As was seen in the rolling of plates, “cold working” raises the strength, lowers the ductility and embrittles steel more than does the regular “hot working” at the cherry-red or white heats which are usual in the forging and rolling processes. If the so-called “cold finishing” at very low red or black temperatures affects the physical properties of plates, it can be readily understood that cold-drawing of wire and of seamless tubes at ordinary temperatures of 70 to 100° F., must considerably accentuate the effects noted. So much is this so that drawn wires and tubes have to be frequently annealed, i.e., heated to a good red heat for a time between successive trips through the dies, annealing having the effect of off-setting the lowered ductility and increased brittleness which would cause the wire to break. In some cases the wire has to be annealed after each pass or draft, but oftener several passes are possible before annealing is necessary. This depends largely upon the quality of the steel used and the amount of “reduction” attempted in each pass.
The raw material, No. 5 soft wire rod, which is about one-fifth of an inch in thickness, comes to the wire-drawing plant from the rod mill. This No. 5 rod is the thinnest which the mills have found it economical to roll, so further pulling down in size can best be done by “drawing.”
Before Drawing into Wire the Scale Must Be Removed from the Rods by “Pickling” in Acid
As all iron and steel materials which have cooled in air from a red heat are covered with a hard, brittle scale of iron oxide, the rod must first be “pickled,” i.e., digested in hot, weak sulphuric acid, which, in 10 or 15 minutes so dissolves and loosens the hard surface that it can be readily jarred loose and washed off. By immersion in a vat of boiling milk of lime, the pickled rod is given a lime coating which neutralizes any acid which remains, and, when dry materially aids in the lubrication of the wire while it is going through the dies. Thorough drying, called “baking,” is accomplished in the dry house at 300 or 400° F., from which the rod with soft, scaleless, lime-coated surface goes to the “drawing” benches. After rinsing free from the pickling acid, the wire is often allowed to acquire a soft film of rust by spraying it with water and keeping it wet in the air for a short time before going to the lime vat. This rust or “sull” coat itself assists in the lubrication. However, the color of the product is not as good as when the “sull” coat is not used, and such wire usually goes into articles for which darker color is no drawback.
Single Wire-Drawing Block
A Wire-Drawing Die
The dies with more or less funnel-shaped holes of accurate diameter are set vertically. They are of extremely hard material in order to stand as long as possible the severe service without excessive wear, which sooner or later so enlarges the holes that the dies become useless. Then they must be removed and either discarded, or, in the case of steel dies, the holes reduced by hammering and redrilled. Often the holes of worn cast iron and steel dies are enlarged to the next larger size and so used.