THE MANUFACTURE OF PIPE AND TUBES
Tubes of some sort have been in use by man since very early times.
Nature provided the first ones in the way of hollow stems of shrubs,
such as alder and bamboo. Some of these we saw in use by the early
smiths conveying the blast of air from their goat-skin bellows into the
crude clay furnaces built in the hillside.
Tubes made of clay, stone, lead and bored logs were also used. Much
later pipes made of cast iron came into rather extended use for the
conveyance of water. The general use of gunpowder greatly accelerated
the manufacture of small tubes which the smiths produced for gun barrels
by hammering and welding together long, red-hot strips of wrought iron
about round rods or “mandrels.” About 1815 illuminating gas came to be
used in England for house lighting. This brought a demand for tubes of
considerably greater length, which were first made by screwing or
otherwise fastening together old gun barrels which were very plentiful
at that period.
The first patents for the making of welded pipe as we know it were taken
out in 1824 and 1825, the latter for the butt-welding method of pulling
a narrow iron plate, called “skelp,” through a bell-shaped orifice which
curled it and welded the edges together very much as is done to-day by
this process.
Our modern pipe of both butt-weld and lap-weld varieties is manufactured
either from wrought iron or from soft steel.
The Butt-Weld Process
[Illustration:
EARLY WATER PIPES
]
[Illustration:
HOW BUTT-WELD PIPES ARE MADE
]
From the double refined puddled iron, in the case of wrought iron, or
from billets of soft Bessemer or open-hearth steel, long narrow plates
are rolled. The width and thickness of these plates, which are known as
“skelps,” are exactly such as will give pipe of the desired diameter and
gauge. In order that the weld may be solid all along, the skelp as it
leaves the rolls has edges not exactly square but very slightly beveled,
so that the surface which is to form the interior of the pipe is
slightly narrower than the other.
After trimming the pieces of skelp so that each has one end with a sort
of point where the tongs are to take hold, they are laid side by side in
a heating furnace and left there until they have become white-hot.
Just in front of the furnace is the “bell,” with a second and slightly
smaller one in front. With strong tongs the workman reaches into the
furnace and fastens onto the pointed end of a piece of the white-hot
skelp. Hooking the handle of his tongs into the draw chain the skelp is
drawn through the first and second bell, the first bending it almost
into tube shape, the second completing the operation and pressing
together the edges of the plate in the top of the bell so tightly that
they weld.
[Illustration:
PLATES CALLED “SKELP” ARE FIRST ROLLED
]
The pipe now goes through what are known as cross rolls, the axes of
which are somewhere near parallel with the axis of the pipe. In these
the pipe is rapidly spun around, surface-cleaned and straightened. Going
up a cooling incline it goes to tables where the ends are cut off and
the product inspected.
A very important part of the inspection is the hydrostatic or water
test. One at a time the pieces are tightly fitted in between two
water-tight caps, water is turned into the pipe and gradually brought up
to the testing pressure of 600 pounds or more per square inch according
to specifications.
[Illustration:
CHARGING SKELP INTO THE HEATING FURNACE
]
[Illustration:
DRAWING BUTT-WELD PIPE
]
Pipes of diameters between ⅛″ and 3″ are usually made by the butt-weld
process.
Lap-Welded Pipes
Pipes larger than 3″, and boiler tubes or other particularly high grade
welded tubes of 2″ and over are usually “lap-welded.” This process gives
a considerably more reliable product than does the butt-welding process,
for reasons which are readily seen.
[Illustration:
HOW LAP-WELD PIPES ARE MADE
]
[Illustration:
BENT SKELP FOR LAP-WELDING BEING CHARGED INTO FURNACE
]
Skelp for lap-welding is rolled in just the same way as is skelp for
butt-welding except that the edges are “scarfed” or decidedly beveled so
that the two edges can make a considerable lap without increase of
thickness of that part of the wall. These pieces of skelp are charged
into the heating furnace just as occurred in the butt-weld process and,
after coming to a white heat, they are drawn through a sort of bell or
die which curves them so that one edge considerably overlaps the other.
Back they go into the furnace to regain any heat that has been lost,
for, to weld properly, the skelp must be hot enough that any scale which
had covered it drips off.
