EP2623616A1 - Expansion controlled autofrettage - Google Patents

Abstract

The method involves applying high pressure of 700 bar at interior (2) of a metal pipe (1). A plastic deformation of metal or of metal wall is occurred from the inner surface (3) of the metal pipe. The pressure is increased gradually during the plasticization treatment. The outer diameter of metal pipe is measured during plasticization treatment. The pressure increasing process is terminated while reaching critical outside diameter of metal pipe, and then pressure is reduced again.

Description

The invention relates to an autofrettage method for increasing the strength of metal pipes, wherein the interior of a metal pipe with a high pressure or internal pressure of at least 700 bar is applied.

Such methods are known in practice in different embodiments. In this case, a metal pipe with a high pressure, which is above the later operating pressure, applied. The pressures used in autofrettage methods are usually in the range of several thousand bar. Due to the action of such a high pressure, the metal of the inner wall portions of the metal pipe is plasticized. This results in internal stresses in the pipe wall, which counteract the forces acting later during operation due to internal pressures. In Autofrettage treatment, the pressure is usually applied by means of a liquid or by means of a liquid of low compressibility, such as with hydraulic oils or ionic liquids. For this purpose, a metal tube is closed at both ends and filled with the liquid. Subsequently, the pressure is built up by pumping to the respective final pressure. With increasing pressure, the plasticization forms starting from the inner surface of the tube. So far, to control the autofrettage treatment, the building up or the increasing pressure is measured. The previously used autofrettage methods initially have the disadvantage that the extent of the autofretting or the degree of plasticization of the tube can not be controlled with sufficient precision and functionally reliable. This often results in faulty pipes or pipes with unsatisfactory material properties. Furthermore, many known autofrettage methods have the disadvantage that - not least because of the insufficient precision in the control of the process - relatively many Pipes are damaged and thus the rejects are relatively high or pipes are insufficiently biased and thus the durability is endangered over the life.

Accordingly, the technical problem underlying the invention is to specify an autofrettage method of the type mentioned at the outset, with which the above-described disadvantages can be avoided.

To solve this technical problem, the invention teaches an autofrettage method for increasing the strength of metal pipes, wherein the interior of a metal tube with a high pressure or internal pressure of at least 700 bar - preferably at least 3,000 bar - is applied, so that starting from the inner surface of the Metal tube takes place a plasticization of the metal or the metal wall,
wherein the pressure or internal pressure is increased during the Autofrettage treatment, wherein further during the treatment increasing outer diameter d _{a of} the tube is measured and wherein the pressure is subsequently reduced again. - It is within the scope of the invention that the pressure increase is terminated upon reaching a critical outer diameter d _{ak of} the tube and the pressure is then reduced again.

The metal pipes are in particular steel pipes. These steel tubes can be provided with a coating or with coatings. In the autofrettage treatment according to the invention, the pressure is expediently increased from 0 to the final pressure which corresponds to the critical outer diameter d _ak . It is within the scope of the invention that after reaching the final pressure of this final pressure is kept constant for a short time, for example, several tens of seconds.

The critical outer diameter d _ak is an empirical value determined by appropriate preliminary tests. The critical outer diameter d _ak depends on the material of the pipe wall, on the thickness of the pipe wall and also on the original outer diameter of the pipe. Appropriately, the critical outer diameter d _{ak is} determined in preliminary tests for a particular metal tube, in which the desired plasticization of the metal tube is achieved and in particular no damage to the metal pipe, for example, by bursting or bursting occurs.

According to the invention, the outer diameter d _{a of} the tube or the changes in the outer diameter d _{a are} measured or determined. According to a preferred embodiment, the measurement of the outer diameter d _a takes place continuously. It is recommended that the outer diameter d _a be measured by a laser interferometric method. In principle, all measuring methods which allow a measurement of changes in the outer diameter d _a in the range from 0 to 50 μm are suitable for the measurement of the outer diameter d _{a of} the metal tubes. Preferably, the outer diameter d _{a is determined} by a non-contact measuring method. In principle, non-contact measuring methods can also be used.

A particularly preferred embodiment of the method according to the invention is characterized in that in addition to the outer diameter d _{a of} the tube and the applied pressure or internal pressure is measured. Conveniently, a continuous measurement of the pressure takes place. According to this preferred embodiment of the invention, therefore, both the pressure built up or changes in pressure and the outer diameter d _a or changes in the outer diameter d _a measured. - The dependence of the outer diameter d _a on the applied pressure is determined for a specific metal tube or for a specific pipe type in preliminary tests. It can be determined in which tolerance range the ratio outer diameter d _a / pressure must lie with error-free metal tubes. For further measurements on metal pipes of this type, material deviations in the pipe wall can then be detected in the event of deviations from the tolerance range. In this way, faulty pipes can be detected in a simple and inexpensive way and be removed from the production process. This embodiment thus allows effective quality control during autofrettage treatment.

