The present invention relates to a method of rolling a tubular metal blank to a tube section of a reduced outer diameter and an increased length.

When a relatively short and thick-walled tubular blank has been made from a metal block in a diagonal rolling mill, this tubular blank is further treated in a so-called pilger mill. The length of the tubular blanks which can be rolled in such a pilger mill depends on the size and type of the mill. However, for a certain mill the length of the tubular blanks must be within certain limits. If the length of a tubular blank exceeds the maximum length, the mill cannot perform the desired rolling, and if the length of the tubular blank is less than the minimum length stated it is necessary to feed the tubular blank into the mill manually and, further, the tubular blank cannot be retained and guided correctly during rolling which involves a risk of accidents. The minimum length of the tubular metal blanks which can be rolled in a pilger mill is normally 1500-2000 mm, while the maxi¬ mum length of the blanks may be 5000-6000 mm depending on the size of the mill.

When a tubular metal blank with a certain diameter is to be trans¬ formed into a tube with a considerably smaller diameter by means of rolling it may be necessary to perform the rolling process step-wise. After each rolling step the tube section with increased length which was produced by the rolling process must be divided into two or more new tubular blanks whose lengths must be within said limits in order that they can be subjected to a renewed rolling process in the pilger mill. This division of the rolled tube sections often results in residue lengths or end tube sections which do not attain the minimum length required for use in the rolling mill. Consequently, such tube sections have so far been scraped for remelting.

When the tubular metal blanks in question are made from stainless steel or other kinds of difficultly malleable metal the necessity of scraping the tube sections whose lengths do not attain the relatively

large minimum length, involves a considerable increase in the cost of the rolling process. It has been proposed to make such too short tube sections fit for continued rolling by connecting two or more tube sections by welding and thus produce a tubular blank whose length exceeds the minimum length of the tubular blanks which can be treated in the rolling mill in question. Such a welding process is labour consuming as weel as expensive because it must be ensured that the tube sections which are welded become absolutely coaxial, and the welding seam must be ground. Furthermore, it has been found that such welded tubular blanks easily break either during feeding into the mill or during the rolling itself which may involve damage of the rolls.

The present invention provides a method which renders it possible to further roll such tube sections which are in themselves too short for treatment in the rolling mill used, and thus, scraping of such too short tube sections is avoided.

The method according to the invention is characterized in that the tubular blank is made by axially joining two or more tubular blank parts having substantially the same radial dimension, by means of interengaging complementary connecting parts formed on adjacent ends of the tubular blank parts. It has been found that a tubular blank which has been made by joining two or more blank parts or tube sec¬ tions by means of such interengaging complementary connecting parts can be subjected to a rolling process without these connecting parts disengaging or breaking, as long as they have been properly shaped.

The finished tube section produced in the rolling process can then be divided in such a way that the parts around the joints formed by the connecting parts are cut off and scraped.

Even though the rolling may be performed while the tubular blank has been preheated or is being heated the method according to the inven¬ tion is especially suited for cold-rolling. In principle, the tubular blank may be made from any kind of metal or metal alloy. Especially, the tubular blanks may be made from stainless steel or another alloy steel type, for example of the type used in the production of annular traces for ball bearings or roller bearings. Although the tubular

blank parts from which the tubular blank is composed are normally made from the same metal or metal alloy, it is in principle possible that the tubular blank parts consist of different metals or metal alloys. During the rolling process and in a manner know per se , the tubular blank is preferably provided with a cylindrical core having an outer diameter corresponding substantially to the desired inner diameter of the tube section produced in the rolling process, which tube section is composed of the two rolled tubular blank parts.

The complementary connecting parts by means of which the tubular blank parts are joined prior to rolling may be of any suitable type. In a preferred embodiment, however, the complementary connecting parts comprise a pivot member provided on one of the tubular blank parts and having a circularly cylindrical outer surface with a redu¬ ced outer diameter, and a socket member having a cylindrical inner surface with a corresponding inner diameter formed on the other tubular blank part. The two tubular blank parts may then be joined by inserting the pivot member into the complementarily formed socket member, and the tubular blank thus formed will then have an inner surface as well as an outer surface which are smooth and substantial- ly cnotinuous. Instead of such pivot and socket members with com¬ plementary cylindrical surfaces the connecting parts may comprise pivot and socket members with complementary conical surfaces. When the complementary surfaces are cylindrical or conical they are prefe¬ rably made with such tolerances that the pivot member may be inserted into the socket member with an interference fit. However, the fit may be less tight and the two tubular blank parts may then while they are joined be rotated in relation to each other around their longitudinal axes, and this will generate such a frictional heat between the outer surface of the pivot member and the inner surface of the socket member that the two surfaces are welded together. The two surfaces may possibly also be held together by other means, such as braze metals, adhesives, binders or mechanical fastening means. These mechanical fastening means may for example comprise cooperating threads or similar cooperating fastening means provided on the outer surface of the pivot member and on the inner surface of the socket member. As an example, the complementary surfaces of the pivot and

socket members may be slightly conical and provided with cooperating cylindrical threads.

