The Heat Treatment Simulation Solution from ESI GROUP

Transcription

1 The Heat Treatment Simulation Solution from ESI GROUP An Economic and Predictive Simulation Solution to Compute the Heat Effects of Welding Courtesy: INA Dr. Frederic Boitout Dr. Damian Dry Yogendra Gooroochurn Philippe Mourgue Harald Porzner

2 1 Overview Heat treatment is an indispensable step in the manufacture of steel products. By deliberate manipulation of the chemical and metallurgical structure of a component, mechanical properties such as hardness, static and dynamic strength and toughness are selectively controlled. However, apart from the desired effects, the heat treatment process can be accompanied by unwanted effects, such as component distortion, high material hardness, low material strength, a lack of toughness (which can lead to crack formation) and inadequate hardness depth (which can lead to fatigue failure). Therefore, success or failure of heat treatment not only affects manufacturing costs but also determines product quality and reliability. Heat treatment must therefore be taken into account during development and design, and it has to be controlled in the manufacturing process. Part designers and heat treatment practitioners are looking for: Process feasibility A high resistance of contacting surfaces against wear A specific micro-structure fitting to the in-service requirements A minimum amount of distortion A dedicated distribution of residual stresses With regard to simulation based design and manufacturing, it is desirable to calculate the effects of heat treatment in advance and to optimize them by varying materials and workpiece shape. Once the part shape is designed, it is of utmost importance to make sure that the heat treatment process is correct and that the process window is safe against process parameter variation. With the aid of the finite element analysis software SYSWELD, such calculations can be carried out for all generally applied heat treatment processes, taking all significant physical effects into account. Thus the part designer/heat treatment practitioner can have a deliberate influence on the minimization of manufacturing costs and the optimization of product reliability and quality. SYSWELD is a powerful tool, which can be used to judge the heat treatment process on a real part geometry, and it can efficiently provide answers to these basic questions: Is the selected heat treatment process feasible? Is the selected steel feasible? Is the selected quenching media suitable? Is the process window safe against process tolerances? Is the part hard where it should be hard? Is there any crack risk occurring during the process? Are the obtained distortions acceptable? Are the residual compressive stresses high enough and well positioned? 2

3 2 Technical Background The technical background of heat treatment is quite complex. It involves heat transfer, phase transformations and mechanics including phase transformation. Figure 1: Physical effects and their interaction in thermochemical-metallurgical simulation of case hardening. Figure 2: Interrelated physical phenomena Download a paper from to learn all about the technical background. 3 Simulation Engineering With the help of the Heat Treatment Advisor, the set-up of a numerical computation is extremely fast. This does not mean that the simulation engineering is simple. The physics behind a heat treatment simulation is quite complex, and a user-friendly intuitively driven graphical user interface will not change that. In order to avoid obtaining incorrect results from an incorrect model and bearing in mind a remark from Albert Einstein, One should never do too much but never less than necessary, SYSWELD provides a very detailed training course for the simulation of heat treatment that covers all real life situations. An extended user guide for best simulation engineering practice gives the best way to transfer practical problems into a

4 heat treatment simulation and the advisor primer shows mouse-click by mouse-click how to use the software. Using SYSWELD, heat treatment simulation engineering is now a straightforward and efficient task. 4 Software and Applications Presentation SYSWELD for heat treatment provides dedicated solutions for heat treatment practitioners as well as for part designers. Heat treatment practitioners will focus on the feasibility of the heat treatment process and need answers to their questions instantly. Consequently, a dedicated packaged solution is available for them, fitting well to the needs of a heat treatment job shop. Part designers focus on the design of parts and try to find the optimum between cost, part shape, material, and heat treatment process. Consequently, a dedicated package is available for the design engineer, providing unmatched meshing and computation capabilities, both on PC and UNIX based computers. 4.1 CAD Data Import and Export Visual Mesh for heat treatment provides graphical modeling capabilities for manipulating Finite Element meshes. Native CAD data is imported, automatically cleaned, and meshed by a batch-meshing algorithm dedicated to heat treatment applications. It is important to notice that SYSWELD performs simulation on the real geometry. It is not necessary to work with simplified objects. Figure 3: Batch meshing for heat treatment 4.2 Meshing Capabilities and Group Concept Specific technical capabilities are provided for the Finite Element modeling of the heat-treated structure. The demand for high quality in computed results requires a refined layered mesh from the surface through a few millimeters of thickness of the part. For two-dimensional structures as well as for three-dimensional structures, a guided layered mesh generator is available. As a result, SYSWELD drastically reduces the time to mesh parts while offering high quality Finite Element models. The group concept allows simple and complete interfacing to any existing meshing tool, and so the definition phase of the numerical problem is extremely short and simple. 1

