EuroWire January 2007

english

The Radyne continuous heat treatment wire line

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At-a-glance – induction wire heating processes in detail : Heating prior to drawing When wire is drawn down to smaller diameter sections it is advantageous to heat the die enabling less force to be applied to the wire while it is being pulled through for the sizing or reduction process. Attention needs to be given to the potential for thermal expansion of the die resulting in extrusion of the incorrect size and, therefore, the temperatures are generally limited. Heating prior to encapsulation Generally applied to aluminium wires both single and stranded. Wire is preheated as it leaves the take off roll and the induction coil is positioned on the catenary angle of the wire line. The wire passes through the induction coil where it is heated to approximately 250°F (120°C) and then immediately passes to the encapsulation process where the PVC flows evenly over the wire. The induction coil length is dependent upon the speed of the process and upon the depth of heating required through the wire cross section. As it is not essential that the wire be through heated, the induction coil length is in the range (for most applications) of 20 to 40" (0.5 to 1 m). Heat treatment of wire Continuous hardening and tempering of steel wire is particularly important in certain wire applications such as in the production of deformed bar for reinforcing concrete structures. This is achieved by using a horizontal in line process where the wire is heated to an austenizing temperature of 1,742°F (950°C), followed by a quench out with water and then reheated to between 660°F (350°C) and 842°F (450°C) for final temper, the temperature being dependent upon the final product tensile strength require- ments. Radyne has a registered ‘Hi Bond’ process for this specific application.

If the wire size is of large enough section such that current cancellation does not occur at the austenizing temperature, a single power source with a single output frequency may be used. Annealing Steel wires typically of between 0.040" (1mm) to 0.400" (10mm) are heated to a specified annealing temperature dependent on the wire grade of between 600°F (315°C) to1,000°F (540°C) for full body annealing. The output frequency of the induction power source is dependent upon the wire diameter and the power level on the production rate needed. Wires can be annealed as a single strand or in multiple strands, all wires generally running parallel to each other in the horizontal plane. Wires are fed on a centreline distance of 0.61" (15.5mm) to 1.0" (25.4mm) each passing through a ceramic tube for wire guidance through the coil and to ease threading of the line. Heating prior to coating The following processes represent two distinctly separate treatment methods; diffusion or metal coating and insulative surface coating. Diffusion The most common process for this application globally is in the production of tyre cord but, could be equally applied to other markets. In a similar method to the annealing process, steel wires of between 0.031" (0.8mm) to 0.080 (2mm) diameter are heated to 1,112°F (600°C) to melt surface coatings of copper and zinc which then diffuse into the base wire to provide a barrier for rust formation. Wires are heated generally in multiples arranged in a horizontal plane through an induction coil arranged as an oval shape, the wires being fed through individual ceramic tubes. The production rate is determined by the calculation: D x V where D equals the wire diameter and V

the wire velocity or throughput speed. Typical power supplies used are 60 to 240kW at an output frequency of 25kHz, with induction coil lengths of 7 to 8 ft long (2 to 2.5 m). The phenomenon known as current cancellation can be used effectively on the application, in that an induction frequency is chosen that is comparatively low relative to wire section ensuring that if a wire breaks in the line that it cannot be heated above Curie temperature (approximately 1,400°F). This eliminates the need to immediately stop the line in the case of a single wire breakage. The cooling rate is controlled, sometimes by using muffles, which may or may not include gas quenching, or simply in open air. Insulative surface coating In the production of electrical wire using coatings such as enamels, epoxies or heat sensitive tape wrappings, the wire can be heated continuously in-line. This technique can also be used for drying paint on wire. As the temperature requirements are generally low (less than 300°F [280°C]), a small power supply can often be incorporated in an existing coating line, the power source operating at a high frequency as it is only necessary to heat the wire surface (not throughout the wire section). Relaxation This process is applied to the production of wire rope and is similar to a wire tempering process in that the wire is continuously heated to 600/1,000°F (315/980°C). The system generally comprises a single induction power source rated at an output frequency of 10kHz feeding either a static or moving induction coil assembly. If the direction of the process can be reversed the coil system is static, alternatively the coil system is arranged to move. Whentheinductioncoiltravelswiththewire at low wire strand speeds, the coil system is physically adjacent to the quenching head and as the line speed increases the distance between the coil assembly and

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EuroWire – January 2007