Bernhard Sonderegger

Computational analysis of the precipitation kinetics in a complex tool steel

Abstract A novel steel grade has been developed recently exhibiting superior thermal stability. This is attributed to the combined precipitation of secondary hardening carbides and intermetallic phases. However, the precipitation behaviour of this steel is not completely understood yet. In this study, computer simulations of the precipitation kinetics during heat treatment of this steel are performed and compared to a complementary experimental characterisation of the precipitate microstructure, which was carried out previously. The simulations employ a novel model for nucleation, growth and coarsening of precipitates in multi-component, multi-phase systems. It is shown that the combination of experimental investigation and computer simulation provides a most comprehensive picture of the complex processes occurring in these materials during thermo-mechanical treatment, which cannot be obtained by the use of one single technique, experiment or simulation, only.

Computer Simulation of the Precipitate Evolution during Industrial Heat Treatment of Complex Alloys

Precipitates are the key ingredient for the strength of heat treatable alloys. To optimize the mechanical properties of alloys it is important to know the response of precipitates to thermomechanical treatments. In the past, application of computer models to describe the evolution of precipitates in the course of these processes has proven difficult due to the complexity of the problem. In this work, a new model based on a mean-field representation of precipitates in a multicomponent matrix is applied to heat treatments of steels. Example simulations are presented for a 9- 12% Cr ferritic/martensitic heat resistant steel for power plant application and a complex tool steel with both carbides and intermetallic phases using the software MatCalc. The predictions of the model are verified on experimental results and the potential application to industrial heat treatment simulation is discussed.

Compositional characterisation and thermodynamic modelling of nitride precipitates in a 12% Cr steel

Abstract In 9–12% Cr steels, nitrides (MX, M2X) and modified Z-phase ((Cr, V, Nb, Fe)N)) are of special interest because of their different contribution to the creep strength of the material. The changes in the chemical composition of complex nitrides and their crystallography were investigated using transmission electron microscopy. A method based on multiple linear least squares fit procedures was applied to extract the complete element content of the nitrides from the electron energy loss spectra. These results, augmented by X-ray spectroscopic data, were compared with the outcomes of the thermodynamic model implemented in the software package MatCalc. The elemental concentrations of precipitates under study i. e. the M2X, MX and modified Z-phase are in good agreement with simulations.

Calculation of Energies of Coherent Interfaces and Application to the Nucleation, Growth and Coarsening of Precipitates

Interfacial energies are essential in modelling nucleation, growth and coarsening processes in solid materials; especially nucleation rates respond very sensitively to small changes of this quantity. Thus, the prediction of interfacial energies has attracted the interest of many researchers since many years. In this work, a simple concept for the calculation of energies of coherent interfaces in multicomponent systems is presented. The model advances the classical nearest-neighbor-broken-bond concept for arbitrary interface orientations and interface curvature. The obtained result is simple enough to be expressed in a single, closed equation. Consequently, it can be easily implemented in the framework of classical nucleation theory, or in complex simulation tools for precipitate evolution based on Kampmann-Wagner type models. In this paper, the theoretical background of the model is discussed, and the results are compared to experimental data. Furthermore, a size correction function for small precipitates is presented and applied to the prediction of nucleation rates. Despite the simplicity of the model, the predictions of the model are found to be in satisfactory agreement with experimental evidence.

Combination of Microstructural Investigation and Simulation during the Heat Treatment of a Creep Resistant 11% Cr-Steel

This work deals with the microstructural evolution of creep resistant martensitic/ferritic 11% Cr-steel during thermomechanical treatment from an experimental as well as modeling point of view. The creep resistance of this material group is highly dependent on the precipitate status. The initial precipitate status is controlled by the chemical composition of the alloy and the heat treatment after casting or hot rolling. It is therefore of utmost interest to understand and model the precipitate kinetics during this process. Once the microstructural evolution has been modeled successfully, only minimum effort is required to computationally test variants in the composition or heat treatment in order to optimize the process. In this work, the material was hot rolled, austenitized and subsequently annealed. All heat treatments have been performed during dilatometry tests. In order to investigate the microstructural evolution during the process, specimens were extracted at definite stages of the treatment. The specimens were then investigated applying various microscopical techniques in order to quantify the microstructural features (grain size, martensite lath width and precipitate data). The experimental data were then compared to thermodynamic simulations (MatCalc). General data such as nucleation sites for precipitates were taken from literature, grain size and martensite lath widths from the experimental data. Simulations include equilibrium calculations and precipitate kinetic simulations. In general, the simulations showed good agreement with the experimental findings, with minor room for improvements. The work thus lays a solid ground for future improvements of the heat treatment process.

3D Simulation of Laser Assisted Side Milling of Ti6Al4V Alloy Using Modified Johnson-Cook Material Model

During machining of hard materials, one approach to reduce tool wear is using a laser beam to preheat the material in front of the cutting zone. In this study, a new concept of laser-assisted milling with spindle and tool integrated laser beam guiding has been tested. The laser beam is located at the cutting edge and moving synchronously with the cutter. In experiment, a reduction in the resulting process cutting forces and tool wear has been observed in comparison to milling without laser. A three-dimensional finite element model in DEFORM 3D was developed to predict the cutting forces in the milling process with and without an additional laser heat source, based on a Johnson-Cook-type material constitutive model adapted for high strains and strain rates. Both in experiment and simulation, the deformation behavior of a Ti-6Al-4V workpiece has been investigated. The comparison of the resulting cutting forces showed very good agreement. Thus the new model has great potential to further optimize laser assisted machining processes.