[Illustration:
THE LAP-WELDING ROLLS WITH MANDREL IN POSITION
]
The welding rolls are very short rolls, almost “sheaves” or wheels, with
concave edges of exactly the outside diameter of the pipe to be formed.
Between these two rolls, at the end of a long straight bar, is a mandrel
or projectile-shaped ball of high-speed steel over which the white-hot
tube must be pushed.
The reheated, curved skelp is pulled from the furnace and the forward
end forced into the rolls which shoot it through and over the mandrel at
high speed, forcing together and welding under heavy pressure the
overlapping edges of what was formerly the plate. Amid the noise and the
shooting sparks an unsuspecting bystander is quite startled by the
suddenness of it all.
While still hot the pipes pass to “sizing” rolls which correct any
variation in inside and outside diameters. The cross or straightening
rolls next smooth and clean their surfaces while straightening the pipes
or boiler tubes.
After the first trip through the welding rolls, boiler tubes and certain
other high grades of pipe go back into the furnace where they are
reheated. They are again put through the welding rolls to make
absolutely sure of a tight weld.
After cooling, the ends of each pipe are cut off. Because of the
“scarfing” of the edges and the great pressure of the rolls, it is
difficult to tell where the welds occur, the thickness of the walls
being practically uniform all around. Lap-welded pipe of as great as 36″
diameter has been made in this way.
[Illustration:
PIPES IN SIZING AND CROSS ROLLS
]
The water-pressure test is given to all lap-welded pipe as are certain
tests for tensile and torsional strengths, and for ability to flatten
without breaking. In the case of boiler tubes, a piece is cut from each
end of each tube, which must stand flanging or spreading “cold” and also
must crush down endwise under the heavy pressure applied in the testing
machine without fracture or opening of the welds.
The pipe may be “threaded” to order or shipped as it comes from the
testing bench.
[Illustration:
THE FINISHING END
]
As remarked, both butt and lap-welded pipe is regularly manufactured
from wrought iron and from steel.
[Illustration:
HYDROSTATIC TEST OF THE PIPES
]
It was suggested during the discussion of the manufacture of wrought
iron, that, owing mainly to high labor costs, wrought iron was with
difficulty competing with the soft steels. Wrought iron is noted for its
welding properties and it has always had its loyal admirers. Aside from
its application as “bar iron” which always has been and still is in
favor with many metal workers for miscellaneous purposes and its use as
Swedish bar iron or low phosphorus melting bar by makers of crucible
steel as a base for their product, wrought iron probably finds its next
most favored place as a material for pipe as is shown by the table given
in Chapter VI.
While not as strong as steel pipe under hydrostatic test, many pipe
users insist that, presumably on account of its slag enclosures and
cinder films which are supposed by some to surround and protect the
fibers, wrought iron pipe outlasts steel pipe when used under conditions
which induce corrosion. Others are as strenuous in their denial of this
assertion and this subject of comparative wrought iron pipe and
steel-pipe corrosion is still a very live issue. For many years this
matter has been under investigation. Hundreds of tests have been made
and discussed by learned societies and their committees. The
laboratories and testing departments, too, of the large pipe
manufacturers and their customers, have made extended investigations.
However, the conditions under which pipe is used are so varied and the
time required for any true and decisive test is so long that really
conclusive results have not been forthcoming. With other materials, each
condition and corrosive influence is largely a “law unto itself,” and
one wonders if such may not prove to be the case with these materials
also. As suggested, a great quantity of published information giving
comparative service tests is available for those who are particularly
interested in this subject. How much of the decline in tonnage and in
percentage of the total skelp produced, is due to the approximately 30%
greater cost of wrought iron pipe and how much to satisfactory
performance of its competitor must be left to you to judge.
Fortunately pipe of both kinds is available, meanwhile, and one can get
whichever he prefers.
The uses of pipe are almost innumerable. Great quantities are used for
conveyance of water, oils and gases, for ice-making and refrigeration,
the heating and draining of buildings, for dry kilns, hospital beds and
apparatus, electric light, railway and telegraph poles, pipe railings,
for conduit work, etc. For many of these applications, the seamless
variety is now utilized, however.
For many purposes coated pipe is highly desirable. This may be by hot
asphalt, or other liquid dip, by surface electro-galvanizing or by the
hot galvanizing method of dipping in molten zinc, by which method
probably the greater portion of coated pipe is treated. Certain other
protective coatings are used to a limited extent.