5 4.3 Heat Transfer Coefficient Wizard Where information for a specific quenching medium is not yet accessible in the SYSWELD database, it is necessary to adjust the convective heat transfer coefficient of the quenching medium as a function of the temperature. Measurements of cooling rate and temperature over time performed with ISO or JIS probes are usually available through the vendor of the quenching media. Even so the quality-assured European and Japanese measurement specifications have been included in SYSWELD. By performing a few simple recursive computations, the convective heat transfer coefficient of the quenching media can be easily evaluated for the defined specimen. It is important to notice that the values obtained will then give a good starting point to compute good behavior tendencies on the real parts. The higher the BIOT numbers typical for the real part the more precise will be the computations using the evaluated heat transfer coefficient. The lower the BIOT numbers, the higher will be the sensitivity of the computed results against variations in the convective heat transfer coefficient depending not only on temperature but also on space coordinates. It is important to notice that the adjustment has to be done only once for each quenching media. The results can be stored in a database and are then accessible for further computations via a mouse-click. Figure 4: Heat exchange coefficient fitting Graphical user interface Figure 5: Heat exchange coefficient fitting Comparison of measured data with computed results 2

6 4.4 Fitting the Martensite Transformation Where information is not yet accessible in the SYSWELD database, it is necessary to adjust (especially for case hardening simulations)- the martensite transformation depending on the carbon content, the martensite start temperature depending on the carbon content and the retained austenite proportion at room temperature depending on the carbon content. The martensite start temperatures (dependent on the carbon content) and the retained proportions of austenite (dependent on the carbon content) are usually known. Based on this data, fine-tuning of the parameters relation to the Koistinen Marburger law (which describes the martensite transformation by mathematical means) can be performed, making use of the SYSWELD PHASE module. It is important to notice that the adjustment has to be done only once for each steel. The results can be stored in a database and are then accessible for further computations via a mouse-click. Figure 6: Results of the martensite transformation fitting Retained proportion of martensite depending on the temperature 4.5 Fitting the Continuous Cooling Diagram If specific information is not yet available in the SYSWELD database, it is necessary to adjust the continuous cooling transformation diagram of the steel, extracting basic parameters from an ITT diagram and parameters for the fine-tuning from the CCT-diagram. For numerical reasons, it is preferable to describe the cooling behavior of steel by differential equations rather then by pairs of temperature-proportion values. Those differential equations have been defined, for example by Johnson-Mehl-Avrami and Leblond. They contain phenomenological parameters that have to be adjusted individually for each CCT diagram. Using the PHASE module and the ITT / CCT display tool, the adjustment of a CCT-diagram is a straightforward and simple task. It is important to notice that the adjustment has to be done only once for each steel. The results can be stored in a database and are then accessible for further computations via a mouse-click. The major steels used in heat treatment are already available in the SYSWELD database. 3

