Step Heating Suitable for Extrusion Processing

Induction heating is widely used to prepare billets for extrusion forming. Its principle is no different from the heating principles described earlier in this chapter. It can also heat “large stock,” namely very long billets. When a hot shear is used to cut a long billet into the required number of billets, these billets can still be maintained at the required temperature.

The extrusion forming process requires the billet to have a soft “head” so as to reduce the starting force of extrusion. Then, heat is generated during extrusion in the die, and the extrusion energy is mechanically applied to the billet. Therefore, the most ideal method is to apply “step heating” to the billet, so that the head temperature is 100-120°C higher than the tail temperature.

If this heating method is not used, there is a risk of dimensional differences and uneven metallurgical quality between the two ends of the finished product. It should be noted that uniformly heated billets may also be used, with the ram speed changed during the extrusion cycle, but this method is not ideal.

The design of induction coils for step heating will not be discussed in detail here. Since the temperature rise is proportional to billet density, and for non-magnetic materials the power density is proportional to H², it is clear that the magnetic field must vary with:

√(θ head / θ tail)

where θ represents the temperature rise.

For an extrusion temperature of 500°C with a gradual reduction of 100°C, this value is approximately:

√(480 / 380) = 1.1

A more accurate calculation must include the variation of the constant-temperature portion.

As discussed in Section 13.10, for a billet with uneven axial temperature distribution, the temperature difference can be completely equalized by means of the time constant:

(cγl²) / (π²k)

For a 0.3-meter-long aluminum billet at 500°C, the time required to equalize the temperature difference is 2.5 minutes. Example 13.7 shows that when the soaking time after heating is 40 seconds, the temperature difference between the two ends of the billet must be 172°C in order to obtain a 100°C temperature difference at the extrusion press.

The calculation method can be adjusted according to actual production conditions. Since the above time constant is generally longer than the time required to obtain a uniform temperature across the cross-section, it is clear that a longer soaking period conflicts with the purpose of step heating.