Induction forging refers to the use of an induction heater to pre-heat metals prior to deformation using a press or hammer. Typically metals are heated to between to increase their malleability and aid flow in the forging die.
Process
Induction heating is a non-contact process which uses the principle of electromagnetic induction to produce heat in a workpiece. By placing a conductive material into a strong alternating magnetic field, electric current is made to flow in the material, thereby causing Joule heating. In magnetic materials, further heat is generated below the Curie point due to hysteresis losses. The generated current is predominantly in the surface layer, the depth of this layer being dictated by the frequency of the alternating field and the permeability of the material.
Power consumption
Power supplies for induction forging vary in power from a few kilowatts to many megawatts and, depending on the component geometry, can vary in frequency from 50 Hz to 200 kHz. The majority of applications use the range between 1 kHz and 100 kHz.
In order to select the correct power it is necessary to first calculate the thermal energy required to raise the material to the required temperature in the time allotted. This can be done using the heat content of the material, which is normally expressed in KW hours per tonne, the weight of metal to be processed and the time cycle. Once this has been established other factors such as radiated losses from the component, coil losses and other system losses need to be factored in. Traditionally this process involved lengthy and complex calculations in conjunction with a mixture of practical experience and empirical formula. Modern techniques utilise finite element analysis and other computer aided modeling techniques, however as with all such methods a thorough working knowledge of the induction heating process is still required.
Output frequency
The second major parameter to be considered is the output frequency of the power source. As the heat is predominantly generated in the surface of the component it is important to select a frequency which offers the deepest practical penetration depth into the material without running the risk of current cancellation. It will be appreciated that as only the skin is being heated time will be required for the heat to penetrate to the centre of the component and that if too much power is applied too quickly it is possible to melt the surface of the component whilst leaving the core cool. Utilising thermal conductivity data for the material and the customer's specified homogeneity (physics) requirements regarding the cross sectional ∆T it is possible to calculate or create a model to establish the heat time required. In many cases the time to achieve an acceptable ∆T will exceed what can be achieved by heating the components one at a time. A range of handling solutions including conveyors, in line feeders, pusher systems and walking beam feeders are utilised to facilitate the heating of multiple components whilst delivering single components to the operator at the required time cycle.
Advantages
Process controllability – Unlike a traditional gas furnace the induction system requires no pre-heat cycle or controlled shutdown. The heat is available on demand. In addition to the benefits of rapid availability in the event of a downstream interruption to production the power can be switched off thus saving energy and reducing scaling on the components. Energy efficiency – Due to the heat being generated within the component energy transfer is extremely efficient. The induction heater heats only the part, not the atmosphere around it. Rapid temperature rise – High power densities ensure that the component reaches temperature extremely rapidly. Scale is reduced as are surface defects and undesirable effects on the surface metallurgy. Process consistency – The induction heating process produces extremely uniform consistent heat. This improves accuracy of the forging and can in extreme cases reduce post forging machining allowances and have a positive effect on die life. No harmful byproducts – Induction heating does not generate any environmental waste products and is a clean process as opposed to more traditional heating methods that generate smoke and toxic emissions.
Types
Bar end heating
Bar end heating is typically used where only a portion of the bar is to be forged. Typical applications of bar end heating are:
Hot heading of bolts
Anti roll bars
Mining tools
Subject to the required throughput, handling systems can vary from simple two- or three-station pneumatic pusher systems to walking beams and conveyors.