the phase field method to diffusive transformations leaving much desired for models that can incorporate displacive transformations. Using the current model, the formation of martensite, which is formed via a displacive transformation, is simulated. The existence of a transformation matrix in the free energy expression along with cubic symmetry operations enables the reproduction of the 24 grain variants of martensite. Furthermore, upon consideration of the chemical free energy term, the model is able to utilise both the displacive and diffusive aspects of bainite formation, reproducing the autocatalytic nucleation process for multiple sheaves using a single phase field variable. Transformation matrices are available for many steels, one of which is used within the model.
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Authors: T.Y. Hsu
Abstract: In order to diminish the industrial pollution to maintain the sustainable development and to reduce the cost of the steel production, a unified technology combining plastic forming and heat treatment for some steel parts production is suggested. This article mainly concerns part theoretical foundation of such technology, i.e. the thermodynamic and kinetic models of the ferrite and pearlite transformations under external stress. Simulation of the ferrite fraction after continuous cooling under stress in a low-alloyed steel is presented. The effects of stresses on bainitic and martensitic transformations are also briefly introduced. The unified technology seems favorable to be realized in manufacturing practice.
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Authors: D.V. Edmonds
Abstract: Recent decades have witnessed some remarkable advances in engineering steels driven by the need to respond to challenges posed, for example, by recovery and transmission of oil and gas, or enhanced vehicle safety and fuel economy. Foremost amongst these must surely be the extended application of carbon steels, achieved principally through ferrite grain refinement by the practice of microalloying coupled with controlled thermomechanical processing. Limitations to strengthening ferrite/pearlite structures further by grain refinement or precipitation, however, has focused attention back to acicular forms of microstructure. One of the most interesting advances in this area has been the development of bainitic steels, which have been almost dormant since the mid-20th century. This resurgence may partly be attributed to a better appreciation of the bainite transformation mechanism, and the experimental work for this which unexpectedly spawned some interesting bainitic microstructures which have seen further development and application. These are the so-called ‘carbide-free’ bainites, which employ alloying to replace carbides, principally cementite, with carbon-stabilized retained austenite. Particularly noteworthy has been the emergence of the transformation induced plasticity (TRIP) sheet steels with enhanced properties principally targeted for automotive use. It is worth mentioning also that a parallel development has produced similar microstructure in austempered ductile irons (ADI), another important ferrous alloy which has seen recent expanding interest in its application. Even more recently, as we proceed into the 21st century, the concept of employing steel microstructures containing carbon-enriched retained austenite, has been developed further by combining both alloying and novel heat treatment procedures to exchange ‘bainitic’ ferrite with ‘martensitic’ ferrite. Interestingly, this non-equilibrium ‘quenching and partitioning’ process route also offers the possibility to increase the retained austenite carbon concentration to very high levels, potentially revealing new and previously unobtainable properties.
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Authors: Harshad K.D.H. Bhadeshia
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Authors: Aleksandra A. Kuklina, Mikhail V. Maisuradze, Yury V. Yudin
Abstract: The most widely used equation for analytical description of the transformation kinetics of the metastable solid solutions (the steel austenite in particular) is Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation [1]. However the practical analysis of the experimental isothermal bainite transformation kinetics often gives significant deviation from the conventional theory [2]. This problem can be solved by the derivation of an analytical function which would provide the best fit of the experimental results. Two analytical approaches describing the kinetics of bainite transformation in steels 300M and D6AC are proposed. The first one is based on an approximation of the experimental ln (-ln (1-Р)) vs. ln τ dependence by a second order polynomial function. The second approach is based on the solution of the differential equation y(x) = ay’(x)+b, where x= ln τ, y(x) = ln(-ln(1-P)). A comparison between the proposed approaches and Kolmogorov - Johnson - Mehl – Avrami equation is conducted. The adequacy of the two analytical models is estimated using Fisher ratio test.
