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.     

Heat transfer during solidification of chemically modified Al–Si alloys around a copper chill

Abstract

The solidifying metal/chill contour will significantly affect the boundary heat transfer coefficients, and solidification modellers should be aware of the casting conditions for which the heat transfer coefficients are determined. The previous work carried out on solidification of Al–Si alloys in a metallic mould and solidification against bottom/top chills has shown that modification and chilling have synergetic effect resulting in a significant increase in the heat flux transients at the casting/chill interface. In the present work, the heat transfer during solidification of unmodified and chemically modified Al–Si alloys around a cylindrical copper chill was investigated. Heat flux transients were estimated using lumped heat capacitance method. Lower peak heat flux was obtained with chemically modified alloy. This is in contrast to the results reported for alloys solidifying against chills and in metallic moulds. The chill thermal behaviour and heat transfer to the chill material when surrounded by modified and unmodified alloys were explained on the basis of the decrease in the degree of undercooling in the case of modified alloy as compared to unmodified alloy and the change in contact condition and shrinkage characteristics of the alloy due to the addition of chemical modifiers.     

Changes in γ-TiC stoichiometry during heat treatment of TiC reinforced Ti composites

Abstract

A liquid phase technique (casting) has been successfully employed for the production of Ti/TiC in situ metal matrix composites. The γ-TiC phase produced by this process is highly substoichiometric. Changes in chemistry of the TiC phase in these composites are noted during annealing at elevated temperatures. The degree of stoichiometry of TiC increases during annealing at 1050°C and further during post-annealing aging at 750°C. This increase in the degree of stoichiometry (C/Ti atomic ratio) is accompanied by lattice expansion and reduction in microhardness values of the TiC particles. The change in TiC stoichiometry is accompanied by the precipitation of soft Ti particles within the TiC phase. The results are compared with those obtained from composites fabricated by a powder metallurgy route where the carbide chemistry is insensitive to heat treatment.     

Computer model of unidirectional solidification of single crystals of high temperature alloys

Abstract

I t has been common practice to use mould withdrawal unidirectional solidification to produce single crystal castings. To grow single crystals successfully, it is important to control several solidification parameters, such as the morphology of the solidification front (solid/liquid interface), thermal gradient, and growth rate during solidification. It is the aim of this study to develop a solidification model that can predict such solidification parameters for various design and operating conditions. The solidification phenomena in the process modelled are basically controlled by two heat transfer mechanisms: conduction and radiation. A set of heat transfer equations and boundary conditions were employed to describe mathematically the heat transfer phenomena. Then the finite difference method was used numerically to solve these equations for specified boundary conditions to obtain the temperature distribution and temperature variation in the casting. The solidification parameters can subsequently be deduced from these temperature data. Several thin plate castings were tested using the model developed. The following design and operating conditions were evaluated: susceptor temperature (power input), withdrawal speed, changes of cross-sectional area in the casting, and geometrical arrangement of the casting tree.

MST/1422     

Modelled heat transfer coefficients for Al–7 wt-%Si alloy castings unidirectionally solidified horizontally and vertically downwards

Abstract

A model is described for the calculation of the interfacial heat transfer coefficient during the unidirectional solidification of Al–7 wt-%Si alloy castings against a water cooled copper chill. The model includes the deformation of the initial solidified skin of the casting into a convex shape within the first seconds of solidification. Thereafter, heat transfer from the casting to the chill takes place through a central contact area and an outer annulus where local separation has occurred. Modelled heat transfer coefficients for solidification horizontally and vertically downwards are compared with experimentally determined values and show broad agreement. Some limitations of the model which prevent better agreement with the experimental values are discussed.     

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.     

CONTINUOUS COOLING OF WET FOUNDRY SAND USING A FLUIDIZED BED

Abstract

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.     

Estimation of air gap and heat transfer coefficient at different faces of Al and Al–Si castings solidifying in permanent mould

Abstract

In the casting processes, the heat transfer coefficient at the metal/mould interface is an important controlling factor for the solidification rate and the resulting structure and mechanical properties. Several factors interact to determine its value, among which are the type of metal/alloy, the mould material and surface conditions, the mould and pouring temperatures, casting configuration, and the type of gases at the interfacial air gap formed. It is also time dependent. In this work, the air gap formation was computed using a numerical model of solidification, taking into consideration the shrinkage and expansion of the metal and mould, gas film formation, and the metallostatic pressure. The variation of the air gap formation and heat transfer coefficient at the metal mould interface are studied at the top, bottom, and side surfaces of Al and Al–Si castings in a permanent mould in the form of a simple rectangular parallelepiped. The results show that the air gap formation and the heat transfer coefficient are different for the different casting surfaces. The bottom surface where the metallostatic pressure makes for good contact between the metal and the mould exhibits the highest heat transfer coefficient. For the sidewalls, the air gap was found to depend on the casting thickness as the larger the thickness the larger the air gap. The air gap and heat transfer coefficient also depend on the surface roughness of the mould, the alloy type, and the melt superheat. The air gap is relatively large for low values of melt superheat. The better the surface finish, the higher the heat transfer coefficient in the first few seconds after pouring. For Al–Si alloys, the heat transfer coefficient increases with increasing Si content.     

