WIRE AND WIRE DRAWING
It may be rather disconcerting to some enthusiastic ones who assume that
we Moderns have made all the progress that is worth while to learn that
so many of our supposedly new products were far antedated. In the case
of wire, again, we were antedated as much as 30 centuries. The wires
which were produced by the Ancients, however, were usually of the noble
metals, gold and silver. They were not drawn through dies as are the
wires that we know, but were hammered into shape from long, thin strips
of metal. The earliest use of our “drawplate” method of which we find
authentic mention was in the 14th century in Germany. The wire was
hand-drawn. Machine-drawn wire was being produced in England as early as
1565.
In the United States the wire drawing industry had become pretty well
established by the middle of the 17th century. As Cort had not at this
time invented the rolling process for bars and rods, very uneven strips
of metal only were available from which to draw the wires. But, even so,
with our highly developed rod mills and our present very satisfactory
No. 5 wire rod to begin with, our wire drawing methods are yet seemingly
crude and show small advance compared with the very great progress which
has been made in other lines of the iron and steel industry.
Unlike the processes of forging, rolling, etc., drawing of wire is done
cold. In this condition steel has its highest strength to stand the
strain. The rod or wire is pulled through very hard cast iron or steel
dies, the general process being well likened to pulling a rope through a
small knot hole.
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.”
[Illustration:
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.
[Illustration:
SINGLE WIRE-DRAWING BLOCK
]
[Illustration:
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.
[Illustration:
THE DRAWING OF WIRE
]
The loose end of each coil of rod of wire is hammered or otherwise made
smaller for a length long enough to be threaded through the hole in the
die and firmly grasped by tongs or “pliers.” The die being firmly
fastened, the draw head, of which the pliers are a part, is drawn away
until a sufficient length has come through that it can be attached to a
revolving block or drum which thereafter continuously pulls the rod
through the die and winds it into a coil about the block as it comes, at
speeds as high as 400 feet a minute.
As the rod passes through the box in which the die is fastened, besides
its lime or sull coating it is sometimes provided and takes up other
lubricants, such as soap, grease, or tallow.
The coil of wire is lifted off the drum and tied, or is redrawn to wire
of a smaller size.
After from one to several drafts, depending upon conditions, the wire
must be annealed for several hours, and again pickled to remove the
scale formed, washed, lime-coated and dried. For wire which must undergo
many reductions or passes, as must all small wires, several drawings,
annealings and cleanings must be gone through with before it is down to
the desired size.
In order to work properly and not cut the die too fast, which would
rapidly enlarge the hole and cause the rear end of the wire to be of
different thickness than the forward end, the rod or wire must be of
maximum softness. Either under-cleaning or over-cleaning will make the
metal harsh, as, also, will under-baking in the dry ovens. These they
endeavor to avoid in preparing the rod.
Other lubricants may be used and certain finishes or colors given to the
wires. Such are the various white to red coatings which come from using
tin and copper sulphate solutions instead of lime coatings. This is more
often done with fine wire than with thicker. Certain “patenting”
processes make use of various methods of tempering wires by cooling in
air after a last annealing.
Not only are wires ordered to size but many times also of certain temper
or grades of hardness and stiffness. These various tempers are
determined largely by the chemical analysis of the steel and the number
of drafts after the last annealing.
Piano wire is well known to have a very high strength, sometimes as high
as 400,000 pounds per square inch. Spring wire is rolled and drawn from
higher carbon steel than is ordinary wire, and, through heat treatment,
the best properties are imparted to it.
Besides being in this form used for miscellaneous purposes, thousands of
tons of wire are annually made into wire nails, staples, wire fence,
barbed wire, etc., all of which products are of American derivation,
with the possible exception of wire nails. Even this, however, if not
originally ours we have made ours through our great production. All are
very speedily turned out by automatic machines into which the wire or
wires feed. The product drops out into boxes below or is rolled into
coils as is the case of the wire fence and barbed wire. The speed of the
machines is so great that the eye cannot begin to follow the operation.
Wire being so well known it is not necessary to speak of its uses, but
it will be interesting to call attention to the very extensive
production of this steel product. Of the 32,000,000 tons of steel
produced in 1915, for instance, over 3,000,000 tons were rods and wire,
and when we think of the length of small or even average sized wire that
it takes to make a pound, a hundred weight, or a ton, some slight
conception of the amount of wire produced and used comes to us. As the
material for wire gauze and cloth, wire rope, cable, piano strings,
springs of all sorts, pins, needles, nails, fence, barbed wire, and the
myriad of other things, wire really is one of our great products.
It was mentioned that piano wire had been made which had a tensile
strength as high as 400,000 pounds per square inch, which is six or
seven times the strength of an equivalent cross-section of steel rail or
plate. Wire is undoubtedly the strongest product which is made from
steel. This is why wire rope or cable is so strong. It is made by
twisting together many small wires.
Though extremely recent from the standpoint of our world’s history, the
building of the famous Brooklyn Bridge by the Roeblings was far enough
back that we likely have forgotten that each of the four big 15¾–inch
cables is made up of 5,296 separate wires tied into a bundle.
Undoubtedly the best preservative against corrosion for iron and steel
is what is known as “hot galvanizing.” Much wire is hot galvanized. In
this process the wire is “pickled” in weak acid to remove scale, given a
soft red coating by dipping in weak muriatic acid and drying. The
strands of wire 20 or 30 abreast are run through a kettle of molten
zinc. The wires are wiped smooth and free from excess zinc by pulling
them through asbestos pads. A continuous coating of zinc is thus
permanently left upon the surfaces of the wires which very effectively
keeps them from rusting. Wires which have to stand severe weather
conditions may not have the excess zinc wiped from them. Telegraph and
telephone wires often have the thicker or unwiped coating.
Very naturally the drawing of wire requires use of much greater power
than the rolling of rod and the speed of drawing is nowhere near as
great. Therefore the cost of wire is comparatively much greater than the
cost of rolled products. In the case of watch springs it was once
computed that the product had to bring 50,000 times the cost of the
steel from which the wire was drawn.
“Continuous” wire drawing has not been so successful nor advantageous as
was the continuous rolling of rods. It has been possible in a small way
and with certain grades of the product to apply the continuous process,
but, mainly on account of mechanical difficulties, continuous drawing of
wire seems to be comparatively unimportant.