Comparative Analysis of Wire Annealing Methods

V. P. Kazmin

(Presented by the scientific seminar of the Department of Automation and Telemechanics)

In the manufacture of various kinds of wires and electrical cables, one of the intermediate technological operations is the drawing operation, during which the material is subjected to plastic deformation.

The result of plastic deformation is a change in the external forms of the metal, accompanied also by a change in its mechanical and physicochemical properties. In addition, the internal structure changes: the metal grains become refined and are oriented in the direction of deformation. This phenomenon has been given the name of work hardening of the metal [1, 2]. A change in the internal structure of the metal leads to a decrease in its ductility and to an increase in electrical resistance. The latter is explained by the impeded motion of free electrons in the deformed crystal lattice. Hard, work-hardened wire cannot be used for electrical-engineering purposes. Therefore, the task arises of removing the work hardening [3]. The process of removing work hardening consists in heating the metal to a temperature at which the mobility of the atoms increases so much that restoration of the equilibrium state disturbed by plastic deformation becomes possible. After the work hardening is removed, the mechanical properties of the metal and its electrical conductivity are restored [1, 2, 3]. The task of heating the wire in order to remove work hardening, or, as we shall call it below—annealing—is currently solved by several methods: electrocontact, induction, or convection in stationary heating furnaces.

The essence of the electrocontact method of annealing is as follows. An electric current of low voltage, supplied from a heating transformer through contact rollers, is passed through a small section of moving wire [8]. The annealing regime for different speeds of wire motion through the contact rollers is automatically regulated by changing the supplied voltage. Electrocontact annealing is usually combined with the process of drawing or enameling. At low drawing or enameling speeds, this method of annealing gives good results and has a number of advantages over other methods. Such advantages may include:

1) continuity of the process;
2) great possibilities for automation;
3) elimination of any intermediate operations (for example, transportation, etc.);
4) high annealing speed.

However, modern drawing machines make it possible to achieve such a wire-drawing speed at which regulation of the annealing regime becomes difficult. This is explained by the fact that, at high drawing speeds, the wire-heating rate exceeds 15,000 deg/sec and regulation becomes unstable. In addition, heating at such a rate worsens the quality of the annealed wire. With the electric-contact method of annealing, hidden annealing defects and nonuniform heating are possible; starting and braking of the drawing mill are substantially complicated (because of the difficulty of controlling annealing in the acceleration and braking sections) [5].

The method used for controlling annealing—control by indirect indicators (current, voltage), and not by temperature, with fundamentally less accurate control by disturbance rather than by deviation—is employed. Elongation of the wire during heating and the impossibility, in this case, of applying high tension make it difficult to eliminate wire vibration in the annealing section. These same causes do not make it possible to ensure good contact, as a result of which the wire undergoes undesirable point erosion and mechanical deformation [5]. Thus, at high drawing speeds the electric-contact method of annealing proves to be of little suitability.

In the induction method of annealing, which is usually also combined with drawing or enameling, a short-circuited turn is formed from the moving wire, in which an electric current is induced [4, 6, 7]. At high speeds this method has the same disadvantages as the electric-contact method. In those cases where the wire is being sent to enameling, both of the above-mentioned annealing methods can be combined with enameling and give good results, since the speed of wire motion in these cases is considerably lower than the drawing speed, and the determining factor is the enameling speed, not the annealing speed, as in drawing.

The third method of annealing is annealing wire on coils or in bundles in heating furnaces [9, 10]. This method has a number of disadvantages compared with the methods described above, namely:

1) it requires auxiliary transport operations;
2) it occupies considerable production space;
3) it has low efficiency;
4) a long cycle time (4–5 hours), i.e., the time from the moment the wire is charged into the furnace until the annealed wire is obtained.

The low efficiency of the latter method is explained by the fact that a considerable part of the energy is spent on heating the furnace itself, which has a large mass, and on radiation into the surrounding space. The long cycle time is due to the prolonged holding of the wire in the furnace, necessary for uniform heating. Shortening the cycle time by intensifying the heating (increasing the temperature in the furnace) is impossible, since nonuniform heating of the wire appears—the upper layers become overheated relative to the inner ones.

Despite the indicated disadvantages, annealing in furnaces is currently the most widespread. The chief determining factors here are the separation of the drawing and annealing processes and the good quality of the annealed wire. In this case, they are not limited—

the speed capabilities of drawing machines. Therefore, greater attention is being given to improving separate wire-annealing installations. The improvement of such installations should be aimed primarily at increasing the efficiency of the installation and reducing the heating time.

REFERENCES

1. A. A. Bochvar. Metallurgy. Moscow, Metallurgizdat, 1956.

2. M. E. Blanter. Metallurgy and Heat Treatment. Moscow, Mashgiz, 1963.

3. Yu. M. Lakhtin. Metallurgy and Heat Treatment. Moscow, Mashgiz, 1961.

4. Industrial Application of High-Frequency Currents. Moscow–Leningrad, “Mashinostroenie,” 1964.

5. Yu. S. Melnikov. Problems of Automation of Electric Annealing of Moving Wire. Dissertation for the degree of Candidate of Technical Sciences. Tomsk Polytechnic Institute, 1965.

6. V. I. Rodimov. Device for bright annealing of wire in the process of high-speed drawing. “Metallurgy and Heat Treatment of Metals,” 1960, No. 1.

7. Z. Brockhaus. Device for continuous heating of strips, wire, etc. FRG Patent, cl. 21, 8/20, No. 971925, 1959.

8. N. Z. Dnestrovsky, S. N. Pomerantsev. Brief Handbook on the Processing of Nonferrous Metals and Alloys. Moscow, Metallurgizdat, 1958.

9. Modern elevator furnaces and their application for annealing wires. TsKB for ultrasonic and high-frequency installations, Technical-Economic Bulletin, 1961, No. 2 (20).

10. A. Sibilev. Heating of strip and wire in “through-type” electric furnaces. “Nonferrous Metals,” 1963, No. 4.