CONTINUOUS COOLING OF WET FOUNDRY SAND USING A FLUIDIZED BED

In order to establish the optimum cooling system for hot returned sand in metal casting processes, a mathematical model including simultaneous heat and mass transfer has been constructed. The present study is especially directed towards clarifying the dynamical behavior of sand cooling in a one-pass fluidized bed. A series of continuous cooling experiments has been made for various conditions. Characteristics of both transient and steady-state bed temperature are explained reasonably by the present model. Furthermore, a possible proposal has been given on the optimal operation, which raises heat exchange efficiency in fluidization cooling of hot molding sand.     

Modelling thermal and microstructural evolution on runout table of hot strip mill

Abstract

A mathematical model has been developed to predict the thermal and phase transformation response of a 0·34 and a 0·05 wt-%C steel during cooling on the runout table of a hot strip mill. The model incorporates the cooling characteristics of laminar water bar sprays, the austenite–ferrite plus pearlite phase transformation kinetics as a function of the austenite grain size, and the heat of transformation. Overall heat transfer coefficients for the laminar water banks were determined from data obtained from in-plant trials carried out at the Stelco Lake Erie Works (LEW) hot strip mill. Isothermal and continuous cooling diametral dilatometer tests were performed on a Gleeble 1500 thermomechanical simulator at temperatures and cooling rates that simulate LEW hot strip mill conditions. The isothermal data were used to establish the phase transformation kinetics as afunction of austenite grain size and temperature. The continuous cooling results were used to obtain the relationship between cooling rate, transformation start temperature, and fraction of ferrite formed. The model was tested and validated by simulating the LEW cooling conditions while monitoring the phase transformation behaviour and by comparison of predicted and measured microstructural detail.

MST/1331     

Mechanical characterisation and quality index of A356-type aluminium castings heat treated using fluidised bed quenching

Abstract

The current study aimed at investigating the effect of fluidised sand bed quenching on the mechanical performance and quality index of A356·2 aluminium cast alloys. Traditional water and conventional hot air quenching media were used to establish a relevant comparison with fluidised sand bed quenching. Quality charts were generated using two models of quality indices to support the selection of material conditions on the basis of the proposed quality indices. The use of a fluidised sand bed for the direct quenching aging treatment of A356·2 casting alloys yields greater UTS and YS values compared to conventional furnace quenched alloys. For the same aging conditions (170°C/4 h), the fluidised bed quenched aged 356 alloys show nearly the same or better strength values than those quenched in water and then aged in a CF or an FB. Based on the quality charts developed for alloys subjected to different quenching media, higher quality index values are obtained by water quenched T6-tempered A356 alloys. Using hot sand as a quenching medium at different temperatures, namely 170, 190 and 210°C, reduces both the strength and the quality with increase in quenching temperature for the alloys investigated. The regression models indicate that the eutectic silicon modification factor has the most significant effect on the quality results of the alloys investigated, for all heat treatment cycles, as compared to other metallurgical parameters.     

Influence of Ti on hot ductility of Nb containing HSLA steels

Abstract

The influence of a low Ti addition (～0·01%) on the hot ductility of Nb containing HSLA steels has been examined. For conventional cooling conditions in which an average cooling rate from the melting point to the test temperature was used, the ductility decreased markedly with the addition of Ti. However, when cooling conditions after melting were more in accord with the thermal heat treatment undergone by the strand during continuous casting, i.e. cooling is fast to begin with, reaches a minimum and then reheats, after which the temperature falls more slowly to the test temperature, the Ti addition was found to be beneficial.     

Preparation of Essential Oils Loaded Granule by Melt Granulation

Abstract

A comparative study on three granulation methods; melt granulation, fluidized bed granulation and wet granulation was performed to fabricate an essential oils loaded granule. The granule properties such as particle size distribution and the loading efficiency of anethole from fennel and cinnamaldehyde from cinnamon showed that the melt granulation in a high shear mixer was the most feasible method among the three methods.

