Seamless tubes of stainless steel can be extruded using glass as a lubricant in the Ugine-Sejournet process. The process is performed at high temperature and is associated with large deformations and high strain rates.The use of finite element modeling (FEM) in the analysis and design of extrusion and other metal forming processes is constantly increasing. Computer models that with adequate accuracy can describe the material behavior during extrusion can be very useful for product and process development. The process development in industrial extrusion today is, to a great extent, based on trial and error. This often involves full size experiments which are expensive, time consuming and interfere with the production. It would be of great value if these experiments could be performed in a computer. In this work, FE models of the stainless steel tube extrusion process were developed and used. Simulations were carried out for different tube dimensions and three different materials: two austenitic stainless steels and one duplex (austenitic/ferritic) stainless steel. The models were validated by comparing the predicted values of extrusion force with measurements from production presses. A large number of input parameters are used in a FE analysis of extrusion. This includes boundary conditions, initial conditions and parameters that describe the mechanical and thermal properties of the material. The accuracy of the extrusion simulation depends, to a large extent, on the accuracy of these parameters. Experimental work, both in the form of material testing and production trials, was performed in order to give an accurate description of the input parameters in these extrusion models. A sensitivity analysis was performed for one of the models and the results showed that the initial billet temperature is the parameter that has the strongest impact on the extrusion force. In order to study the temperature evolution in the billet during manufacturing, the entire process chain at extrusion of stainless steel tubes was simulated using FEM. This process flow model includes sub-models of induction heating, expansion and extrusion.The work includes the use of a dislocation density-based material model for the AISI type 316L stainless steel. It is expected that this physically based model can be extrapolated to a wider range of strains, strain rates and temperatures than an empirical model, provided that the correct physical processes are described by the model and that no new phenomena occur. This is of interest for steel extrusion simulations since this process is carried out at higher strains and strain rates than what are normally used in mechanical laboratory tests.The developed models have given important contributions to the understanding of different phenomena that occur during extrusion of stainless steel tubes, and can be used to analyze how different process parameters affect the extrusion process.

The manufacturing process chain at glass-lubricated extrusion of stainless steel tubing is simulated using the finite element method. The developed model includes sub-models of induction heating, expansion and extrusion. An in-house mapping tool is used to transfer the temperature fields between the electromagnetic-thermal and thermo-mechanical analyses. Using the combined model it is possible to study the influence of different process parameters on the temperature distribution in the billet, and how this affects the final extrusion properties. In this study, the model is applied to two cases of tube extrusion, one using an austenitic stainless steel and one using a duplex, austenitic/ferritic, stainless steel. It is shown that the induction heating model successfully predicts the temperatures obtained experimentally from thermocouples placed in the steel billets during heating. The agreement between models and experiments regarding extrusion force and expansion force is satisfactory.

The manufacturing process chain for extrusion of AISI 316L tubes is simulated using the finite element method. Models of induction heating and expansion is included and the temperature field in the billet before extrusion is calculated. It is shown that a correct initial temperature of the billet is needed in order to predict the extrusion force curve in the initial stage of the process.

A physical based model for the evolution of flow stress of AISI 316L from room temperature up to 1300 °C, strains up to 0.6 and strain rates from 0.0005 up to 10 s-1 is developed. One set of tests have been used for model calibration and another more complex set of tests for its validation. The model is based on a coupled set of evolution equations for dislocation density and (mono) vacancy concentration. Furthermore, it includes the effect of diffusing solutes in order to describe dynamic strain ageing (DSA). The model described the overall flow stress evolution well with exception of the details of the effect of the DSA phenomenon. Its numerical solution is implemented in a format suitable for large-scale finite element simulations.

The simulation of hot extrusion processes is a difficult and challenging problem in process modeling. This is due to very large deformations, high strain rates and large temperature changes during the process. Computer models that with sufficient accuracy can describe the material behavior during extrusion can be very useful in process and product development. Today, the process development in industrial extrusion is to a great extent based on trial and error and often involves full size experiments. Numerical simulations can most likely replace many of these experiments, which are often both expensive and time consuming. The motivation for this research project is a request for accurate finite element models that can be used in process design and development of stainless steel tube extrusion. The models will be used to investigate the effect of different process parameters on the quality of the extruded tube. In the work presented in this thesis, thermo-mechanically coupled simulations of glass-lubricated tube extrusion were performed. Extrusion models in two and three dimensions were developed. Only extrusion problems with radial symmetry were considered. Simulations were carried out using the commercial code MSC.Marc, which is a Lagrangian finite element code. Frequent remeshing was therefore needed during the analyses. The models were validated by comparing predicted values of extrusion force and exit surface temperature with measurements from an industrial extrusion press. The two- dimensional model was shown to provide good and fast solutions to extrusion problems with radial symmetry. A two-dimensional model is sufficient for many applications and this model is planned to be used for solving process problems further on. For the three-dimensional model it was concluded that a very fine mesh would be needed to successfully predict the extrusion force using four-node tetrahedrons. This would result in unacceptably long computational times. The future work will be aiming at improving the three- dimensional model in order obtain accurate results within a reasonable time. To obtain reliable simulation results a good constitutive model is crucial. This work has focused on the use of physically based material models, which are based on the underlying physical processes that cause the deformation. It is expected that these models can be extrapolated to a wider range of strains, strain rates and temperatures than more commonly used empirical models, provided that the correct physical processes are described by the model and that no new phenomena occurs. Physically based models are of special interest for steel extrusion simulations since the process is carried out at higher strain rates than what are normally used in mechanical laboratory tests. A dislocation density-based material model for the AISI type 316L stainless steel was used in the finite element simulations included in this thesis. The material model was calibrated by data from compression tests performed at different temperatures and strain rates.