GB2155836A

GB2155836A - Increasing the strength of single-wall tubular components

Info

Publication number: GB2155836A
Application number: GB08506584A
Authority: GB
Inventors: Christof Rabofsky
Original assignee: Voestalpine AG
Current assignee: Voestalpine AG
Priority date: 1984-03-14
Filing date: 1985-03-14
Publication date: 1985-10-02
Also published as: ATA85184A; AT380417B; DE3507293A1; GB8506584D0; DE3507293C2; GB2155836B

Abstract

The outside of the tube 1 is subjected to heating over a limited axial area, the heating being carried out progressively in the axial direction of the tube. At the same time a radially outwardly directed pressure is applied to the inside of the tube, the inner surface of the tube preferably being cooled at the same time as the application of pressure. For heating purposes a high-frequency induction coil 2 is displaced in the longitudinal direction of the tube 1. A cylindrical filling body 5 is inserted in the tube and has a region of smaller diameter between end portions bearing seals 6 cooperating with the inner wall of the tube. The annular space 7 formed in this way is filled with oil under pressure. Alternatively (Figure 2) water in this space may be frozen. <IMAGE>

Description

SPECIFICATION Increasing the strength of single-wall tubular components The invention relates to a method of increasing the strength and the internal-pressure-resistance of single-wall tubular components, such as for example hydraulic cylinders, and to apparatus for performing this method.

In a tube under internal pressure the tangential stresses are at their maximum on the inner surface, while the tangential stresses on the outer surface diminish as the wall thickness increases. In the case of thick-walled tubes in particular, the strength of the material cannot be fully utilized in the outer zones.

In order to improve the utilization of material it has already been suggested that a plurality of tubes be slid one inside another, the outer tube in each case being slid pre-heated over an inner tube.

After cooling, the outer tube contracts on to the inner tube and tangential tensile stresses arise in the outer tube, while compressive stresses arise in the inner tube. In this way the stresses occurring in the operating state can be better distributed over the cross-section.

In addition, it is already known to load and permanently deform the inner zone of a tube by mechanical deformation, in particular by fluid or gas compressive stressing beyond the yield point of the material. In such mechanical deformation of the inner zone of a tube the outer zone undergoes expansion. The stresses in the outer zone generally remain just below the yield point and the resiliently deformed outer zone subsequently causes compressive prestressing of the permanently deformed inner zone. In order to apply the compression loading in the inner zone of the tube, it is already known in particular to roller-burnish the inner surface with a special tool, to force through a plug of excess size, or to perform hydraulic cold drawing by high pressures in the interior of the tube.In this connexion, hydraulic cold drawing using elevated pressures or forcing through a plug with an excess dimension are suitable fro substantial wall thicknesses, but these methods having the disadvantage that very great forces or pressures are necessary, which require a relatively large outlay in terms of apparatus. In tubes which an offset or stepped wall thickness, however, these methods can be used only with increased outlay, since with tubes of this type separate pressures or different plugs or internal bore stepping must be used for the different tube sections.

What is desired is a method of the type described above, by which the desired stress characteristic can be better controlled over the crosssectional area of the wall of the tube, and when prestresses applied by the method are added to the subsequent operating stresses a more uniform stress distribution over the cross-section of the wall of the tube is achieved.

The present invention provides a method in which the tubular component is subjected simultaneously on its outside to an axially advanced heating which is limited in the axial direction and the entire tube length or a partial region thereof is subjected on its inside to a compressive stress directed radially outwards.

