The Working of Steel Part 16

The equations for the different shapes are as follows:

For flats _XY_ = 7.062 For rounds _XY_ = 14.124 For spheres _XY_ = 21.185

It will be noted that the constants increase in a ratio of 1, 2, and 3, and the three bodies in question will increase in hardness on being quenched in the same ratio, it being understood that the diameter of the sphere and round and thickness of the flat are equal.

Relative to shape, it is interesting to note that rounds, squares, octagons and other three axial bodies, with two of their axes equal, have the same surface for the unit weight.

For example:

Size Length Surface Weight Surface for 1 lb.

2 in. Sq. 12 in. 96.0 sq. in. 13.60 lb. 7.06 sq. in.

2 in. Round 12 in. 75.4 sq. in. 10.68 lb. 7.06 sq. in.

Although this discussion is at present based upon mathematical a.n.a.lysis, it is hoped that it will open up a new field of investigation in which but little work has been done, and may a.s.sist in settling the as yet unsolved question of the effect of size and shape in the heat treatment of steel.

HEAT-TREATING EQUIPMENT AND METHODS FOR Ma.s.s PRODUCTION

The heat-treating department of the Brown-Lipe-Chapin Company, Syracuse, N. Y., runs day and night, and besides handling all the hardening of tools, parts of jigs, fixtures, special machines and appliances, carburizes and heat-treats every month between 150,000 and 200,000 gears, pinions, crosses and other components entering into the construction of differentials for automobiles.

The treatment of the steel really begins in the mill, where the steel is made to conform to a specific formula. On the arrival of the rough forgings at the Brown-Lipe-Chapin factory, the first of a long series of inspections begins.

ANNEALING METHOD.--Forgings which are too hard to machine are put in pots with a little charcoal to cause a reducing atmosphere and to prevent scale. The covers are then luted on and the pots placed in the furnace. Carbon steel from 15 to 25 points is annealed at 1,600F. Nickel steel of the same carbon and containing in addition 3-1/2 per cent nickel is annealed at 1,450F. When the pots are heated through, they are rolled to the yard and allowed to cool.

This method of annealing gives the best hardness for quick machining.

The requirements in the machine operations are very rigid and, in spite of great care and probably the finest equipment of special machines in the world, a small percentage of the product fails to pa.s.s inspection during or at the completion of the machine operations. These pieces, however, are not a loss, for they play an important part in the hardening process, indicating as they do the exact depth of penetration of the carburizing material and the condition of both case and core.

HEAT-TREATING DEPARTMENT.--The heat-treating department occupies an L-shaped building. The design is very practical, with the furnace and the floor on the same level so that there is no lifting of heavy pots. Fuel oil is used in all the furnaces and gives highly satisfactory results. The consumption of fuel oil is about 2 gal.

per hour per furnace.

The work is packed in the pots in a room at the entrance to the heat-treatment building. Before packing, each gear is stamped with a number which is a key to the records of the a.n.a.lysis and complete heat treatment of that particular gear. Should a question at any time arise regarding the treatment of a certain gear, all the necessary information is available if the number on the gear is legible. For instance, date of treatment, furnace, carburizing material, position of the pot in the furnace, position of gear in pot, temperature of furnace and duration of treatment are all tabulated and filed for reference.

After marking, all holes and parts which are to remain uncarburized are plugged or luted with a mixture of kaolin and Mellville gravel clay, and the gear is packed in the carburizing material. Bohnite, a commercial carburizing compound is used exclusively at this plant.

This does excellent work and is economical. Broadly speaking, the economy of a carburizing compound depends on its lightness. The s.p.a.ce not occupied by work must be filled with compound; therefore) other things being equal, a compound weighing 25 lb. would be worth more than twice as much as one weighing 60 lb. per cubic foot. It has been claimed that certain compounds can be used over and over again, but this is only true in a limited way, if good work is required. There is, of course, some carbon in the compound after the first use, but for first-cla.s.s work, new compound must be used each time.

