CALCULATION OF STEEL DATA USING JMATPRO. Uwe Diekmann

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1 CALCULATION OF STEEL DATA USING JMATPRO Uwe Diekmann Metatech GmbH, Kamen, Germany Abstract In order to find the optimum balance of product performance, alloying and heat treatment costs the calculation of materials properties of steels as a function of chemical composition and processing can be helpful. JMatPro is a commercial software package which is based on CalPhaD and extended by various models which allow calculation of materials properties. It is now widely used e.g. in steel-industry and also for generation of materials property data for FE-simulation, like casting-, forming- and heat-treatment simulation. Calculation of materials properties extends the knowledge which is available in leading steel databases, like StahlDat SX of the German Steel Institute. Keywords: Calphad, StahlDat SX, JMatPro, Materials Property Data, CCT, TTT, Flow curves 1. INTRODUCTION Materials impact cost, safety, quality, and performance. In virtual product development environment, design and simulation engineers perform detailed modeling of the performance and service life of components and assembled products. Materials databases which compile information from standards or tests have been developed since many years and can now give excellent general information to the end-user. An example is the new StahlDat SX database for steels which was developed by Metatech using the Granta-MI standard software for this purpose [1]. For detailed modeling of processes and development of new materials it is necessary to get consistent materials data, which also take into account the variations of chemical composition and different processing routes. For cost and time reasons it is practically not possible to get this information from tests or databases. Missing data have to be calculated by appropriate tools - JMatPro was designed for this. 2. CALCULATION OF MATERIALS PROPERTIES Thermodynamic modeling tools for exploring the equilibrium and phase relationships in complex materials have become increasingly used in actual industrial practice especially for materials engineers. JMatPro is a tool which bridges the gap in translating this information into the materials properties being targeted by the CAE-user, e.g. mechanical properties, thermo-physical properties. 2.1 Introduction to JMatPro The software tool JMatPro, which is an acronym for Java-based Materials Properties software, is a suite of computer programs that has been developed by Thermotech Ltd. and Sente Software Ltd. for the prediction of a broad range of materials properties. JMatPro started in 1998 as a project funded by a consortium of companies and institutions interested in expanding their materials properties modelling capacity. The CALPHAD-book [2] can be seen as a starting point and scientific foundation of the system. The initial commercial release of JMatPro was available 2002, current release of JMatPro is V.6.2. JMatPro has been designed so that it can be used by any engineer or scientist that requires materials properties as part of their everyday work. It offers: ease of use due to an intuitive user interface

2 fast and robust calculations, sound models for predictions of materials properties data management interface in order to browse through calculated properties. The following properties are calculated by JMatPro: Stable and metastable phase equilibria Solidification behaviour and properties, e.h. Scheil calculations Mechanical properties, e.g. tensile strength, hardness, flow curves Thermo-physical and physical properties, e.g: Young s (E), Shear (G) and Bulk (K) moduli, Poisson s ratio, Thermal conductivity, Thermal expansion coefficient, Density, Specific heat. Phase transformations, e.g.: TTT/CCT/TTA diagrams, Jominy diagrams JMatPro has different modules for each material group. Each module is pre-configured with certain preference settings so that the end-user can select only the relevant options for calculation of data. Thermodynamic calculations are a basis for the calculation of materials properties. JMatPro utilises core minimisation routines developed for the PMLFKT software programme by Lukas et al. [3] and extended by Kattner et al. [4]. This basis was extended by comprehensive new subroutines which provide a variety of benefits, including setting automatic start points, checking for miscibility gaps, finding phase boundaries. Fig. 1. JMatPro user interface showing the different Material Types (left) and the calculation modules for General Steel. With JMatPro the influence of changing chemical compositions, e.g. scattering within the allowable compositional range, and the other parameters can be analyzed. The complexity of the underlying materials models is hidden by an intuitive user interface (Fig. 1) so that they can be easily applied. Use cases of JMatPro are both the development of materials as well as the generation of materials property data for simulation packages Equilibrium calculations For development of steels and also for tuning of heat treatments thermodynamic calculation of phase equilibria is a good starting point. One example is the calculation of the adequate Ti-content in a Boron-steel in hardening steel. Boron has to be in solid solution in austenite for this case and Ti is used to bind Nitrogen in order to prevent the formation of BN. Fig 2 shows the result of a concentration stepping of a real multicomponent chemistry and indicates the highest amount of free Boron in austenite at a certain Ti-level.

3 Fig. 2. Exemplary equilibrium calulation: Free Boron in austenite as function of Ti-concentration 2.3 Phase transformations (TTT, CCT, TTA) Properties of many steels depend more on microstructure constituents like ferrite, pearlite, bainite and martensite rather than on stable phases. JMatPro uses a combined approach of thermodynamics and physical based models to describe the phase transformation in general steels. In a first step the amount and composition of austenite at austenisation temperature is calculated. The transformation-times for ferrite, pearlite and bainite are then calculated using a model based on [5] so that CCT and TTT diagrams for a wide range of steels can be calculated. The model takes into consideration austenite grain size which is an important factor in order to compare different diagrams of the same steel. Fig. 3a shows a typical TTT for a low alloyed steel. For high alloyed grades like duplex steels and other materials, e.g. Ni-based superalloys, Al-alloys, structure stability over time and precipitation of phases are important. Precipitiation is calculated by using the Johnson-Mehl-Avrami equation as a basis so that TTT/CCT diagrams can be calculated [6]. Fig. 3b shows a TTT digram for duplex steel showing the evolution of different phases, e.g. Sigma, Chi, Laves, Alpha-Cr. In extension to TTT/CCT diagrams JMatPro is now also able to calculate TTA diagrams which were validated against the heat treatment atlas [7], which is now available online in StahlDat SX [1]. The calculation takes into account the initial microstructure and uses again a modified Johnson-Mehl-Avrami approach to calculate re-austenisation which kinetics is limited by C diffusion.

