The Working of Steel Part 17

The gear comes out uniformly hard all over and of the same degree of hardness as when tempered in an open tank. The output of the machine depends on the amount of metal to be cooled, but will average from 8 to 16 per hour. Each machine is served by one man, two furnaces being required to heat the work. A slight excess of oil is used in the firing of the furnaces to give a reducing atmosphere and to avoid scale.

[Ill.u.s.tration: FIG. 64.--Hardening and shrinking sleeves.]

CARBURIZING LOW-CARBON SLEEVES.--Low-carbon sleeves are carburized and pushed on malleable-iron differential-case hubs. Formerly, these sleeves were given two treatments after carburization in order to refine the case and the core, and then sent to the grinding department, where they were ground to a push fit for the hubs. After this they were pushed on the hubs. By the method now employed, the first treatment refines the core, and on the second treatment, the sleeves are pushed on the hub and at the same time hardened.

This method cuts out the internal grinding time, pressing on hubs, and haulage from one department to another. Also, less work is lost through splitting of the sleeves.

The machine for pus.h.i.+ng the sleeves on is shown in Fig. 64. At _A_ is the stem on which the hot sleeve _B_ is to be pushed. The carburized sleeves are heated in an automatic furnace, which takes them cold at the back and feeds them through to the front, by which time they are at the correct temperature. The loose mandrel _C_ is provided with a spigot on the lower end, which fits the hole in the differential-case hub. The upper end is tapered as shown and acts as a pilot for the ram _D_. The action of pus.h.i.+ng on and quenching is similar to the action of the Gleason tempering machine, with the exception that water instead of oil is used as a quenching medium. The speed of operation depends on a number of variables, but from 350 to 500 can be heated and pressed on in 11 hr.

CYANIDE BATH FOR TOOL STEELS.--All high-carbon tool steels are heated in a cyanide bath. With this bath, the heat can be controlled within 3 deg. The steel is evenly heated without exposure to the air, resulting in work which is not warped and on which there is no scale. The cyanide bath is, of course, not available for high-speed steel because of the very high temperatures necessary.

DROP FORGING DIES

The kind of steel used in the die of course influences the heat treatment it is to receive, but this also depends on the kind of work the die is to perform. If the die is for a forging which is machined all over and does not have to be especially close to size, where a variation of 1/16 in. is not considered excessive, a low grade steel will be perfectly satisfactory.

In cases of fine work, however, where the variation cannot be over 0.005 to 0.01 in. we must use a fine steel and prevent its going out of shape in the heating and quenching. A high quality crucible steel is suggested with about the following a.n.a.lysis: Carbon 0.75 per cent, manganese 0.25 per cent, silicon 0.15 per cent, sulphur 0.015 per cent, and phosphorus 0.015 per cent. Such a steel will have a decalescent point in the neighborhood of 1,355F. and for the size used, probably in a die of approximately 8 in., it will harden around 1,450F.

To secure best results care must be taken at every step. The block should be heated slowly to about 1,400F., the furnace closed tight and allowed to cool slowly in the furnace itself. It should not soak at the high temperature.

After machining, and before it is put in the furnace for hardening, it should be slowly preheated to 800 or 900F. This can be done in several ways, some putting the die block in front of the open door of a hardening furnace and keeping the furnace at about 1,000F.

The main thing is to heat the die block very slowly and evenly.

The hardening heat should be very slow, 7 hr. being none too long for such a block, bringing the die up gradually to the quenching temperature of 1,450. This should be held for 1/2 hr. or even a little more, when the die can be taken out and quenched. There should be no guess work about the heating, a good pyrometer being the only safe way of knowing the correct temperature.

The quenching tank should be of good size and have a spray or stream of water coming up near the surface. Dip the die block about 3 in.

deep and let the stream of water get at the face so as to play on the forms. By leaving the rest of the die out of the water, moving the die up and down a trifle to prevent a crack at the line of immersion, the back of the block is left tough while the face is very hard. To overcome the tendency to warp the face it is a good plan to pour a little water on the back of the die as this tends to even up the cooling. The depth to which the die is dipped can be easily regulated by placing bars across the tank at the proper depth.

After the scleroscope shows the die to be properly hardened, which means from 98 to 101, the temper should be drawn as soon as convenient.

A lead pot in which the back of the die can be suspended so as to heat the back side, makes a good method. Or the die block can be placed back to the open door of a furnace. On a die of this size it may take several hours to draw it to the desired temper.

This can be tested while warm by the scleroscope method, bearing in mind that the reading will not be the same as when cold. If the test shows from 76 to 78 while warm, the hardness when cold will be about 83, which is about right for this work.

S. A. E. HEAT TREATMENTS

The Society of Automotive Engineers have adopted certain heat treatments to suit different steels and varying conditions. These have already been referred to on pages 39 to 41 in connection with the different steels used in automobile practice. These treatments are designated by letter and correspond with the designations in the table.

HEAT TREATMENTS

_Heat Treatment A_

After forging or machining: 1. Carbonize at a temperature between 1,600F. and 1,750F.

(1,650-1,700F. desired.) 2. Cool slowly or quench.

3. Reheat to 1,450-1,500F. and quench.

_Heat Treatment B_

After forging or machining: 1. Carbonize between 1,600F. and 1,750F. (1,650-1,700F.

Desired.) 2. Cool slowly in the carbonizing mixture.

3. Reheat to 1,550-1,625F.

4. Quench.

5. Reheat to 1,400-1,450F.

6. Quench.

7. Draw in hot oil at 300 to 450F., depending upon the degree of hardness desired.

_Heat Treatment D_

After forging or machining: 1. Heat to 1,500-1,600F.

2. Quench.

3. Reheat to 1,450-1,500F.

4. Quench.

5. Reheat to 600-1,200F. and cool slowly.

_Heat Treatment E_

After forging or machining: 1. Heat to 1,500-1,550F.

2. Cool slowly.

3. Reheat to 1,450-1,500F.

4. Quench.

5. Reheat to 600-1,200F. and cool slowly.

_Heat Treatment F_

After shaping or coiling: 1. Heat to 1,425-1,475F.

2. Quench in oil.

3. Reheat to 400-900F., in accordance with temper desired and cool slowly.

_Heat Treatment G_

After forging or machining: 1. Carbonize at a temperature between 1,600F. and 1,750F.

(1,650-1,700F. desired).

2. Cool slowly in the carbonizing mixture.

3. Reheat to 1,500-1,550F.

4. Quench.

5. Reheat to 1,300-1,400F.

6. Quench.

7. Reheat to 250-500F. (in accordance with the necessities of the case) and cool slowly.

_Heat Treatment H_

After forging or machining: 1. Heat to 1,500-1,600F.

2. Quench.

3. Reheat to 600-1,200F. and cool slowly.

_Heat Treatment K_

After forging or machining: 1. Heat to 1,500-1,550F.

2. Quench.

3. Reheat to 1,300-1,400F.

4. Quench.

5. Reheat to 600-1,200F. and cool slowly.