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Metal Casting Technologies : March 2009
Back to the Tin-bronze foundry practice A By Prof. John H. D. Bautista, Technical Consultant, Philippine Metalcasting Association, Inc. brass is basically an alloy of copper and zinc; while a bronze is basically an alloy of copper and another element instead of zinc – like tin, silicon, phosphorus, antimony, aluminum, beryllium, and nickel. Then, it is my experience that most brasses are formed (mechanically-worked: rolled, drawn, or forged) and most bronzes are cast. Thus, we would prefer to call a copper-alloy foundry a bronze foundry, rather than a brass foundry. It is interesting to note that it takes much less capital to start a bronze foundry business than any other foundry business. Besides, the possible choices of alloys are so many that it becomes quite interesting in trying to serve other industries better. Because of this myriad of choices, it is very challenging, but fully rewarding, for the skillful foundryman who can short-list this very long list of alloys. The primary consideration is the application of the planned casting. Controlling aspects would be: mechanical properties like strength – tensile, yield, compressive, and impact – elongation, and hardness; and physical properties like corrosion resistance, abrasion resistance, thermal conductivity, electrical conductivity, heat-treatability, weight, and aesthetics. Both mechanical and physical properties are controlled by the chemical composition. Hence, a customer should be asked to give his service requirements and the foundry should be the one to determine what chemical composition to use; the Tin Bronzes 86-12 88-10- 2 82- 18 Chemical Analysis: Copper, % Tin, % Lead, % Zinc, % Tensile Properties: Tensile strength, psi Yield strength, psi Elongation, % Compressive Properties, psi Brinell hardness, 500kg Izod impact, ft-lb Uses: 86.0 12.0 0.5 1.5 88.0 10.0 --- 2.0 82.0 18.0 --- --- 45.000 40,000 30,000 20 0.5 15,000 15,000 25,000 80 5 80 12 Special purpose bearings General, high quality usage; gun metal. 135 0.5 Valve slides; bells, parts resistant to crushing Table 1. Suggested standard tin bronzes (Gleaned from Harold J. Roast: Cast Bronze, ASM; edited by Prof. J. H. D. Bautista, PMAI.) 54 www.metals.rala.com.au 77 -23 87-2 77.0 23.0 --- --- --- 24,000 18,000 18,000 --- 20 --- --- --- --- Valve faces 87.0 6.0 2.0 5.0 customer should never, never specify chemical composition as this should be the foundry’s prerogative. Would you believe that there is a bronze composition that can match, if not surpass, any given ferrous-metal composition? But there are bronze compositions that cannot be matched by any ferrous metal composition! Bronze is so versatile, and it takes less to get started in business, so that most enterprising foundrymen start here. Management savvy and skill should take care of the economics. We contend that the primary reason or motive for ever going into business is to provide either a product or a service; profit is the incentive and reward. Therefore, choose the product or service that you like to provide and learn how to cut down operating costs to give you a bigger profit. That is the way to personal happiness! Now for starters, let’s talk about the tin-bronzes which have the widest applications in any industry. Table 1 shows a short-list of tin bronzes and leaded tin bronzes that could satisfy the requirements in most of the applications in industry. The tin bronzes are usually selected for their mechanical properties; while the leaded tin bronzes are usually selected generally for their lubricating properties, that is, lead imparts sliding properties to the metal, so that the higher lead contents usually have higher sliding properties. Leaded Tin Bronzes 81-7 81.0 3.0 7.0 9.0 75-15 67-25 80-5 75.0 7.0 15.0 3.0 67.0 5.0 25.0 3.0 34,000 28,000 25,000 21,000 16,000 12,000 14,000 11,000 22 15 50 6 10 65 7 7 12,000 10,000 16,000 9,000 60 12 42 7 High grade valves Low pressure valves; same as 85-5-5-5 Loco- motive bearings Railway car bearings 80.0 15.0 5.0 --- --- --- --- --- --- --- Seal ring pots for aircraft