Materials Science Forum Vols. 638-642

Abstract: Today the numerical simulation of hot deformation processes is very advanced. But it requires mathematical models for metalphysical processes as for microstructure development, which take place during the deformation. Until now such models were developed for many steel grades and non-ferrous materials. For new steels as multi-phase steels laboratory investigations are required, in order to determine the optimal processing technologies of these materials. This applies also to the modelling. So far it is impossible, to calculate sole by mathematical solutions the manifold parameters of metalphysical processes and microstructure, for this reason laboratory trials and simulations are needed implicitly. Even for well known materials such procedures can be essential and useful. Using the multi-functional simulation system Gleeble HDS-V40 it is shown, which possibilities a physical simulation offers today. Starting with the annealing conditions, followed by microstructure development up to cooling, selected examples reflect the results of property development during hot deformation processes. The differences between conventional deformation after re-heating and deformation after direct-charging will be presented. The last-mentioned concept offers in its combination of near-netshape casting and direct charging special benefits, especially saving of energy.

Abstract: A model for anisotropic Ostwald ripening was developed using a chemical potential (weighted mean curvature) difference as a driving force for mass-transport. Based on this model, grain growth simulations of silicon nitride during the phase transformation and Ostwald ripening were performed. Comparison with experimental results during the phase transformation suggests that grain growth be controlled by interfacial reaction. Simulations of Ostwald ripening predict that the growth exponent be 3 for the reaction-controlled case, and increases up to 5 as the growth kinetics shifts from reaction-controlled to diffusion-controlled. It was reported that the mean aspect ratio of silicon nitride crystals increased during the phase transformation, and decreased during Ostwald ripening. These behaviors were successfully simulated by this model. The concave depression at the tip of silicon nitride crystal that was experimentally observed. Simulations by the Ostwald ripening model demonstrated that it could be developed when the liquid phase was super-saturated, and further that the tip shape was a function of the liquid concentration.

Abstract: The article presents the results of physical simulations of the process of rolling plates of steel in the strength category 350÷460MPa on the finishing stand of a reversing mill. The simulations were performed for three steel grades and final plate thicknesses 40mm. The purpose of the simulations was to select the conditions of plate deformation to obtain, directly after rolling, structure of material corresponding to the structure obtained from normalizing annealing. Thus, the simulations reflected the normalizing rolling process. The research carried out has allowed to elaborate the new technology of production of rolling plate without an additional thermal treatment like normalization. It allow to decrease a time and total cost of production for this sort of product.

Abstract: In the present work, low-carbon ultra grained constructional low-alloyed steel were subjected to thermo-mechanical treatment for modification of microstructure. It shows that microstructure after thermo-mechanical treatment is quite dependent on the alloy composition, conditions of hot deformation, grain size of austenite and cooling rate. The research was provide by using the computer program for thermo and thermo – mechanical treatment. The most optimal variant of heat treatment and thermo – mechanical deformation were obtained. The verifications were provided by the dilatometer with possibility of deformation DIL 805A/D.

Abstract: The article describes the behaviour of coke in the blast furnace. Factors, which cause weakening and degradation of coke lumps at temperatures above 1300°C have been analyzed. On the basis of preliminary testing of samples taken from a blast furnace at different distances from the tuyère outlet and tests for thermo-abrasion ξ, the advisability of using the pre-tuyère chamber for the assessment of coke quality at high temperatures has been indicated. Thermodynamic calculations for the determination of the chemical composition of the products of reaction of coke ash mineral substances with elementary carbon and air, as well as the behaviour of coke at high temperatures under inert gas conditions are presented. The number of compounds forming from coke ash components during heating in a stream of gases of highly differential reductiveness reflects the complexity of the physicochemical phenomena.

Abstract: The concept of combining the latest finite element (FE) and discrete element (DE) multiscale numerical technologies for modelling of the tool/workpiece interface during high shear processing is described. The potential of FE tools and techniques merged with DE based transient dynamics is highlighted. Linking of the modelling scales is based on transferring the corresponding boundary conditions from the macro model to the representative cell, considered as the meso- level model. This meso- model consists of a large number of bodies that interact with each other. The transfer processes are described by the system of diffusion and motion equations including contact detection and interaction solutions for particles integrated in time. Modelling of the tool/workpiece interface including both mixing of the oxide particles into the subsurface layer during hot rolling of aluminum and heat generation during friction stir welding (FSW) are considered.

