Drawing Furnace

a heat-treating furnace for working metal strips continuously drawn through the working chamber on rollers or on a pneumatic cushion. One or several drawing furnaces are included in production lines, on which the strip undergoes thermal and thermochemical treatment and is cleaned, pickled, coated (galvanized or coated with aluminum, plastic, or electrical insulation), painted, dried, and straightened.

Drawing furnaces are classified on the basis of their function—for example, hardening, normalizing, annealing, or tempering—and their design, which may be either horizontal or vertical (see Figure 1). Horizontal types reach 300 m in length and have hourly outputs of up to 75 tons. In these furnaces, strips with widths up to 2.5 m and thicknesses from 0.2 to 6 mm are treated at rates up to 4 m/sec. To improve output efficiency and save working space in the plant area, two-story to five-story furnaces are sometimes used. The height of vertical drawing furnaces varies from 15 to 45 m, and the metal strip may be arranged in open loops numbering from one to fifty. The total length of the strip in the furnace may reach 1 km. Vertical drawing furnaces that treat strips in this arrangement can handle strips with thicknesses of 0.05 to 1.5 mm at rates up to 10 m/sec with hourly outputs up to 100 tons; vertical furnaces in which the strip is treated in a single loop can process thicknesses up to 3 mm. The rollers that support the strip are made of heat-resistant steel and are electrically driven.

Most drawing furnaces have multiple chambers, including heating, holding, slow- and fast-cooling, hardening, and drying chambers, which are arranged consecutively and are joined by seals or separated by partitions. The number, size, function, and temperature and gas modes of the chambers depend on the technology of the treatment. The thermal conditions of drawing furnaces are constant over time and variable over the length of the furnace. The heating and cooling elements are arranged in the furnace on both sides of the metal strip.

Drawing furnaces without a protective atmosphere operate with direct-flame heating and air or water cooling of the metal strip. Furnaces with a protective atmosphere are heated by radiant tubes or electric-resistance heaters; the air or water is isolated from the metal strip in the cooling chamber, and heat is transferred through the walls of steel tubes or muffles. To accelerate the heating and cooling of the metal strip, the furnace air is circulated.

Proposed types of drawing furnaces feature rapid heating and cooling systems using gas jets, electron beams, and metal and salt melts. Pneumatic-cushion transport of the metal strip and jet heating and cooling are used for special applications.