STEAM TEMPERING OF BALL BEARING PARTS

B. V. Volkenau

Before grinding, ball bearing parts undergo heat treatment. The heat treatment consists of the following operations: 1) heating for hardening to 780–870°C; 2) quenching in oil (water); and 3) tempering at 130–180°C [1; 3].

Tempering is carried out in oil or in air with natural circulation or with circulation produced by a fan.

The transfer of parts from one operation to another is carried out in different ways. From the first operation to the second, transfer is always by conveyor, i.e., without the participation of a worker. Transfer from the second operation to the third is carried out both by conveyor and by manually reloading the parts from one device into another.

The source of heat in all tempering operations is electric power. Air and oil are heated by electricity.

In engineering practice, to obtain temperatures up to 180°C, as is the case in tempering ball bearing parts, it is not rational to use electric power, especially that obtained at thermal power stations.

The production of electric power at thermal power stations is associated with large heat losses. Of the heat of the fuel, only 21% is used at power stations. If losses in the network are also taken into account, then at the point of consumption only a small share of the heat of the fuel expended in the boiler house can be used, considerably less than the indicated 21%.

In most technological processes where a temperature up to 180°C is required, other, non-electric methods of heating are used, for example, heating by steam. With steam heating, very precise temperature control is achieved.

In the heat treatment of ball bearing parts, heating by electric power has traditionally become established. This is the case both where such heating is rational—in annealing and heating for hardening—and where such heating is not rational—in tempering.

The use of steam for tempering parts, in comparison with tempering in air, is advantageous because of the rapid heating of the parts. This is theoretically easy to see from a comparison of the heat-transfer coefficients: $\alpha$ for air varies from 5–200

\[ \frac{\text{kcal}}{m^{2}\,\text{h}\,\text{deg}}, \]

while $\alpha$ for condensing steam varies from 6000–15000

\[ \frac{\text{kcal}}{m^{2}\,\text{h}\,\text{deg}}. \]

Compared with tempering in oil, a considerable advantage is obtained in that it is not necessary to consume oil, and the washing of parts to remove oil is also eliminated.

Tempering is of great importance for the quality of bearings. During tempering, the brittleness of hardened parts is reduced and plasticity is increased. The influence of the duration of tempering at 150°C was investigated by P. A. Dvoryanov [2].

To determine the service life of a bearing, the theoretical life under a given load is 164 hours.

The results of the endurance test as a function of tempering duration were as follows:

With steam tempering, the tempering time can be increased considerably. The autoclave takes up little space, and it is possible to install two autoclaves and double the tempering time. This cannot be done either when tempering in air or when tempering in oil.

The experiments were carried out at 1GPZ in 1933–34. A vertical boiler (autoclave) was made of sheet iron; its internal diameter was 600 mm and its height 1150 mm. The steam inlet was made at the top of the cylinder, and the outlet for steam and condensate in the lower bottom.

For loading from above, a cover on 8 bolts was made. Such a boiler has sufficient capacity to load, in a cylindrical cassette with holes, 500–450 kg of 13/16″ balls and rollers, and a smaller quantity of rings (320–290 kg).

Such a boiler is too small for production purposes. Fig. 1 shows a large boiler for loading 500 kg of medium rings, i.e. for two hours’ operation of the hardening furnace.

In the cylindrical part of the boiler, two holes with nuts and rubber plugs were made for thermocouples (Fig. 1). One thermocouple shows the steam temperature; the other, inserted into a hole drilled in the part to the middle, shows the temperature of the part.

To regulate the temperature it is necessary to install a reducing valve, but the experiments were conducted with manual regulation.

Fig. 1. Autoclave (boiler) for tempering ball-bearing parts.

The experiments conducted showed that the parts undergo no corrosion, and after tempering leave the boiler dry. The oil remaining after the hardening tank is almost completely washed off. The surface of the parts was so freed from oil that the parts could be sent for grinding. Increasing the blowing time yields parts completely free of oil.

