Forging with Induction Heating
The need to develop efficient, non-polluting sources of renewable energy has created a number of new industries and, with them, special requirements for components. Among these are the numerous wind farms and the need to develop components to meet the special requirements of the windmill generating systems.
These components include the large, high-stress bolts that anchor the towers and the numerous special bolts used in the overall assembly (Fig. 1). A key supplier to this industry is Dokka Fasteners, which has facilities in Dokka, Norway, and Auburn, Mich.
Dokka’s requirements for the hot forming of bolts covers diameters from 20-62 mm (0.78-2.44 inches) with various lengths from 50-106 mm (1.96-4.17 inches). To meet this requirement, Ambrell, an Ameritherm Company based in Scottsville, N.Y., provided six systems designed specifically for their operations. Two systems are rated at 200 kW/10 kHz and process parts from 20-40 mm in diameter. A 375-kW/3-kHz power supply is used with each of four other systems, handling diameters of 30-62 mm.
When specifying induction power supplies for forging, two key criteria are the power rating and frequency of the power supply. Power required is based on the maximum temperature required for the operation and the weight of the material processed per hour. The standard calculation is:
P = M x SH x (T2 - T1)
Where P = power in BTU; M = pounds/minute; SH = specific heat of material; T1 = initial temperature in ˚F; T2 = final temperature in ˚F
Using a factor of 57 BTU/minute/kW, we can determine the required power at 100% efficiency. This figure then has to be modified for coupling efficiency and power losses due to radiation, conduction and convection.
With regard to frequency selection (Fig. 2), the initial depth of penetration is based on the operating frequency of the induction field. The lower the frequency, the greater the initial depth of magnetic penetration. Thus, lower frequencies are utilized as the bar diameter increases. Subsequent heating to provide a uniform temperature through the bar is the result of conduction from this outer surface and is dependent on both the power input and the thermal conductivity of the material.
Frequency also determines the minimum bar size that can be heated efficiently. If the depth of penetration at a particular frequency is too deep, field cancellation can occur and only a portion of the power used will be converted to useful heat.
As the diameter of the bar increases, the time required to get uniformity from surface to core will increase accordingly. The ideal condition is to have the surface temperature as close as possible to the core temperature when the part reaches the press. Accordingly, the surface temperature should be somewhat above the core temperature when the part is removed from the induction coil. Radiation loss from the bar surface will cause its temperature to drop, while thermal inertia continues to increase the core temperature. These should cross at the ideal forging temperature as the part is put in the press (Fig. 3). If the heat is put in at a rate slower than the thermal conductivity, however, uniformity can be achieved with minimal variation.
In this instance, bar-end heating provides the opportunity for a low power input rate to achieve good thermal uniformity. Since, in the case of the larger bars, a 30-second delivery cycle was required, it was decided to provide an eight-station heating arrangement (Fig. 4). As one bar achieves temperature, it is replaced by a cold bar. In this way, each bar is heated for seven cycles of 30 seconds each, or 3.5 minutes. This slow heating approach provides excellent uniformity from surface to core but utilizes a low heat input to avoid overheating of the bar surface. This also reduces chances of surface decarb and formation of scale.
When choosing a power supply, frequency is generally selected on the basis of the smallest-diameter part since larger parts can be heated via conduction from the penetration depth. Power, on the other hand, is selected by the largest kW demand as noted previously. For these reasons, two different power supplies were chosen.
Based on the use of a low power input per bar and the subsequent heating time for uniformity, an eight-coil configuration is optimal. The uniqueness of this system is in its approach to providing the eight-station heating (seven stations heating and one station loading/unloading).
Ordinarily, the eight individual coils would be connected in a series configuration requiring a high power-supply voltage. It would need a costly transformer to drive this coil, and the overall voltage across the system would present a number of problems. In this instance, the coils work in sets of four even though they are driven by a common power supply, which requires half the driving voltage of the eight-station system. The novel electrical system provides the ability to perform this function.
The operator removes one bar every 30 seconds and replaces it with a cold bar in the coil sequence 1-5-2-6-3-7-4-8. This sequence is simple for the operator to recognize by looking at the relative bar-temperature colors.
Another innovation, based on Dokka’s experience with earlier systems, was the use of a modular coil system. Previously, the coils used were a single assembly that required considerable downtime when a coil failure was experienced. The entire assembly had to be removed when a single section required maintenance.
To overcome this problem, a modular coil-assembly system was utilized. The coils for each station are identical, with matching entrance and exit fittings (Fig. 5).
This allows each coil to be removed as an individual unit, which, in turn, reduces downtime during changeover or repair and greatly reduces the number of spare coils and the cost to maintain them. The individual coils are encapsulated with high-temperature cement and are fitted with easily removable skid bars, further reducing downtime when being replaced.
As the diameters vary, so does the overall heated length required for forging. Generally, this length is expressed as:
L = 2D + 10 mm
Where L = length of heating in mm and D = bar diameter in mm
Each coil position is fitted with an adjustable end-stop to accommodate the length of the bar inserted into the coil and match its specific requirements.
This article was written by Stanley Zinn.