GB2086283A - Tube-forming process - Google Patents

Abstract

This invention relates to a process of forming a seamless metal tube in which a cylindrical workpiece having a closed or partially closed bore is pushed through a die or dies by means of a mandrel engaging the closed or partially closed end of the bore, wherein the workpiece (14) is initially pierced to provide the closed or partially closed bore therein by feeding it axially, by means of a plurality of driven rolls or discs (10 and 11), over a piercing plug (13), movement of the workpiece over the plug being stopped before complete penetration of the plug through the billet has taken place. <IMAGE>

Description

SPECIFICATION Tube-forming process This invention relates to a process of forming seamless metal tubes which is of the kind generally known as the push-bench process. In this process a cylindrical workpiece in which a bore having one closed or "partially closed" end has previously been formed is pushed, by means of a mandrel which is inserted into said bore so that its leading end engages the closed end of the bore or a shoulder in the case of a partially closed end, through at least one and usually a succession of dies having orifices of decreasing diameter so that the workpiece is progressively elongated and reduced in external diameter.

Hitherto, the hollow workpiece has commonly been produced by piercing an initially solid billet with a punch. Usually the solid billet is of square cross-section (although it can be of octagonal or circular cross-section) and after being heated it is placed in an open container of circular crosssection. The punch, which is also of circular crosssection is then pushed into the billet through the open end of the container but is only allowed to penetrate part-way into it so that the pierced billet will be left with one closed end or partially closed (i.e. shouldered) end when the punch is removed.

This piercing process has the effect of changing the originally square cross-sectional solid billet into a workpiece having a closed or partially closed bore as previously explained and an external cross-sectional shape which corresponds to the internal cross-section shape of said container -- i.e. the hollow workpiece will, after piercing, be of circular cross-section externally.

The piercing process suffers however from the disadvantage that it is not possible to support the leading end of the punch as it penetrates the heated billet. Consequently, any asymmetric lateral forces applied to the punch during its working stroke could lead to a lateral deflection of the punch to a degree which would produce an undesirable degree of eccentricity in the pierced hole relative to the longitudinal axis of the billet unless the punch is made sufficiently strong to resist such asymmetric lateral bending forces.In practice, this means that a limit has to be placed on the ratio L/D, where L is the length of the working part of the punch (which is equal to the length of the pierced hole) and D its diameter, and hitherto such ratio has usually fallen within the range of 6-10. Such a relatively low ratio means for example that considerable elongation has to be achieved in the push-bench itself in order to produce tubing having thin or medium wall thickness in adequate lengths for subsequent processing (usually stretch reducing), whereas it is not easy to produce relatively thick-walled tubing at all with the above described process.

In order to overcome the disadvantages associated with punch piercing the alternative or rotary cross-roll piercing can be used. This permits the aforementioned L/D ratio to be substantially increased with substantially better concentricity than that provided by punch piercing, enables longer thick-walled tubes to be produced by the push bench, and reduces the amount of elongation necessary on the push-bench to produce tubing of adequate length.

However the rotary pierced shell is throughpierced and hence does not have a shoulder for the mandrel bar to push against.

In order therefore for cross-roll piercing to be usable with the push bench process some means of closing or partially closing the end of the rotary pierced shell is necessary. One such means is provided by a separate machine which grips the shell after piercing and partially closes one end by squeezing it between specially shaped tools.

The object of the present invention is to eliminate the need for a separate end closing machine and, if desired to enable the plug support bar on the piercer to be used as the push bench mandrel bar. This will enable the advantages of cross-roll piercing to be obtained at less expense, and in addition will provide improved internal quality, the opportunity to optimise the distribution of elongation between the piercer and the push-bench, and an opportunity to minimise cycle time.

In accordance with the invention there is provided a process of forming a seamless metal tube in which a cylindrical workpiece having a closed or partially closed bore is pushed through a die or dies by means of a mandrel engaging the closed end of said bore, characterised in that the hollow workpiece is formed from a solid cylindrical billet by forcing said billet over a piercing plug by means of a plurality of driven rolls or discs which are arranged to engage the exterior of the billet so as to rotate it and at the same time feed it axially over said piercing plug, and by stopping relative axial movement between the pierced billet and the piercing plug before the latter has penetrated the billet completely.

The invention will now be more particularly described with reference to the accompanying drawing which is a schematic diagram illustrating one step in an example of a method in accordance with the invention.

Referring to the drawing there is shown therein a billet piercing apparatus which comprises a plurality (for example 3) of rolls such as are indicated by reference numerals 10 and 11. Each of the rolls comprises a pair of frusto-conical portions arranged in a back-to-back relationship so that the largest diameter is approximately at the centre of the length of the roll. Furthermore each roll is arranged so that its longitudinal axis is slightly inclined to the longitudinal axis of the piercing apparatus and the rolls are connected to suitable gearing and an electric motor (not shown) whereby they may be positively driven about their longitudinal axes.

There is also provided a piercer bar 12 arranged so that its length extends along the longitudinal axis of the piercing apparatus, said bar having a piercing plug 13 at that end of the bar which is nearer to said rolls. The bar and the pierced shell are supported externally by a series of guide roll assemblies (not shown). Said piercing plug 1 3 is disposed in a position in which it projects between one set of ends of the rolls and rotation of the rolls by the aforementioned electric motor is used both to rotate and feed axially a workpiece 14 through the rolls and over the piercing plug 13. Said workpiece 14 will thus be fed into the rolls from the left-hand end thereof as viewed in the drawing -until the leading end of the workpiece engages the rolls whereupon rotation of the workpiece and axial feeding thereof will occur.The workpiece will then be forced over the piercing plug 13 so that a bore having a closed or partially closed end will be produced in the workpiece as indicated by reference numeral 1 5 in the drawing.