Development and Improvement of 9-12%Cr Steels by a Holistic R&D Concept

The research activities on ferritic / martensitic 9-12% Cr steels at the Institute of Materials Science, Welding and Forming (IWS) are represented by a network of interacting projects focusing on mechanical properties of base and weld metal, microstructural characterisation of creep and damage kinetics, weldability, microstructure analysis in the course of creep, modelling of precipitation and coarsening kinetics, simulation of complex heat treatments and the deformation behaviour under creep loading. The individual projects are briefly described and the conceptual approach towards a quantitative description of the creep behaviour of 9-12% Cr steels is outlined.

Thermomechanical investigation of the production process of a creep resistant martensitic steel

During the production process of creep resistant martensitic steels, the microstructure of the material undergoes a number of transformations due to thermomechanical loading. The final microstructural features have direct influence on the mechanical properties of the alloy such as creep, fatigue and corrosion resistance, as well as toughness. In order to study the effect of each production step, the thermomechanical history of the material is reproduced in a controlled manner at lab scale for detailed examination of the flow curves during hot deformation. In addition, microstructural investigations are applied to samples after each step of the simulated production process. This procedure provides an overview of the influence of processing parameters on the material’s microstructure and allows the improvement of the processing steps. The objective of this work is to reproduce parts of the production and manufacturing process of forged parts and tubes in a controlled way and to study the microstructural evolution with respect to phenomena such as recrystallization and strengthening. For this purpose hot-rolled experimental 11%Cr steel is investigated using the thermomechanical simulator Gleeble®3800. The deformed samples are investigated via LOM, SEM and EBSD. For comparison, as-received samples are included in the investigations. The interpretation of the microstructural investigation and of the obtained flow curves during the hot compression tests allow conclusions on dynamic recrystallization and recovery. Results indicate dynamic recovery as main softening process for both tested temperatures, whereas the higher temperature leads to a significant formation of delta ferrite. These results allow for improved precipitation kinetic simulations, and for further optimizing the thermomechanical treatment with respect to improved microstructure.During the production process of creep resistant martensitic steels, the microstructure of the material undergoes a number of transformations due to thermomechanical loading. The final microstructural features have direct influence on the mechanical properties of the alloy such as creep, fatigue and corrosion resistance, as well as toughness. In order to study the effect of each production step, the thermomechanical history of the material is reproduced in a controlled manner at lab scale for detailed examination of the flow curves during hot deformation. In addition, microstructural investigations are applied to samples after each step of the simulated production process. This procedure provides an overview of the influence of processing parameters on the material’s microstructure and allows the improvement of the processing steps. The objective of this work is to reproduce parts of the production and manufacturing process of forged parts and tubes in a controlled way and to study the microstructural evo...

Application of Thermo-Calc TCFE7 to High-Alloyed Mottled Cast Iron

The thermodynamic modeling of alloy systems consisting of stable and metastable phases e.g. high-alloyed mottled cast iron can be problematic. Thermodynamic databases are rather well-developed for low, medium and high alloyed steels (e.g. HSS) but the application of those databases is not yet very common for high-alloyed (mottled) cast irons. The Thermo-Calc software together with the TCFE7 database is used to calculate isopleth and property diagrams, using the CALPHAD method. Additionally Scheil-Gulliver calculations are performed to simulate the effects of microsegregation during solidification. The results from the thermodynamic calculations are compared with measurements on own samples and with literature values. Those measurements include quantitative light-optical analysis, SEM with BSE detector, EDX measurements for the distribution of the alloying elements as well as XRD and DSC measurements. The investigations show the possibilities which are offered by thermodynamic calculations for high-alloyed mottled cast iron as well as the limitations and the compromises which have to be taken into account when calculating stable and metastable phases existing next to each other.

Modeling Particle Distances of Coherent Prolate- and Oblate-Shaped Precipitates in bcc Systems

In a wide range of materials, precipitation hardening is the key for optimizing properties such as strength or creep performance. In order to model strengthening effects with physically based concepts, precipitate kinetic simulations have to be linked to micromechanical models. Part of this link is the precipitate distance distribution in the glide planes of dislocations. Recently, a new model for the calculation of distance distributions has been introduced, which is specially designed for arbitrary size distributions and, thus, capable of handling more realistic microstructures when compared to classical approaches. Up to now, this model has been restricted to spherical precipitates. In this work, the model is advanced to account for all kinds of spheroids, that is, ellipsoids with rotational symmetry. Any prolate, oblate or globular precipitate shape can be represented by a specific shape factor, or aspect ratio, and an effective radius. The result is represented in the form of a multiplicative factor for particle distances depending on the aspect ratio only, and can be expressed as a single explicit formula. It is shown, that prolate shape is most effective for minimizing particle distances in glide planes, followed by oblate shape and finally spheres. Since numerous precipitate types feature needle-or platelike shapes, the present model offers a wide field of applications.