Effect of plate orientation on quenching characteristics

Abstract

The effect of position on the face of a steel plate on the quenching process during cooling from a typical austenitisation temperature has been examined. In addition, the effect of plate orientation on the quenching mechanism has been investigated. The most significant effect of these parameters on the quenching process relates to the length of the vapour blanket stage, which is very sensitive to position on the surface of the specimen. This has considerable importance in relation to the production of unhardened regions. A markedly different mechanism of heat transfer during nucleate boiling at different positions on the face did not give rise to corresponding differences in the surface heat transfer coefficients at this stage in the quench. The investigation of heat transfer in the surface of the plate has been supported by still photographs that clearly show the different processes at different positions and orientations of the plate. In particular they show violent disruption of the surrounding quenchant as large packets of vapour are nucleated in quick succession on the underside of a horizontal plate.

MST/3032     

J. D. Llewelyn and his family circle

Abstract

At the beginning of the nineteenth century, Swansea was a small South Wales town of some 6000 inhabitants which could not quite make up its collective mind as to whether it wished to become a busy industrial seaport or a leisurely seaside resort. Commercially well situated at the mouth of the River Tawe, it provided harbour facilities for the increasingly industrialised Swansea Valley and overlooked a beautiful bay that Walter Savage Landor compared favourably with the bay of Naples. The town might have become either another Bristol or another Brighton, but in the event its thriving coal and copper industries soon converted Swansea into ‘Copperopolis’, one of the principal centres of metal-working in the world.¹     

Matrix chemistry optimisation of Al/Altex composites preserving both bres and matrix alloy integrity

Abstract

The mechanical properties of continuous fibre reinforced metals (CFRMs) are known to be very sensitive to the selection and composition of the metallic matrix; for aluminium based CFRMs, commercial alloys are not suitable for this purpose. In the present study, the chemical composition of the matrix of unidirectionally Altex fibre reinforced aluminium composites was adjusted to meet the requirements previously established for a maximum exploitation of the fibre strengthening potential in CFRMs, particularly accounting for the peculiarities of the aluminosilicate Altex fibre and of the squeeze casting process. It was found that a matrix made from high purity elements based on the ternary Al–Zn–Mg system confers the best tensile strength properties to the CFRM, provided the Zn and Mg additions are tightly controlled to prevent the formation of interfacial intermetallic compounds during processing. The optimum composition was shown to be Al–6Zn–(0.3–0.6)Mg (wt-%); in this case Zn traps the Mg which leads to the formation of fine matrix precipitates indentified as Zn₂Mg (γ' phase), thus preventing a deleterious reaction of Mg at the fibre/matrix interface. The resulting composite material (fibre volume fraction ~ 54%) does not require additional heat treatment after casting and yields a tensile strength up to 1116 and 257 MPa in the axial and transverse directions respectively.     

Casting methods for the production of rotationally symmetric copper bimetals

ABSTRACT

Multiple-material products are characterised by a complex property profile which is achieved by combining the particular advantages of at least two different materials. Bimetal casting is an energy- and material-efficient technology for the production of multi-metallic objects. This paper describes the development of a semi-continuous casting process for the formation of a rotationally symmetric bimetal with a cohesive bonding character at the interface of a copper–tin alloy (CuSn6) and pure copper (Cu99.5). Initial experiments are conducted by static casting to evaluate the thermal process window. Based on the results of the initial experiments, a vertical semi-continuous compound casting process is developed. A stable cohesive bond between the joining partners is accomplished by forming a solid solution at the interface.

This paper is part of a Thematic Issue on Copper and its Alloys.     

Calculation of transformation sequences in quenched steel components to help predict internal stress distribution

Abstract

In this paper are described briefly the foundations of an on–line information system developed mainly for optimizing heat treatment technologies and steel selection by predicting the progress of transformation during quenching at various points in engineering components of different chemical compositions. The flexible generation of the recently introduced workpiece transformation diagrams and the distribution of the free specific dimensional changes through the cross–section of the components are presented. Finally, the practical adaptability of the computerized system for judging incidental distortions and quenching cracks, and for providing input data for calculations of internal stresses by the finite–element method is discussed.

MST/87     

Modelling and simulation of the electric slag remelting guide vane casting (ESRC) process and its application

Modelling and experiment work is carried out for a large stainless steel casting with variable cross-section and a curved surface which is produced by the electric slag remelting casting process. The casting is part of a hydraulic runner generator at a power station of a big river.

