During continuous casting mould powder forms a pool of liquid flux which infiltrates into the solidifying shell/mould gap and forms a flux film containing liquid and/or solid layers. Subsequent crystallisation of the film results in the formation of an air gap at the mould wall. An air gap can also be formed by the shrinkage of the solidified shell. In practise, no air gap is formed by shell shrinkage in the upper mould since molten flux will flow immediately into and fill any gap formed. However, in the lower mould, if the shell temperature falls below the break temperature of the flux there is no liquid to fill any gap formed by shell shrinkage and hence an air gap can form. A model was developed which determines the effect of the various layers (including the air gap) on the horizontal heat transfer between shell and mould. A fully coupled, heat transfer/stress analysis was used to calculate: (i) the thickness of the various layers of the flux film; (ii) horizontal heat flux; and (iii) the resultant stress in the solidified shell.The model predicted that: (a) the hoop stress increased as the interfacial thermal resistance decreases (i.e. the heat flux increases); (b) the maximum hoop stress at the exit decreased with increasing flux film thickness; (c) the ferro-static pressure tended to hinder the formation of an air gap at mid-face positions whereas in the corners the thicker shell tended to counteract the ferro-static pressure resulting in air gap formation; (d) the thickness of the liquid film at various positions in the mould was computed for fluxes with optimum break temperatures to determine if they provided liquid lubrication throughout the mould; and (e) the maximum stress occurred in the corner when casting with mould fluxes but at the mid-face when using oil.KEY WORDS: continuous casting; mould flux; heat transfer; finite element; stress analysis; billet; oil casting. 95© 2007 ISIJ metal. The molten flux forms a liquid pool on the top of the mould, infiltrating the mould/shell gap and partially solidifies near the mould. The flux film formed initially is glassy in nature but in time, crystallisation occurs. 17,18) Since the density of the crystalline phase is greater than that of the glass phase, crystallisation results in shrinkage of the solid flux layer and the formation of an air gap at the copper mould/flux interface. Thus the crystal/glass distribution has a significant effect on the horizontal heat transfer.A second type of air gap can also be formed by shell shrinkage. However, in the upper part of the mould liquid flux will flow into and fill any gap formed. In the lower part of the mould if the shell temperature is lower than the break temperature of the flux there will be no liquid to fill the gap (due to shell shrinkage or inadequate mould taper). Consequently an air gap can be formed under these circumstances. In this paper the air gap formed by crystallisation of the flux film is referred to as an interfacial contact resistance, whereas the air gap formed by shell shrinkage is referre...
Powder consumption, Q s , is an important process variable in the continuous casting of steel but there is no agreement on the factors affecting Q s . This study seeks to identify those factors which have an effect on Q s . A large database containing powder consumption values and casting conditions, mould flux and steel composition data from a large number of steelworks has been analysed using statistical techniques to identify those factors which have significant effect on powder consumption. The following parameters, in decreasing order, were found to be significant (at the 95% confidence level): casting speed, viscosity, stroke length, oscillation frequency and break temperature.
High pressure high temperature (HPHT) design is a significant new challenge facing the subsea sector, particularly in the Gulf of Mexico. API 17TR8 provides HPHT Design Guidelines, specifically for subsea applications. This paper presents the results of a fatigue based fracture mechanics assessment case study conducted on a fully clad subsea HPHT component. The component was assumed to be constructed from F22 low alloy steel internally clad with alloy 625 and exposed to 20ksi (137.8MPa) and 400°F (204°C) internal pressure and temperature. A number of different assessment methods were evaluated as part of this study, including standard failure assessment diagram (FAD) based assessment methods, such as those found in API 579-1/ASME FFS-1 and BS 7910, as well as finite element (i.e. crack mesh) methods. A detailed description of the finite element analysis (FEA) of the uncracked and cracked component is provided. An internal surface flaw assumed to be exposed to sour production fluids was evaluated. The results of the fatigue and fracture assessments are summarized along with the key differences between the assessment methods adopted. The sensitivity of the assessment results to other variables such as welding residual stresses is also discussed.
Environmentally assisted sub-critical static crack growth can occur in offshore pipelines exposed to aggressive production environments. Recent advances in fracture mechanics testing methods have shown that slow static crack growth rates can be reliably measured in sweet and sour environments under constant stress intensity factor (K) conditions. This has potential implications for the engineering critical assessment (ECA) of pipe girth welds subject to low cycle fatigue loading with long periods of operation under constant static load between cycles, e.g. lateral buckling. This paper demonstrates the influence of including static (i.e. time dependent) crack growth as well as fatigue crack growth in a modified pipeline ECA approach.
Engineering Critical Assessment (ECA) procedures generally use the Failure Assessment Diagram (FAD) concept for integrity assessment of components containing flaws. An FAD assessment is described by Kr and Lr values, where Kr reflects the toughness of the material while Lr measures the proximity to plastic collapse. Nowadays pipeline girth welds generally have high fracture toughness (Kmat) and it can be argued that plastic collapse is the governing failure mode. The definition of plastic collapse can affect the determination of the Lr parameter and should be carefully chosen. In the present work finite element analysis has been carried out to evaluate the collapse load under local collapse and global collapse conditions. These have been compared with the solutions available in the BS7910 and R6 procedures and the differences have been highlighted. The impact of the choice of plastic collapse solution on the crack driving force has been analysed.
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