By virtue of the fact that the outside of the tube is subjected to heating over an axially limited area, the increase in volume during heating is resisted in the longitudinal direction. The increase in volume on account of heating thus results mainly in an increase of the external diameter. If, at the same time as internal pressure is present, this increase in the external diameter causes the elastic limit of the material in the inner zone to be exceeded, this results in a permanent deformation, which during cooling produces biaxial tensile stresses in the outer zone and biaxial compressive stresses in the inner zone. By simultaneously using compressive stressing directed radially outwards on the inside of the tube, the heat stressing on the outside can be kept low, so that the heating does not cause any adverse effects from the metallurgical point of view.In principle the outer zone can be heated without exceeding the elastic limit therein, the pressure built up inside and directed radially outwards producing a tensile stress in the tube wall, which depending upon the depth of penetration of the heating produced from outside can be built up to a greater or lesser extent into the wall cross-section. By simultaneous application of heat on the outside of the tube and compressive stressing on the inside of the tube, a prestressing may be set over the entire wall thickness which ensures substantially uniform stressing of the entire wall thickness under operating pressure.In this connexion the method is preferably carried out in such a way that the inside of the tubular component is cooled at the same time as the compressive stressing, so as to provide a further possibility of controlling the depth of penetration of the pressure or heating treatment.

The heating of the outside can be selected to be in the temperature range of 100cm to 600 C, preferably from 100"C to 350 C, ensuring that no undesired structural changes occur in the material of the tube wall. For better control of the depth of penetration of the heating, this is preferably carried out inductively, thereby permitting a greater or lesser depth of penetration of the heating to be determined by variation of the current frequency or the velocity of the induction coil advance. In this connexion the depth of penetration is advantageously limited to 20 to 50 % of the cross-section of the material of the tubular component from the outside to the inside.

The internal pressure can be built up with simultaneous cooling of the inner wall in a particularly simple manner by freezing water in the interior of the tube.

The invention also provides apparatus for performing this method, comprising a high-frequency induction coil (having an internal diameter substantially corresponding to the external diameter of the tubular component) and a substantially cylindrical filling body which can be joined to the inner wall in a sealed manner and the outer surface of which is offset to a smaller diameter between two ends bearing seals, it being possible to fill with water the annular space formed and to cool it andl or to act upon it with fluid under pressure.

A filling body of this type permits the use of the desired internal pressure over a predetermined axial length of the tube, so that different wall thicknesses over the axial length of the tube can also be taken account of in a simple manner by varying the internal pressure. A further possibility of taking into consideration different wall thicknesses over the axial length of the tube is provided by using an induction coil, the inductive heating effect of which is substantially limited to the axial length of the coil, as a result of which a variable setting of the heating is also possible over the axial length of the tube.

In a particularly simple manner the apparatus may be designed in such a way that the feed for the fluid under pressure is constructed as an axial bore which passes through one end face and to which is joined at least one radial bore opening into the region of the outer surface of smaller diameter, it being possible to force pressure medium over an axially limited region with an apparatus of this type.

In order to utilize the internal pressure by freezing water, the apparatus is particularly advantageously designed in such a way that axially orientated bores opening in the cross-sectional region outside the outer surface of the region offset to a smaller diameter are provided at both end faces of the filling body for the supply of water and for venting the annular space, and a substantially central, axial bore is provided for passing a cooling medium through.

In all cases it is advantageous if the intensity and/or the frequency of the current source for the high-frequency induction coil can be varied.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic view of a first embodiment of apparatus according to the invention; Figure 2 is a view similar to the view in Fig. 1 of a modified embodiment of the apparatus; Figure 3 is a graph of the temperature characteristic which can be attained over a tube wall thickness by an induction coil; Figures 4 to 8 are graphs of the tangential tensile and compressive stress characteristics when using various treatments; and Figure 9 shows a tangential voltage characteristic which under operating pressure.

In Fig. 1 an induction coil 2 which is displaceable along the axis of a tube 1 in the direction of the arrow 3 is disposed on the outside of the tube.

Leads 4 connect the coil 2 to a high-frequency source 4 whose current intensity and frequency are variable in a manner known per se.

A filling body 5 which sealingly bears on the inner surface of the tube 1 by way of seals 6 is inserted into the tube 1. Over an axial region a the body 5 is of reduced diameter b, so as to produce an annual space 7. The body 5 has an axial feed bore 8 for oil under pressure, which opens into the annular space 7 by way of a radial bore 9 (or bores). By forcing oil under pressure into the blind axial bore 8 and thus into the annular space 7, pressure is built up on the inside of the tube 1 and the cold deformation of the inner zone of-the tube resulting from this pressure extends over a crosssectional area of the tube wall which can be largely controlled by the application of different current intensities or different frequencies to the induction coil 2.