THE PACKING DEPARTMENT.--In Fig. 56 is shown the packing pots where the work is packed. These are of malleable cast iron, with an internal vertical f.l.a.n.g.e around the hole _A_. This fits in a bell on the end of the cast-iron pipe _B_, which is luted in position with fireclay before the packing begins. At _C_ is shown a pot ready for packing. The crown gears average 10 to 12 in. in diameter and weigh about 11 lb. each. When placed in the pots, they surround the central tube, which allows the heat to circulate. Each pot contains five gears. Two complete sc.r.a.p gears are in each furnace (_i.e._, gears which fail to pa.s.s machining inspection), and at the top of front pot are two or more short segments of sc.r.a.p gear, used as test pieces to gage depth of case.

[Ill.u.s.tration: FIG. 56.--Packing department and special pots.]

After filling to the top with compound, the lid _D_ is luted on.

Ten pots are then placed in a furnace. It will be noted that the pots to the right are numbered 1, 2, 3, 4, indicating the position they are to occupy in the furnace.

The cast-iron ball shown at _E_ is small enough to drop through the pipe _B_, but will not pa.s.s through the hole _A_ in the bottom of the pot. It is used as a valve to plug the bottom of the pot to prevent the carburizing compound from dropping through when removing the carburized gears to the quenching bath.

Without detracting from the high quality of the work, the metallurgist in this plant has succeeded in cutting out one entire operation and reducing the time in the hardening room by about 24 hr.

Formerly, the work was carburized at about 1,700F. for 9 hr. The pots were then run out into the yard and allowed to cool slowly.

When cool, the work was taken out of the pots, reheated and quenched at 1,600F. to refine the core. It was again reheated to 1,425F.

and quenched to refine the case. Finally, it was drawn to the proper temper.

SHORT METHOD OF TREATMENT.--In the new method, the packed pots are run into the case-hardening furnaces, which are heated to 1,600F.

On the insertion of the cold pots, the temperature naturally falls.

The amount of this fall is dependent upon a number of variables, but it averages nearly 500F. as shown in the pyrometer chart, Fig. 61. The work and furnace must be brought to 1,600F. Within 2-1/2 hr.; otherwise, a longer time will be necessary to obtain the desired depth of case. On this work, the depth of case required is designated in thousandths, and on crown gears, the depth in 0.028 in. Having brought the work to a temperature of 1,600F.

the depth of case mentioned can be obtained in about 5-1/2 hr. by maintaining this temperature.

As stated before, at the top of each pot are several test pieces consisting of a whole sc.r.a.p gear and several sections. After the pots have been heated at 1,600F. for about 5-1/4 hr., they are removed, and a sc.r.a.p-section test-piece is quenched direct from the pot in mineral oil at _not more than_ 100F. The end of a tooth of this is then ground and etched to ascertain the depth of case.

As these test pieces are of exactly the same cross-section as the gears themselves, the carburizing action is similar. When the depth of case has been found from the etched test pieces to be satisfactory, the pots are removed. The iron ball then is dropped into the tube to seal the hole in the bottom of the pot; the cover and the tube are removed, and the gears quenched direct from the pot in mineral oil, which is kept at a temperature not higher than 100F.

THE EFFECT.--The heating at 1,600F. gives the first heat treatment which refines the core, which under the former high heat (1,700F.) was rendered coa.r.s.ely crystalline. All the gears, including the sc.r.a.p gears, are quenched direct from the pot in this manner.

The gears then go to the reheating furnaces, situated in front of a battery of Gleason quenching machines. These furnaces accommodate from 12 to 16 crown gears. The carbon-steel gears are heated in a reducing atmosphere to about 1,425F. (depending on the carbon content) placed in the dies in the Gleason quenching machine, and quenched between dies in mineral oil at less than 100F. The test gear receives exactly the same treatment as the others and is then broken, giving a record of the condition of both case and core.

