US2274581A - Method of producing seamless tubes - Google Patents

Description

' Feb 24, 1942. B. BANNlsTER Erm. 2,274,581

METHOD OF PRQDUCING SEMLESS TUBES Filedneo. 20, 1959 Patented Feb. 24, 1942 2,274,581 METHOD OF PRODUCING SEAMLESS TUBES Bryant Bannister and George J. Kirchner, Mount Lebanon, Pa., assignors to National Tube Company, a corporation of New Jersey Application December 20, 1939, Serial No. 310,252

2 Claims.

This invention relates to the manufacture of seamless pipe and, more specifically, to a method of producing a Wide range of pipe sizes of desired K wall thickness from billets of a single diameter.

More particularly, the invention relates to a method of piercing solid cylindrical billets to obtain pierced shells of Wide diameter and wall thickness range from billets of one diameter. The length of such billets depends, of course, on the Wall thickness, length and size of pipe desired.

One of the objects of the present invention is to provide a method of producing a large variety of pier-ced shell sizes from a very limited number of billet sizes. Another object of the present invention is to provide a method of producing a number of different pierced shell sizes from billets of the same size.

At the Ipresent time, operating under the best available conditions and using the most modern and eilicient piercing mills, it is necessary to use `a great variety of billet sizes (diameters) to obtain a finished product of the desired diameter. For instance, one tube company in its operations uses thirty-four different diameters of billets to make seamless tubing ranging in size from 2%" to '26" O. D. and of various wall thicknesses. Within certain limits, products of various diameters can be'produced from one size of billet by subjecting the pierced and rolled shell to a sinking (reducing) or cold drawing operation. However, it is readily understandable that these are quite expensive as they involve additional heating and manufacturing steps.

Existing piercing mills are, s far as we are aware, designed to slightly expand the billet while piercing the same. Pierced shells having a thick wall can be produced on these mills which are slightly smaller in diameter than the billet but at the expense of quality in the pierced shell.

By using the present invention on a piercing mill of the present design, reductions up to 25% are readily obtainable without detriment to the Workpiece. Thus, we are enabled to produce the full range of pipe sizes above stated from a maximum of eight billet sizes. This results in a tremendous saving in billet cost due to the fact that frequent resettings of the bar mill are eliminated, rolling of small lots is done away with and, it is no longer necessary to stock a large variety of billets, bar mill and piercing mill tools.

The early Mannesmann patents and others indicate large diameter reductions during piercing, but such reductions are impossible of commercial attainment with the apparatus as shown in these patents because the-improper speed relationship between the rolls and the billet causes severe twisting of the workpiece. Subsequently, skilled workers in the art recognized this diiiiculty and attempted to correct it by providing theoretically true rolling relationship between the roll surfaces and the billetl surfaces as the latter are reduced.v

It was believed that the roll and billet should approximate a bevel gear and pinion in speed relationship, i. e., the roll should have the same diameter ratio to the billet at all transverse sections of the pass. However, We have found that mills providing this true rolling relationship'subject the workpiece to severe twisting in practice and are, therefore, not suitable for the present purposes.

Careful experimentation has developed that this relationship is far from the correct one to obtain no twist or to control twisting within operable limits. It has been found that the speed of. the roll relative to the surface speed of the billet should increase as the cross sectional area of thepierced portion of the billetl is decreased. This is true largely because as the billet section is reduced, the wall thickness is decreased and the metal of the billet exerts greater and greater pressure against theguide shoes. Naturally, the tendency for slippage between the roll and billet increases as rotational resistance increases, and to compensate for this increased roll surface speed is necessary. If the pass decreases the billet diameter, the twist radius decreases as the billet section is reduced and less increase in roll speed is required for compensation than would be required for an expanding pass.

As to why an increase in roll speed will compensate for slippage, it might be well to consider y that portion of the billet being acted upon by the roll as a series of thin discs such as would be made if the billet were cut by transverse planes,l closely spaced. In this case, if the roll diameter contacting each of the discs bore a constant diameter relationship or, stated in another Way, if the diameter of the roll at each section contacting the discs divided by the diameter of the disc in questiony was the same for all sections, true r rolling relationship would exist and, if the discs were free to rotate without resistance, each would rotate the same number of revolutions per revolution of the roll. However, as the outlet end of the pass is approached, the resistance to rotation increases and, as a consequence, under the conditions above described the rotation of the disc would progressively decrease towards the outlet of the pass. In order to compensate for this, the roll diameter should progressively increase from the inlet to the outlet of the rolling surface by an amount which will overcome the tendency for the billet discs to lag.

