Improving Performance - Billet Casting Improving ... - Pyrotek

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Page 8 Improving Performance - Billet Casting Pyrotek’s range of Perlube lubricants offer superior lubrication matched to each specific process. Most billet casters will consume up to ten times less Perlube than other vegetable or synthetic lubricants while not contaminating the cooling water system as much. METAL TRAnSFER Getting metal from the metal distributor into the mould usually happens in three ways. Certain hot top technologies simply flood an orifice through a narrow channel between the metal distributor and the mould. Hydro and Showa are examples of this type of system. In conventional tables, a downspout, or dip tube, is commonly used in conjunction with a float to control the metal flow into the mould. As stated earlier, these systems usually generate more oxides. Controlling the metal temperature is critical to prevent the spouts from freezing. Pyrotek continues to offer a wide range of float and spout designs manufactured from calcium silicate to accommodate this process. These materials insulate and offer long service life for maximum operating economy (see Figure 20). Figure 20 - Pyrotek Floats and Pins Wagstaff hot top systems (see Figure 21), however, utilize a thimble, located between the metal distribution refractory and the mould. The thimble directs the molten metal into the mould so it must have a very smooth internal diameter, as well as being capable of withstanding metal erosion, thermal shock, and any damage associated with maintenance and cleaning between casts. Since the outer diameter of the thimble mates with the internal diameter of the T-plate with a soft seal gasket made from ceramic paper between them, the machined tolerances of both parts are critical. Pyrotek’s unique mould process during thimble manufacture allows the bottom outer diameter of the thimble to have very tightly controlled concentricity, thereby ensuring a perfect fit between the thimble and the T-plate. In addition, pyrotek supplement each production batch of thimbles is produced to a very strict manufacturing standard utilising electronic and physical inspection techniques. The techniques identify basic porosity and dimentional tolerances, which are directly related to the strength and life of the thimble. Figure 21 - Pyrotek Hot Top Systems for Wagstaff MOULD LUBRiCATiOn Lubrication inside the mould plays a very important part in casting product with good surface quality. Most conventional systems utilise a mould bore grease or brushon lubricant. During the length of the cast, however, this lubricant is dissipated which results in billet surface defects. Most hot top systems employ some type of continuous lubrication system. These systems can range from a simple serrated copper plate between the mould bore and header plate, where lubricant oozes down the mold bore, to a sophisticated pressurized, timed injection system. There is a tendency to use too much oil when trying to correct surface defects usually caused by other variables. This in turn contaminates casting machines and cooling water, causing more chemicals to have to be added to the cooling water to maintain its quality, which often affects the water’s ability to adequately quench the metal in the mould. In many casthouses, the billet cooling water system is a common one shared with other pits where sheet ingot is cast, which is a very big oil contaminator of the water. Pyrotek’s range of Perlube lubricants offer every type of billet caster a superior lubricant matched to his specific process. The advantage is that most billet casters will consume up to ten times less Perlube than other vegetable or synthetic lubricants whilst the Perlube oils do not contaminate the cooling water system anywhere near as much because they
Improving Performance - Billet Casting Figure 22 - Cooling Water Figure 23 - Mould and Hoses are >95% biodegradable (Figure 22). This solution can also save considerable chemical treatment costs. COOLing WATER QUALiTY Cooling water quality plays a critical role in good billet casting. Some conventional systems today still use a mould table, which has external cooling hoses to each of the moulds (see Figure 23). Most hot top systems use a Wagstaff-type mould table with integral water headers that feed cooling water directly from inside the tabletop to the moulds. A baffle system, with water screens, is usually used to prevent large pieces of debris from plugging the water slots or holes in the mould bore. This again becomes critical because, if the cooling water system becomes oil contaminated, then the water screens quickly become clogged, with debris from algae and bacteria, thereby causing hot tears and bleed-outs during the cast. Many cooling systems are at or near maximum capacity during casting, and when more chemicals are added to the cooling water, the effect of cooling is diminished. As discussed above, the biodegradable and economical Pyrotek Perlube product range can greatly reduce oil contamination in the cooling water and the consequent chemical treatment costs. BiLLET CASTing COATingS Coatings also play an important role in producing good quality billet: in particular the application of boron nitride and graphite coatings for T-plates. A good non-wetting coating will minimize oxide adhesion at cast start, allowing the billet to get into steady-state casting mode quickly. This can help to reduce bottom butt scrap. Pyrotek has developed a premium, proprietary coating for this purpose. This boron nitride based coating, applied at the Pyrotek factory, withstands 30–40 casts before recoating of the T-plate becomes necessary. It also has superior lubricity for good oxide mobility. T-Kote, an “easy-to-use” companion coating was developed to enhance protection of the T-plate. T-Kote comes premixed and has a slight tint, allowing the operator to see visually where any spots may have been missed. Combined, these two products also offer superior life to the surface of the transition plate. Graphite coating is used in some transition plate and header coating applications. It does not offer the lubricity of boron nitride, but provides more penetration of the hot face of the transition plate and resistance to some metal alloy constituents less compatible with boron nitride. Pyrotek offers a Naphtha-based graphite coating called Pyroslip 325/350 (see Figure 24). Since molten aluminium is very aggressive, a release agent should be applied to protect the launder and table surfaces coming into contact with the metal. Boron nitride coatings and bone ash are utilised in such applications. In contrast the binders utilised for the BN coatings adhere strongly to the various refractory materials and assist in reducing metal erosion of the refractory shapes, reducing the damage associated with maintenance and cleaning between casts, and with the addition of a blue colorant, provides an additional assistance in helping to monitor the effectiveness of the coating practice. Traditionally, bone ash was used because of its release properties. However, due to its poor adhesion, bone ash must be repeatedly replenished and may become entrained by the flowing of molten aluminium. Customers report that in contrast to bone ash, major reductions in residues and quicker turn around of casting tables helped Figure 24 - Pyroslip Products pyrotek supplement John Blasen global Market Development Manager, ZYP - Pyrotek Coatings PYROTEK’S MiSSiOn “Providing innovative solutions to customer needs utilizing our global resources.” Page 9
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