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 pa.s.sed 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 3/4-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 sc.r.a.p gear being also subjected to the test. Finally, the sc.r.a.p gear is securely clamped in the straightening press shown in Fig. 57. With a 3-1/2-lb. hammer and a suitable hollow-ended drift manipulated by one of Sandow's understudies, teeth are broken out of the sc.r.a.p 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 pa.s.sing 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 350F., 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: FIG. 58.--Gas heated drawing plate for tooth ends.]

The temperatures of the reheating furnaces, numbered from 1 to 26 and shown on the board to the left in Fig. 59, are taken every 5 min.

Each furnace has a large metal sign on which is marked the temperature at which the furnace regulator is required to keep his heat. As soon as any variation from this is posted on the board outside the pyrometer room, the attendant sees it and adjusts the burners to compensate.

[Ill.u.s.tration: FIG. 59.--Pyrometer recording room.]

[Ill.u.s.tration: FIG. 60.--Inside of Pyrometer switch room.]

DIES FOR GLEASON TEMPERING MACHINES.--In Fig. 62 is shown a set of dies for the Gleason tempering machine. These accurately made dies fit and hold the gear true during quenching, thus preventing distortion.

[Ill.u.s.tration: FIG. 61.--Carburizing furnace record.]

Referring to Fig. 62, the die _A_ has a surface _B_ which fits the face of the teeth of the gear _C_. This surface is perforated by a large number of holes which permit the quenching oil to circulate freely. The die _A_ is set in the upper end of the plunger _A_ of the tempering machine, shown in Fig. 63, a few inches above the surface of the quenching oil in the tank _N_. Inside the die _A_ are the centering jaws _D_, Fig. 62, which are an easy fit for the bore of the gear _C_. The inner surface of the centering jaws is in the shape of a female cone. The upper die is shown at _E_. In the center (separate from it, but a snug sliding fit in it) is the expander _G_, which, during quenching, enters the taper in the centering jaws _D_, expanding them against the bore of the gear _C_. The faces _F_ of the upper die _E_ fit two angles at the back of the gear and are grooved for the pa.s.sage of the quenching oil. The upper die _E_ is secured to the die carrier _B_, shown in Fig. 9, and inside the die is the expander _G_, which is backed up by compression springs.

[Ill.u.s.tration: FIG. 62.--Dies for Gleason gear-hardening machine.]

HARDENING OPERATION.--Hardening a gear is accomplished as follows: The gear is taken from the furnace by the furnaceman and placed in the lower die, surrounding the centering jaws, as shown at _H_ in Fig. 62 and _C_ in Fig. 63. Air is then turned into the cylinder _D_, and the piston rod _E_, the die carrier _B_, the top die _F_ and the expander _G_ descend. The pilot _H_ enters a hole in the center of the lower die, and the expander _G_ enters the centering jaws _I_, causing them to expand and center the gear _C_ in the lower die. On further advance of the piston rod _E_, the expander _G_ is forced upward against the pressure of the springs _J_ and the upper die _F_ comes in contact with the upper surface of the gear. Further downward movement of the dies, which now clamp the work securely, overcomes the resistance of the pressure weight _K_ (which normally keeps up the plunger _A_), and the gear is submerged in the oil. The quenching oil is circulated through a cooling system outside the building and enters the tempering machine through the inlet pipe _L_. When the machine is in the position shown, the oil pa.s.ses out through the ports _M_ in the lower plunger to the outer reservoir _N_, pa.s.sing to the cooling system by way of the overflow _O_. When the lower plunger _A_ is forced downward, the ports _M_ are automatically closed and the cool quenching oil from the inlet pipe _L_, having no other means of escape, pa.s.ses through the holes in the lower die and the grooves in the upper, circulating in contact with the surfaces of the gear and pa.s.ses to the overflow. When the air pressure is released, the counterweights return the parts to the positions shown in Fig. 63, and the operator removes the gear.

[Ill.u.s.tration: FIG. 63.--Gleason tempering machine.]