A further very preferred embodiment of the method according to the invention is characterized in that a metal tube already subjected to at least a first autofrettage treatment is subjected to at least a second autofrettage treatment and by measuring the outer diameter d _{a of} the tube during the second autofrettage treatment the first autofrettage treatment or the degree of plastification of the tube wall in the first autofrettage treatment is determined or controlled. The first autofrettage treatment can be an autofrettage treatment according to the invention or else an autofrettage treatment according to the prior art. With the aid of the second autofrettage treatment, it can be recognized to what extent the metal tube has already undergone an autofrettage treatment and whether the degree of plasticization has taken place as desired or desired. This embodiment of the method according to the invention can thus be used to control metal pipes which have already undergone an autofrettage process. The embodiment will be explained in more detail below. It is within the scope of the invention that the pressure or internal pressure is built up in the metal tube by means of a liquid. Expediently, this is a liquid of low compressibility, preferably a hydraulic fluid, for example a hydraulic oil. Empfohlenermaßen the metal tube is closed at both ends and the build-up of the internal pressure by means of at least one pump.

Appropriately, the admission of the inner surface of the tube with a pressure or internal pressure of 700 to 15,000 bar, preferably from 2,500 to 9,000 bar and more preferably from 2,800 to 8,500 bar. A recommended embodiment is characterized in that the applied pressure or internal pressure is in the range between 5,000 and 8,500 bar. - Preferably, the pressure increase or the pressure build-up over a period of 2 to 25 s, preferably 2 to 20 s and preferably 2 to 10 s. After reaching the final pressure, this is expediently kept constant for a short time, in particular kept constant for 3 to 40 s.

Bei einem Druck p₂ von $$p_2=\frac{2{\mathrm{\sigma }}_F}{\sqrt{3}}\ln \left(r_a/r_i\right)$$

wäre theoretisch die gesamte Rohrwandung (100 %) plastifiziert. σ_F steht im Übrigen für die Spannung, bei der die elastischen Eigenschaften des Materials in Fließeigenschaften übergehen. r_i ist der Innenradius des Metallrohres und r_a ist der Außenradius des Metallrohres.A particularly preferred embodiment of the invention is characterized in that the pressure or internal pressure is increased with the proviso that a plastification of the inner wall portions of the tube is achieved according to a Plastifizierungsradius r _p , said Plastifizierungsradius r _p 50 to 92%, preferably 70 is up to 92% and preferably 75 to 92% of the outer radius r _{a of} the tube. The radii are measured from the center or from the longitudinal central axis of the tube. As already stated, the plasticization begins on the inner surface of the metal tube and propagates on further pressure increase to the outside. The plastication radius r _p indicates the extent of plasticization in the radial direction of the tube. - The plasticization of the metal begins by the way at an internal pressure p ₁ of: $${\mathrm{<figure-callout\; id="1"\; label="p"\; filenames="imgf0002.png"\; state="\{\{state\}\}">p</figure-callout>}}_1=\frac{{\mathrm{\sigma }}_F}{\sqrt{3}}(1-{\left(r_i/r_a\right)}^2$$

At a pressure p ₂ of $${\mathrm{<figure-callout\; id="2"\; label="p"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">p</figure-callout>}}_2=\frac{2{\mathrm{\sigma }}_{\mathrm{<figure-callout\; id="3"\; label="F"\; filenames="imgf0001.png"\; state="\{\{state\}\}">F</figure-callout>}}}{\sqrt{3}}\ln \left(r_a/r_i\right)$$

theoretically, the entire pipe wall (100%) would be plasticized. Incidentally, σ _F stands for the stress at which the elastic properties of the material change into flow properties. r _i is the inner radius of the metal tube and r _a is the outer radius of the metal tube.

It is within the scope of the invention that straight and / or bent metal pipes or metal pipes are treated with at least one bent pipe section with the autofrettage method according to the invention. Especially in bent pipes or bent pipe sections, the inventive method has proven particularly useful.

The invention is based on the finding that a particularly precise and functionally reliable control of the degree of plasticization is possible with the autofrettage method according to the invention. The process is characterized by simplicity and low effort. Incidentally, with simultaneous measurement of outside diameter and pressure, material defects or defective tubes can be detected in a very simple way and be removed from the production process. The method according to the invention also makes it possible to easily control already autofretted pipes. It should also be emphasized that the measures according to the invention with relatively low Cost can be realized. Compared to known from the prior art autofrettage method significantly less pipes are damaged in the inventive method and the inventive method is thus characterized by an advantageously low committee.

Fig. 1

einen Schnitt durch ein mit dem erfindungsgemäßen Verfahren behandeltes Metallrohr mit zwei unterschiedlichen Plastifizierungsgraden und

Fig. 2.

die Änderung des Druckes und die Änderung des Außendurchmessers d_a eines Metallrohres während der erfindungsgemäßen Autofrettage-Behandlung.

