Materials Science Forum Vols. 706-709

Abstract: Thermal processes in a thin metal film subjected to a cyclic external short-pulse heating are considered (axially-symmetrical 3D problem). The heat transfer proceeding in domain analyzed is here described by the dual phase lag model (DPLM). According to the newest opinions the DPLM constitutes a very good description of real heat transfer processes proceeding in the micro-scale domains subjected to the strong external heat flux. The base of DPLM formulation is a generalized form of Fourier law (GFL) in which two times τ_q, τ_T appear (the relaxation time and thermalization one, respectively). The acceptation of GFL leads to DPLM equation [1, 2]. Thermal processes proceeding in a thin metal film subjected to a cyclic external short-pulse heating are considered (axially symmetrical 3D problem). In the paper the thermal interactions between cyclic external heat source q_b and cylindrical micro-domain are analyzed. The external heat source is the function dependent on spatial co-ordinates and time. On the remaining parts of the boundary the no-flux conditions are assumed. It should be pointed out that the DPL model requires the adequate transformation of boundary conditions which appear in the typical macro heat conduction models. The initial conditions are also known (initial temperature of domain and initial heating rate). Numerical model of the process discussed bases on a certain variant of finite differences method and in the final part of the paper the examples of computations are shown.

Abstract: The problem of transverse crack formation in continuously cast steel has been an issue for over 30 years. The crack susceptibility depends not only to the steel composition, but also to the temperature history during casting. To improve the surface quality, two key solutions have been suggested; adjusting the straightening stage to an appropriate temperature range, out of hot ductility trough, and/or alloy modification. These solutions have certain limitations and difficulties still exist for grades such as peritectic steels containing Nb or V. In this article, the hot ductility of a peritectic steel grade with Nb has been studied under direct cast conditions. This was performed using a Gleeble 3800 thermo-mechanical simulator under two critical strain rates. In addition, the transverse crack formation and appearance, as well as the application of infrared camera to study the surface temperature profile of the slabs is presented.

Abstract: The weld heat affected zone (HAZ) of HPS 100W was characterized in terms of microstructure and mechanical properties. HAZ simulation was carried out in a GLEEBLE 3500 along with in-situ dilation tests at different peak temperatures of 850°C, 1100°C, 1300°C and 1350°C with heat input of 2kJ/mm. Subsequently, impact toughness data were obtained at different temperatures by means of Charpy tests. The microstructures were characterized using optical microscope, SEM and TEM to correlate the structure-property relationship of the HAZ. Under the tested conditions, all the simulated HAZs exhibited higher impact toughness than the base metal.

Abstract: We present the results from first principle calculations of H₂O adsorption on oxygen-covered Fe (100) surface. The calculations are based on a density-functional theory, surface modeling which uses supercell slab models. As a surface oxygen coverage increases, the surface is not activated, which makes the adsorption of water molecules on Fe surface unfavorable. It has been found that the surface covered oxygen exerts an influence on the adsorption of H₂O molecule on Fe surface.

Abstract: A new approach for case hardening of powder metallurgical steels is surface densification prior to heat treatment, hence avoiding hardening to the core caused by open porosity. With regard to this process chain a porosity and carbon dependent model of the transformation kinetics is essential. In powder metallurgical materials the transformation behavior is mainly influenced by the chemical composition, homogeneity and porosity. Using a prealloyed powder, e.g. Astaloy 85 Mo, a homogeneous distribution of alloying elements after sintering can be assumed and the transformation behaviour is mainly determined by pores and the carbon profile caused by case hardening. The effect of carbon is widely known but up to now, only a few details about the effect of porosity on the transformation can be found in literature. It is reported that a decreasing relative density causes a reduction of incubation and overall isothermal transformation time. In the present study, the transformation kinetics of a powder metallurgical steel based on Astaloy 85 Mo were investigated for the carbon levels 0.5 and 0.8 wt% as well as the relative densities 6.8, 7.2 and 7.8 g/cm³. The investigations were carried out using a high-speed quenching dilatometer. The isothermal time temperature transformation diagrams for this powder-metallurgical alloy are presented and Avrami-type equations are fitted to the measured data. A good correlation can be found for the transformation model and the experimental results verifying the used modeling approach showing the potential to be applied within case hardening simulations.

Abstract: The thermomechanical properties of an AlMg0,8Si1 (corresponds to EN AW-6181) alloy in T4 state (Ecodal^® 608) were investigated under coldArc^® welding conditions using Gleeble 3500 concerning the numerical simulations of residual welding stresses and distortions. Thereby, tensile tests were carried out during the heating phase, cooling phase and after natural aging up to 10 days. Thus, the effect of weld cycle and corresponding dissolution of β''(Mg₂Si)- precipitates on the mechanical properties was physically simulated. Furthermore, two phase transformation models were used to simulate the dissolution of hardening precipitates as a result of weld temperature cycles. The used models were compared, considering their capabilities and accuracy.

