Inductive heating is being used more in the forging industry to heat forging stock because it offers precise, stable temperature control to match material properties, hot forming resistance and tool wear.

Inductive heating is being used more in the forging industry to heat forging stock because it offers exact, stable temperature control, which permits optimum matching of material properties, hot forming resistance and tool wear. Significantly higher power density compared with other heating technologies allows small, compact systems with high throughput rates. Typical problems, such as oxidation, scaling loss, decarburization and grain growth, can be significantly reduced due to the short heating time. Improved automation and low reject rates guarantee high productivity. Predominantly used round or square forging stock size ranges from 20 to 150 mm (~0.75 to 6 in.), and occasionally up to 300 mm (~12 in.). Necessary frequencies for this size range vary from 300 to 4,000 Hz and up to 30 kHz for smaller sizes.

Fig. 1. Applications of inductive heating equipment in the forging industry

Converter technology

Today's transistor converters operate under load control in a parallel resonant circuit consisting of an inductor and correction capacitors. Fast, switched power semiconductors enable modern protection concepts, offering improved reliability and availability. The converter handles critical load situations, such as when filling and emptying the inductor, which achieves automatic adaptation to the load over a wide frequency range. Microprocessor-based control permits free parameterization of the converter and provides a variety of communication interfaces (field bus, Ethernet) for high-level process control. The Ethernet interface permits simple remote diagnosis and maintenance, and further enhances the ease of servicing through modular configuration. Converters with single outputs up to 4,500 kW are currently available. Higher outputs can be achieved by connecting several converters to the load in parallel.

The converter can normally be installed separately from the actual heating line. When lower outputs are used, the converter is integrated into the substructure for a compact design and short, high-efficiency energy routing between capacitors and inductor. The inductors and drive units are mounted on the base frame. The choice of machine type and configuration is determined primarily by material form (round or square billets or bars), material dimensions, throughput rate, cycle time, and final temperature (Fig. 1). In the case of products formed by presses and hammers (e.g., axle journals, connecting rods, crankshafts and steering components), the initial blanks are predominantly shear-cut billets in loose bulk form.

Billet heating plant BTH

Billets are lifted from the magazine onto the main transport line via a step feeder or spiral conveyor. Depending on stock form and size, either a plate or vibration conveyor is used to transport billets (without any gaps) to the stock column at the inductor where a roller billet pusher moves the stock through the heating section. A fixed bottom roller and either a manual or automatic top roller on the pusher advance the stock column at the required speed (Fig. 2). The drive normally runs in a cyclic mode, whereby continuous transport is selected for short cycle times. Billets glide on water-cooled skid rails. Inductors can be changed by removing the power and water connections.

In case of an interruption at the forming station, the furnace switches to a reduced throughput rate and a reduced output, and the temperature distribution in the stock column can be reasonably maintained, with only a few billets in the transition zones rejected. A more complex "stop and-go" technology can be used, whereby the outputs of the individual inductors are controlled so the stock column holds its temperature while stopped, allowing almost all billets to be forged after restarting. Billet temperature is measured using a pyrometer after the inductor. Billets are finally moved by chain to a multi-way switch (Fig. 3). This transport method is limited to billet sizes of about 180 mm diam. by 7 m (7 in. by 23 ft) due to the forces acting on the tubular guide leading to unacceptable wear. Greater throughput rates (from approx. 7 t/h and larger billet diameters) require a plant configuration in which the heating section is spread over several separate inductors, with each pair linked by a set of driving rollers.

Billet heating plant CTH

Large cross-section billets used with this machine type generally precludes the use of shears, so forging stock is transported to the inductor via a bar magazine with a bolt saw and a feed-roller track. Drive rollers in front of the inductor can be used between the 1,800-2,400 mm (71-97 in.) long inductors. Transport is continuous.

In the "go slow" mode, the stock column can be reversed by approximately one inductor length to overcome unwanted cooling, especially in the unheated sections between the inductors. Heated billets are immediately available after the interruption in the forming section is rectified. The throughput of this machine type can be increased by adding further modules consisting of a substructure, inductors, and driving rollers (Fig. 4). Output up to 22 t/h has been achieved. Typical forging cycle times are between 8 and 20 seconds.

Bar heating machine STH

Bar feedstock is heated using a machine with a modular design similar to the CTH system. Each module consists of inductors and drive rollers, and interconnection of individual modules produces overall machine lengths up to 20 m (~66 ft) with a throughput rate of 20 t/h. Compared with an ingot heating machine, drive equipment design is simpler, consisting of only a driven carrying roller. Conversion times to new stock cross sections are very short. Such machines are typically found in combination with hot shears. Working cycles are determined by the forging process; multiple-die hot presses require very short cycle times (down to 0.5 seconds).

Fig. 2. Intake side of BHT billet heating plant with roller ingot pusher; Fig. 3. Discharge side of BTH billet heating plant with three-way switch; Fig. 4. CTH ingot heating machine with throughput rate of 20 t/h

Single-ingot heating machine ETH

Induction systems described above heat the stock in a continuous process, having several ingots in the heating section at any given time, which is suitable for high ingot throughput rates. By comparison, individual heating is preferred in applications requiring only partial heating, such as heating the end of a bar for upsetting, or if very high temperature accuracy is required. In these cases, the ingot or bar is pushed into the inductor and heated in place up to the target temperature. The front edge is positioned accordingly in the inductor to avoid overheating of the end face. Multistage heating is possible if the ingots pass through several inductors in succession.

It also is often necessary to ensure the thorough soaking of ingots of varying lengths. In this case, a short-circuit ring on the rear end face serves to avoid overheating. Single-ingot heating is especially beneficial where irregular cycle times, small batches and varying final temperatures are involved, because it is relatively simple to hold the ingot at its required temperature. Due to the variety of different applications, this machine type is usually tailored to meet the customer's production requirements.

Enhanced process control

Today's control systems offer convenient, graphics-based operator guidance in the user's own language and many opportunities to further improve process reliability. Integrated product data management stores machine settings for processing different products, ensuring that optimized values are available immediately when restarting after a product change. Internal process models generate suggested values for new products, and modern control systems allow the use of parallel process models to monitor the temperature measurements. Statistical temperature correction tracks machine settings so precisely, that medium-term temperature drift is compensated.

Customer-side control interfaces have become more important recently. They permit remote diagnosis and maintenance, as well as complete integration of the control system into the customer's Intranet. This, in turn, enables direct access to a variety of machine data; for example, for batch tracking and for production data acquisition and archiving.

Maintenance and service are supported by fault analysis and evaluation functions. The maintenance manual is integrated into the visualization solution, whereby event-specific information can be displayed regarding maintenance requirements or fault elimination. IH