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1

Numerical modelling and validation of precipitations kinetics in advanced creep resistant austenitic steel

Vujic S., Farooq M., Sonderegger B., Sandström R., Sommitsch C.

Computer Methods in Materials Science

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2012

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Vol. 12, No. 3

175--182

EN

The austenitic steel Sanicro 25 is one of the most promising austenitic steels for the application in superheater tubes in coal fired thermal power plants. In this work, the microstructural evolution of this material during heat treatment and thermal ageing has been investigated. The investigations were carried out by light microscopy (LIMI), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). Scheil calculations were carried out by thermo-kinetic software MatCalc to analyse the solidification process which indicates Nb(C,N), Cr2N and Laves phase in the melt. Long term precipitation calculations predict the formation of five precipitate types in Sanicro 25: M23C6, Z-phase, Nb(C,N), Laves and Cr2N. Phase fractions and mean radii evolution of precipitates were calculated and compared to the experimental results. Calculated precipitate evolution shows good compliance with experimental data.

PL

Stal austenityczna Sanicro 25 jest jedną z obiecujących stali do zastosowań na rury w wymiennikach ciepła w elektrowniach węglowych. W niniejszej pracy przeprowadzono badania rozwoju mikrostruktury podczas obróbki cieplnej i sztucznego starzenia tych stali. Badania wykonano za pomocą mikroskopu optycznego (LOM), skaningowego mikroskopu elektronowego (SEM), elektronowego mikroskopu transmisyjnego (TEM) i spektroskopu (EDS). Obliczenia metodą Scheila przeprowadzono wykorzystując oprogramowanie MatCalc. Analizowano proces krzepnięcia, w którym powstają fazy Nb(C,N), Cr2N oraz faza Lavesa. Obliczenia długoterminowego procesu wydzielania wykazały, że w Sanicro 25 powstaje 5 różnych faz: M23C6, faza-Z, Nb(C,N), faza Lavesa oraz Cr2N. Obliczone ułamki objętości poszczególnych faz oraz średni promień rosnących wydzieleń zostały porównane z wynikami doświadczeń i otrzymano dobrą zgodność.

2

Particle distance distributions and their effect on precipitation strengthening

Sonderegger B., Holzer I., Kozeschnik E., Sommitsch C.

Computer Methods in Materials Science

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2011

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Vol. 11, No. 1

148-153

EN

In this work, a general formulation of particle distances in glide planes is derived for arbitrary size distributions. The approach is designed to use the results of Wagner-Kampmann-type models. Size distributions are represented by a set of precipitate classes, where each class contains precipitates of a specific radius and a corresponding number density. The outputs of the calculation are not only particle distances, but particle distance distributions. This representation provides the convenience that any user-defined criterion for the precipitate-dislocation interaction can be applied; e.g. the side condition that at least one third of all spaces between obstacles have to be "open" for dislocation movement in order to allow plastic deformation. As a result, the combination of distribution function and side condition allows a direct calculation of the precipitates strengthening effect. It is shown, that the accordance of simulation and measurements is very promising. The method thus provides the potential to bridge the gap between the simulation of precipitate parameters and the quantitative calculation of precipitate strengthening.

PL

W pracy sformułowano metodę oceny odległości pomiędzy cząstkami w płaszczyznach poślizgu dla dowolnego rozmiaru cząstek wydzieleń. Metoda jest przystosowana do wykorzystania wyników z modelu typu Wagnera-Kampmanna. Rozkład wielkości cząstek jest reprezentowany przez zbiór klas wydzieleń, w którym każda klasa zawiera wydzielenia o pewnym specyficznym promieniu i odpowiadającej im gęstości. Wynikiem obliczeń są nie tylko odległości między cząstkami, ale także rozkład tych odległości. Zaletą takich wyników jest możliwość zastosowania dowolnie sformułowanego przez użytkownika kryterium interakcji wydzielenie-dyslokacja, na przykład warunek, że aby możliwe było odkształcenie plastyczne, co najmniej jedna trzecia wszystkich odległości między przeszkodami musi być otwarta dla ruchu dyslokacji. W rezultacie kombinacja funkcji rozkładu odległości i podanego kryterium pozwala na bezpośrednią ocenę umocnienia wydzieleniowego. W artykule wykazano, że symulacje dają wyniki zgodne z obserwacjami doświadczalnymi. Metoda jest zatem krokiem w kierunku wy-pełnienia luki pomiędzy symulacjami parametrów procesu wydzieleniowego i oceną umocnienia wydzieleniowego.

3

Hot forming and heat treatment simulation in a steel mill

Redl C., Tian B, Wieser V., Sommitsch C.

