Heating of Manganese Steel.—Another form of heat-treating furnace is that which is used for the heating of manganese and other alloy steels, which after having been brought to the proper heat are drawn from the furnace into an immediate quenching tank. With manganese steel in particular, the parts are so fragile and easily damaged while hot that it is frequent practice to have a sloping platform immediately in front of the furnace door down which the castings may slide into a tank below the floor level. Such a furnace with a quenching tank in front of its door is shown in Fig. 108.
These tanks are covered with plates while charging the furnace and the cold castings are placed in a moderately cool furnace. Since some of these steels must not be charged into a furnace where the heat is extreme but should be brought up to their final heat gradually, there is little discomfort during the charging process. When quenching, however, from a temperature of 1,800° to 1,900°, it is extremely unpleasant in front of the doors. The swinging shield is here adapted to give protection for this work. As will be noted it is hung a sufficient distance in front of the doors, that it may not interfere with the castings as they come from the furnace, and slide down into the tank.
To facilitate the work, and avoid the necessity of working with the bars outside the edges of the shield, the slot-like hole is cut in the center of the shield, and through this the bars or rakes for dragging out the castings are easily inserted and manipulated. The advantage of such a swinging shield is that it may be readily moved from side to side, or forward and back as occasion requires.
FURNACE DATA
In order to give definite information concerning furnaces, fuels etc., the following data is quoted from a paper by Seth A. Moulton and W. H. Lyman before the Steel Heat Treaters Society in September, 1920.
This considers a factory producing 30,000 lb. of automobile gears per 24 hr. The transmission gears will be of high-carbon steel and the differential of low-carbon steel, carburized. The heat-treating equipment required is:
| 1. Annealing furnaces | 1,400 to | 1,600°F. |
| 2. Carburizing furnaces | 1,700 to | 1,800°F. |
| 3. Hardening furnaces | 1,450 to | 1,550°F. |
| 4. Drawing furnaces | 350 to | 950°F. |
All of the forging blanks are annealed before machining, about three-quarters of the machined gears and parts are carburized, all the carburized gears are given a double treatment for core and case, all gears and parts are hardened and all parts are drawn.
The possible sources of heat supply and their values are as follows:—
| 1. Oil | 140,000 | B.t.u. per gallon |
| 2. Natural gas | 1,100 | B.t.u. per cubic foot |
| 3. City gas | 650 | B.t.u. per cubic foot |
| 4. Water gas | 300 | B.t.u. per cubic foot |
| 5. Producer gas | 170 | B.t.u. per cubic foot |
| 6. Coal | 12,000 | B.t.u. per pound |
| 7. Electric current | 3,412 | B.t.u. per kilowatt-hour |