Affinity of Nickel Steel for Carbon.—The carbon- and nickel-steel gears are carburized separately owing to the difference in time necessary for their carburization. Practically all printed information on the subject is to the effect that nickel steel takes longer to carburize than plain carbon steel. This is directly opposed to the conditions found at this plant. For the same depth of case, other conditions being equal, a nickel-steel gear would require from 20 to 30 min. less than a low carbon-steel gear.
From the quenching machines, the gears go to the sand-blasting machines, situated in the wing of the heat-treating building, where they are cleaned. From here they are taken to the testing department. The tests are simple and at the same time most thorough.
Testing and Inspection of Heat Treatment.—The hard parts of the gear must be so hard that a new mill file does not bite in the least. Having passed this file test at several points, the gears go to the center-punch test. The inspector is equipped with a wooden trough secured to the top of the bench to support the gear, a number of center punches (made of ¾-in. hex-steel having points sharpened to an angle of 120 deg.) and a hammer weighing about 4 oz. With these simple tools, supplemented by his skill, the inspector can feel the depth and quality of the case and the condition of the core. The gears are each tested in this way at several points on the teeth and elsewhere, the scrap gear being also subjected to the test. Finally, the scrap gear is securely clamped in the straightening press shown in Fig. 57. With a 3½-lb. hammer and a suitable hollow-ended drift manipulated by one of Sandow's understudies, teeth are broken out of the scrap gear at various points. These give a record confirming the center-punch tests, which, if the angle of the center punch is kept at 120 deg. and the weight of the hammer and blow are uniform, is very accurate.
After passing the center-punch test the ends of the teeth are peened lightly with a hammer. If they are too hard, small particles fly off. Such gears are drawn in oil at a temperature of from 300 to 350°F., depending on their hardness. Some builders prefer to have the extreme outer ends of the teeth drawn somewhat lower than the rest. This drawing is done on gas-heated red-hot plates, as shown at A in Fig. 58.
FIG. 57.—Press for holding test gears for breaking.
Nickel steel, in addition to all the tests given to carbon steel, is subjected to a Brinell test. For each steel, the temperature and the period of treatment are specific. For some unknown reason, apparently like material with like treatment will, in isolated cases, not produce like results. It then remains for the treatment to be repeated or modified, but the results obtained during inspection form a valuable aid to the metallurgist in determining further treatment.
Temperature Recording and Regulation.—Each furnace is equipped with pyrometers, but the reading and recording of all temperatures are in the hands of one man, who occupies a room with an opening into the end of the hardening department. The opening is about 15 ft. above the floor level. On each side of it, easily legible from all of the furnaces, is a board with the numbers of the various furnaces, as shown in Figs. 59 and 60. Opposite each furnace number is a series of hooks whereon are hung metal numbers representing the pyrometer readings of the temperature in that particular furnace. Within the room, as shown in Fig. 60, the indicating instrument is to the right, and to the left is a switchboard to connect it with the thermo-couples in the various furnaces. The boards shown to the right and the left swing into the room, which enables the attendant easily to change the numbers to conform to the pyrometer readings. Readings of the temperatures of the carburizing furnaces are taken and tabulated every ten minutes. These, numbered 1 to 10, are shown on the board to the right in Fig. 59. The card shown in Fig. 61 gives such a record. These records are filed away for possible future reference.
FIG. 58.—Gas heated drawing plate for tooth ends.