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Authors: José Alberto da Cruz, Thiara Francis Mateus Rodrigues, Virgínia Dutra Costa Viana, Dagoberto Brandão Santos
Abstract: A significant amout of stabilized austenite can be obtained in high carbon steel containing high amounts of manganese and silicon (1.5-2 %). At relatively low temperatures the bainite plates formed are extremely thin, making the material very strong. In this study, the influence of the thermal cycle of austempering on the mechanical behavior of a spring steel 0.56C-1.43Si-0.58Mn-0.47Cr (wt. %), with TRIP effect was investigated. The thermal cycle consisted of heating three groups of hot-rolled wire steel at austenite field of 900°C for 300 s, and quickly transferring those to a metallic bath maintained at 200, 220 or 270°C, respectively, for different heat treatment times. The samples were then tested in tension and their microstructures were examined by scanning and transmission electron microscopy. The samples treated at 220°C showed higher elongation, yield strength and tensile strength than those maintained at 200 or 270°C. The high level of strength and ductility is due to a mixture of martensite and very fine bainitic ferrite with interlath film of retained austenite. The temperature has shown a strong influence on bainite formation kinetics. The fracture behavior of the steel was also evaluated using SEM fractography.
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Authors: Si Xin Zhao, Wei Wang, Da Li Mao
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Authors: Vera Wirths, Rainer Wagener, Wolfgang Bleck, Tobias Melz
Abstract: Light-weight design is one of the main drivers for material development in the automotive industry. For optimum weight reduction new materials and their fatigue behavior under real cyclic service loads have to be taken into account (Gassner test). Currently the casted components made from Austempered Ductile Iron (ADI) show better service fatigue life for variable load cases than some traditional forging steels because of it’s inherent retained austenite. The traditional forging steels are the precipitation hardening ferritic-pearlitic steels (PHFP steel) and the martensitic quenched and tempered (Q&T) steels. The next steel generation for forged components in the drive train might be bainitic steels with an optimized microstructure with respect to cyclic behavior. Depending on the chemical composition and the heat treatment it includes a ferritic primary phase and a secondary phase, which consists of either carbides, martensite, retained austenite or M/A constituents. By alloying of more than 1% Si the formation of cementite will be suppressed and a carbide free bainite (CFB) will be formed. The secondary phase of this CFB contains retained austenite, which has the possibility to close crack tips by local compression stresses due to the transformation to martensite. As a result of this CFB exhibits better cyclic properties than the commonly used forging steels. The materials and process design as well as results of the fatigue behavior will be presented.
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Authors: Maxim Shimanov, Grzegorz Korpala, Adnan Terzic, Rudolf Kawalla
Abstract: Abstract. Excellent combination of mechanical properties makes bainitic steels very attractive for commercial application. The most potential benefit of bainitic steels is found in the lightweight design of car bodies. The chemical composition, productions process and the austenite state control the properties of those steels. To reach the target of the correct component design it is important to focus on transformation kinetics. High carbon steels have been investigated, which contain approximately 0.5% C, 1.5% Si, 1.5% Mn, 0.9% Cr and 1.5% Cu. It’s expected to form a carbide free bainitic microstructure due to the Si addition. The residual austenite in the microstructure of high carbon bainitic steels ensures the forming ability besides the high strength. Maximum tensile strength is 1650 MPa and elongation is around 30%. Those steels can be used to produce large scale components.
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Authors: Sang Ho Uhm, Joo Noh Moon, Chang Hee Lee, Ji Hyun Yoon, Bong Sang Lee
Abstract: As a part of a study on modeling the microstructural evolution during the welding process, a prediction model of TTT diagram for bainite transformation was studied. This model consisted of a thermodynamic model for the bainite-start(Bs) temperature and a kinetic model for the bainite transformation. A kinetic model was empirically established for low alloy carbon steels, based on Johnson-Mehl-Avrami(JMA) equation. Reaction constants seemed not to have noticeable tendencies for temperatures and were averaged for each alloy. It was, however, found that the mean reaction constant significantly affected the reproducibility for the isothermal kinetics. Therefore, a calibration method to the kinetic parameter was proposed. From calibrations, rate constants were formulated as a function of alloying element and temperature. And TTT diagrams were calculated and compared with experiments.
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