We have discovered that 'where the surface speed of the rolls varies from that called -for by theoretically true rolling relationship and is at least 10% and not more than 25% faster -at the point of minimum` billet sectional area than at the inlet when the billet v.diameter is increased during piercing, that the objects of the present invention can be accomplished in an expeditious manner without injury to the workpiece. In fact if this relationship is employed, la workpiece of higher physical properties is obtained than is obtained from existing mills where shells varying only slightly in diameter from the billet are produced. This is particularly beneficial where "al1oy steels as distinguished from carbon steels are being forged.

It is obvious that with rolls suited to reducing the billet diameter while piercing, the speed relations would be reversed if the billet is expanded, since the ratio of roll. diameter to billet diameter at the outlet would be less than the same ratio at the point where piercing commenced. When the billet diameter is reduced in the 'piercing operal should be between a value greater than 1 and 1.25 wherein R=radius of roll at point where piercing rolling commences B=radius of billet at point where piercing rolling commences Rp=radius of roll at point where wall reduction ceases Bp=radius of billet at point where wall reduction ceases.

The accompanying drawing schematically shows an application of the above, I being the roll, 2 the billet and 3 the point, the guide shoe being shown at 4. It is to be understood that the guide shoe is projected into the plane of the roll for illustrative purposes.

' Thus, it is seen that working with the pass of the present design, cylindrical billets can be pierced to obtain shells of approximately the same or smaller diameter than that of the billet by varying the position of the plug lto maintain the rolling relationship within these limits as determined by the formula set forth. Since all billets are conditioned, that is to say, inspected and all surface imperfections removed, before piercing, and the larger the diameter of the billet for a given weight the less surface there is to condition, it is desirable from a cost standpoint to obtain pierced shells in a reducing operation rather than from an expanding operation.

This invention lends itself with advantage to either barrel or cone type rolls. In either case the rolls are set for a billet of a certain diameter and from this setting, and using billets of this diameter, a wide variety of pierced shell diameters can be obtained by varying the position of the plug so long as the rolling ratio is maintained within the aforementioned limits. If it is desired to maintain a constant wall thickness,'the l diameter ofv the plug will have to be varied according to the position of the plug with respect to the gorge. However, it will be seen that by using rolls having a speed relationship between entry and gorge as set forth in the foregoing, pierced shells can be obtained over a wide range of sizes by merely varying the position of the plug'or mandrel.

It is to be noted that the guides should be moved a distance corresponding to the change in plug position and an insert used, such as the shoe 4, which conforms somewhat to the configuration of the portion of the rolls being used. That is to say, when the mill is used for a reducingpiercing operation, the guide insert will be tapered inwardly of the pass, being thicker at the outlet end, and in the case of the parallel piercing operation the inserts used will be parallel to the pass, or this elect may be obtained by setting the guides at varying angles to the. pass instead of using inserts of different cross sectional shape. It is to be understood that a parallel piercing operation is one in which the diameter of the piercedv billet equals that of the solid billet entering the' pass, the pierced billet being neither greater nor less in diameter than the solid billet.

We claim:

l. A method of piercing cylindrical billets by helically advancing a billet over a mandrel plug intermediately disposed between at least two metal working rolls, characterized by applying a progressivelyvarying roll surface speed to said billet as its cross sectional area is reduced so that the numerical value of the formula RXBp lies between a value that is greater than unity but does not exceed 1.25 wherein R=radius of roll at point where piercing commences B=radius of billet at point where piercing commences Rp=radius of roll at point where wall reduction ceases Bp=radius of billet at point where wall reduction ceases.

RPXB RXBP lies between a value that is greater than unity but does not exceed 1.25 wherein R=radius of roll at point where piercing commences B=radius of billet at point where piercing commences Rp=radius of roll at point where wall reduction ceases Bp=radlus of billet at point where wall reduction ceases.

BRYANT BANNISTER. GEORGE J. KIRCHNER.

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