Mathematical models of the heat transfer and melted rate from an expendable metallic electrode were established. The melted rate is related to some important parameters, such as electric current and voltage, temperature and the flow rate of the cooling water in a crystallizer. Electrode melting, the moving pattern of the melted pool and the slag pool and the melt solidification in the metal pool were simulated.

In order to make the enmeshment of the variable curved surface of the casting, a co-operation method of extended constructive solid geometry (CSG) from 9 to 21 uniform geometry with B-Spline surface functions is developed. By this method the guide vane casting is enmeshed. The boundary condition between the steel casting and the crystallizer cooled by water was measured.

Based on the above new content the solidification simulation software ESRC3D is developed. Using it to simulate the whole process of ESRC with different parameters, instead of exploring the technological parameters of casting production by the trail-and-error method, the optimized parameters from the simulation for production of the large stainless steel castings with variable cross-section and a curved surface, such as the guide vane castings, have been used to produce them. Economic benefit and good quality of castings are obtained. Guide vane castings are widely applied to the hydraulic runner generator at numerous river power stations.     

Quantum Fluctuations in Equilibrium Superradiance

Abstract

The role of the quantum fluctuations of the order parameter in a second-order phase transition is discussed, for a superradiant model. The mean field equations and the equation of the critical temperature are obtained. The change of the value of the critical exponent of the specific heat α is connected with the supplementary dimension introduced by the frequency dependence of the field variables. The functional integral method is used, most of the calculations being performed within the Gaussian approximation.     

Effects of wetted wall on laminar natural convection in vertical annular ducts

Abstract

A detailed numerical analysis is performed to investigate the effects of latent heat exchange, in connection with evaporation of the liquid film on the wall, on the natural convection heat transfer in vertical concentric annuli. Major governing parameters identified are Gr_T, Gr_M, Pr, Sc, and N. Results are specifically presented for an air‐water system under various heating conditions to illustrate the latent heat transport during the evaporation process. The effects of the channel length, ratio of radii N and wetted wall temperature on the momentum, heat and mass transfer are examined in detail. Tremendous enhancement in heat transfer due to the exchange of latent heat was clearly demonstrated.     

Precipitation of secondary phases in super duplex stainless steel ZERON100 isothermally aged

Abstract

In the present paper the results of secondary phases determination and quantification in ZERON100 duplex steel, heat treated at 850–1000°C for 180–2400 s, are presented. During the isothermal heat treatments, at 850°C, the χ phase is the only phase to precipitate at α/γ boundaries and triple points, while at 900 and 950°C, χ phase is the first phase to precipitate after 180 s, followed by σ phase 300 s later. At all the temperatures the total amount of secondary phases is ～3% after <600 s aging.     

Microscopic analysis of Fe–Cr alloy produced by single roll strip casting

Abstract

In the present investigation, the microstructures and mechanical properties of Fe–Cr alloy prepared by single roll strip casting were studied. Optical microstructure showed subgrain boundaries inside large grains. Cracks were observed along the grain boundary. Scanning electron microscopy, X-ray diffraction study confirms the formation of chromium carbide at the grain boundary in the case of as cast alloy. Electron backscatter diffraction showed preferred orientation of grains in the as cast alloy. Carbides and undesired phases were not observed in heat treated alloy. Pores present in as cast samples expand after heat treatment process. Mechanical properties, like tensile strength, yield strength, elongation and hardness, of Fe–Cr single roll strip casting alloys were improved after heat treatment.     

A note on casting speed of Ohno continuous casting process

Abstract

The Ohno continuous casting process, known as the OCC process, is a heated mould crystallisation process that permits the generation of single crystal or cast products with a unidirectional structure. In this process, solidification takes place at the mould exit. Thus, the understanding of the process parameters is crucial to the successful application of this technology. The present note is aimed at clarifying the often misunderstood factors influencing the casting speed of the process.     

Transient-temperature and residual-stress fields in axisymmetric metal components after hardening

Abstract

A finite-element method is used to analyse the transient–temperature and residual-stress fields in an axisymmetric metal specimen during quenching. In the calculation of transient-temperature fields it is assumed that an unsteady source of latent heat exists in the specimen when a phase transformation occurs. Factors such as the surface heat transfer coefficient, heat conductivity coefficient, linear expansion coefficient, density, specific heat capacity, latent heat, and so forth are all temperature dependent. The elastic-plastic properties of the specimen are modified according to temperature fields, which are determined; the influence of plastic deformation on the temperature fields is neglected. The agreement between the calculated results and the experimental data shows that the numerical analysis method is reliable. The method may also be applied to the analysis of specimens with other than axisymmetric shape.

MST/15