GB712562A - Method and apparatus for thoroughly electrically heating large billets - Google Patents

Abstract

712,562. Heating billets by induction. ALLMANNA SVENSKA ELEKTISKA AKTIEBOLAGET. June 23, 1952 [June 25, 1951], No. 15721/52. Class 39 (3) Large billets having a thickness of at least 10 cms. are electrically heated to forging temperature by induction by currents and frequencies generated in rotating machines, wherein during the first period of heating the surface of the work is heated to the highest permissible value #, while during the second period of heating the work is held at this temperature by controlling the power supplied to the work while heat is conducted from the surface to the interior. The duration of the first period lies between T 1 and 3T 1 where T 1 is the time in which the temperature of the inside of the work and the surface become equal when power supplied to the work has been cut off. The power is reduced at the end of the first heating period to that required for the second period in one step, in the proportion 1: T 0 /T 0 +T 1 {1-e-t0/T 1 } where T 0 is the quotient of the overtemperature #-#m of the surface over the average temperature #m of the work divided by the speed of the increase of temperature d#m/dt at which said overtemperature occurs. When the billets are cylinders with a thickness of at least d min or plates with a thickness of at least # min, the billets are heated by currents of the same frequency lying between v# (350 to 2000) (10/dmin)<SP>2</SP> and v# (350 to 2000) (10/#min)<SP>2</SP>. Several furnace units comprising coils 10 may be operated simultaneously with a control power source comprising a generator or generators connected in parallel to supply a substantially constant voltage Eg. A voltage divider 2 connected in parallel therewith supplies to each of the furnace units voltages Eg, EgX and Eg(1-X) by an individual voltage control device 3 for each furnace unit, whereby an individually controlled co-acting or counteracting voltage ΠEgy is series-connected to said voltage EgX and by separate changeover devices for each furnace unit adapted to connect during the first heating period the coil 10 to a voltage Eg(1#y), the power of the furnace being held constant by controlling the ratio y, and to switch over at the end of the first period the coil 10 to a voltage Eg(x+y), which is decreased continuously or stepwise during the second period. One generator connector tap 4 of the coil 10 is connected to the free and 5 of the winding producing the voltage Eg y while the other connector tap 6 is connected through a change over switch or two switches 7 either to the free end 8 of the voltage divider 2 or to its intermediate pole 9. A capacitance 11 is controlled automatically on resonance with the reaction of the furnace coil. The Specification contains further formulae for the heat control. Billets more than 25 cms. thick may be heated by a frequency of 50-60 cycles. Several furnaces may be employed each with several heating zones with different powers through which the billets are conveyed, or a larger number of furnaces are employed in which the billets are not moved during the whole heating period. To avoid oxidation the furnaces are filled with a protective gas. Specification 637,008 is referred to.