Abstract: The paper presents results of theoretical and experimental studies on the influence of strip temperature reduction at the final stage of the normalizing rolling process in a continuous bar rolling mill on the energy and force parameters. The studies were carried out for 38 mm-diameter round bars of constructional steel S355J2G3 (St52-3 acc. to DIN). At the first stage, numerical modelling of the rolling process was performed for the conditions of the currently used rolling technology. The aim of this stage of the work was to determine the distribution of strip temperature during the rolling process and of strip temperature after the rolling process. The obtained computation results were compared with the measurement results recorded during the actual rolling process using a thermovision camera. A computer program designed for three-dimensional modelling of metal flow during rolling in passes was used for the determination of the distribution of strip temperature during rolling. The second stage of the studies included modelling of the bar rolling technology modified by applying normalizing rolling after introducing accelerated strip cooling at the final stage of rolling. The aim of this stage of the work was to establish the influence of reduced strip temperature on the change in the values of the energy and force parameters of the process. By comparing the computed values of rolling power with those of the permissible power it was found that these values were lower than the permissible power of the rolling machine’s main drives installed in the Rolling Mill under analysis. Thus, it was demonstrated that it was possible to implement the process of normalizing rolling in the conditions of the Shape Mill under study.

Abstract: The influence of compressive stress up to 250 MPa and plastic deformation of austenite on the martensite transformation behavior of a eutectoid steel is studied both experimentally and theoretically. It is demonstrated that martensite formation is assisted by stress but it is retarded when transformation occurs from plastically deformed austenite. With the quantitative modeling of the problem based on the theory of displacive shear transformation, the explanation of contradicting observations is presented.

Abstract: The paper deals with the inverse problems appearing in the thermal theory of foundry processes. In particular, the problems connected with identification of casting and mould thermophysical parameters can be considered. To estimate the parameters determining heat transfer processes in the system casting-mould, the additional information concerning the changes of temperature at the points selected from the domain considered is necessary. The localization of sensors should assure the best conditions of identification process. Generally speaking, the sensor should be located at the point for which the sensitivity of temperature field with respect to parameter estimated achieves maximum. In the case of simultaneous identification of several parameters, the proper choice of sensors localization is more complicated. The main goal of the paper is to discuss a theoretical base connected with the problem formulated, to present the possibilities of numerical methods application, to solve the problem, and finally to show the examples of inverse problems solutions (in particular, the latent heat of binary alloy will be estimated).

Abstract: The simulative accelerated creep test (ACT) on Gleeble thermal-mechanical simulator was developed as a response to an overall need of gaining in a short time useful physical data for estimation of long-term behaviour of materials exposed to operation at elevated temperatures. While most of the design of components and estimation of power plants lifetime is based on long-term creep data, which are generally available for the plate and pipe creep resisting steels, in case of the welded joints on these components such data are not always easily found and e.g. for the repair welds are very often not available. Next to its short duration, demanded from the accelerated creep testing procedure are the following conditions: - The basic temperature and applied strains in the ACT must prevent odd transformations like secondary dissolution of carbides or intensive formation of non-equilibrium phases. - The final deformation at fracture must be like at real creep – just a few pct in total. - The depletion of weld metal or steel matrix in alloying elements must be achieved similar to that of crept steels and the carbide phases at onset of cracks must not be different. To meet these requirements a low-cycle thermal-mechanical deformation-relaxation test was developed, during which the sub-structural processes characteristic of creep occurred much faster, transforming the initial microstructure of the creep resisting steels and welds to the near-to-equilibrium one in less than 30hours. The formation of dislocation configurations and the sequence of carbide precipitation and coagulation, as confirmed by TEM investigations, well resembled these of the conventional creep, so the testing procedure, hence called the simulative accelerated creep test – ACT, has been verified this way.