MACHINEABILITY
Reheating for machine ability was done at 100° less than the drawing temperature, but the time of soaking is more than double. After both drawing and reheating, the blanks were buried in lime where they remain, out of contact with the air, until their temperature had dropped to that of the workroom.
For straightening, the barrels were heated to from 900 to 1,000°F. in an automatic furnace 25 ft. long, this operation taking about 2 hr. The purpose of hot straightening was to prevent any stresses being put into the blanks, so that after rough-turning, drilling or rifling operations they would not have a tendency to spring back to shape as left by the quenching bath.
A method that produces an even better machining rifle blank, which practically stays straight through the different machining operations, was to rough-turn the blanks, then subject them to a heat of practically 1,0000 for 4 hr. Production throughout the different operations is materially increased, with practically no straightening required after drilling, reaming, finish-turning or rifling operations.
FIG. 24.
FIG. 25.
FIGS. 24 and 25.—Roof system of cooling quenching oil.
This method was tested out by one of the largest manufacturers and proved to be the best way to eliminate a very expensive finished gun-barrel straightening process.
FIG. 26.—Details of the cooler.
The heat-treating required a large amount of cooling oil, and the problem of keeping this at the proper temperature required considerable study. The result was the cooling plant on the roof, as shown in Figs. 24, 25 and 26. The first two illustrations show the plant as it appeared complete. Figure 26 shows how the oil was handled in what is sometimes called the ebulator system. The oil was pumped up from the cooling tanks through the pipe A to the tank B. From here it ran down onto the breakers or separators C, which break the oil up into fine particles that are caught by the fans D. The spray is blown up into the cooling tower E, which contains banks of cooling pipes, as can be seen, as well as baffies F. The spray collects on the cool pipes and forms drops, which fall on the curved plates G and run back to the oil-storage tank below ground.
The water for this cooling was pumped from 10 artesian wells at the rate of 60 gal. per minute and cooled 90 gal. of oil per minute, lowering the temperature from 130 or 140 to 100°F. The water as it came from the wells averaged around 52°F. The motor was of a 7½-hp. variable-speed type with a range of from 700 to 1,200 r.p.m., which could be varied to suit the amount of oil to be cooled. The plant handled 300 gal. of oil per minute.
CHAPTER VI
ANNEALING
There is no mystery or secret about the proper annealing of different steels, but in order to secure the best results it is absolutely necessary for the operator to know the kind of steel which is to be annealed. The annealing of steel is primarily done for one of three specific purposes: To soften for machining purposes; to change the physical properties, largely to increase ductility; or to release strains caused by rolling or forging.
Proper annealing means the heating of the steel slowly and uniformly to the right temperature, the holding of the temperature for a given period and the gradual cooling to normal temperature. The proper temperature depends on the kind of steel, and the suggestions of the maker of the special steel being used should be carefully followed. For carbon steel the temperatures recommended for annealing vary from 1,450 to 1,600°F. This temperature need not be long continued. The steel should be cooled in hot sand, lime or ashes. If heated in the open forge the steel should be buried in the cooling material as quickly as possible, not allowing it to remain in the open air any longer than absolutely necessary. Best results, however, are secured when the fire does not come in direct contact with the steel.
Good results are obtained by packing the steel in iron boxes or tubes, much as for case-hardening or carbonizing, using the same materials. Pieces do not require to be entirely surrounded by carbon for annealing, however. Do not remove from boxes until cold.
Steel to be annealed may be classified into four different groups, each of which must be treated according to the elements contained in its particular analysis. Different methods are therefore necessary to bring about the desired result. The classifications are as follows: High-speed steel, alloy steel, tool or crucible steel, and high-carbon machinery steel.