Abstract: The fast development of automotive industry effects significantly on aspirations of designers and constructors to reduces the mass-produced cars, affecting meaningly on fuel consumption and gas emition. From the standpoint of automotive industry materials for modern car-body sheets should have high mechanical properties (primarily high tensil strenght) and very good cupping. The required high mechanical and plastic properties steels used in produce of car bodies are dependent on the type of the obtaining structure, witch be shaped by an appropriate heat and thermo-plastic treatment. The modern steels used in automotive industry are multi-phase steels e.g. dual-phase (DP), complex-phase (CP) and transformation induced plasticity (TRIP) steels. In this paper are presented the results of physical and numerical modeling of heat treatment the experimental complex-phase steel, witch be conducted in the Institute of Modeling and Automation of Plastic Working Processing on Częstochowa University of Technology. The numerical modellig of heat treatment were carried with using the commercial programe TTSteel. Based on the results of computer simulation the changes of steel microstructure during continuous cooling were analyzed and the characteristics temperature and CCT diagram was constructed. Numerical research have been verified by the physical simulation of heat treatment by the dilatometer DIL805. The characteristic temperature of investigated steel and the size of initial austenite grains were determined. On the samples was also metallographic examination and Vickers hardness testing conducted. The obtained results were used to build a real CCT diagram of steel.

Abstract: Dual-phase (DP) steels have a composite-type microstructure consisting mainly of hard martensite islands embedded in a soft ferrite matrix. DP steels exhibit a characteristic combination of high strength, high work hardening rate and good ductility. Mechanical behaviour of DP steels is closely related to their microstructures. Hence, it is necessary to take into account their microstructural parameters in any attempt to estimate their flow behaviour. In this study, the flow curves of low carbon DP steels with 26.4 and 52% martensite produced by intercritical annealing processes at different temperatures were calculated using the finite element method (FEM). According to the results of microscopical observations of steel microstructures, martensite islands were assumed in the model to be spherical in shape. Moreover, in agreement with the experimental results in the literature clearly showing the possibility of martensite plastic deformation in similar steels during straining when its volume fraction is greater than about 30%, the plasticity of martensite islands in the steel microstructures was taken into account in the model by using their experimentally obtained stress-strain relations as a function of their estimated carbon contents. Having estimated the stress-strain behaviour of the ferrite phase in all steel samples using the microstructural parameters corresponding to the starting ferrite+pearlite steel, the flow curves of different DP steel samples with elastic martensite (in low martensite content steels) and elasto-plastic martensite (in high martensite content samples) were calculated. The calculated stress-strain curves exhibited reasonable agreements with the experimental stress-strain curves obtained from the tensile tests.

Abstract: Numerical simulations of phase separation in Fe-Cr-Mo―Ni or Fe-Cr-MoーTi quaternary alloys similar to ferrite phases in dplex stainless steeswere performed by the Cahn-Hilliard equation. We obtained that the asymptotic behaviour of minor element Ni, Mo, Ti in an Fe-Cr-X quaternary alloy along a trajectory of a peak top of the major element Cr is classified into three groups according to the sign of the second derivative of the chemical free energy with respect to the compositions of Cr and X(X=Mo, Ni or Ti]. It is also predicted that that small addition of Mo upto 10% accelerates phase separation of Cr. It seems that the optimum value of Mo exists to enhance the phase separation of Cr in Fe-Cr-Mo ternary alloys. Simulation result indicates that the phase separation of Cr is most enhanced with addition of 1% Mo. The above mentioned simulation results given by the numerical simulation by the Cahn-Hilliard equation were in good agreement with those obtained by the Monte Carlo simulation Theoretical analyses were performed in order to discuss the simulation results. On the basis of theb simulation results Optimum materials design of the duplex stainless steel has been established.

Abstract: Today the barbotage process is commonly used for refining aluminium and its alloys. There are many refining reactors available all over the world. The refining gas can be introduced to the liquid metal by nozzles, ceramic porous plugs and rotary impellers. The last ones become the most popular. The shape of impellers is different and influences the level of gas dispersion in the liquid metal. Physical modelling is quite often used for modelling the aluminium barbotage process. In this way it is possible to observe the phenomena that take place in the whole process. Results obtained from this kind of research can be representative and transformed to real conditions if the test stand is built according to the theory of similarity. The test stand was built for modelling the refining process in the URO-200 reactor. Three different shapes of impellers were tested. The processing parameters like the flow rate of refining gas and the impeller rotary speed changed in the range from 5 to 15 dm³/min and from 0 to 500 rpm respectively. Additionally the research of oxygen removal from water was carried out as an analogy of hydrogen removal from liquid aluminium. The level of oxygen was measured by means of oxygen meter Elmetron CO-401. The obtained results were discussed and graphically presented.