THE MANUFACTURE OF SEAMLESS STEEL TUBES
It is more than likely that the popularity of the bicycle, which created
the recent great demand for strong, light and perfect tubes, was largely
instrumental in developing the seamless tube industry, which may be said
to have “sprung up” within the last twenty-five years. Previously all of
the iron and steel pipes or tubes obtainable were either of the “butt”
or “lap-weld” variety with the exception of those which were made from
long pieces of metal by boring holes lengthwise through them. Tubes by
this boring method are, of course, quite difficult and expensive to
make.
Shorter and thicker billets of steel can more easily be bored. When the
holes have been enlarged by pushing larger and larger-nosed rams through
them in a hydraulic press, they can be rolled down to size over a
mandrel, just as lap-welded pipe is rolled, only in this case they are
put through several times and considerably reduced in diameter. In this
way some of the seamless tubes are made.
The important starting point in all processes for seamless steel tubes
is the piercing operation and it is mostly in the method of getting the
first hole through the billet that they differ, since the hot-rolling
and the cold-drawing processes by which they are finished have long been
known.
One of the most important modern processes for seamless tubes, the
Mannesmann, is based upon the principle that if a white-hot round steel
bar is rapidly rotated between “cross rolls,” a longitudinal rupture
which is almost a hole forms along its center. We may liken the motion
of the bar in the rolls to the whirling of a lead pencil between the
palms of our hands, except, of course, that the bar is kept rotating in
one direction only. Though something like this tendency of a steel bar
to open along the center through pressure applied at two opposite points
on the outside, seems to have been known to forgemen, the Brothers
Mannesmann came upon the similar tendency under action of the rolls, by
accident.
They were German tool steel manufacturers. A critical customer wanted
perfectly round and surface-polished bars of steel. They attempted to
give his bars this perfect shape and smoothness by finishing them
between cross rolls which spun the bars rapidly around while they were
slowly passing along through the machine. The pieces were perfect
outwardly, but, much to the steel makers’ chagrin, the customer reported
that the quality of the steel was not as satisfactory as that which he
previously had been receiving. Upon investigation it was found that this
cross-rolling under pressure tended to form a small hole along the
center of the bar with slight cracks in the metal all around it.
Upon this happenstance discovery is based the Mannesmann process for
piercing the bar, which consists in pushing over a piercing head such a
center-weakened piece as it comes through the cross rolls.
One or two modifications of the Mannesmann piercing method are also in
use.
The material generally used for seamless steel tubes is medium soft
open-hearth steel of .15% to .25% carbon. It is received as billets
which are rolled down and cut at the mill or they are purchased as 3″ to
6″ “rounds” and cut into such lengths as give proper amounts of steel
for the tubes which are to be formed. Usually the bars cut for tubes are
from three to five feet in length.
They are heated in a furnace and after the end has been dented at the
center, they go into the rolls.
[Illustration:
PIERCING A SOLID BILLET BY THE MANNESMANN PROCESS
]
[Illustration:
ROLLING DOWN THE PIERCED TUBE
]
The rolls seize the forward end of the bar and swiftly whirl it as it is
slowly pulled in. A piercing head of high-speed steel at the end of a
stiff mandrel extends between the rolls just as we saw it in the
pipe-rolling process. As the forward end of the rapidly whirling
white-hot steel bar pushes against this piercing head, the piece,
weakened along its center line is pierced. As neither the rolls nor the
piercing head can be resisted, it is forced through the rolls and grinds
its way over the piercing head with the supporting bar, the walls of the
white-hot tube being thinned down and the piece very materially
lengthened.
It comes out a rough tube with thick and irregular walls. After its
removal from the rolling mill bar upon which another and colder piercing
head is placed in readiness for the next tube, it goes to other rolls
through which it is passed, first without a mandrel inside, and later,
with one, until it has become somewhere near the desired size and the
walls have been pulled down to the proper thickness. The mandrel, of
course, determines the size of the interior of the tube, and the rolls,
its outside diameter.
Some are sold in this form as hot-finished tubes after having been
straightened and cut to length by removal of the ends.
A great deal of the seamless tubing made is given the cold finish, i.e.,
it is drawn through dies much as rods are drawn in the making of wire.
For cold-drawing, one end of each tube is reduced in size over a length
of a few inches, by forging or by other means. This is where the
“pliers” are to take hold.
