From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Article discussing the development and metallurgical process of malleableizing castings using continuous conveyor type furnaces.
| Identifier | ExFiles\Box 142\2\ scan0433 | |
| Date | 15th April 1939 | |
| 196 Malleableizing By W. F.{Mr Friese} Ross* INITIATED by a few of the larger producers of castings for the automotive and machine production industry, the trend toward furnace equipment designed to decrease annealing time to a minimum has steadily asserted itself. At the present time malleableizing cycles of over 55 hours are becoming rare and increasingly costly in relation to the newer methods of production in this highly competitive field. Based on production methods of a decade ago, charges, packed in scale or ore filled pots and covered, were placed on stools in box-type kilns and heated to from 1550 deg. Fahr. to 1650 deg. Fahr. in approximately 30 to 40 hours. This heating-up period was followed by a soaking period of from 50 to 65 hours, after which cooling to about 1200 deg. Fahr. for a period of 80 to 90 hours in the kiln permitted the opening of the kiln for relatively fast cooling to 500 deg. Fahr. or 600 deg. Fahr. prior to removing and unpacking the charges. The total cycle was, therefore, from 175 to 225 hours, assuming 5 to 10 hours total handling time. The castings produced by this cycle were uniformly satisfactory in regard to machining qualities, elongation (18 per cent to 20 per cent), and tensile strength (50,000 lb. per sq. in. or better). The retention of these characteristics, plus the beneficial advantages of a relatively short malleableizing cycle, constitutes a metallurgical problem which may briefly be analyzed as follows: The speed of graphitization during the heating-up and soaking periods is in direct relation to the temperature employed. Therefore, the cycle may be materially shortened during this phase of the annealing cycle by employing temperatures somewhat higher than the 1550 deg. Fahr. referred to above. At the higher temperature, however, excessive grain growth or overannealing must be prevented by dropping the temperature of the charge rapidly to a semi-critical temperature, or about 1400 deg. Fahr., a condition manifestly impossible to effect in the kiln or batch type annealing equipment unless the entire charge is withdrawn from the heating chamber. From 1400 deg. Fahr. precipitation of temper carbon is continued by cooling slowly to about 1300 deg. Fahr. before withdrawing the material from the heat. The various steps, including the fast and slow cooling, would indicate the practical utilization of a furnace equipment of continuous type for malleableizing. The elements necessary to design a continuous malleableizing furnace, with suitable provision for all factors influencing the actual time interval, resolve into an equipment broadly described as a continuous conveyor type equipment, suitably divided into separately controlled heating, fast cooling and slow cooling zones, designed with provision for controlled atmosphere to eliminate the scaling of castings by oxidation, thus materially reducing the tare weight of supports, containers and packing. The application of special atmosphere to malleableizing constitutes a major factor in the development of short cycle work. A continuous furnace adapted to the now obsolete malleableizing cycles and arranged to anneal work packed in pots would not only reach inconceivable proportions, but would certainly be grossly inefficient and costly. The elimination of all tare weight except for relatively light conveying trays or boxes may, therefore, be attributed to the utilization of non-oxidizing atmosphere, which surrounds the work while subjected to all temperatures within the oxidizing range and thus prevents the formation of scale. Any furnace equipment to be used in conjunction with a special protective atmosphere must be arranged for effective sealing by means of welded joints in the enclosing shell, and charging and discharge opening. This illustration shows the discharge end of a 340 kw., roller rail hearth, conveyor type malleableizing furnace. The special atmosphere generating equipment is mounted over the aisle. * Furnace engineer, The Electric Furnace Co. April 15, 1939 Automotive Industries | ||