The pivot member on one of the tubular blank parts and the socket member on the other tubular blank part may be formed in any suitable manner, for example by grinding or rolling. In a preferred embodiment the pivot member is formed by externally turning one end portion of one of the tubular blank parts whereas the socket member is formed by intemally turning one end portion of the other tubular blank. Ir¬ respective of the process used, the outer surface of the pivot member and the cooperating inner surface of the socket member are preferably formed with rounded edges or corners which contributes to the fact that the joints between the tubular blank parts can resist the heavy forces to which they are exposed during the rolling process.

The tubular blank parts used in the method according to the invention may be produced in any suitable manner of producing a seamless tube, for example by drawing, rolling, such as hot-rolling, extrusion, welding, drilling or casting. Furthermore, the tubular blank parts may be made from any kind of metal, such as steel, preferably stain¬ less steel, titanium, copper or alloys thereof. Finally, rolling of the tubular blanks does not necessarily have to take place in a pilger mill but may take place in any other kind of tube rolling mill.

The invention will now be further described with reference to the drawings in which Fig. 1 shows the two tubular blank parts having complementary pivot and socket members which have substantially circularly cylindrical cross-sections,

Fig. 2 shows the tubular blank parts shown in Fig. 1 joined together to form a tubular rolling blank Fig. 3 shows a tubular rolling blank like that of Fig. 2 but whose joint is formed by interengaging of conical pivot and socket members, and

Fig. 4 is a schematic view of a tubular blank of the type shown in

Figs. 2 and 3 during rolling in a pilger mill.

Fig. 1 shows two tubular blank parts 10 and 11 of metal, such as stainless steel. These blank parts may have arisen as residue lengths cut off the rolled tube sections to be subjected to a further rolling process in order to reduce the outer diameter and to increase the length. The length of each of these tubular blank parts 10 and 11 is not sufficient to be fit for rolling in a usual tube rolling mill or a usual pilger mill. With the purpose of producing a tubular rolling blank 12 with a length which makes the rolling blank suitable for rolling in a pilger mill, the two blank parts 10 and 11 are joined by means of a pivot-socket joint 13. For this purpose a pivot member 14 having a cylindrical outer surface and rounded convex and concave edges 15 and 16, respectively, is formed on one end portion of the tubular blank part 10 by turning the outer surface thereof along a length "1". A socket member 17 is formed at one end of the other tubular blank part 11 by internally turning said one end of the blank part 11 so as to provide this socket member 17 with an inner shape which is completely complementary with the outer shape of the pivot member 14. The pivot member 14 can thus be axially inserted in and received by the socket member 17 to form the joint shown in Fig. 2, in which practically no air space is present. The outer surface of the pivot member 14 and the inner surface of the socket member 17 are preferably formed with such tolerances that the pivot member 14 can be received in the socket member 17 by an interference fit. Alterna¬ tively, a less tight fit between the pivot member 14 and the socket member 17 can be used, and the joint may then be strengthened by brazing or by any other binding means. In a preferred embodiment, however, a slide fit or a gentle interference fit is used, and the outer surface of the pivot member 14 is connected to the inner sur¬ face of the pivot member 17 by friction welding, the tubular blank parts 10 and 11 being rotated in relation to each other while the pivot and socket members are in engagement. This may for example take place in the lathe used for preparing the pivot and socket members.

The embodiment shown in Fig. 3 corresponds exactly to that of Fig. 2 with the exception that the pivot member 14 shown in Fig. 3 has an outer conical surface while the socket member 17 has a corresponding inner conical surface.

In the method described above, two or more blank parts may be con¬ nected so as to form a rolling blank 12 having a length which is suitable for rolling in a pilger mill. Fig. 4 diagrammatically il¬ lustrates how the tubular blank 12 with the joint 13 car. be inserted between a pair of rollers 18 in a pilger mill, the blank 12 being inserted over a mandrel 19 having an outer diameter corresponding exactly to the desired inner diameter of the end product of the rolling process. When the blank 12 has undergone this new rolling process and its length thus been considerably increased, for example doubled or tripled, this new tube may be cut into suitable lengths depending on the field of application, the joint 13 being cut away and scrapped. Residue lengths, if any, may possibly be subjected to a further rolling step as described above.

The length "1" of the pivot member 14 and the socket member 17 can be adapted to the diameter of the tubular blank parts in such a manner that the length is increased with increasing diameter. Normally, in the case of thin tubes an axial length of 15 mm of the joint will be adequate while in case of bigger tube diameters the axial length of the joint can be 20 mm or more.