In melt granulation, the granule particle size was well controlled by polyethylene glycol 6000 (PEG) content of which the optimum value was found to De 20%. Impeller speed and massing time in high shear mixer had small contribution to the particle growth when PEG content was optimized, while PEG particle size had some effect. Finer PEG powder improved the uniformity of granule size. Moreover, the cooling method of the hot mass affected the final granule properties significantly. The cooling with a fluid bed dryer was the best method.

Both of the retention rates of anethole and cinnamaldehyde in the final granule were more than 95% of initial doses irrespective of cooling method. Further, the adoption of a fluid bed dryer enabled very rapid cooling of hot granule with negligible loss of essential oils.     

Sand bed configurations under 2‐D standing waves

Abstract

Laboratory experiments conducted in a wave basin with movable sand bed have shown that, under the action of 2‐D standing water waves, longitudinal sand bars with small ripples riding on their tops are formed beneath the nodes of standing waves. Wavelengths of the sand bars are approximately equal to one‐half of those of standing water waves. The formation of sand bars under standing waves over an initially horizontal sand bed is explained by the suction and impinging effects of the wave action. A mathematical model for describing the induced density of sediment‐water mixture due to wave action is proposed. By employing the induced‐density model and mass conservation of sediment‐water mixture, a numerical simulation model is constructed. Bed configurations resulting from the actions of 2‐D standing waves are studied by using the simulation model. The simulation results of sand bed configuration under the action of 2‐D standing waves are in good agreement with those obtained from laboratory experiments.     

Model for cooling and phase transformation behaviour of transformation induced plasticity steel on runout table in hot strip mill

Abstract

A mathematical model has been developed considering non-symmetric cooling in the thickness direction of strip on a runout table (ROT). In order to solve a one-dimensional transient heat transfer equation including the heat evolved from phase transformation, a finite element method was applied, coupled with thermodynamic and kinetic analyses. The heat capacities of each phase and heat evolution owing to phase transformation were obtained from thermodynamic analysis of the Fe–C–Mn–Si system using Thermo-Calc. The phase transformation kinetics of a transformation induced plasticity (TRIP) steel were derived by using continuous cooling experiments and thermodynamic analysis. Heat transfer coefficients of strips on the ROT were, by applying an inverse method, determined from actual mill data under various cooling conditions. Using the developed model, temperature–time variations of plain carbon and TRIP steels on the ROT were calculated. The calculated results were in good agreement with the actual mill data. In addition, quantitative phase evolution during cooling could also be predicted by the model. From this analysis, the optimum cooling pattern on a ROT for the production of TRIP steel could be determined.     

Influence of thermomechanical processing and accelerated cooling on microstructures and mechanical properties of plain carbon and microalloyed steels

Abstract

A thermomechanical control process consisting of slab reheating, controlled rolling, and accelerated cooling has been adopted at the plate mill, Bhilai Steel Plant, India for achieving high strength and toughness in C–Mn and microalloyed steels while keeping the mechanical properties and the flatness constant. A mathematical model has been developed to predict the temperature distribution in the plate during accelerated cooling, taking into account the heat generation of the phase transformation. Effects of chemistry and mill parameters on ferrite grain refinement are explained in terms of nucleation and growth rate.     

Particle scale study on heat transfer of gas–solid spout fluidized bed with hot gas injection

ABSTRACT

In this paper, the heat transfer characteristics of a 2D gas–solid spout fluidized bed with a hot gas jet are investigated using computational fluid dynamics-discrete element method. The initial temperature of the background gas and particles in the spouted bed was set to 300?K. The particle temperature distribution after injection of 500?K gas from the bottom, center of the bed, is presented. The simulation results indicate well heat transfer behavior in the bed. Then, statistical analysis is conducted to investigate the influence of inlet gas velocity and particle thermal conductivity on the heat transfer at particle scale in detail. The results indicate that the particle mean temperature and convective heat transfer coefficient (HTC) linearly increase with the increase in inlet gas velocity, while the conductive HTC and the uniformity of particle temperature distribution are dominated by the particle thermal conductivity. The conductive and convective heat transfer play different roles in the spout fluidized bed. These results should be useful for the further research in such flow pattern and the optimization of operating such spouted fluidized beds.     

