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Metal Casting Technologies : March 2009
Back to the The following table shows the most preferred tapping temperatures from the furnace for various castings: ? Light castings, below ½-inch section thickness - 1200° C (2190° F) ? Medium castings, ½ to 1½-inch section thickness - 1160° C (2120° F) ? Heavy castings, over 1½-inch section thickness - 1120° C (2050° F) Color scale guide for temperatures – Molten bronze has a range of temperatures in the crucible of 1000-1250°C. If there is no pyrometer available, molten metal temperature measurements can still be done by plunging a ½-inch steel bar into the melt, holding it there for about five to seven seconds, then looking at the color of the steel bar after withdrawal. The color of the steel bar can be correlated with pyrometer read ings. So, standardize the practice by having the color-temperature relationship calibrated with a pyrometer and, thus calibrated, there would thence be less need for a pyrometer. The color scale below permits a good approximation of such high temperatures: The entries in bold italics are the ranges that can be used in determining molten bronze metal temperatures. Retrieval – in foundry parlance, this is the shakeout process. These castings may be shaken out as soon as they cease to be visibly red in a dark room – that is, below 538° C (1000° F) – but they may equally be left to cool in the mold overnight. Gating and risering – these alloys do not have high shrinkage; risers of very moderate feeding capacity can be used. In making a casting of a new design, it is often best to cast it without any risers and then break it up and see what evidence there is of need for risers as judged by signs of internal or external shrinkage. This method is especially very useful if a great number of the part in question is required. Machining – the leaded tin bronzes machine more easily and at higher speeds; while the plain tin bronzes machine less rapidly although still quite satisfactorily. Heat treatment – these alloys are not normally heat treated. However, the casting process undoubtedly leaves many residual stresses in the finished product and a stress-relieving anneal at a temperature between 260-538° C (500-1000° F) would be required for construction castings. Ventilation – any foundry melting leaded bronzes should pay special attention to ventilation both at the furnace area and during the transfer of metal from furnace to molds and during the period of pouring. It would help to periodically analyze foundry atmospheres for lead content. Historical note – the various ages of mankind’s development have been named after the materials that man used; e.g., stone age, bronze age, iron age, steel age, and today plastic age. Bronze is the oldest of the metal alloys that man used and there have been prominent bronze castings that graced man’s history. The oldest is King Solomon’s Bowl measuring 4.6 meters (15 feet) inside diameter and 2.3 meters (7½ feet) deep, weighing about 45 tonnes, next is the Colossus of Rhodes, a bronze statue of Apollo, towering about 46 meters (105 feet) over the harbor of Rhodes with one foot on each shore (this was shattered by an earthquake in 224BC and no trace remains), and finally Benvenuto Cellini’s Perseus with Medussa’s head, a splendid tribute to Cellini’s skill both as an artist-sculptor and a bronze foundryman. These prove that metalcasting is both an art and a science, which makes it very interesting indeed! But the real wonder here is how they did them with their more primitive methods and resources at the time! Can anybody today do better? ¦ References: (1) Harold J. Roast, Cast Bronze, American Society for Metals, Cleveland, Ohio. (2) The FOSECO Foundryman’s Handbook, FOSECO International, Limited, London, England. (3) Bruce L. Simpson, Development of the Metal Castings Industry, American Foundryman’s Society. Colour of steel Lowest visible red to dark red Dark red to cherry red Cherry red to bright cherry red Bright cherry red to orange Orange to yellow Yellow to light yellow Light yellow to white 56 www.metals.rala.com.au Temp, 0 C 475 to 650 650 to 750 750 to 815 815 to 900 900 to 1090 1090 to 1315 1315 to 1540 Temp., 0 F 880 to 1202 1202 to 1382 1382 to 1500 1500 to 1652 1652 to 1994 1994 to 2400 2400 to 2800