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 |
For the heat treatment specified only comparatively low temperatures are required. No difficulty will be experienced in attaining the desired maximum temperature of 1,800°F. with any of the heating medium above enumerated; but it should be noted that the producer gas with a B.t.u. content of 170 per cubic foot and the electric current would require specially designed furnaces to obtain higher temperatures than 1800°F.
| Assuming | |
| Cost of oil- and gas-fired furnaces installed as | $100.00 per square foot of hearth |
| Cost of coal-fired furnace installed as | 150.00 per square foot of hearth |
| Cost of electric furnace 100 kw. capacity installed as | 90.00 per kilowatt |
| Cost of electric furnace 150 kw. capacity installed as | 70.00 per kilowatt |
Output 3,000 lb. charge, 8 hr. heat carburizing, 2 hr. heating only. Annual service 7,200 hr. Fixed charges including interest, depreciation, taxes, insurance and maintenance 15 per cent. Extra operating labor for coal-fired furnace 60 cts. per hour, one man four furnaces.
| Class fuel | Fuel per charge | Unit fuel cost | Installation cost | Efficiency per cent | Fixed charges | Cost per charge | |
|---|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | |
| Carburizing | |||||||
| 1 | Oil | 52.0 gal. | $0.15 gal. | $2,400.00 | 12.6 | $.40 | $8.20 |
| 2 | Natural gas | 4.4 M | 0.50 M | 2,400.00 | 18.8 | 0.40 | 2.60 |
| 3 | City gas | 8.3 M | 0.80 M | 2,400.00 | 17.0 | 0.40 | 7.04 |
| 4 | Water gas | 18.7 M | 0.40 | 2,400.00 | 16.4 | 0.40 | 7.88 |
| 5 | Producer gas | 37.3 M | 0.10 M | 2,400.00 | 14.5 | 0.40 | 4.13 |
| 6 | Coal | 814.0 lb. | 6.00 ton | 3,600.00 | 9.4 | 0.60 | 3.98 |
| 7 | Electricity | 500.0 kw-hr. | 0.015 kw. | 9,000.00 | 53.0 | 1.50 | 9.00 |
| Heating | |||||||
| 1 | Oil | 30.8 gal. | 0.15 gal. | 2,400.00 | 21.4 | 0.10 | 4.72 |
| 2 | Natural gas | 2.61 M | 0.50 M | 2,400.00 | 32.0 | 0.10 | 1.40 |
| 3 | City gas | 4.9 M | 0.80 M | 2,400.00 | 28.8 | 0.10 | 4.02 |
| 4 | Water gas | 11.1 M | 0.40 M | 2,400.00 | 27.6 | 0.10 | 4.54 |
| 5 | Producer gas | 22.1 M | 0.10 M | 2,400.00 | 24.6 | 0.10 | 2.31 |
| 6 | Coal | 348.0 lb. | 6.00 ton | 3,600.00 | 22.0 | 0.15 | 1.38 |
| 7 | Electricity | 329.0 kw-hr. | 0.015 kw. | 10,500.00 | 81.75 | 0.44 | 5.38 |
This shows but two of the operations and for a single furnace. The total costs for all operations on the 30,000 lb. of gears per 24 hr. is shown in Table 29.
NOTE.—Producer plant fixed charges are included in the cost of gas and are charged as "heat" in column 5, so they are omitted from column 4.
CHAPTER XII
PYROMETRY AND PYROMETERS
A knowledge of the fundamental principles of pyrometry, or the measurement of temperatures, is quite necessary for one engaged in the heat treatment of steel. It is only by careful measurement and control of the heating of steel that the full benefit of a heat-treating operation is secured.
Before the advent of the thermo-couple, methods of temperature measurement were very crude. The blacksmith depended on his eyes to tell him when the proper temperature was reached, and of course the "color" appeared different on light or dark days. "Cherry" to one man was "orange" to another, and it was therefore almost impossible to formulate any treatment which could be applied by several men to secure the same results.
One of the early methods of measuring temperatures was the "iron ball" method. In this method, an iron ball, to which a wire was attached, was placed in the furnace and when it had reached the temperature of the furnace, it was quickly removed by means of the wire, and suspended in a can containing a known quantity of water; the volume of water being such that the heat would not cause it to boil. The rise in temperature of the water was measured by a thermometer, and, knowing the heat capacity of the iron ball and that of the water, the temperature of the ball, and therefore the furnace, could be calculated. Usually a set of tables was prepared to simplify the calculations. The iron ball, however, scaled, and changed in weight with repeated use, making the determinations less and less accurate. A copper ball was often used to decrease this change, but even that was subject to error. This method is still sometimes used, but for uniform results, a platinum ball, which will not scale or change in weight, is necessary, and the cost of this ball, together with the slowness of the method, have rendered the practice obsolete, especially in view of modern developments in accurate pyrometry.