Flow pattern and alloy dissolution during tapping of steel furnaces

Abstract

Alloying of steel during tapping from BOFs and EAFs has been studied by computational fluid dynamics in two-dimensional axisymmetric models of two ladles. The flow patterns and particle trajectories have been computed for six different levels of steel in the tapping ladle, five different alloy sizes, two alloy injection points, and three types of bulk alloy (FeMn, SiMn, and FeSi75). Based on the fluid dynamics in the ladle and a definition of good alloying practice, conclusions with regard to alloy sizing and timing of alloy addition have been established. The computational results support findings in plant tests, which show the benefit of using small sized alloys. Furthermore, a method that allows us to estimate the optimum feeding rate for alloy additions during steelplant operation has been developed. Results from full scale tests in steelplants are shown.     

Diffusion bonding of alumina reinforced 6061 alloy metal matrix composite using Al–Li interlayer

Abstract

This article describes a study of the application of a solid state diffusion welding technique to an aluminium alloy (6061) matrix composite reinforced with alumina particles in two different percentages (10 and 20 vol.-%) using an Al–Li alloy interlayer. The influence of bonding parameters on joint formation and the effect of the reinforcement in the bond line were determined by microstructural study of the joints. Shear tests using single overlap joints were used to evaluate the strength of these bonds.     

Behaviour of surface oxide scale before roll bite in hot rolling of steel

Abstract

Hot stalling rolling tests using mild steel slabs with thin and thick scale layers, and cold stalling rolling simulation tests using lead slabs coated with brittle lacquer layers, have been conducted to investigate the behaviour of the surface oxide scales before hot rolling. The effects of rolling reduction, temperature, scale thickness and structure, and slab geometry were examined. The experimental results indicate that the oxide scale can either adhere and deform with the parent steel, or delaminate from the parent steel, or suffer from cracking before hot rolling, depending on the hot strength of the scale and the stress status in the scale, which depend on rolling reduction and temperature, and scale thickness and structure. Therefore, the cracks observed in the oxide scales on the steel slab after hot rolling are generally the combined results of cracking before rolling and further cracking in the roll gap. On the basis of comparative analysis, the prerolling behaviour of the oxide scales in a laboratory scale rolling mill and in an industrial hot strip mill is discussed.     

Development of high performance Mg–Al2O3 composites containing Al2O3 in submicron length scale using microwave assisted rapid sintering

Abstract

In the present study, magnesium composites reinforced with different volume fraction of submicron size Al₂O₃ particulates were synthesised using powder metallurgy technique incorporating an innovative microwave assisted rapid sintering technique. The sintered materials were subsequently hot extruded for characterisation in terms of microstructural, physical and mechanical properties. Microstructural characterisation results revealed a reasonably uniform distribution of Al₂O₃ particulates, minimal porosity and good matrix reinforcement interfacial integrity. The average coefficient of thermal expansion (CTE) value for Mg–Al₂O₃ composites was found to decrease with increasing amount of submicron Al₂O₃ particulates. Mechanical characterisation of the composites revealed an increase in hardness, elastic modulus, 0·2% YS and ultimate tensile strength (UTS) with the increase in amount of alumina particulates. Ductility exhibited the reverse trend. An attempt is made in the present study to correlate the effect of the presence of submicron alumina and its increasing amount with the microstructural, physical and mechanical properties of magnesium.     

On the determination of the surface energetics of porous polymer materials

The solid surface tension γsv of hydrophobic polymer powders has been determined using the capillary penetration technique. By plotting Kγlv cos ζ, where K is a geometric factor, versus the liquid surface tension γlv, the following values of γsv were directly derived from the curves: poly(tetrafluoroethylene) γsv = 20.4 mJ/m², polypropylene γsv = 30.2 mJ/m², polyethylene γsv = 34.4 mJ/m², and polystyrene γsv = 27.5 mJ/m². These values are in good agreement with the γsv values obtained from contact angle measurements on flat and smooth solid surfaces of the same materials. If the contact angles were first calculated from the capillary penetration experiments, which is the usual procedure applied in the literature, distinctly higher contact angles were obtained. Obviously these angles are affected by the powder morphology and are therefore meaningless contact angles in terms of a surface energetic interpretation.     