7 Figure 7: CCT-diagram of a 100Cr6 steel source [12] 4.6 Database for Thermal and Mechanical Material Properties The thermal, metallurgical and mechanical material properties of a heat-treated steel are quite complex and depend on temperature, phases and carbon content. SYSWELD features a comprehensive material database including the major steels that are used for case hardening, surface hardening and through hardening. It is important to notice that the values given in the SYSWELD material database are average values extracted from experiments and literature; missing values have been completed by best simulation engineering practice. It is important to note that properties of steel depend on the manufacturer, the year, the country etc. The material properties in the SYSWELD material database therefore represent an average material that will give good tendencies. In no case, the data will fit precisely to an individual steel. 4.7 The Heat Treatment Advisor The Heat Treatment Advisor is a graphical user interface that allows an intuitive and process-driven methodology to set-up simulations. Once a dedicated project is defined and stored, parts, process, and material parameters can be exchanged with a few mouse-clicks within the project and in less then 1 minute a computation of a variant can be started. With the help of the advisor, case hardening and through hardening processes can be fully defined. In case of surface hardening, a few more simple operations with the standard capabilities of the software are needed to adjust the energy input through the surface. Delivered with the software is an illustrated advisor primer that shows, systematically, how to perform an industrial heat treatment study. Set-up of computations with the Advisor is therefore efficient. 4

8 Figure 8: Intuitive and straightforward set up of a heat treatment simulation with the Heat Treatment Wizard 4.8 Automatic Solver The SYSWELD solver provides an automatic solution for heat treatment problems, covering all related complex mathematics and material physics. Depending on temperature, phase proportions, and proportion of chemical elements, thermal and mechanical properties are computed, including large strains. Isotropic and kinematic hardening (including phase transformations), transformation plasticity, nonlinear mixture rules for the yield stress of phases, phase dependent strain hardening, restoring of strain hardening during diffusion controlled phase transformations, melting and solidification of material, material properties depending on temperature, phases and proportion of chemical elements and all features dedicated to the methodology of finite elements are taken into account. The solver is unique and a result of about 50 men-years of development work. It is important to notice that the user does not need to be familiar with the mathematics involved in this solver in order to perform heat treatment computations. The only work needed to perform a computation is to load the project and to start the solver. Figure 9: Launching a computation the only work necessary is to load the project name 4.9 Multi-Physics Post-Processor The multi-physics post-processing capabilities provide instantaneous process information for the evolution of 5

9 Temperature field Heating and cooling rates Metallurgical structure of the material Distortions Stresses Yield stress of the modified material Plastic strains SYSWELD provides a variety of techniques for reviewing process results including Contour plots Iso-lines and iso-surfaces Vector-Display X-Y diagrams Symbol plots Numerical presentation Cutting planes Animations Figure 10: Case hardening of a splined shaft 6

10 Figure 11: Simulation of a gear component as simulated in the C.A.S.H. project made from nodes and elements on a single processor computer. Courtesy DaimlerChrysler AG Figure 12: Computed hardness of a through hardened train wheel 7

11 Figure 13: Distortions after quenching Figure 14: Computed final yield stress (the yield stress depends on the composition of phases) displayed on cross-sections through the structure 8

12 Figure 15: Cooling rates displayed over the CCT-diagram examination of critical points where the hardness is too low or missing Of specific interest is the capability to review movies on the evolution of results, step by step, for all important results on the surface or through the structure. The simultaneous display of the evolution of results gives a deep understanding of process and computed results. Figure 16: Step by step display of generated movies of important results in JASC Animation Shop 9

13 Figure 17: Temperature field at the beginning of the quenching Courtesy VSTC Figure 18: Distortion of a large gear after quenching - Courtesy VSTC 10

14 4.10 The Jominy Test In SYSWELD, the jominy test is implemented as predefined ready-to-run simulation project. The user has to define only the chemical composition of the steel, the computation of the jominy test is done fully automatically. At the end of the computation, the most important results like for example the hardness profile are displayed. The jominy test is the key to a precise heat treatment simulation: Once the computed hardness coincides well with the measured hardness, it is secured that the CCT-diagram of the steel under examination is numerically well implemented for the full bandwidth of possible cooling rates. In case of discrepancies, the CCT diagram can be modified in order to meet precisely the measured hardness profile. Due to the fact that the formulas used for the hardness computation are empirically approved, existing CCT diagrams can be tuned following recent hardness measurements. Based on the optimized CCT-diagram, the core hardness of complex parts can be precisely predicted, which is of utmost importance for the lifetime of parts and components under dynamic loads. Figure 19: Comparison of computed and measured hardness of a jominy test of 16MnCr Expert Modus Based on the open architecture of SYSWELD, experts can perform computations of arbitrary complexity, including programming user defined elements, material laws, phase transformations, and so on. Even basic researchers will find an excellent foundation to work. It is important to note that for standard heat treatment simulations, there is no need to use anything other than the Heat Treatment Advisor graphical user interface Users Guide A Heat Treatment User s guide has been added to the set of documentation. It covers the usage of the Welding Wizard as well as all the engineering knowledge related to steady state and transient welding. It includes the following chapters: Usage of the Heat Treatment Wizard Messages Managed by The Heat Treatment Wizard How to Choose Numerical Parameter Files 11

