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Modelling of microstructure evolution in hot work tool steels during service

Krumphals F., Wlanis T., Sommitsch Ch., Holzer I., Sonderegger B., Wieser V.

Computer Methods in Materials Science

2009

Vol. 9, No. 2

228-233

Hot work tool steels are commonly in use as tools for manufacturing processes of metallic materials at elevated temperatures. To establish a reliable lifetime prediction of the tool, it is necessary to characterize the initial microstructure as well as its evolution during service since the material properties depend on the microstructural configuration. The investigated X38CrMoV5-1 hot work tool steel, which has a body centered cubic lattice structure, forms a distinct dislocation cell and subgrain structure, respectively. In this work a dislocation model for thermal creep using the rate theory with particular consideration of the subgrain boundary behaviour is applied [1]. The subgrains limit the dislocation movement and their diameter is a key parameter in determining the creep rate under many conditions. Prior computations using a dislocation dynamics technique show that the emergence of an organized subgrain structure results as a consequence of elastic energy minimization. The choice of a dislocation model has many advantages over phenomenological models and equations of state with variables not identified with microstructural features in hot work tool steels. For a detailed description of the dislocation density evolution under thermal and cyclic mechanical loads, the dislocation model is adapted to consider both creep and concurrent appearing of low cycle fatigue. Several impacts of different load cases on the microstructure evolution are demonstrated. Simultaneously, the influence of precipitation size evolution and distribution on dislocation movement is considered. The model is compared with experimental investigations of inelastic strains in tools used for hot extrusion applications, i.e. for both container [2] and die [3]. [1] N.M. Ghoniem et al.: "A dislocation model for creep in engineering materials", Res Mechanica 29, 197-219, 1990 [2] C. Sommitsch et al.: "Modelling of creep-fatigue in containers during aluminium and copper extrusion", Computational Materials Science 39, 55-64, 2007 [3] W. Mitter et al.: "Lifetime prediction of hot work tool steels", Lab. Report, Journal of Heat Treatment and Materials Science (HTM), 52, 1997

Tematem pracy jest przewidywanie czasu pracy narzędzi w warunkach eksploatacyjnych. Końcowe własności wyrobu zależą od jego początkowej mikrostruktury oraz zmian tej mikrostruktury podczas wytwarzania. Dlatego za główny cel pracy postawiono sobie modelowanie rozwoju mikrostruktury podczas procesu obróbki cieplnej oraz pracy narzędzi w warunkach eksploatacyjnych. Modelowanie ewolucji mikrostruktury ze szczególnym uwzględnieniem kinetyki wydzieleń wykonano z wykorzystaniem pakietu MatCalc. W pracy analizie poddano stal narzędziową X38CrMoV5-1 o strukturze bcc, która tworzy wyraźną strukturę dyslokacyjną oraz podziarnową. Wykonane obliczenia numeryczne poddano również weryfikacji doświadczalnej.

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

2007

Vol. 7, No. 1

47-53

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.

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.