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.
How Lap-weld Pipes Are Made
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.
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.
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.
The Finishing End
As remarked, both butt and lap-welded pipe is regularly manufactured from wrought iron and from steel.
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.
CHAPTER XXI
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.
Piercing a Solid Billet by the Mannesmann Process
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.
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.
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.
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.
CHAPTER XXII
TRANSFORMATIONS AND STRUCTURES OF THE STEELS
It was “Ali Baba” who is quoted as saying, “Those who do not know how to take the Philistine, better hadn’t!” or words to that effect.
Now through these chapters we have attempted to discuss in an entirely non-technical manner the subjects presented. On this account we were compelled to forego discussion of many things which are highly important and interesting but which are more or less difficult of explanation without the use of scientific terms and theories. One such has been the “mechanism” of the hardening of steel and its opposite, its softening by annealing. For those who may desire to get a glimpse into this “wonderland” it is hardly fair to refrain from brief discussion of the subject just because it is technical and difficult and so may prove to be tedious to some who have little reason to be interested.
It seems desirable, therefore, to impose this more technical chapter or two that the subject of the real metallurgy of iron and the steels may at least be “hinted at.” We say “hinted at” advisedly for it is a long, long story, and, even now, after a great many years of serious study no one has yet read it to the end. We are not saying this in a discouraging way, however, for there seems little reason to doubt that the multitude of facts which have been disclosed through the tireless experiments and the study of hundreds of investigators have put us well on our way to the solution of this one of Nature’s great problems.
To those, however, who are not interested in the known details of “how” and “why” hardening and softening of steel is possible and why hardening of pure iron and mild steels does not and cannot take place, we must say as would “Ali Baba”:—“Those who do not care to study it better hadn’t.” Anyway, the study of this rather intricate subject is conducive of “headaches,” and perhaps it is not extremely important when viewed from the non-technical standpoint of these articles.
We have several times referred to the debt which civilization owes to iron and steel structural materials, machinery and tools and particularly to those tools which have hardened cutting edges. Almost every one knows that hardened cutting edges are imparted to tools by sudden cooling in water or oil from a good red heat. Probably most of us, too, know that the blacksmith can again soften such tools by reheating to the same red heat and allowing them to cool slowly. This he calls annealing. In this softened or annealed condition a piece can readily be sawed or filed, while in its hardened state a saw or file produces no result upon it.
Now what are the facts, meaning and the cause of this dual life of the alloy, steel, without which we would be so greatly handicapped.
To be better prepared to understand the answer, let us consider three or four accompanying and closely allied phenomena which close observation of the habits of steel has disclosed.