Mould thermal monitoring: a window on the mould

Abstract

Corus R,D&T at Teesside Technology Centre has developed over a number of years a mould thermal monitoring (MTM) system based on an array of thermocouples in the mould copper plates. The system is installed on the Corus slab casters in the UK, on slab casters at Outokumpu (UK), Sidmar (Belgium) and Kosice (Slovakia) and, at the time of the 4th European Continuous Casting Conference, was also on the medium thickness slab caster at Tuscaloosa (USA), which was sold to Nucor in 2004. The MTM system was also under development on the thin slab caster at Trico (USA) before plant closure (subsequently bought by Nucor), and aspects are currently being developed on the Corus DSP (direct sheet plant) thin slab caster at IJmuiden (The Netherlands). While a prime function is detection and prevention of sticker type breakouts, the MTM system allows real time assessment of thermal conditions, provides a valuable input for online grading and, most important, enables modifications to mould powder practices to be assessed. The present paper briefly outlines recent developments to the MTM system and presents examples of the use of the system to assist mould powder developments.     

Physical properties of selected brazing filler metals

Abstract

A suitable selection of the filler metal is vital for producing satisfactory brazed joints. The wettability of brazing alloys with base metals depends on physical properties such as surface tension, density, melting point, and viscosity. Thermal conductivity and electrical resistivity are also important since the filler metal is frequently required to have similar values to those of the base metal. In the present paper, the physical properties of liquid alloys relevant to brazing have been evaluated. Six different filler metal systems were analysed, comprising alloys based on Ag, Al, Au, Cu, Ni, and Ti. Results show that the viscosity values for most binary brazing filler alloys are of the order of 2–8 mPa s, with Cu and Al alloys exhibiting the lowest viscosities. The surface tensions of brazing alloys vary from 800 to 1800 mN m^-1, with the lowest surface tension values corresponding to the Ag and Al alloys. Thermal conductivity and electrical resistivity values fall in the range 10–200 W m^-1 K^-1 and 17–300 μΩ cm, respectively.     

Effects of latent heat of fusion on thermal processes in laser welding of aluminium alloys

Abstract

Based on the Green's function method, a mathematical model allowing for the latent heat of fusion and solidification is developed to describe the steady state, two-dimensional heat flow during welding of thin plates. It is demonstrated that the latent heat has a pronounced effect on shape and size of the weld pool and mushy zone. The thermal efficiency of base metal fusion by a line heat source η _t can exceed 0·4839 considerably if the latent heat is taken into account. It is shown that the known simplified approaches for considering the latent heat can introduce large errors into the estimation of η _t. The calculated and experimental weld pool shapes are compared.     

Residual stresses and distortions in welded structures: a perspective for engineering applications

Abstract

In this paper, some of the historical developments and recent advances in understanding welding induced residual stresses and distortions are discussed in the context of their impact on today's engineering applications. With recent rapid advances in computational simulation techniques, the complex thermomechanical phenomena associated with typical welding processes can be effectively dissected into solvable problem sets for both fundamental understanding and specific engineering applications. Although a great deal of research is currently still ongoing to advance our fundamental understanding of the complex thermophysical and thermomechanical phenomena, an engineering perspective is provided to demonstrate how an engineer can make use of the current state of knowledge to derive effective practical solutions when dealing with day to day problems in the areas of residual stresses and distortions in welded structures. First, some of the fundamental mechanical considerations associated with residual stress and distortion developments are presented. Then, some of the computational modelling requirements for engineering applications are discussed in the light of recent developments. Finally, application examples are presented to demonstrate how an effective engineering solution can be sought by taking advantage of today's advanced modelling techniques with appropriate engineering assumptions.     

Effect of section thickness and modification on thermal analysis parameters of A357 alloy

Abstract

Thermal analysis technique relies on the cooling curve obtained when the sample is cooled in a sampling cup. This may not represent the cooling behaviour of the real casting. The microstructure developed during solidification depends not only on the nucleation and modification potential of the melt but also on the thermal gradient imposed during solidification by the mould. The factors affecting the thermal gradient are the mould material and casting section thickness. In the present investigation the effect of modification melt treatment, cooling rate and casting section thickness on the thermal analysis parameters of A357 alloy was studied. It is found that the dimensionless heat flux parameter is high for small section thickness castings. The metal/mould interfacial heat flux is high in a copper mould. Thermal analysis parameters of A357 alloy are found to be affected significantly by the combined action of modification, chilling and section thickness.     