The invention will be explained in more detail with reference to a drawing showing only one exemplary embodiment. In a schematic representation:

Fig. 1

a section through a treated with the process according to the invention metal tube with two different degrees of plasticization and

Fig. 2.

the change in pressure and the change in the outer diameter d _{a of} a metal tube during the autofrettage treatment according to the invention.

The invention relates to an autofrettage method for increasing the strength of metal pipes 1. The interior 2 of the metal pipe 1 is subjected to a high internal pressure p and this pressure or internal pressure p is increased during the autofrettage treatment. In the exemplary embodiment, the internal pressure p is increased from 0 to about 5,500 bar (s. Fig. 2 ). Due to this pressure increase takes place starting from the inner surface 3 of the metal tube 1, a plasticization of the metal or the metal wall instead. The plasticized inner regions of the tube wall of the metal tube 1 are in the Fig. 1a and 1b characterized by a cross-hatching.

According to the invention, during the autofrettage treatment, the increasing outer diameter d _{a of} the metal tube 1 is measured ( Fig. 2 ). According to the invention Furthermore, the pressure increase after reaching a critical outer diameter d _{ak is} terminated ( Fig. 1b ) and the pressure is then reduced again ( Fig. 2 ). The FIGS. 1a and 1b show a treated with the novel autofrettage method metal tube 1 with two different degrees of plasticization. In the state of the metal pipe 1 according to Fig. 1b after pressure increase, the critical outer diameter d _ak has been reached, as well as the desired maximum degree of plasticization. The degree of plasticization can be specified by the plastication radius r _p . In the state of the metal pipe 1 according to Fig. 1b may the Plastifizierungsradius r _p 90% of the outer radius r _{a of} the metal tube 1 amount. Incidentally, r _i stands for the inner radius of the metal pipe 1. All radii are measured from the center M of the metal pipe 1.

Fig. 2 shows the pressure build-up or the course of the pressure p and the course or the change of the outer diameter d _a during the autofrettage treatment according to the invention. In the embodiment according to Fig. 2 A first autofrettage treatment is performed as well as a second autofrettage treatment as a check-up. The Fig. 2 can be taken that when pressure is applied to a metal tube 1, the changes in the outer diameter d _{a can be divided} into an elastic portion E and a plastic portion P. In view of this, with a second autofrettage treatment (right side of Fig. 2 ), to what extent the metal tube 1 has already been subjected to an autofrettage treatment and whether the degree of plasticization has been carried out according to or as desired. This embodiment of the method according to the invention can thus be used to control metal tubes 1 which have already undergone an autofrettage treatment.

Claims (9)

Autofrettage method for increasing the strength of metal pipes (1), wherein the interior (2) of a metal tube (1) with a high pressure or internal pressure p of at least 700 bar is applied, so that starting from the inner surface (3) of the metal tube (1 ) takes place a plasticization of the metal or the metal wall,
wherein the pressure or internal pressure p is increased during the treatment, wherein further during the treatment increasing outer diameter d _{a of} the tube is measured and wherein the pressure is subsequently reduced again. Autofrettage method according to claim 1, wherein the pressure increase is terminated upon reaching a critical outer diameter d _ak and the pressure is then reduced again. Autofrettage method according to one of claims 1 or 2, wherein in addition to the outer diameter d _{a of} the metal tube (1) and the applied pressure or internal pressure p is measured. Autofrettage method according to any one of claims 1 to 3, wherein an already at least a first autofrettage treatment subjected metal tube (1) at least a second autofrettage treatment according to one of claims 1 or 2 is subjected and wherein by means of the measurement of the outer diameter d _a of Metal tube (1) in the second autofrettage treatment, the extent of the first autofrettage treatment or the degree of plastification of the tube wall in the first autofrettage treatment is determined. Autofrettage method according to one of claims 1 to 4, wherein the pressure or internal pressure p in the metal tube (1) is constructed by means of a liquid. Autofrettage method according to one of claims 1 to 5, wherein the application of the inner surface (3) of the metal tube (1) with a pressure or internal pressure p of 700 to 15,000 bar, preferably from 2,500 to 9,000 bar and particularly preferably from 2,800 to 8,500th bar is performed. Autofrettage method according to one of claims 1 to 6, wherein the pressure increase or the pressure build-up over a period of 2 to 25 s, preferably 2 to 20 s and preferably 2 to 10 s. Autofrettage method according to one of claims 1 to 7, wherein the pressure p is increased with the proviso that a plastification of the inner wall portions of the metal tube (1) is achieved according to a Plastifizierungsradius r _p , said plastication radius r _p 50 to 92%, preferably 70 to 92% and preferably 75 to 92% of the outer radius r _{a of} the metal tube (1). Autofrettage method according to one of claims 1 to 8, wherein curved tubes or tubes are treated with at least one bent pipe section.

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