Induction heating increases productivity during ring rolling

During the ring rolling process, temperature losses can be counteracted with an induction heating system.

On a radial-axial ring rolling machine, a pierced piece of basic material (blank) is formed to a seamlessly rolled ring in a continuous rolling operation. The cross-section of the ring is reduced in its wall thickness and height. Consequently, the diameter is enlarged. The driven main roll and the mandrel from the radial roll pass, thereby reducing the wall thickness.

Both driven axial rolls – the axial roll pass – serve to reduce the ring height. During the rolling process, a ring centering device retains the rotating ring in a defined position. Rings with rectangular cross-section and profiled rings can be rolled for different applications. Depending on the application of the finished product, most varied materials are rolled on the ring rolling machines. This encompasses all common steel grades as well as titanium and nickel-base alloys, copper and aluminium alloys, etc.

During the rolling operation, the ring slides over a table and is reduced in its cross-section by means of the above-described rolls. For the heat balance of the ring, heat is generated through the forming operation and dissipated by means of convection and radiation through the ring’s surface. At the contact points, heat is also withdrawn from the ring. The exposed ring surface is also growing as the ring diameter increases, which in turn leads to higher heat losses.

Low forming temperatures result in higher process forces, poorer material flow and lower productivity when rolling is performed at full capacity of the ring rolling machines. This makes one or even several reheating operations necessary to ensure that a loss of quality is prevented when a minimum temperature is reached. For such a reheating step, the actual rolling process has to be interrupted, the machine is opened so that the ring can be removed and transported to a furnace. These interruptions lead to considerable logistical efforts and reduce the productivity of the plant.

Temperature losses can be counteracted with an induction heating system. For that reason, an induction unit is installed on each controlled motion axis at one or several points of the ring rolling machine. The unit mainly comprises a conventional frequency converter (not included in the image), a condenser cabinet providing the required reactive power for inductor operation and a transformer unit with the inductor directly mounted on it. The transformer unit is connected with the condenser cabinet via air- and water-cooled cables providing electric adaptation of the induction voltage to the output voltage of the converter. The frequency converter operates in a frequency range between 4 and 10 kHz.

During ring rolling the temperature level is increased which may particularly result in enhanced productivity of approx. 10 percent when larger rings are employed (with regard to plant size). The advantages arising from the fact that reheating operations can be dispensed with are even significantly greater, for example tool wear and loss of scale are decreased and maintenance measures are required less often.

The challenge is to position the inductor closely to the workpiece (coupling distance) making sure that this is maintained throughout the entire process (ring increase, ring height, iameter). To do this, the transformer is electromotively supported on a flanged plate in a flexible and adjustable manner. The motion enables both a translational and a rotational movement of the transformer unit on the supporting arm. Consequently, the supporting arm has to move the inductor only to near the workpiece. A precise contactless positioning is then realized via optical sensors and motors facilitating the extension of the induction unit from the machine area for loading and unloading purposes.