The heating time of the parts is shown in Fig. 2 and does not exceed 10 minutes. It is interesting to compare this with the heating of balls in the rotating “Fiat” furnace, where heating is achieved after 100 minutes or more (Fig. 4).

Comparing heating by steam (Fig. 3) and heating by air (Fig. 5), it is important to note that in the former case the temperature of the parts follows the steam temperature, and how much the temperature of the parts lags behind the air temperature.

In a steam boiler it is impossible to imagine uneven heating; in an air conveyor furnace, however, uneven heating is a common phenomenon and, as a consequence, rejects occur.

Fig. 2. Tempering of rollers R.R.9. Charge 480 kg; I—temperature in the boiler, II—temperature at the middle of the roller, III—pressure.

After tempering, the parts come out of the boiler, as indicated, dry. After tempering, the boiler is connected to the drainage line, where the pressure is 1 atm. The temperature of the parts and of the boiler walls at the end of tempering is above 100°C, and when opened

Fig. 3. Steam tempering of outer rings 318N. Charge 290 kg.

…of the autoclave, the heat of the boiler walls and of the parts is expended on evaporation of particles of condensate retained between the parts. The quality of steam tempering

Fig. 4. Tempering by electric power in an air atmosphere in a Fiat furnace for 1″ balls. I — temperature in the furnace; II — temperature at the center of the ball.

Fig. 5. Tempering of outer rings 318N in an air furnace without air circulation. Temperature at the middle of the body of the ring.

was checked by the plant laboratory and proved to be entirely satisfactory. These experiments were repeated at GPZ in 1951–52, with the same satisfactory results.

The steam consumption per 1 metric ton of parts during tempering is as follows: balls for 5 hours to 135°C, 43 kg; rollers for 2 hours to 150°C, 52 kg; and rings for 2 hours to 150°C, 49 kg—an average of 50 kg. The consumption of electric power per ton in an air furnace is 131 kWh for balls, 69 kWh for rollers, and 87 kWh for rings.

When steam tempering is introduced on a hardening line, an autoclave is installed at some distance from the quenching tank (Fig. 6). Under the discharge chute of the quenching-tank conveyor, a perforated cassette is installed on a tray to receive the dripping oil.

Fig. 6. Diagram of hardening with steam tempering.

1. Electric hardening furnace. 2. Quenching tank. 3. Perforated cassette for rings. 4. Autoclave for steam tempering. 5. Telpher.

The dimensions of the cassette, if its outside dimensions are approximately equal to the inside dimensions of the boiler, are sufficient to receive more than two hours’ output of the hardening furnace—500 kg.

The chute has a hinged plate at the bottom in order to receive 50 kg of rings when the cassettes are being changed.

For moving the cassettes from the chute into the autoclave and then on to sorting, a telpher on a beam, or a hand-operated slewing crane for individual servicing of each hardening unit, or a travelling beam for servicing several units, is installed.

The technological process will proceed as follows: after a day off, when the unit begins operating, a cassette is placed at the chute of the quenching tank, and quenched rings enter it for 2 hours. During this time the autoclave is not operating. When the cassette is filled, it is moved by telpher into the autoclave, and a new cassette is placed at the chute. In the autoclave the cassette is held for 2 hours and is unloaded for inspection; the telpher loads a second cassette into the autoclave, and so on. Three cassettes are required for each unit.

From the beginning of delivery of rings from the quenching tank to the delivery of rings to sorting at the beginning of each week, 4 hours will elapse. Thereafter the rings will be delivered every 2 hours.

Comparing (Fig. 7) the hardening unit with an air furnace and with an autoclave for steam tempering, in comparison with a washing machine and an air hardening furnace, it is evident how little space the autoclave occupies.