In the embodiment according to Fig. 2 the filling body 5 has a continuous axial bore 10, by way of which a cooling medium can be passed through the filling member 5. The annular space 7 can be connected to a water feed line and a water discharge line or vent respectively by way of two bores or feed lines 12 passing through the end walls 11 of the body 5. The annular space 7 can therefore be filled with water by way of one of the bores or feed lines 12. After the annular space 7 has been filled with water, the water can be frozen by passing a cooling medium through by way of the central bore 10, as a result of which the radial pressure can be built up with simultaneous cooling of the inner wall of the tube. The induction coil 2 is displaceable at a variable velocity of advance in the direction of the arrow 3.

The internal pressure can instead be applied by forcing through a plug with a slight excess dimension with respect to the internal diameter of the tube. In this case the plug and the induction coil must be moved forward synchronously. Where there are gradations in the wall thickness, stepped fitting differences in the bore can be dispensed with.

Fig. 3 shows the effect of inductively heating the outside of the tube. Over the wall cross-section c, within the wall thickness of the tube 1 the application of high frequency produces a temperature gradient which is associated with a correspondingly variable depth of penetration for heating the outer zone of the tube from the outside. The corresponding tangential stress characteristic in the condition of heating, but without the application of pressure, is illustrated in Fig. 4.

Fig. 5 shows plastic extension in the inner partial region of the tube wall and thus diagrammatically explains the conditions which occur when applying pressure without heating.

The conditions which occur with simultaneous heating and application of pressure with respect tot he tangential stress are shown in Fig. 6. Fig. 7 shows the tangential stress characteristic after cooling under pressure. Fig. 8 shows the tangential stress characteristic achieved by the treatment, in the cooled state without pressure. If operating pressure is now applied on the inside of the tube, the inside of the tube under compressive prestress is substantially capable of withstanding the operating pressure, the uniform tangential stress characteristic shown in Fig. 9 being produced as a whole over the entire wall thickness c of the tube 1.

Claims

1. A method of increasing the strength and the internal pressure resistance of a single-wall tubular component, comprising the simultaneous steps of subjecting the outside of the tubular component to axially advancing heating which is limited in the axial direction and subjecting the inside of the tubular component, over all or part of its length, to a compressive stress directed radially outwards.

2. A method as claimed in claim 1, in which the inside of the tubular component is cooled at the same time as the compressive stressing.

3. A method as claimed in claim 1 or 2, in which the outside is heated to within a temperature range of 100"C to 600 C.

4. A method as claimed in any preceding claim in which the heating is carried out inductively.

5. A method as claimed in any preceding claim, in which the depth of penetration of the heating from the outside is 20 to 50 % of the cross-section of the material of the tubular component.

6. A method as claimed in claim 5, in which the depth of penetration is kept in the said range by varying the current intensity and/or frequency and/ or the velocity of the advance of a high-frequency induction coil.

7. A method as claimed in any preceding claim, in which the internal compressive stress is applied by fluid pressure, by freezing water, or by forcing a plug through the tubular component.

8. Apparatus for performing a method according to any preceding claim, comprising a high-frequency induction coil and a substantially cylindrical filling body having end portions with seals for engaging the inside of the tubular component, the outer surface of the body having a region of smaller diameter between the end portions, thereby defining an annular space, means being provided for filling the annular space with water and cooling it and/or for acting upon the annular space with fluid under pressure.

9. Apparatus as claimed in claim 8, in which the body has an axial bore which passes through one end portion and communicates with at least one radial bore opening into the annular space.

10. Apparatus as claimed in claim 8, in which the end portions have through-bores opening into the annular space for supplying water and for venting the annular space, and the body has a bore for passing a cooling medium through.

11. Apparatus as claimed in any of claims 8 to 10, including a current source of variable intensity and/or frequency for the induction coil.

12. A method as claimed in claim 1, substantially as described with reference to the accompanying drawings

13. Apparatus as claimed in claim 8, substantially as described with reference to, and as shown in, Figure 1 or Figure 2 of the accompanying drawings.