Computer Methods in Materials Science

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2007

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Vol. 7, No. 1

5-10

EN

Numerical simulation is nowadays becoming a standard tool in industrial processes. This paper provides an insight about the usage of finite element calculations in the development and production process in the steel mill of Böhler Edelstahl GmbH, Kapfenberg, Austria. Simulation methods are used all along the value chain at the mill starting with melting and casting of the steel. Nevertheless this paper focuses on the simulation of hot forming and heat treatment as well as on the simulations done as a service for our customers. The activities of finite element simulation of hot rolling concentrate on one side on the basic principles, for example, widening, strain distributions, grain models etc. On the other side, typical chains of hot rolling have been simulated and the results verified by comparing measured parameters with calculations. As an example the widening of a cold work steel at different temperatures is presented and compared with other steel grades. Considering as example a hot work tool steel of type 1.2343 the production chain from forging over the pre heat treatment at the mill to the application as a tool for aluminium extrusion is surveyed. The forming of the casted and remelted steel block is done on a radial forging machine of the type GFM SX55. The simulation takes into account five passes and delivers strains and temperatures within the work piece. Afterwards the pre heat treatment cycle of the part consisting of heating, quenching in polymer solution and subsequent air cooling is modelled. Simulation services are provided to the customers to optimize the material selection for their specific applications. The aforementioned tool steel is used as liner or die material in the extrusion industry. By a combination of a visco-plastic material model and a damage model it is necessary to predict the lifetime of the tools in use. In order to obtain the residual stresses and phase distribution after nitrogen quenching a heat treatment simulation of the die is performed. Afterwards a simulation of the extrusion process is carried out to get the distribution of effective stress and temperature in the die. The calculated life time as well as the predicted location of the maximum damage agrees well with observations from real extrusion processes.

4

Creep fatigue of multi-part container during hot extrusion of copper - Simulation and experimental comparison

Krumphals F., Wlanis T., Redl C., Sommitsch C.

Computer Methods in Materials Science

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2007

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Vol. 7, No. 1

47-53

EN

Extrusion tools exhibit a complex strain-time pattern under a variety of cyclic loading conditions and thus are prone to failure by creep-fatigue interactions. Elevated temperature failure by creep-fatigue processes is time dependent and often involves deformation path dependent interactions of cracks with grain boundary cavities. The extrusion industry tries to accelerate the process by increasing the billet temperature and/or by accelerating the press speed that raise the loading of the tools. On the other side the tool steel producers develop enhanced more homogeneous and cleaner materials in order to increase tools lifetime. Finite element simulation of the extrusion process to get the temperature and stress evolution in the container, coupled with constitutive equations as well as lifetime consumption models in order to calculate both the inelastic strains and the tools lifetime, help to optimise the extrusion process and to compare the operating times of different hot work tool steels. Viscoplastic constitutive models were developed in the past to take into account the inelastic behaviour of the material during creep-fatigue loads. In the present study the Chaboche model was selected and calibrated to the material response of a hot work tool steel between 470°C and 590°C. To extend the prediction capability of Chaboche’s model for non-isothermal processes a temperature-rate term was added to the isotropic hardening rule. Additionally, a creep-fatigue lifetime rule for complex processes was investigated that is independent of single loading parameters, like stress or strain ranges or corresponding maxima, for the description of an entire cycle. Instead this rule evaluates the total damage in each time increment and accumulates that to the lifetime consumption. The present paper shows the development of temperatures, stresses and lifetime consumption during three copper extrusion cycles in a two-part container. The simulation of the heat treatment and the resulting state of the container used was the basis for the subsequent modelling of the cyclic loads during the press cycles. The numerical FEM extrusion simulation consists of the plastic simulation of the billet extrusion with rigid tools as well as of the subsequent simulation of several cycles of the same process, only considering the elastic container and using the time dependent temperature and pressure boundary conditions at the contact surface billet-liner. The reason for this procedure is the much shorter calculation time for the elastic container model with specified boundary conditions in comparison to the plastic extrusion process, especially for several extrusion cycles. Both a constitutive law and a lifetime consumption rule were coupled to the elastic container model in order to get the local inelastic strain rates and the damage rate, respectively. To verify the calculated temperature and pressure boundary conditions at the contact surface billet-liner, a experimental extruding plant was constructed. To obtain a pressure distribution, three holes at different levels were drilled into the container, with only a thin container wall thickness left. The pressure force is transmitted through a plug gauge with a ceramic temperature isolator to a load cell. The system plug gauge /load cell sustains the container wall against damage. The same drilled holes are also used for temperature measurements, measuring points are positioned near the inner wall of the liner, in the middle of the container and for heat control at the outer container region. For the chosen extrusion examples, the simulations led to maximum lifetime consumption in the region of relatively high both temperature and equivalent stresses. These results seem to be reasonable in comparison to real lifetime of copper extrusion containers.

PL

Artykuł przedstawia rozkład pól temperatur, naprężeń oraz zużycia materiału podczas trzech cykli wyciskania miedzi w dwuczęściowym pojemniku. Symulacja procesu obróbki cieplnej oraz ostateczny stan pojemnika zostały wykorzystane jako podstawa do opracowania kolejnych kroków zużycia materiału podczas cyklicznych etapów wyciskania. Symulacja numeryczna MES obejmuje modelowanie plastycznego wsadu i sztywnych narzędzi oraz kilku cykli tego samego procesu dla elastycznego pojemnika przy wykorzystaniu zależnych od czasu warunków brzegowych dla temperatury oraz nacisku. Powodem takiej procedury obliczeniowej jest krótszy czas symulacji niż w przypadku plastycznego procesu wyciskania. W celu weryfikacji obliczonych warunków brzegowych temperatury i nacisku na powierzchni styku, skonstruowano specjalną maszynę laboratoryjną do prowadzenia procesu wyciskania.