Comparison of UF6 packages under certification test and accident conditions

Abstract

Currently there are three packages approved by the NRC for US domestic shipments of fissile quantities of UF₆: NCI-21PF-1, UX-30, and ESP30X. For approval by the NRC, packages must be subjected to a sequence of physical tests to simulate transportation accident conditions as described in 10 CFR part 71. The primary objective of this project was to compare conditions experienced during these tests to conditions potentially encountered in actual accidents and to estimate the probabilities of such accidents. Comparison of the effects of actual accident conditions to 10 CFR part 71 tests was achieved by means of computer modelling of structural effects on the packages due to impacts with actual surfaces, and thermal effects resulting from tests and other fire scenarios. In addition, the likelihood of encountering bodies of water during transport over representative truck routes was assessed. Modelled effects and their associated probabilities, accident rates, and other characteristics gathered from representative routes were combined with existing event tree data to derive generalized probabilities of encountering accident conditions comparable to or exceeding the 10 CFR part 71 test conditions. This analysis suggests that the regulatory conditions are unlikely to be exceeded in real accidents.     

Tools and techniques for use in development of mould powders

Abstract

The tools and techniques used within Corus UK to improve mould powder performance and product quality are reviewed. Mould thermal monitoring, which allows real time assessment of heat transfer, is a critical tool in the development of mould powders. Thermal analysis, viscosity measurements, flux–mould plate interactions, steel velocity profiles in the mould and slag film properties also play a part in continuing studies. Case studies of improvements in process performance achieved using these methods are presented.     

UV irradiation and aging effects on nanoscale mechanical properties of ultra high molecular weight polyethylene for biomedical implants

Abstract

As a result of its relatively high strength, high chemical resistance, low creep and low wear rate, ultra high molecular weight polyethylene (UHMWPE) has been widely used as the 'soft' articulating surface for total hip and knee arthroplasty. However, for long term artificial joint replacements, accelerated wear as a result of aging of UHMWPE is one of the most important problems that can lead to joint failure. Therefore, the present work is focused on investigating the effects of thermal and serum aging and UV irradiation dose on the nanomechanical properties (elastic modulus, hardness and visco-elasticity) of UHMWPE (type GUR410) specimens under different deformation rates. The continuous stiffness measurement (CSM) nanoindentation technique is used in the present work to measure the nanomechanical properties. The results show a considerable increase in the nanomechanical properties with increasing deformation rate. It is also demonstrated that the nanomechanical properties of the thermally and serum aged UHMWPE specimens decrease compared to the virgin specimens, while their visco-elastic behaviour increases. For the UV irradiated specimens, the nanohardness and nano-elastic modulus show an increase with irradiation dose especially for small penetration depths. Moreover, a considerable decrease in visco-elastic behaviour was observed for the UV irradiated specimens as a result of the crosslinking effect of the radiation.     

A Method to Evaluate Water Resistance of Acrylic Adhesives

A number of two-part acrylic adhesives were examined for the effect of immersion time in water at 23°C, 80°C and 100°C on the tensile lap shear strength. The mechanism of acrylic bonded joint failure in water was discussed based on the observed results. The relative water resistance between these adhesives was compared from shear strength retention values. The dependence of the water resistance of these adhesives at 23°C and at elevated temperatures was discussed. It has been found that for acrylic adhesives, the water resistance at room temperature can be evaluated on the basis of the shear strength retention values after aging in water at 80°C.     

Prediction of the tensile load-bearing capacity of a co-cured single lap joint considering residual thermal stresses

In this paper, stress distributions in a co-cured single lap joint subjected to a tensile load were investigated using the finite element analysis. Residual thermal stresses, which resulted from the curing process of the co-cured single lap joint, were also considered. Since the adhesive layer in the co-cured single lap joint was about 10 μm thick, very thin compared with the thickness of both adherends, the interface between the steel and composite adherends was assumed to be perfectly bonded. The co-cured single lap joint was analyzed with respect to several bond parameters such as the bond length and stacking sequence of the composite adherend. The failure mechanism of the co-cured single lap joint was partial cohesive failure in the composite material, which was significantly affected by the interfacial tensile stress at the free edge of the co-cured single lap joint. Interfacial tensile stress was a primary factor that caused interfacial delamination between the steel and composite adherends in the co-cured single lap joint. Finally, tensile load-bearing capacities calculated from the Ye-delamination failure criterion were compared with the experimental results, and relatively good agreement was found.