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![Figure 1.3: Typical distortion of plates after cooling [YM03]](profile/Yalcin-Kaymak/publication/45666270/figure/fig3/AS:669515591458844@1536636364977/Typical-distortion-of-plates-after-cooling-YM03_Q320.jpg)
A complex thermo-mechanical model and its finite element implementation for simulating the transient fields of the temperature, micro structure, stress, strain, and displacement during a heat treatment process is introduced. At the integration points, the temperature, phase fractions, Scheil's sum, plastic strain, transformation induced plastic str...
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Citations
... The simulation model was developed using the COMSOL Multiphysics® software. The modelling approach presented in the PhD thesis [7] for long profiles has been extended for flat strips and re-implemented using the COMSOL Multiphysics® software. The temperature field and the displacement field are computed using the built-in heat transfer in solids and solid mechanics physics in the software. ...
... Two state variables are defined per integration point, the Scheil's sum, ss in Equation [7] and the bainite volume fraction, in Equation [8]. The bainite transformation starts at time ( ), which is a function of temperature and taken from the TTT-diagram. ...
... The calibration values used in the 3MA system were from tensile test machine validation campaign, described in Appendix D4.2. The residual stress profile in rolling direction were also measured under different velocities V = [6,7,9,12,5.5] min/s for strip reference 871605. ...
During the last decades European steel strip and plate producers have focused their production on producing Advanced High Strength Steels to a much higher share. One reason is that downstream customers e.g. automotive manufacturer and construction market have a growing need for inventing light weight solutions to reduce emissions etc.
It has though been clear that by introducing a greater share of AHSS also the residual stress related problems have increased dramatically. For many grades the yield stress limits have increased by a factor four or even more which in turn means that a product/material could have elastic stresses that are several times higher than before.
From a production point of view, many strip and plate processes looks the same, are equipped the same and are controlled almost in the same way as when mild steel was standing for the bulk production. This unfortunately introduces problem with residual stress.
To improve competitiveness a lowering of residual stress related problems is of great need. To reach there, consequently, new process control approaches are needed which are “residual stress” oriented and not “flatness” oriented.
The main objective of this project is therefore to illuminate, to test and to evaluate if there are techniques, methods and models already available to be applied in creation of a “residual stress” focused process control.
... Während der Abkühlung aus der Umformwärme entwickeln sich die finalen Eigenspannungen im Material in Abhängigkeit von den thermischen und umwandlungsinduzierten Verzerrungen. Die Arbeiten [12,13] und [14] haben gezeigt, dass eine Spraykühlung mit einem Luft-Wasser-Gemisch zur gezielten Steuerung des Temperaturverlaufes nach dem Schmieden geeignet ist. Das Ziel lag darin, eine sogenannte Best-Yield (BY) Behandlung durchzuführen, bei der die gewünschten Werkstoffeigenschaften über den maßgeschneiderten Temperatur-Zeit-Verlauf eingestellt werden. ...
Zusammenfassung
Ziel dieser Arbeit ist die Einstellung eines vorteilhaften Druckeigenspannungsprofils in warmumgeformten Bauteilen durch intelligente Prozessführung mit angepasster Abkühlung aus der Schmiedewärme. Die Machbarkeit und das Potenzial werden an einem Warmumformprozess, bei dem zylindrische Proben mit exzentrischer Bohrung bei 1000 °C umgeformt und anschließend aus der Schmiedewärme im Wasser abgekühlt werden, aufgezeigt. Vorige Arbeiten zeigen, dass sich Zugeigenspannungen in den derartig umgeformten Proben aus dem Material 1.3505 einstellen. Mittels der vorgestellten mehrskaligen FE-Modelle, wird in dieser Arbeit eine alternative Prozessvariante analysiert, mit der vorteilhafte Druckeigenspannungen anstelle von Zugeigenspannungen durch eine angepasste Abkühlung aus der Umformwärme in den Proben erzeugt werden können. Die angepasste Kühlung wird durch eine partielle Beaufschlagung der Proben mit einem Wasser-Luft-Spray erreicht. Auf diese Weise kann die lokale Plastifizierung durch inhomogene Verzerrungen aufgrund thermischer und umwandlungsinduzierter Effekte beeinflusst werden, um letztlich das Eigenspannungsprofil individuell zu gestalten. Die wissenschaftliche Herausforderung dieser Arbeit besteht darin, unterschiedliche Eigenspannungen in der Oberfläche der Proben zu erzeugen, während die geometrischen und mikrostrukturellen Eigenschaften gleichbleiben. Es wird nachgewiesen, dass eine Beeinflussung der Eigenspannungen und sogar die Umkehr des Spannungsvorzeichens allein durch eine geschickte Prozessführung beim Abkühlen möglich ist.
