0.15 to 0.25 per cent carbon, straight carbon steel.—Heat to 1,650°F. Hold at this temperature until the work is uniformly heated; pull from the furnace and cool in air.
0.15 to 0.25 per cent carbon, 1½ per cent nickel, 1/2 per cent chromium steel.—Heat to 1,600°F. Hold at this temperature until the work is uniformly heated; pull from the furnace and cool in air.
0.15 to 0.25 per cent carbon, 3½ per cent nickel steel.—Heat to 1,575°F. Hold at this temperature until the work is uniformly heated; pull from the furnace and cool in air.
Care in Annealing.—Not only will benefits in machining be found by careful annealing of forgings but the subsequent troubles in the hardening plant will be greatly reduced. The advantages in the hardening start with the carburizing operation, as a steel of uniform and fine grain size will carburize more uniformly, producing a more even hardness and less chances for soft spots. The holes in the gears will also "close in more uniformly," not causing some gears to require excessive grinding and others with just enough stock. Also all strains will have been removed from the forging, eliminating to a great extent distortion and the noisy gears which are the result.
With the steels used, for the heat-treated gears, always of a higher carbon content, treatment after forging is necessary for machining, as it would be impossible to get the required production from untreated forgings, especially in the alloy steels. The treatment is more delicate, due to the higher percentage of carbon and the natural increase in cementite together with complex carbides which are present in some of the higher types of alloys.
Where poor machining conditions in heat-treated steels are present they are generally due to incomplete solution of cementite rather than bands of free ferrite, as in the case of case-hardening steels. This segregation of carbon, as it is sometimes referred to, causes hard spots which, in the forming of the tooth, cause the cutter to ride over the hard metal, producing high spots on the face of the tooth, which are as detrimental to satisfactory gear cutting as the drops or low spots produced on the face of the teeth when the pearlite is coarse-grained or in a banded condition.
In the simpler carburized steels it is not necessary to test the forgings for hardness after annealing, but with the high percentages of alloys in the carburizing steels and the heat-treated steels a hardness test is essential.
To obtain the best results in machining, the microstructure of the metal should be determined and a hardness range set that covers the variations in structure that produce good machining results. By careful control of the heat-treating operation and with the aid of the Brinell hardness tester and the microscope it is possible to continually give forgings that will machine uniformly and be soft enough to give desired production. The following gives a few of the hardness numerals on steel used in gear manufacture that produce good machining qualities:
0.20 per cent carbon, 3 per cent nickel, 1¼; per cent chromium—Brinell 156 to 170.
0.50 per cent carbon, 3 per cent nickel, 1 per cent chromium—Brinell 179 to 187.