Experimental and numerical modelling of the flow field in a CuxSny direct chill caster

Abstract

To investigate the influence of the casting speed on the flow field and the shape of the solidification front in an industrial bronze caster, numerical calculations have been performed and experimental flow field measurements were used to validate the numerical model. Both numerical and experimental model represented 1:1 the real caster geometry of 820 × 250 × 800 mm³. A steady-state calculation, considering solidification and turbulent flow, was performed. Furthermore, a 1:1 water model of the industrial bronze caster has been built and combined with a Particle Image Velocity (PIV) setup to measure the apparent flow fields. Amongst other parameters, the numerical model provided information on the influence of the casting speed on the solidification front shape. The water model gave the experimentalist the facility to adjust the shape of the solidification front to the one predicted by the numerical model and compare the resulting flow field with the numerical prediction. This work presents a comparison of the results of both numerical and experimental models for different casting speeds with the same numerical parameters and boundary conditions.     

Electrochemical synthesis of ferritic stainless steels alloyed with 1 wt-%Ti,V and Nb in sulphuric acid solution

Abstract

Corrosion behaviours of stainless steel alloys containing corrosion resistant elements were investigated. Ferritic stainless steel electrodes were synthesised by the application of a scan rate of 1 mV s^–1. Stainless steels were unalloyed and alloyed with approximate 1 wt-% Ti, V and Nb elements. The samples were obtained from casting and forging. The steels were exposed to different heat treatments. Heat treatment was not applied to the first group of samples. The second and the third group of samples were rapidly cooled after annealing at 1100°C for 30 and 180 min respectively. The corrosion performances of ferritic stainless steels were investigated in 0·1 M H₂SO₄ solution, by use of electrochemical impedance spectroscopy. Scanning electron microscopy (SEM) investigations were performed. Scanning electron microscopy micrographs showed generalised pitting on the surface. Corrosion resistance was calculated by Stearn-Geary equation. It was determined that titanium has the best effect on the corrosion resistance of ferritic stainless steels homogenised for 180 min.     

Modelling molten flux layer thickness profiles in compact strip process moulds for continuous thin slab casting

Abstract

The important functions promoted by powdered flux added over the liquid steel surface in continuous casting moulds are strongly affected by the thickness of the liquid layer that forms as a result of the heat absorbed. The present work discusses the results of a three-dimensional steady state model, developed to represent the coupled fluid flow and heat transfer phenomena that determine thickness profiles of the liquid flux layer. Since the laminar flow of the liquid slag layer depends on the shearing imposed on it by the turbulent motion of the liquid steel beneath it, and since additionally this motion is strongly influenced by the flow characteristics of the steel stream poured into the mould through the submerged entry nozzle (SEN), separate turbulent flow models for the liquid steel in the SEN and the mould were also developed. The consistency among the models and their accuracy was judged by comparing thickness and temperature flux profiles measured in plant against predicted ones; the comparison showed good agreement. The effects of casting speed, mould width, and flux viscosity and heat of melting on the liquid layer thickness were investigated. The last variable was found to exert the most marked influence. Different from conventional casting moulds, where the liquid layer thickness increases with increasing casting speed, in compact strip process moulds the thickness remains almost constant with increasing casting speed. This difference is well accounted for by the model, which suggests that this behaviour stems from the different slag flow patterns generated in straight, wide moulds and in thin moulds having a central upper funnel shaped section.     

Results of full scale CONSTOR® V/TC prototype 9 m horizontal drop test

Abstract

In the context of the research on the mechanical safety of packages for radioactive material, full scale drop tests with spent fuel and high activity waste transport and storage casks have been performed by the Federal Institute for Materials Research and Testing (BAM). The research reflects national and international interest in acquiring comparative knowledge of full and reduced scale model drop tests as well as in finite element calculations. This paper presents the experimental, analytical and first numerical results of the full scale drop test with the full scale CONSTOR^® V/TC prototype, manufactured by GNS, Gesellschaft für Nuklear-Service mbH, Germany. The prototype was tested by BAM in a 9 m horizontal drop test onto the unyielding target of the BAM drop test facility in Horstwalde, Germany.     

Simple model for austenite grain growth in microalloyed steels

Abstract

Previous models for grain growth are usually based on Beck's formula, which are inadequate for quantitative prediction of austenite grain growth during reheating of as cast microstructures in microalloyed steels. The applications of these empirical grain growth models are limited to some particular categories of steels, such as Nb, Nb–Ti and Ti–V microalloyed steels, etc. In this study, a metallurgically based model has been developed to predict the austenite grain growth kinetics in microalloyed steels. This model accounts for the pinning force of second phase particles on grain boundary migration, in which the mean particle size with time and temperature is calculated on the basis of the Lifshitz–Slyozov–Wagner (LSW) particle coarsening theory. The volume fraction of precipitates is obtained according to the thermodynamic model. The reliability of the model is validated by the agreement between theoretical predictions and experimental measurements in the literature.     