... A more precise adjustment of the heat-transfer coefficients and thus of the temperature gradients is achieved in [29] and [30] by means of a cooling medium with added polymer solutions in customised concentrations. In addition, the works [31,32] and [33] show that the controlled cooling using a two-phase fluid flow on the basis of a water-air mixture has proved to be very flexible, since both comparatively low heat transfers as in gas quenching and high cooling rates like those in water cooling can be achieved. Thus, a surface tempering of austenitised components like gears [34] and pinion shafts [35] could be realised by controlled spray cooling, with the surface being quenched and tempered by means of the residual internal heat. ...
The aim of this work is to generate an advantageous compressive residual stress distribution in the surface area of hot-formed components by intelligent process control with tailored cooling. Adapted cooling is achieved by partial or temporal instationary exposure of the specimens to a water–air spray. In this way, macroscopic effects such as local plastification caused by inhomogeneous strains due to thermal and transformation-induced loads can be controlled in order to finally customise the surface-near residual stress distribution. Applications for hot-formed components often require special microstructural properties, which guarantee a certain hardness or ductility. For this reason, the scientific challenge of this work is to generate different residual stress distributions on components surfaces, while the geometric as well as microstructural properties of AISI 52100 alloy stay the same. The changes in the residual stresses should therefore not result from the mentioned changed component properties, but solely from the targeted process control. Within the scope of preliminary experimental studies, tensile residual stresses in a martensitic microstructure were determined on reference components, which had undergone a simple cooling in water (from the forming heat), or low compressive stresses in pearlitic microstructures were determined after simple cooling in atmospheric air. Numerical studies are used to design two tailored cooling strategies capable of generating compressive stresses in the same components. The developed processes with tailored cooling are experimentally realised, and their properties are compared to those of components manufactured involving simple cooling. Based on the numerical and experimental analyses, this work demonstrates that it is possible to influence and even invert the sign of the residual stresses within a component by controlling the macroscopic effects mentioned above.
... The simulation model was developed using the COMSOL Mul-tiphysics® software. The basics of the modelling approach can be found in the PhD thesis [1] and further details about Comsol implementation can be found in [2]. The temperature field and the displacement field are computed using the built-in heat transfer in solids and solid mechanics physics in the software. ...
... A comprehensive modelling of the complex phenomena to estimate the residual stress and deformation is essential for developing an optimal process control. The basis of the complex quenching model presented in this paper is developed within author's PhD thesis [1]. The model consists of a series of coupled physics, which are summarized in Figure 2. ...
Heat treatment of the advanced steel grades (like micro-alloyed steels or AHSS steel grades) is a challenging process as the residual stress/deformation are very pronounced and the customer quality requirements are getting striker. Instead of a trial and error based process control, a model based optimal process control is essential to reduce waste material costs and to stay competitive.
A comprehensive COMSOL® model was developed to simulate the complex heat treatment phenomena to estimate the residual stresses and deformation as an initial step. The model consists of strongly coupled physics of heat transfer, micro-structure change, and deformation fields. The major couplings are: the latent heat of phase transformation, the volume change (due to temperature and micro-structure change), and transformation induced plasticity (trip). Moreover, the nonlinearities like: creep, large deformation and buckling are included too.
The constitutive model parameters as well as the isothermal and martensitic transformation kinetic parameters are calibrated and validated by dilatometry tests for a certain type of steel.