4 Fig. 3 Different TTT diagrams for steel in JMatPro. a) left - decomposition of austenite in a heat treatable steel, b) right - precipitation of phases in a Duplex steel 2.4. Thermo-physical properties Having the amount of microstructure constituents and phases present, properties for each phase as a function of temperature and composition are calculated. JMatPro uses internal property databases to calculate the properties of each phase in dependency of composition and temperature. Materials properties are finally calculated as a function of amount of phases and morphology using a mixture model. Based on the calculation of phase transformation JMatPro can calculate microstructures for complex cooling curves as they occur in practical applications. JMatPro's capability to calculate physical and thermo-physical properties has been well documented elsewhere for various metallic systems [8, 9]. JMatPro is capable of providing either the overall property or that for each phase. The influence of processing on properties, i.e. different quenching rates and austenisation can be calculated (Fig. 4). Fig. 4 Thermal conductivity of a steel, dependent on temperature and quenching rate at fixed composition and grain size Mechanical properties For calculation of yield or proof stress of materials the standard Hall-Petch equation and solid solution hardening is considered. Phase transformation data are used to calculate the strength for each

5 microstructure dependent on grain-size and cooling rate. Fig. 5a shows the evolution of microstructure of a 3310 steel quenched from 900 C at a constant cooling rate of 10 K/s. Fig. 5b shows the corresponding proof stress data for each element and also for the entire material as function of temperature. Fig. 5 Quenching of a steel a) left - Evolution of microstructure, b) right corresponding proof stresses The calculation of flow stress curves in steels follows similar procedures as described in [10] for titanium alloys. The present approach models materials flow behaviour based on fundamental deformation mechanisms in operation and is predictive in nature. It automatically determines whet her deformation should be dominated either by dislocation glide or by dislocation climb, depending on the temperature, strain and strain rate regime. The strength model includes creep as a controlling factor at high temperatures. This has significant advantages, in that process models do not need to include additional models for recovery and recrystallization. Fig. 6 shows an exemplary Flow-Stress-Analysis for a stainless steel. Fig. 6 Exemplary flow stress analysis of a steel at a strain-rate of 1/s 2.6. Export of data A very important use case for JMatPro is the export of consistent property data to FEA analysis tools. JMatPro is now widely used in casting, forming and heta treatment simulation. For many important tools

6 specific exporters for data are included in the system. FEA-users can easily generate their specific material model using JMatPro and import the data directly into their favorate system: Casting simulation: Magmasoft, Procast, Anycast, etc. Forming simulation: Deform, Simufact, Forge Heat treatment simulation: Deform HT, Simufact premap General purpose: Ansys and use of clipboard, Excel, Ascii. 3. SUMMARY Even leading materials databases for steel, like StahlDat SX Pro, cannot deliver all the thermo-physical and mechanical properties which are necessary for process simulation. Calculation of materials properties can generate missing data. Although calculation of materials property data will be subject of further research work for future decades to further extend the capabilities it is possible today to get useful results for practical engineering tasks. JMatPro is a tool which is now widely accepted in steel industry for development of new grades and for generation of data in process simulation. Key advantages of the system are ergonomics, speed and robustness: It can be used in workshops and by occasional users so that materials simulation can now be a standard task in product and process development. LITERATURE [1] Stahldat SX: The European Database of Steels, available online: (2012) Verlag Stahleisen, Düsseldorf [2] N. Saunders, A.P. Miodownik, CALPHAD Calculation of Phase Diagrams, Pergamon Materials Series vol.1, ed. R.W. Cahn (1998), Oxford: Elsevier Science. [3] H.L. Lukas, J. Weiss, E.-Th. Henig, Strategies for the Calculation of Phase Diagrams, CALPHAD, 6, (1982), [4] U.R. Kattner, W.J. Boettinger, S. R. Coriell, Application of Lukas Phase Diagram Programs to Solidification Calculations of Multicomponent Alloys Z.Metallkde., 87, (1996), [5] J.S. Kirkaldy, D.Venugopolan, Phase Transformations in Ferrous Alloys, eds. A.R. Marder and J.I. Goldstein, AIME, (Warrendale, PA: AIME, 1984), 125. [6] A. P. Miodownik, N. Saunders, Modelling of materials properties in duplex stainless steels, Materials Science and Technology, Bd. 18, Nr. 8, S , [7] J. Orlich, A. Rose, P. Wiest, Atlas zur Wärmebehandlung der Stähle. (Band 3: Zeit-Temperatur-Austenitisierungs- Schaubilder). (1973) Verlag Stahleisen mbh, Düsseldorf [8] N. Saunders, Z. Guo, X. Li, A. P. Miodownik, J.P. Schillé: Using JMatPro to model materials properties and behavior. JOM, 55 No. 12, ( 2003) [9] Z. Guo, N. Saunders, P. Miodownik, J.-P. Schille, Modelling phase transformations and material properties critical to the prediction of distortion during the heat treatment of steels, International Journal of Microstructure and Materials Properties, Bd. 4, Nr. 2, S , Jan [10] Z. Guo, N. Saunders, J. P. Schillé, A. P. Miodownik, Modelling high temperature flow stress curves of titanium alloys, in MRS International Materials Research Conference, 2008, S