Influence of B on hot ductility of high Al,TWIP steels

Abstract

The influence of B on the hot ductility of high Al, Ti containing twinning induced plasticity (TWIP) steels has been examined. It was established that provided the B was fully protected by adding sufficient Ti to combine with all the N, then B could segregate to the austenite grain boundaries and improve ductility. This improvement was particularly marked for the temperature range of 700–900°C, the range in which the straightening operation often takes place in continuous casting. Of most importance in the present work has been the detection of B at the boundaries using a secondary ion mass spectrometry technique. The cooling rate from the reheating temperature of 1250°C to the tensile testing temperature range of 700–1200°C was 60 K min^?1, but it is likely that slower cooling rates ≤25 K min^?1, more in keeping with the secondary cooling rate on continuous casting, will give even better ductility. Ti additions in themselves are beneficial to the hot ductility of these steels as precipitation of AlN at the austenite boundaries is avoided, but only if the cooling rate is sufficiently slow to allow the TiN particles to coarsen. However, to ensure freedom from cracking, an addition of B is also required.     

Mathematical model coupling phase transformation and temperature evolution during quenching of steels

Abstract

To calculate the internal stresses in steels during quenching it is necessary to be able to predict accurately the evolution of phase transformations during the cooling process. After a review of published papers, a mathematical model coupling phase transformations with temperature field predictions at each instant during the cooling process is proposed. Incubation and growth periods are treated separately, using Scheil's method and the Johnson–Mehl–Avrami formulation, respectively. Martensitic growth is considered separately using the Koistinen and Marburger law. The temperature field calculation is carried out by the resolution of the heat equation using an implicit finite–difference method. An internal heat source term has been included in the heat equation to take into account the heat generated by the phase transformation. The results of the theoretical calculations are compared with those obtained by experiment, and the validity of the model and the effect of internal stresses on the theoretical predictions are discussed.

MST/13     

Numerical modelling of permeability variation with composition in aluminium alloy systems and its relationship to hot tearing susceptibility

Abstract

A numerical micromodel has been developed to simulate the evolution of equiaxed primary phase grains during the solidification of alloys in the systems Al–Cu, Al–Si, Al–Mg, and Al–Zn. The microstructures generated have then been used to model liquid permeability as a function of composition for each system, for a given solid fraction and cooling rate. In all systems a marked minimum occurred in the permeability curve at a value < ～1 wt-% solute. The composition corresponding to the minimum permeability tended to increase with increasing equilibrium partition coefficient. It is argued, for each system, that the composition displaying minimum permeability would correspond to that composition exhibiting maximum susceptibility to hot tearing. Comparison of the permeability data with experimental hot tear test data for the same systems reveals the limitations of most hot tear tests.     

Modelling and simulation of bulk metallic glass production process with suction casting

Abstract

An integrated model, which coupled nucleation and crystalline growth with the heat transfer process, is presented in the present paper. The temperature, temperature gradient, cooling rate and the crystalline fraction of Zr₆₅Al_7·5Cu_17·5Ni₁₀ in suction casting have been calculated with this model. The results show that the metallic glass can be obtained at the bottom and the radial boundary of the rod sample, and that the crystalline phase precipitates in the centre of the sample. The crystalline fraction reaches the highest value of 0·0128 in the centre of the sample. Comparatively lower nucleation rate, as well as the higher viscosity and the cooling rate account for the formation of the bulk metallic glass.     

Effective cooling of work rolls in strip rolling

Abstract

An examination has been made of the effectiveness of roll cooling as applied to strip rolling, taking into account practical considerations such as the spray location and positioning, coolant flowrate, and rolling temperature. It is indicated by the results that, for cold rolling, the most effective cooling can be achieved by commencing spray cooling at the exit side of the roll gap and by providing a large spray contact angle. However, for hot rolling, cooling should commence at least 45° from the roll gap exit and a high spray flux density with relatively small spray contact angle is more appropriate.

