Transformer to Furnace

Stick your hand into a powerful electric induction coil. Nothing happens. Then stick a steel rod into it--the metal becomes red hot almost at once. This phenomenon is the basis of the fast-growing industry of induction heating: more induction equipment (using over 175,000,000 watts) was installed in the U.S. last year than in any previous three-year period. And today nearly all of it is used for smithing the weapons of war in arsenals, navy yards, private plants. Induction heating--with welding and substitution of casting for many forging operations--is one of the three big technological developments which gives U.S. arms-making a pace in 1942 which was unthinkable in 1917.

Induction currents, engineers are finding, provide a quick, simple, easily controlled way of heating or melting metals and of concocting alloys. An induction furnace is simple--it is merely a transformer whose core (or "secondary") is replaced by the metal "charge" to be heated. The coiled wires surrounding the charge carry alternating current, usually of high frequency--changing the direction of its flow not 60 times a second (as in ordinary household AC) but several thousand times a second. As the secondary current induced in the metal charge stabs back & forth, the metal's resistance creates great heat. A small box-shaped furnace (using 600 kilowatts) will melt a ton of steel an hour all day long--a rate of production much exceeding that of coke-fired furnaces.

Yet U.S. engineers were slow in adopting induction heating since its invention in 1916. The inventor was the late Dr. Edwin Fitch Northrup, who was exploring for any method of electrical heating which scientists might have overlooked. Early induction furnaces used low power with frequencies of 20,000 to 80,000 cycles. This limited them to laboratory and small-scale work until development in the '20s of generators capable of producing strong currents of 1,000 to 12,000 cycles--good enough for most industrial purposes.

Whirling Cannon. Induction heating contributes to one of the most ingenious practices in modern arsenals--the centrifugal casting of gun barrels. Steel is induction-melted, then poured into a horizontally rotating mold which continues to spin until the casting hardens. Thus formed is a hollow, easy-to-bore barrel instead of the solid ingot from which cannon were formerly forged and drilled. Slag (formed by the oxidation of the molten metal) is forced to the inner surface of the casting, where it can easily be tooled away.

Further advantages of induction melting:

> Furnaces are so compact that they also serve as ladles.

> Magnetic forces keep the molten metal stirring about, so that the several components of an alloy remain well mixed.

> Alloys of exacting recipe can be accurately blended. Temperatures can be closely controlled.

Hot shells. Induction heating is used in forging thick metal disks into shells. Its speed and adaptation to automatic work contribute to the mass production demanded in munitions making. The quick-heated metal forms only a soft scale which does not wear out the dies as fast as the hard scale of slow-heated shell material.

The skin effect. Induction's alternating current, unlike DC, concentrates on the surface of the conductor. Hence the outside of a metal object heats up rapidly while the inside stays cool. Given a skin-blistering dose of high-frequency induction current and then quenched in water, metals are hardened on the surface, yet remain resilient and tough inside. Typical parts thus hardened are airplane bearings, tank treads, engine cylinder walls. Often they last ten times longer than without such surface hardening. Noses of armor-piercing shells and bombs are induction-hardened without affecting the rest of the projectile. Among other applications of induction:

> Heating radio vacuum-tubes during sealing to drive gases out of the metal parts.

> Heating vatfuls of chemicals where a flame, spark or arc would set off an explosion.

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