Simulation and application of dynamic heat transfer model for improvement of continuous casting process

Abstract

Numerical simulation is being increasingly used to improve the existing cooling systems. In order to attain highest quality strand, a two-dimensional dynamic mathematical heat transfer model of billet continuous casting of low carbon steel has been presented. This model can be used to compute the billet temperature distribution and shell thickness, especially it can be used to simulate the solidification process which is caused by frequently variational casting conditions. The fluctuation of measured temperature has been reduced to <10°C with thermal imaging system. The online model can monitor surface temperature and shell thickness in the casting process. So it provides the possibility for the online process control. For the validation of the dynamic model, a lot of billet surface temperature and shell thickness measurement were carried out on an actual casting machine. Finally, the dynamic model has been used for adjusting the operating parameters to improve the casting speed.     

Numerical modelling of small disc creep test

Abstract

The small disc creep test is described. The disc creep curves observed need to be related to the creep properties of the material obtained by conventional testing. This relationship should include a means of assessing the creep damage which is initially present in the material and therefore to an estimate of remanent life. There are difficulties with such correlations and these are shown to be related to an imperfect understanding of the complex nature of the small punch test. This understanding can be improved by suitable physical modelling of the deformation occurring. The present paper describes a suitable finite element model and verifies the model against experimental observations of disc creep tests. The model uses a realistic creep deformation law which includes strain hardening, thermal softening and damage accumulation for 0·5Cr0·5Mo0·25V steel. It is shown that the results obtained from the test are sensitive not only to the initial condition of the specimen but also to the conditions of the test (e.g. the values of frictional forces at the punch/specimen interface). The paper makes suggestions of how the model may be used to optimise the disc creep test.     

Tensile properties of thermally exposed aluminium borate whisker reinforced 6061 aluminium alloy composite

Abstract

The effect of thermal exposure on the tensile properties of aluminium borate whisker reinforced 6061 aluminium alloy composite was studied. The interfacial reaction was investigated by TEM and the mechanical properties were studied using tensile tests. The results indicated that the interfacial reaction had an influence on the mechanical properties of the composite, so that the maxima of Young’s modulus and ultimate tensile strength of the composite after exposure at 500°C for 10 h were obtained for the optimum degree of interfacial reaction. The yield strength, however, was not only affected by the interfacial state but also by many other factors.     

Role of dislocation density and structure in grain boundary segregation of phosphorus and carbon in model Fe-P-C alloy

Abstract

The present study has investigated the effects of dislocation density, produced by tensile prestraining, on the grain boundary segregation of phosphorus and carbon in a Fe-0.06P-0.002C (wt-%) alloy during stress free isothermal annealing at 500°C for periods up to 1800 h. Changes in grain boundary segregation were followed using Auger spectroscopy, while changes in dislocation density and structure were observed using transmission electron microscopy techniques. The segregation of phosphorus (but not carbon) was enhanced, compared with unstrained specimens, during initial aging. Analysis of diffusion rates required to cause the observed increase in phosphorus segregation suggested that the kinetics of phosphorus segregation was enhanced by pipe diffusion. At intermediate aging times, desegregation of phosphorus was observed, an effect attributed to a reduction in intragranular solute levels resulting from phosphorus precipitation on dislocations. In the case of carbon this process continued to the longest aging times examined and, essentially, complete desegregation ensued. In contrast, at these long aging times, phosphorus segregation resumed, and this was associated with an increase in the binding energy of phosphorus to the carbon denuded grain boundary.     

Adhesively Bonded Patch Repair of Composite Laminates

Damaged composite laminates repaired using adhesively bonded patches have been studied. A special adhesive element is developed to examine the stress distribution in the bonded region. Utilizing the adhesive element, one is able to incorporate the regular elements in the laminate and patch. It has the advantage of reducing the adhesive bonding problem to a two-dimensional in-plane problem, and avoiding the need for refined meshes in the adhesive. The special adhesive element is derived based on the assumption of constant shear stress through the thickness of the adhesive. The damaged area of the composite laminate is simulated as a hole. The repair efficiency is evaluated by comparing the stress concentration factor in the damaged hole before and after repair. The effects of the thickness, size and material properties of both patch and adhesive on the stress distribution are presented through a parametric study. Numerical results indicate that a stiffer and thicker patch is able to carry higher loads, and, consequently, reduce the load across the damaged area yielding less stress concentration in the damaged hole. For a high shear modulus and thin thickness of the adhesive layer, less loads are transferred to the patch resulting in a high stress concentration in the damaged hole.