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Accounting for the random character of nucleation in the modelling of phase transformations in steels

Poloczek Łukasz, Kuziak Roman, Foryś Jakub, Szeliga Danuta, Pietrzyk Maciej

Computer Methods in Materials Science

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2023

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Vol. 23, No. 2

17--28

EN

In our earlier work, a stochastic model of multi-stage deformation at elevated temperatures was developed. The model was applied to calculate histograms of dislocation density and grain size at the onset of phase transformation. The histograms were used as input data for the simulation of phase transitions using the traditional deterministic model. Following this approach, microstructural inhomogeneity was predicted for different cooling conditions. The results obtained, showing the effect of dislocation density and inhomogeneity of austenite grain size on the microstructural inhomogeneity of the final product, can be considered reliable as they are based on material models determined in previous publications and validated experimentally. The aim of the present work was to extend the model by taking into account the stochastic nature of nucleation during phase transitions. The analysis of existing stochastic models of nucleation was performed, and a model for ferritic transformation in steels was proposed. Simulations for constant cooling rates as well as for industrial cooling processes of steel rods were performed. In the latter case, uncertainties in defining the boundary conditions and segregation of elements were also considered. The reduction of the computing costs is an important advantage of the model, which is much faster when compared to full field models with explicit microstructure representation.

2

Thermal-mechanical finite element simulation of flat bar rolling coupled with a stochastic model of microstructure evolution

Szeliga Danuta, Czyżewska Natalia, Kusiak Jan, Kuziak Roman, Morkisz Paweł, Oprocha Piotr, Pietrzyk Maciej, Piwowarczyk Michał, Poloczek Łukasz, Przybyłowicz Paweł, Rauch Łukasz, Wolańska Natalia

Computer Methods in Materials Science

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2022

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Vol. 22, No. 3

137--148

EN

It is generally recognized that the kinetics of phase transformations during the cooling of steel products depends to a large extent on the state of the austenite after rolling. Austenite deformation (when recrystallization is not complete) and grain size have a strong influence on the nucleation and growth of low-temperature phases. Thus, the general objective of the present work was the formulation of a numerical model which simulates thermal, mechanical and microstructural phenomena during multipass hot rolling of flat bars. The simulation of flat bar rolling accounting for the evolution of a heterogeneous microstructure was the objective of the work. A conventional finite-element program was used to calculate the distribution of strains, stresses, and temperatures in the flat bar during rolling and during interpass times. The FE program was coupled with the stochastic model describing austenite microstructure evolution. In this model, the random character of the recrystallization was accounted for. Simulations supplied information about the distributions of the dislocation density and the grain size at various locations through the thickness of the bars.

3

Stochastic model describing evolution of microstructural parameters during hot rolling of steel plates and strips

Szeliga Danuta, Czyżewska Natalia, Klimczak Konrad, Kusiak Jan, Kuziak Roman, Morkisz Paweł, Oprocha Piotr, Pietrzyk Maciej, Poloczek Łukasz, Przybyłowicz Paweł

Archives of Civil and Mechanical Engineering

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2022

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Vol. 22, no. 3

art. no. e139

EN

Enhancing strength-ductility synergy of materials has been for decades an objective of research on structural metallic materials. It has been shown by many researchers that significant improvement of this synergy can be obtained by tailoring heterogeneous multiphase microstructures. Since large gradients of properties in these microstructures cause a decrease of the local fracture resistance, the objective of research is to obtain smoother gradients of properties by control of the manufacturing process. Advanced material models are needed to design such microstructures with smooth gradients. These models should supply information about distributions of various microstructural features, instead of their average values. Models based on stochastic internal variables meet this requirement. Our objective was to account for the random character of the recrystallization and to transfer this randomness into equations describing the evolution of dislocations and grain size during hot deformation and during interpass times. The idea of this stochastic model is described in the paper. Experiments composed of uniaxial compression tests were performed to supply data for the identification and verification of the model in the hot deformation and static recrystallization parts. Histograms of the grain size were measured after hot deformation and at different times after the end of deformation. Identification and validation of the model were performed. The validated model, which predicts evolution of heterogeneous multiphase microstructure, is the main output of our work. The model was implemented in the finite element program for hot rolling of plates and sheets and simulations of these processes were performed. The model’s capability to compare and evaluate various rolling strategies are demonstrated in the paper.

4

Influence of heat treatment on thermo-mechanical fatigue and high temperature creep properties for 316L steel

Poloczek Łukasz, Kuziak Roman, Pidvysots’kyy Valeriy

Journal of Metallic Materials

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2021

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Vol. 73, no. 3

33--43

EN

The main purpose of the work was to develop the characteristics of high-temperature structure degradation processes under the synergistic effects of thermo-mechanical fatigue and high temperature creep for samples made of 316L steel in the delivery condition and after heat treatment. The use of heat treatment consisting of solution treatment at 1100°C for 45 minutes and water cooling improved the mechanical properties of 316L steel. Moreover, in fatigue tests, in every case, increasing the strain value in a single cycle leads to a faster sample rupture. An additional aim of the research was also to develop and verify a new innovative research methodology concerning the combination of fatigue cycles with the creep process at elevated temperature, the aim of which is to better reflect the behaviour of the material in real working conditions.

PL

Głównym celem pracy było opracowanie charakterystyk wysokotemperaturowych dla procesów degradacji struktury pod wpływem synergicznego oddziaływania zmęczenia cieplno-mechanicznego i pełzania w podwyższonej temperaturze dla próbek wykonanych ze stali 316L w stanie dostawy i po obróbce cieplnej. Zastosowanie obróbki cieplnej polegającej na przesyceniu w 1100°C przez 45 minut i chłodzeniu wodą poprawiło właściwości mechaniczne stali 316L. Ponadto w testach zmęczeniowych w każdym przypadku zwiększenie wartości odkształcenia w jednym cyklu prowadzi do szybszego rozerwania próbki. Dodatkowym celem badań było również opracowanie nowej, innowacyjnej metodyki badawczej dotyczącej połączenia cykli zmęczeniowych z procesem pełzania w podwyższonej temperaturze, której celem jest lepsze odzwierciedlenie zachowania materiału w rzeczywistych warunkach pracy.