End-to-end induction heating of metal billets

Why does the surface heat up before the core

The induced current is concentrated closer to the surface of the metal, and heat to the center is transmitted by thermal conductivity. As the frequency increases, electromagnetic penetration decreases; as the diameter or thickness increases, the heat transfer path becomes longer. Therefore, an attempt to accelerate the process with only high power can give an overheated surface at a cold core. For massive cross-sections, a lower frequency range, an extended active zone or a stepwise mode with exposure is usually required.

AVN 1–20 kHz is considered primarily for large steel blanks, deep and through heating and long inducers. AVZ and other AVN implementations can be rational for medium cross sections and a more compact line. ABB and ABC are more often used on smaller parts or where the desired area should not go deep. LN is suitable for manual inductor feeding and single operations. The boundary between the series is not determined by a single table of diameters: it is specified by the material, temperature, time and the result of the test heating.

Baseline data for thermal calculation

The wording "heat steel to 900 °C" is insufficient. It is necessary to determine where and how the temperature is measured and what difference between the surface and the center is permissible at the time of unloading.

the brand of material and the state of supply;

shape and dimensions of the section, length and weight of the workpiece;

initial, final and permissible maximum surface temperature;

allowable temperature difference by section;

number of parts per hour or mass of metal in kg/h;

piece-by-piece, cassette or continuous loading;

allowable cycle time and distance to the next operation;

network parameters, shift mode and liquid cooling conditions.

Indicative energy example

The calculation below shows the order of magnitude and does not establish uniform heating. A steel cylinder Ø80 × 120 mm has a volume of approximately 0,000603 m³ and a mass of approximately 4,73 kg at a density of 7850 kg/m³. For heating from 20 to 900 °C, assume an average specific heat capacity of 0,62 kJ/(kg·°C). Useful heat: 4,73 × 0,62 × 880 ≈ 2580 kJ, or approximately 0,72 kWh per workpiece.

With conventionally accepted overall efficiency 55 % from the network will require about 1,30 kW·h for one part. For 30 parts per hour, the average electrical load will be around 39 kW. Preliminary check the ability to bring the order of 50–60 kW, taking into account the stock, but this value can not be directly taken as the nominal output power of the converter. The result depends on the inductor, pauses and the required temperature difference; it is confirmed by testing with the measurement of the surface and core.

How to achieve uniformity

For piece mode, the workpiece can be rotated, moved through several turns or heated stepwise: the energy supply alternates with a short shutter speed when the temperature is leveled due to thermal conductivity. In a continuous line, the uniformity is regulated by the length of the active zone, the step of the turns, the feed rate and the distribution of power. The pyrometer controls the surface, but the core temperature during the development of the mode is checked by contact measurement or cutting the control billet according to the agreed method.

The inductor is calculated under the geometry and working gap. A simple inductor is usually included. Complex, profiled and replaceable inducers are coordinated separately. The exact result - temperature, performance and gradient - is fixed only after testing on the customer's parts or an agreed analogue.

From concept to commission

UralInduktor defines the layout, forms the concept of the complex and the technical solution. The Chinese plant designs and manufactures the installation in detail. Management and documentation are delivered in Russian. After assembly, the plant conducts a check with the inductor ordered; then the equipment is rechecked in Miass with full connection without load.

Commissioning is carried out on the territory of the customer with his blanks. Specialists adjust the mode, conduct trial tests, train operators and draw up acts. The cooling circuit is selected according to a separate heat calculation; with its compliance and compliance with the operating mode, round-the-clock operation is possible. Warranty - 18 months under the contract, with a countdown from the date of shipment or the signing of the commission act.

What to send for selection

Direct the drawing, the metal brand, the required core temperature and performance. We will perform the primary heat calculation and offer the frequency range, power, inductor and supply circuit. Price on request.