Water based processing of iron powder utilising starch consolidation

Abstract

The effects of water based shaping, by means of starch consolidation (SC), of an iron powder system regarding oxygen/carbon content and sintering performance were evaluated. Specifically, the influence of the drying conditions and the use of two different thickeners, xanthan gum and cellulose ether, were studied. The results showed that cellulose ether gave lower sintered density than xanthan gum, mainly because of less favourable rheological impact and air/gas entrapment at mould filling and consolidation. Due to less oxidation at drying and less removal of carbon at sintering, freeze dried specimens sintered to a higher density than room temperature air dried ones. The degree of oxidation and removal of carbon also influenced the as sintered microstructure. Ferrite grains surrounded by iron phosphide were found in both air dried and freeze dried specimens. However, the higher carbon content in freeze dried specimens also resulted in a significant amount of iron carbide grains (inclusions), which can be a potential strength limiting factor.     

Powder metallurgy of Udimet 720 for nuclear industry application: optimisation of the alloy,mechanical testing and modelling

Abstract

A specific treatment to produce powder metallurgy (PM) alloy 720 meeting the high temperature creep requirements for turbine discs in the future high temperature reactor is proposed. The resulting microstructure consists of large grains and grain boundaries that are uncorrelated with prior particle boundaries. The tensile and low cycle fatigue properties measured are in the range observed for cast and wrought (C&W) alloy 720 with the same precipitation state. The creep resistance is significantly increased compared with classical PM alloy 720, and approaches that of C&W alloys. This improvement is believed to be due to the reduction of damage sites at the grain boundaries and to a low secondary creep rate. A model has been developed that can reproduce the observed behaviour in relation to metallurgical variables.     

Investigation on metal powder production efficiency of new convergent divergent nozzle in close coupled gas atomisation

Abstract

In the present study, a new designed close coupled annular slit type nozzle is discussed for the most efficient powder production and the availability of the finest particle size. Pure tin powder was produced by using nitrogen gas at the Dumlupinar gas atomisation unit. It was found that the finest particle size with the highest efficiency was obtained at the atomising pressure where the maximum aspiration pressure is formed. Above a certain gas/melt flow ratio, the resultant particle sizes cannot be considerably reduced by further increasing atomising pressure. At that point, the applied nozzle system reaches a capacity limit, which also marks the point of the most economic work for the production of fine powders with that system. The main achievement of the nozzle discussed in the present study is that the most efficient powder product is accomplished at relatively low atomisg gas pressures.     

Bulk high hardness Al90Ce2Mn8 alloy prepared by powder metallurgy

Abstract

Using powder metallurgy, bulk high strength Al₉₀Ce₂Mn₈ alloy 25 mm in diameter by 10 mm with near nil porosity has been obtained under certain pressing and heating conditions. The conditions for the best mechanical properties are a pressing temperature of 753-793 K, a pressing time of 30 min, and pressing stress of 1·2 GPa. The compression strength reaches 895 MPa with a hardness of 26 HRC when the alloy is pressed at 753 K. The strength increase is attributed to second phase strengthening and fine grain strengthening.     

Process simulation of powder injection moulding: identification of significant parameters during mould filling phase

Abstract

Simulating and optimising the powder injection process are complex problems since a number of linked material, geometry and process variables have to be considered. In addition, it is very difficult to identify critical parameters for designing binder systems, feedstocks, parts, moulds and processing conditions owing to the fact that multiple objective functions have to be considered. Towards the goal of identifying the level of significance of various material, process and geometry parameters during powder injection moulding, a systematic procedure for sensitivity analysis has been successfully developed for the mould filling phase of the PIM process. In this sensitivity analysis, all input parameters were defined for the mould filling simulation and all output parameters for optimum design of part, mould and processing conditions and dimensionless sensitivity values for all input and output parameters were calculated, which allow parameters with different units to be compared quantitatively. The sensitivity analysis procedure developed will be an invaluable tool for both the design engineer in the PIM industry who has to determine the critical input parameters for given design targets, as well as for the production engineer who has to optimise and monitor the production stage.