For steels and aluminum alloys, liquid quenching is the most effective method to achieve fast cooling rates. In the example of water quenching an aluminum 319 cylinder head, the temperature drops rapidly within 30 seconds from solutionizing temperatures at 495 °C to water pool temperature below 100 °C. In this temperature range, the water boiling in the quenching process goes through three boiling regimes, film boiling, transition boiling and nucleate boiling, before reducing to convection heat transfer. Since each boiling regime has unique heat transfer characteristics that are governed by different physics, modeling the water quenching processes by computer simulation requires a heat transfer framework, instead of just a few equations, that can describe all the boiling regimes. Among several heat transfer frameworks found in the literature, we had successes in developing a CFD methodology to simulate the boiling process by adapting a heat transfer framework based on Leidenfrost point (LFP), minimum heat flux (MHF) and critical heat flux (CHF). This CFD methodology, when integrated with FEA structural analysis, is the key enabler for virtual process verification. This is achieved by first calculating temperature histories and profiles in CFD and then applying the temperature data as thermal load to FEA to predict thermal residual stress and distortion. Although the LFP, MHF and CHF framework have been proven useful to model the water quenching process, these parameters are not constants and they have to be calibrated through experiments for each quenching condition. The objective of this paper is to develop a consistent method to calibrate the boiling heat transfer framework using cooling curves obtained by the ASTM D6200 quenchometer. Also included in this paper is a preliminary discussion on broadening the standard in order to support: (1) generic cooling curve characteristics for any quenchant, (2) the analytical cooling curve for computation model calibration.
Hochleistungsbauteile wie Zahnräder, Ritzelwellen oder Kurbelwellen unterliegen im Betrieb einem komplexen Belastungsprofil. Bei miteinander im Eingriff stehenden Verzahnungen kommt es bedingt durch die Zahnnormalkräfte zu drei primären Belastungsarten am Zahn: Zahnfußbiegung, Flankenpressung und Gleitbeanspruchung [Breuer et al. Verknüpfung von Wärmebehandlung und Formgebung beim Präzisionsschmieden von Verzahnungen, 2000]. Diesen kann durch ein gradiertes Eigenschaftsprofil von einer verfestigten Randschicht hin zu einem duktilen Kern entgegengewirkt werden. Deswegen besitzen Technologien der Randschichtverfestigungen durch Härten sowie Vergüten einen hohen Stellenwert in der Zahnradfertigung [Breuer et al. Verknüpfung von Wärmebehandlung und Formgebung beim Präzisionsschmieden von Verzahnungen, 2000].
The cooling process after aluminum extrusion is crucial for the strengthening of AlMgSi-type alloys but can induce distortions caused by fast cooling, especially for complex or critical sections. The optimum cooling conditions fall within the so-called quench window between two limitations (high strength and lower distortion). Appropriate modeling, using both physical and numerical methods, can guide us to achieve a better cooling strategy in extrusion plants. In this paper, we focus on the study of the optimal strategy of the initial cooling for selected cases. A simple extruded section in the actual scale was simulated by a reasonably realistic numerical model. The section was cooled by a water spray from one side (top side) and still air on the other side. The effect of the initial cooling was evaluated by defining a measure called the "front width", and the distortion mechanism for different front widths was studied. The effect of some selected parameters (e.g., the thickness, the stop time and the extrusion speed) were also taken into consideration. The simulation results showed that a combination of distortion stages (concave or convex) played a main role in determining the final shape and distortion magnitude. If the process parameters (e.g., the front width, extrusion speed and others) are set in such a way that could restrict the accumulation of distortions at subsequent zones, then the section suffers fewer distortions. For example, if the cooling begins sharper the extrudate suffers smaller distortion. A longer stop time is beneficial for less distortion when it is long enough to ensure that the section in the cooling zone is cooled to room temperature. The extrusion speed has varying effects and could be optimized for any profile thickness. However, even when optimizing the other parameters, the thicker section might be distorted more than the thinner section.