It might appear that a disadvantage of steam tempering is the violation of the conveyor principle. With steam tempering in 4 autoclaves, there will be occupied

only one worker is needed. The simplicity of the autoclave design in comparison with a conveyor creates considerable convenience and reliability in operation. All troubles associated with electric heaters, with rings falling under the conveyor, and with the conveyor being pulled out are eliminated. All this leads to shutdowns for several hours, since repairs have to be carried out inside the furnace. The galvanometers regulating the furnace temperature—complex devices that often require repair—are eliminated.

In the autoclave there is only one place where repair may be required—the cover gasket; but it is located outside and can be quickly replaced together with the cover. The regulating device—a reducing valve with a pressure gauge graduated according to temperature—is considerably simpler than contact galvanometers.

Fig. 7. Diagram of hardening with tempering in an air furnace with circulating air.

1. Electric hardening furnace. 2. Hardening tank. 3. Washing machine. 4. Air hardening furnace with circulation. 5. Fans.

Steam for tempering was taken from the heating main. At that time, the use of steam for production was not yet permitted, and the return of condensate was required. This was one of the reasons why steam tempering was not introduced. In the absence of a main, a vertical Shukhov boiler can be installed outside the shop.

Fears of an increase in the humidity of the air in the shop are unfounded. When the autoclave is opened, it will contain \(0.5\ \text{m}^3\) of steam. The specific weight of steam at \(1\ \text{ata}\) is \(0.6\ \text{kg}/\text{m}^3\), i.e., there will be \(0.3\ \text{kg}\) of steam. Normally, air contains 8–10 g of water vapor per cubic meter. With 10 openings of the autoclaves, 3 kg of water vapor will enter the shop. With a shop cubature of \(6\text{–}8\ \text{thousand m}^3\), the increase in absolute humidity in the shop will be approximately \(1/2\ \text{g}/\text{m}^3\) with a single air exchange per hour. This should not cause difficulties. If it is desired to avoid even such an increase in humidity, then by installing a steam ejector on the drainage line the admission of steam into the shop can be reduced to zero.

All that has been set forth can be summarized in the following propositions:

1. Under conditions in which electric power is obtained from a thermal power station, it is irrational to heat to temperatures on the order of 150–180°C by means of electric energy.

2. Under the indicated conditions, it is economically more advantageous to carry out heating with steam.

3. Steam for tempering is used at a low pressure, up to \(5.3\ \text{ata}\).

4. The tempering temperature is easily and accurately regulated by a reducing valve.

5. The heat-transfer coefficient in steam tempering is much higher than in tempering in an air furnace; therefore, heating of the parts proceeds faster.

1. Heating of the parts proceeds through the entire thickness of the loaded mass of parts during steam tempering. During tempering by electric power in air, heating is nonuniform, depending on the position of the part in the mass.

2. During steam tempering, the conveyor principle of movement of parts in the hardening-and-tempering unit is violated.

3. In steam tempering, the tempering boiler (autoclave) is simpler than a conveyor tempering furnace, and more reliable in operation, since stretching of the conveyor, parts getting under the belt, and burning out of the electric heaters are eliminated.

4. The boiler occupies less space than a conveyor air or oil bath, and repairs are simpler.

5. Servicing the boiler does not require more workers than a conveyor furnace and, taking into account the absence of malfunctions, even fewer.

6. In steam tempering, the boiler makes it possible to increase the tempering time without limit, without increasing the space occupied by the boiler; lengthening the tempering improves the quality of the bearings and extends their service life [2].

7. The quality of parts tempered with steam is entirely satisfactory.

8. In steam tempering, the operation of washing off the oil from the quenching tank is eliminated.

9. In steam tempering, no corrosion is observed, and the parts come out of the boiler dry.

References

1. Ya. R. Rauzin. Heat Treatment of Chromium Steel, p. 188, 1950.
2. P. A. Dvoryanov. “The Effect of Duration during Tempering on the Service Life of Bearings.” Podshipnik, No. 7, pp. 12—16, 1938.
3. G. B. Lurie. Technology of Rolling-Bearing Production, p. 448, 1949.