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Induction Through Heating Quenching And Tempering Process

  The steel rod is quenched through induction heating and then tempered by induction heating at high temperatures. The induction through heating quenching and tempering process can be completed in a continuous production line. This induction and penetration heating quenching and tempering process are suitable for all kinds of medium carbon steel, low alloy steel rod, pipe, shaft parts. Domestic petroleum machinery enterprises Φ 280 x 9000 ~ 12000 mm round steel conditioning treatment is conducted on the device. This kind of equipment has the characteristics of high production efficiency, low energy consumption per unit, small distortion of the workpiece, less oxidation and decarburization, no pollution, easy automation, small floor area, and so on, especially suitable for mass production.

Bar induction material tempering production line field picture

▲ Bar induction material tempering production line field picture

1. Selection of induction penetration heating frequency

Under the condition of induction heating, the effective penetration depth of induced current h is

640 (10)

In the formula:

640 (11)

When the ratio of the heated round rod diameter D to the current penetration depth H is 4:1, the corresponding power supply current frequency is called the critical frequency. When the equipment frequency is lower than the critical frequency, the efficiency of induction heating drops sharply; when the equipment frequency is higher than the critical frequency, the electrical efficiency does not increase much, but the equipment cost increases obviously.

The relation between effective critical frequency of several materials and workpiece size is shown in Figure 1. The frequency of inductive diathermy power supply should be close to critical frequency as far as possible.

Relation between effective heating critical frequency of several materials and workpiece size

▲ FIG. 1 Relation between the effective heating critical frequency of several materials and workpiece size

2. Selection of induction penetration heating equipment power

During induction penetration heating, the diathermy of the workpiece section is realized by heat conduction from the outer layer to the inner layer, and an appropriate temperature gradient is needed to prevent the section from overheating. Therefore, the selected energy density should not be too high, and too low will significantly reduce heating efficiency. Table 1 shows the power density required for steel through heating.

▼ Table 1 Power density required for steel through heating (kW/in)

Power density required for steel through heating

In continuous induction heating quenchants, the power used by the penetrating heating power supply equipment should be considered in terms of production capacity. At this point, power P can be expressed by the following formula:

P=ηGQ

In the formula:

η — Heating efficiency

G — Capacity per hour (kg/h)

Q — Energy required to heat workpiece per unit weight

(kW•h/kg, or kW•h/t) is shown in Figure 2.

Heating temperature and energy consumption of single weight for induction diathermy of workpiece

▲ FIG. 2 Heating temperature and energy consumption of single weight for induction diathermy of the workpiece

The diathermy process is related to the thermal conductivity of the metal (W/ (mm•℃)) and the induction thermal coefficient KT (W/ (mm•℃)). The relation between d/ H (diameter to dire heat depth ratio) value of the circular rod, KT, and thermal conductivity is shown in FIG. 3.

Relation between d H value of steel rod KT and thermal conductivity

▲ FIG. 3 Relation between d/ H value of steel rod, KT and thermal conductivity

The steps of calculating diathermy power by using the figure above are as follows:

(1) Select heating frequency and calculate the d/ H value of bar material (D /h is between 1 and 4).

(2) The thermal conductivity and D/H value during heating with the metal, check the corresponding induction heat coefficient K in Figure 3.

(3) The power PL required by the workpiece per unit length is determined by the following formula:

PL=KT(ts-tc)

In the formula: ts — surface temperature (℃)

tc — Central temperature (℃)

(4) Consider the efficiency η of each power loss and divide it by PL to get the power needed to heat the workpiece per unit length.

3.Induction heating and tempering continuous production line

Figure 4 shows an example of an automatic four-head helical drive rolling system used for online induction and tempering.

Four-head helical driving rolling system induction tempering production line

▲ FIG. 4 Four-head helical driving rolling system induction tempering production line

The automatic induction heating line includes an automatic processing system, a programmable controller, and a fiber optic sensor. After being sent to the hot working area by the transmission system, the workpiece is processed by the four-head tilting drum system QHD. The rolling driver is connected to the head disk so that one side of the workpiece rotates around the axis and moves forward along the axis direction. Once the workpiece is in the system, the optical fiber sensor detects its position and begins the austenitizing heating. The sensor can also detect abnormal operation, such as improper feeding, alarm, and automatically shut off.

Physical picture of tilting and rolling transmission device

▲ The physical picture of tilting and rolling transmission device

Heating end feeding process

▲ Heating end feeding process

The induction frequency of the HQD system during quenching is usually 500kHz or 3~10kHz.In each case, a temperature transfer controller automatically checks the temperature of the workpiece to prevent improperly austenitized workpiece from passing through the system. The workpiece is cooled to about 95℃ after quenching the ring, and the martensite transformation is completed in the returned firearm. When the workpiece is transported to the tempering system, the optical fiber sensor detects the workpiece and starts tampering with a low frequency (300Hz) current. The temperature is generally in the range of 400~600℃, and the tempering process is completed.

Pipe induction diathermy and tempering production line

▲ FIG. 5 Pipe induction diathermy and tempering production line

Note: The pipe enters from the right side, after heating austenitizing, it goes through quenching, output, and tempering. It is then sent to the machine to be cooled.

Bar tempering heating in progress

▲ Bar tempering heating in progress

END

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