It is however desired to ensure that the piercing plug 1 3 (which is longitudinally fixed) shall not pass completely through the trailing end of the workpiece 14 so that a closed end or at least a shoulder will be formed at said trailing end to provide an abutment which can be engaged by a mandrel bar for subsequently pushing the workpiece through a die or dies, various methods can be adopted to ensure that said piercing plug 1 3 does not pass completely through the trailing end of the workpiece. Thus in one arrangement means are provided to detect the changes in load on the electric motor, and therefore the electric current taken by the motor, when the tip of the piercing plug 13 reaches the trailing end of the workpiece.Detection of such changes in motor current can then be used either to stop the motor and/or initiate withdrawal of the piercer bar and workpiece in an axial direction away from the rolls.

Alternatively the position of the trailing end of the workpiece can be monitored so that again the piercing operation will be halted when such trailing end has reached a predetermined position just before the piercing plug 13 will break through the trailing end. As yet a further alternative a positive stop may be provided for example at the leading end of the workpiece so that again the piercing operation will be halted when the piercing plug has penetrated to the desired depth and such a stop may for example be mounted on the piercer bar itself.A still further alternative method involves measuring the load applied to the piercer bar during the piercing operation and detecting when such load decreases as the tip of the piercing plug 1 3 reaches the trailing end of the workpiece so that the decrease in such load on the piercer bar can then be used to initiate withdrawal of the bar and the workpiece.

After the workpiece has been pierced to provide a closed or partially closed bore as aforesaid the piercer bar 12 can if desired be removed and a separate mandrel bar (not shown) then inserted and the hollow workpiece can then be elongated by means of a push-bench process in a conventional way. Alternatively and preferably the piercer bar 12 can be left in position and after moving the assembly of pierced workpiece and piercer bar from the piercing apparatus, the piercer bar itself can be used as a mandrel bar in a similar manner in which the piercer bar is pushed at its trailing end to force the workpiece through a die or succession of dies in order to elongate it and reduce its external diameter.

Furthermore instead of using rotating rolls to carry out the piercing operation as abovedescribed, rotating discs can be used instead.

Some advantages of the above described process are as follows: 1. It utilises a rotary piercing which produces a more concentrically pierced shell than the hitherto used process of forcing a punch into a billet mounted in a container. Furthermore a considerable increase in the aforementioned L/D ratio can be achieved. For example with the above described process it is possible to achieve such a ratio lying within the range of 50/70. These longer (and thinner-walled, if desired) pierced shells reduce the amount of elongation required in the push-bench process itself and also enable thickwalled tubes to be produced if desired.

2. In an alternative known method of piercing as previously mentioned, a solid billet may be completely pierced by a rotary process with a through parallel (i.e. bore without a shoulder) bore but in this case one end of such hole has to be closed if use is to be made of the push bench process. This of course requires additional equipment and is therefore more costly.

3. If the same bar is used for supporting the piercing plug and for pushing the pierced shell through the push bench, this results in a shorter operating cycle and a further reduction in equipment in that no separate bar threading device is required and one mandrel bar "circuit" can be used for both piercer and push bench. A reduction in internal scale formation is also achieved if the same bar is used for both operations which will result in an improved internal finish on the tube.

4. When the piercer bar itself is used as the pushing mandrel on the push bench it is possible to operate with a reduced clearance between the interior of the pierced workpiece and the exterior of the piercer bar, thus reducing the amount of diameter reduction of the workpiece required to obtain intimate contact of the piercer bar with the interior of the workpiece and this will improve the flow conditions on the push bench.

5. When the piercer bar and pierced shell are discharged from the piercer as a single unit without withdrawing the piercer bar, the total piercer cycle time can be reduced and hence the cycle time of the total process can be reduced with a potential increase in output.

Claims (8)

1. A process for forming a seamless metal tube in which a cylindrical workpiece having a closed or partially closed bore is pushed through a die or dies by means of a mandrel engaging the closed end of said bore, characterised in that the hollow workpiece is formed from a solid cylindrical billet by forcing said billet over a piercing plug by means of a plurality of driven rolls or discs which are arranged to engage the exterior of the billet so as to rotate it and at the same time feed it axially over said piercing plug, and by stopping relative axial movement between the pierced billet and the piercing plug before the latter has penetrated the billet completely.

2. A process as claimed in Claim 1 wherein relative axial movement between the pierced billet and the piercing plug is stopped at the desired position by detecting changes in the load on an electric motor which is used to drive said rolls or discs.

3. A process as claimed in Claim 1 wherein relative axial movement between the pierced billet and the piercing plug is stopped at the desired position by monitoring the position of the trailing end of the workpiece as it passes through said rolls or discs.

4. A process as claimed in Claim 1 wherein relative axial movement between the pierced billet and the piercing plug is stopped at the desired position by means of a stop which is arranged to be engaged by the leading end of the workpiece.

5. A process as claimed in Claim 1 wherein relative axial movement between the pierced billet and the piercing plug is stopped at the desired position by measuring the load applied to the piercing plug during the piercing operation.

6. A process as claimed in any one of the preceding claims wherein the piercing plug is left in the pierced billet after piercing has been carried out and is then used as a mandrel for pushing the billet through a die or dies.

7. A process of forming a seamless metal tube as claimed in Claim 1 and substantially as hereinbefore described with reference to the accompanying drawing.

8. A seamless metal tube when formed by a process as claimed in any one of the preceding claims.