EuroWire January 2007

english

Interactive power control under closed loop conditions

the quenching head increases, thereby monitoring a constant dwell time between the heating and quenching processes. Preheating Induction heating systems have been incorporated on existing wire lines to pre- heat the wire and increase the production capability of existing processes. For example, consider a single wire being preheated prior to entering an electric furnace or fluidised bed. In a particular project, two lines each incorporating 80kW/25kHz induction power source coupled to 10 ft (3 m) long induction coils were utilised to preheat a single wire from 70°F (25°C) to 1,292°F (700°C) prior to an electric furnace. Production rates were raised from 400 ft/min (122 m/min) to 656 ft/min (200 m/min) on wire sizes from 0.031 to 0.200" (0.8 to 5.0mm) in diameter. Hardening and tempering – the ‘Hi-Bond’ process In line hardening and tempering is a common application where the wire is heated to 1,742°F (950°C), quenched to harden and reheated to between 660°F (350°C) – 1,200°F (650°C) to temper. Currently this approach is being success- fully applied to the treatment of deformed bar for reinforcing concrete structures to give a low relaxation high yield strength to the wire – the Radyne registered Hi-Bond process. The heating for hardening is carried out in two stages using 10 kHz to raise the wire to 1,382F (750°C) with a single coil and 50 kHz or 200kHz to raise the wire from 1,382°F (750C) to 1,742°F (950°C) with two or more coils sizes, subject to wire range, throughput and efficiency requirements. Typically coils are 6 ft (1.8 metres) in length for each stage and powers of 280KW (at 1,0kHz) and 180KW at 50kHz are used. Immediatelyafterheatingto1,742°F(950°C) the product is sprayed with high-pressure water jets to reduce the temperature to approximately 80°F (30°C) and dried via an air wipe. Tyre cord diffusion This application requires simultaneous heating of typically 10 to 24 wires running in parallel and heated to approximately 1,112°F (600°C) to melt surface coatings of copper and zinc which diffuse into the base wire to produce tyre cord wire. The wires typically have an inter axis dimension of 15.5mm to 25.5mm and range from 0.021" (0.8mm) to 0.080" (2.0mm) diameter. Typical throughput is based on a DV=70 (D= diameter and V= velocity). The number of wires within a given heating coil are generally determined by the inter axis dimension; the coil assembly becoming unwieldy for a large number of wires at high inter axis dimensions.

Compared to processes such as gas and electric furnaces, infrared heaters, resistance heaters, and fluidised beds, induction heating responds extremely quickly to changes in process operating parameters. A small change in power or line speed almost instantaneously affects the resultant temperature of the product being processed. For this reason, the control of the line to achieve consistent results needs to be carefully considered. The two standard methods used are feedback from temperature sensing devices (such as infrared pyrometry) and from line speed. Temperature sensors In the case of heating magnetic steel to austenizing temperature for a hardening process, unless an atmosphere is incor- porated, scale can form on the wire surface, which can affect readings by both single and dual colour infrared pyrometer systems. Therefore the elimination of scale and the accuracy of positioning and focus of the pyrometry system will determine the resultant signal fed back to the induction power supply. Airborne contamination such as smoke fumes can also affect the signal from pyrometers. Unless particular care is taken in the cleanliness of the wire and exactness of other process parameter feedback and closed loop control using pyrometry is unlikely to be effective. Temperature sensors also need to be focused on the wire being heated and, particularly in the case of small diameter wire, these wires canmove vertically during the process and being removed from the field of view of the pyrometer give false signals to the induction process. Line speed Computation of line speed relative to wire size and induction heater power level is a viable process and feed forward controllers have been used successfully. Nonferrous materials So far, our considerations have concerned the induction heating of carbon steel wires. Nonferrous materials such as aluminium and brass can be equally heated with induction heating, however, not as efficiently. For example, consider brass wire 0.080" (2.0mm) diameter and the requirement to heat it from ambient 70°F (20°C) to 1,200°F (650°C) at a rate of 985 ft/min (300 m/min). This will require a total of 540kW of output power at a frequency of 50kHz with a total 10 ft (3 metre) long induction coil. A brass wire of 0.24" (6.00 mm) diameter being heated from 70°F (20°C) to 1,200°F (650°C) at a rate of 985 ft/min (300 m/min) will require 1,500kW of output power

Multi wire furnace technology

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at a frequency of 10kHz with a total 20 ft (6 metre) long induction coil. The resultant overall efficiencies are, respectively, 6% on the 0.080" (2.0mm) diameter wire in the first example and 20% for the 0.240" (6.0mm) diameter wire in the second example. When compared to overall efficiencies of up to 80% in heating magnetic steel, it can be seen why induction heating is not used extensively for nonferrous materials. That said, very successful installations are in operation at low efficiencies due to other benefits such as the working environment offered by the induction process. Looking to the future Concludes Martin Wagstaff, “Clearly inductionheatingwill continue tobewidely used in the wire industry, particularly for steel wires. There will be increased interest and a greater number of systems used to complement and enhance the productivity of existing conventional heating systems. Development will continue in the heating of very thin wires and in heating of special alloys composite metals and materials such a titanium and tungsten. The physical size of induction power supplies will decrease while their performance will increase. There will be future developments in control techniques and systems to ensure very close tolerances and consistency of the wire products and enhancements will be made via inline quality control.” “As further processes where induction heating can be utilised are revealed, each must be considered with regard to its own merits. Our own experience tells us that sometimes the most unlikely application or the one that appears at first to be nonviable, may result in a successful and financially viable installation.” n

Radyne Basingstoke,

Hants RG24 8NA, UK Tel : +44 118 978 3333 Fax : +44 118 977 1729

Email : sales@radyne.co.uk Website : www.radyne.co.uk

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EuroWire – January 2007