MST/642     

Damping capacity of sand cast magnesium alloy AZ91

Abstract

Measurements of damping properties have been carried out on sand cast specimens of the most widely used commercial magnesium alloy AZ91. Damping properties of specimens in both as cast and heat treated (T4 and T6) conditions were determined by measurements of the logarithmic decrement of free bending beam vibration at various frequencies. The best damping properties were found in the as cast specimens. This is directly related to the microstructure, which consists of α magnesium, large inclusions of β phase (Mg₁₇Al₁₂), as well as lamellar eutectoid Mg₁₇Al₁₂ precipitating as a result of low cooling rate. For homogenised and annealed specimens, the maxima of the hardness and of the damping capacity as a function of annealing time was found. The measurements of the logarithmic decrement can be explained using a model of Granato and Lücke.     

Model for extracting phase transformation kinetics from dilatometry measurements for multistep transformations

Abstract

A detailed analysis has been made of the length change of a hypoeutectoid steel during a continuous cooling or an isothermal transformation. A model has been developed in which the transient dilatation is calculated based on the fraction of the phases present. The model accounts for the carbon partitioning effects and is applicable to the extraction of the phase change kinetics from a dilatation curve for a multistep phase transformation. It has been validated by comparing the model results with the experimental results of an interstitial free steel. Isothermal transformations and continuous cooling experiments have also been carried out with a bainitic steel. The model was used to extract the phase transformation kinetics from the dilatation curves of this steel. Excellent agreement between the calculated results and the experimental ones has been obtained.     

Evaluation of semiphysical phase transformation model for engineering steels using measured continuous cooling transformation diagrams

Abstract

A semiphysical model has recently been constructed by Parker that calculates the phase transformation behaviour of engineering steels as a function of time for an arbitrary cooling path. To compare the model with measured diagrams from the literature for linear cooling, a large number of continuous cooling transformation (CCT) diagrams have been calculated. The quantified comparison has been used to construct heuristics that indicate the accuracy of the model as a function of the steel composition. For 10 alloying elements the domain for which the model is valid has been established.     

Cyclic overaging heat treatment for ductility and weldability improvement of nickel based superalloys

Abstract

A new cyclic overaging heat treatment for ductility and weldability improvement of advanced nickel based superalloys has been investigated. The heat treatment consists of intermittent heating cycles during the cooling stage to check the nucleation of fine γ' in the material during the overaging. Using Rene 80 alloy as a model material, the effects of intermittent heating rate and duration were studied and optimised. Quantitative metallography was carried out to compare the microstructures produced by the proposed heat treatment with those produced by other conventional pre-weld heat treatments. Tensile tests and welding tests both showed that the proposed cyclic over-aging heat treatment is effective in improving the tensile ductility and weldability of Rene 80 alloy.     

Effect of modification melt treatment and chilling on eutectic arrest temperature and time during solidification of A357 alloy

Abstract

Thermal analysis technique has been recognised as an efficient non-destructive tool to assess the degree of modification in Al–Si alloys. Apart from chemical modification, chilling refines the microstructure. This is particularly significant as majority of Al–Si alloys are cast in metallic moulds. In the present study, the interaction between chilling and modification melt treatment is investigated to assess their effect on thermal analysis parameters using computer aided cooling curve analysis. For modified alloys, the depression of the eutectic arrest temperature was significant at higher cooling rates. The eutectic arrest temperature and time were correlated with the cooling rate using a power law. High cooling regime in thermal analysis plots was attributed to the combined effect of chilling and modification melt treatment on heat transfer.     

Instrumentation and simulation of industrial steel wire rod cooling line

Abstract

A coupled thermal and metallurgical mathematical model has been developed to simulate the Stelmor process for controlled cooling of steel wire rod. The model predicts the through section microstructure, which is subsequently used to estimate the mechanical properties, in terms of ferrite fraction, ferrite grain size, and interlamellar pearlite spacing. Plant trials were conducted to evaluate heat transfer coefficients for the water and forced air cooling stages of the process in terms of the process parameters. To enable the development of a control model, the effect of individual process parameters, including their extreme settings, on rod temperature distribution was studied. An inverse modelling approach has been used to optimise the value of the heat transfer coefficient. The model results were compared with plant trials for a range of rod diameters, rolling speeds, number of active water boxes, and the operation ratio of the Stelmor fans. This validation was performed for three carbon–manganese steel grades (0.07–0.67%C) with rod diameters of 5.5–14.0 mm. A good agreement between the predicted and measured thermal behaviour of the rod was achieved.