Now we can never heat steel without forming upon it a brittle oxide or
scale which is much harder and harsher than the metal itself. During its
sojourn in the heating furnace and its journey through the rolls,
therefore, each of the tubes acquired a hard brittle surface which must
be removed before the tube can be “drawn.” The most practical way of
removing this scale is by “pickling” the tube in some weak acid, usually
sulphuric (oil of vitriol). The acid dissolves some of the scale and
loosens the remainder so that it can be washed off. To neutralize any
excess acid which clings to the tube and to aid lubrication, it is
dipped into lime-water and then dried.
The tube now goes to the drawing benches which are long steel frames
along which a heavy steel draw chain is continuously traveling from the
center toward one end. Anchored at the opposite end of the bench is a
long bar upon which is fastened the mandrel or ball which is to
determine the inside diameter of the tube in the drawing as did the
mandrel between the rolls in the rolling.
[Illustration:
TUBE-DRAWING BENCH
]
The tube to be drawn is threaded over the long rod which is anchored in
place, and the forged-down small end is pushed through the “die,” very
firmly fastened to the bench near its center. The pliers take hold of
the forged-down end of the tube with a vise-like grip, and are then
hooked into the draw chain. The tube is thus slowly drawn through the
hole in the die. As these dies are of very hard material, either hard
cast iron or hardened steel with hole a little smaller than outside of
the tube, they compress the tube upon the mandrel inside and the
thickness of wall is thus regulated, the excess metal being squeezed out
so that the tube is very materially lengthened. Tallow or grease with
the lime-coat lubricate the tube, a little being continually drawn into
and through the somewhat funnel-shaped die.
As was the case with the “cold finishing” of plate and the drawing of
wire, this cold working increases the elastic limit and tensile strength
of the steel. So cold-finished tubes are stronger than hot-finished. For
many purposes such increase in strength is highly desirable. The
exterior of the tube is also made very smooth and uniform in diameter by
the drawing.
[Illustration:
HOW TUBES ARE DRAWN
]
The cold-drawing has a disadvantage, however. It somewhat embrittles the
steel, as may be inferred from the increase in strength. This is not a
serious matter, however, unless the cold-drawing has been overdone.
But for smaller sizes of tubing many drawings have to be resorted to, to
reduce the steel to the size required. Sometimes ten or even fifteen
passes are required before the tubes reach their final size. In such
case the tubes have to be annealed and repickled, limed and dried after
each pass or two in order to restore to the steel its ductility. If this
were not done the tube would eventually break in the die.
The last pass is through an accurate “sizing” die which corrects any
variation in inside or outside diameter.
As the pulling strain which the steel will stand is limited, too much of
a reduction in size in any single pass must not be attempted.
As annealing, pickling, drying, etc., have to be done after every pass
or two, a considerable period of time elapses between the piercing of
the billet and its final pass as a small tube. For economy in handling,
tubes cannot be considered or handled singly, but must be treated in
quantity, so this period between billet piercing and the final pass may
be as much as two weeks, possibly more.
The tubes must next be straightened. This is done in cross rolls as has
been mentioned under the manufacture of lap-welded pipe, or in various
other types of machines.
[Illustration:
CUPPING AND DRAWING SEAMLESS TUBES FROM PLATES
]
Much seamless tubing goes into automobile, bicycle, and various other
products for which very high grade and perfect material is desirable.
One of the many interesting applications of seamless tubing is its use
in very fine sizes for hypodermic needles.
Seamless tubes are easily bent, swaged, upset, spun or otherwise changed
in form, as the material is ductile and there are no welds to open.
Very large tubes are not made in the way just described. They are rather
made by “cupping” flat, round steel plates through a die. A cup is then
in several successive drawings put through smaller dies, under which
treatment it grows longer each time and gets a thinner wall until it has
become a long tube with the one end still closed. For open ended tubes
this and the upper, open end are cut or trimmed off.
Cold-drawing here necessitates annealing to restore ductility just as it
does elsewhere and each annealing operation is necessarily followed by
pickling for removal of the scale formed.
By rapidly spinning large tubes in lathes or other machines and the
application of pressure with the proper tools and lubrication, the walls
of the tubes may be deformed. In this way the ends may be expanded, made
smaller, or completely closed. By such “spinning” operations large tubes
are made into articles of various shapes.
By this same “cupping” or hydraulic drawing of flat, well lubricated
sheets of soft steel, seamless high-pressure gas cylinders, steel drums,
barrels, and the like are made.