Fig. 74 illustrates a heat treating furnace which is used by many large manufacturers of machinery. This furnace is arranged with single and double compart ments, the heating chambers being separated by a solid fire brick wall in such a manner that each cham ber can be heated entirely independently of the other. At the side of the heating chamber is the combustion space, into which the burners fire. The heat passes from this space over a bridge wall and along the arch of the heating chamber. Circling this arch, the heat is .drawn across the hearth and into the flues, which criss-cross under the hearth, thereby heating the floor of the furnace from above and below, and saving some of the waste heat that would otherwise have been lost.
In the main flue is located the air blast pre-heater pipe ; the air passing through this pipe is pre-heated to almost the flue temperature. This effects an econ omy of fuel, for it lowers the temperature of the gases leaving the furnace, and returns to the furnace a large amount of heat which would otherwise have been lost. The working openings are covered by heavy fire brick doors, encased in cast iron frames flanged and ribbed to prevent warping and cracking. These doors are raised and lowered by a direct counter-balancing arrangement, operating on roller bearings, and when lowered are held firmly against the furnace front by door checks. An opening with a cover is provided in the door for a peep-hole. This furnace is properly de signed for all classes of heat treating, as it amounts to a semi-muffle furnace. It gives an even heat throughout the chamber, without any danger from ox idation. Suitable pyrometer holes are provided in the walls and arch. No stack is required, and either oil or coal may be burned.
Fig. 75 illustrates a portable semi-muffle furnace for annealing or general hardening. It is mounted on a self contained oil burning system, requiring only a small amount of compressed air for operation. This air is used to force the oil to the burner, and also con stitutes an atomizing agent for the oil. Self-contained furnaces of this type are installed in shops where the work does not warrant the installation of a complete oil burning system.
Fig. 76 illustrates a portable fuel oil burner which gives an intense flame. It is used in many machine shops, boiler and locomotive shops and foundries, for pipe bending, plate bending, starting cupolas, drying moulds and general heating. It is self-contained, and requires but a small amount of compressed air for operation.
The average jobbing shop uses pit furnaces, the crucibles holding about 200 lbs. A few large manu facturing plants, outside of the rolling mills, still use pit furnaces, but these are, almost without exception, those whose castings are so small or so thin that the drop in temperature due to pouring into a ladle from a tilting or tapping furnace cannot be allowed. The great majority of the manufacturing plants whose aim is large production use tilting or tapping furnaces on account of their greater speed. Few of the users of tilting crucible oil furnaces, tilting oil flame furnaces or oil fired reverbatory furnaces come into the small jobbing class. Most of the large manufacturers, ex cept the rolling mills, use oil on account of the great speed of melting possible with its use, and in this connection it should be noted that the open flame, oil furnace seems to come nearest to meeting the needs of the large manufacturer who must melt large quan tities of red brass.
A reverbatory furnace 5 x 2% ft. was brought to a temperature of 2700 degrees F. with 2.8 gallons of Wagon tires from 2 in. to 4 in. wide require about 5 gal. of oil per hour when treating 30 tires.
In a crucible furnace, three 150 lb. pots were brought to a temperature of 3200 degrees F. with 5 gallons of oil. A brass furnace melted 40 lb. of brass in 30 minutes on 6 lb. of oil.
A forging furnace has handled 1 ton of material ready for the hammer on 22 gallons of oil. This was average shop work.
A bolt heading furnace turned out 600 1% in bolts per hour on 6% gallons of oil. The bolts were 6 in. long, consequently the fuel consumption was only 10.4 gallons of oil per ton of material. The same furnace, operating two machines, heated 44 in. bolts at the rate of 1380 per hour. A large rivet heat ing furnace has heated 600 rivets an hour on a con sumption of 3 gallons of oil, and an annealing furnace treated 1 ton of material on 10.5 gallons of oil.
In an air furnace, the time of getting out a heat was greatly reduced, and the capacity of the plant in creased by the use of oil. One malleable iron foundry which used 1000 lb. of coal per ton of iron and took ten hours to a heat is now using 50 gallons of oil per ton of iron and running heats in less than five hours In general blacksmith practice at the Mare Island Navy Yard an economy of 60 per cent in fuel cost was effected, 40 per cent more work was done by the same number of men with oil instead of coal fuel. A piece of armor plate 10 ft. long, 4 ft. 6 in. wide, and 6 in. thick. was heated in eighty minutes. the job requiring eight hours when coal was used as a fuel Pyrometers are of great value in connection with the heat treatment of steel or other metals, as they make possible the accurate determination of the high temperatures encountered. When heating for harden ing, the temperature can be maintained steadily at the point which has given the best results in practice. The correct hardening temperature for any carbon steel can be determined accurately by the use of a pyrometer.
When testing a piece of steel with this apparatus, the temperature indicated by the meter rises uniformly until the metal is heated to a certain point, at which it continues to absorb heat without appreciably rising in temperature. This is called the "point of decal escence." At this temperature, the indicating pointer of the meter remains stationary, the added heat being consumed by internal changes. When these changes are completed, the temperature again rises. The point of decalescence should be carefully noted, and as soon as temperature begins to rise above it the steel should be removed from the furnace and allowed to cool slowly, for overheating at this critical point has a ten dency to lessen the hardness of the steel. When it has cooled to the proper color, it is cooled by plunging into oil.
The purpose of annealing steel is to soften it for machining, and also to remove all stress due to roll ing or hammering. The temperature for annealing should be slightly above the critical point, which varies with different steels. This temperature should be maintained just long enough to heat the entire piece evenly; care should be taken not the steel much above the hardening point for when steel is heated above this temperature it becomes coarse. Overheated steel that is not actually burned can be partly restored by heating it to the proper tem perature and allowing it to cool slowly in hot ashes or sand; after this process it must be hardened again at the proper tempering heat.
The presence of scale on the surface of hardened steel is due to oxidation when hot; consequently, to prevent scale the heated steel must not be exposed to the action of the air. When using an oven heating furnace, the flame should be so regulated that it is not visible in the heating chambers.