15 Frequently Asked Questions Way to Work The Most Important Tips and Tricks Access to Electronic Manuals Getting Info from Manuals Advanced Information How to Present Results in an Effective Format Step by Step Example Case and Through Hardening Getting it Right Assessment of Case Hardening Simulation A Diffusion Based Case Hardening Simulation A Tutorial Figure 20 Heat Treatment User s Guide 5 How to Get Further Information Further information about SYSWELD for Heat Treatment is available through all subsidiaries of the ESI Group or directly from the ESI Group web-page Or simply contact Harald Porzner, Harald.Porzner@esi-group.com 12

16 References [1] Denis S., Gauthier E., Simon A. & Beck G., "Stress/phase transformation interactions: basic principles, modelization and their role in the calculation of internal stresses", Proc. Int. Symp. on the Calculation of Internal Stresses in Heat Treatment of Metallic Materials, Vol. 1, pp , Linkoping (Sweden), 1984 [2] Bergheau J.M. & Leblond J.B., "Coupling between heat flow, metallurgy and stress-strain computations in steels - The approach developped in the computer code SYSWELD for welding or quenching", Proc. of Vth Eng. Found. Conf. on Modeling of casting, welding and Advanced Solidification Processes Davos (Switzerland), 1990 [3] Leblond J.B. & Devaux J.C., "A new kinetic model for anisothermal metallurgical transformations in steel including effect of austenite grain size", Acta Metallurgica, Vol. 32, n 1, pp , 1984 [4] Fernandes F., "Modélisation et calcul de l'évolution de la température et de la microstructure au cours du refroidissement des aciers", Thèse de Doctorat de l'inpl, Nancy, 1985 [5] Leblond J.B., Mottet G. & Devaux J.C., "A theoretical and numerical approach to the plastic behavior of steels during phase transformation, I: Derivation of general relations, II: Study of classical plasticity for ideal-plastic phases", Jour. of the Mech. and Phys. of Solids, Vol. 34, n 4, pp , 1986 [6] Leblond J.B., Devaux J. & Devaux J.C., "Mathematical modelling of transformation plasticity in steels, I: Case of ideal-plastic phases, II: Coupling with strain-hardening phenomena", Int. Jour. of Plasticity, Vol. 5, pp , 1989 [7] Fortunier R., Leblond J.B., Pont D. & Bergheau J.M., "Récents développements dans la simulation numérique des traitements thermochimiques des aciers", 9th Int. Conf. on Heat Treatment and Surface Engineering, Nice (France), 1994 [8] Großer Atlas Schweiß-ZTU-Schaubilder: Fachbuchreihe Schweißtechnik, DVS-Verlag GmbH Düsseldorf, 1992 [9] Dieter Liedke, Rolf Jönsson: Wärmebehandlung, Expert Verlag, 1996 [10] Karl Heeß und 14 Mitautoren: Maß- und Formänderung infolge Wärmebehandlung, Expert Verlag, 1997 [11] Ruth Chatterjee-Fischer und 8 Mitautoren: Wärmebehandlung von Eisenwerkstoffen, Nitrieren und Nitrocarburieren, Expert Verlag, 1995 [12] Atlas zur Wärmebehandlung der Stähle, herausgegeben vom Max-Planck Institut für Eisenforschung, in Zusammenarbeit mit dem Werkstoffaussschuss des Vereins Deutscher Eisenhüttenleute, Band 2 von Adolf Rose und Hans Hougardy, Verlag Stahleisen m.b.h., Düsseldorf 13