US20140373585A1

US20140373585A1 - Method of and Apparatus for Shot Peening Stainless Steel Tubing

Info

Publication number: US20140373585A1
Application number: US13/922,280
Authority: US
Inventors: Lani Ache
Original assignee: Foster Wheeler North America Corp
Current assignee: Amec Foster Wheeler North America Corp
Priority date: 2013-06-20
Filing date: 2013-06-20
Publication date: 2014-12-25
Also published as: WO2014203141A1

Abstract

A method of and an apparatus for shot peening an inner surface of a stainless steel tube. A set of at least two electromagnets is arranged around the stainless steel tube and has an iron core with an air gap adjacent to the stainless steel tube. At least one magnetic shot particle is provided inside the stainless steel tube. A controller controls the activation of the electromagnets according to a predetermined sequence, so as to cause the at least one magnetic shot particle to repeatedly hit the inner surface of the tube so as to shot peen the inner surface.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a method of and an apparatus for shot peening stainless steel tubing. The present invention especially relates to a method of and an apparatus for shot peening stainless steel superheater tubing of a supercritical or an ultra-supercritical steam generator.

DESCRIPTION OF THE RELATED ART

Improving the efficiency of steam generating power plants, especially, of supercritical or ultra-supercritical steam generator plants, by increasing the steam pressure and temperature is desired, for example, to minimize greenhouse gas emissions. Higher steam parameters give rise to higher demands for the quality of the superheater tubing, especially, for their high temperature strength and steam-side oxidation/corrosion resistance. Steam-side oxidation and the resultant exfoliation of iron-based scales cause unplanned shutdowns at power generation facilities. Stainless steel tubing with application dependent amounts of different alloy additions is usually used to achieve the desired properties. Sufficient chromium content in the alloy is important for improving the oxidation/corrosion resistance of the steam-side of the tube. Most of the improved oxidation resistance can be attributed to the presence of an FeCr₂O₄spinel oxide layer combined with a continuous Cr₂O₃(chromia) oxide layer formed near the steam-touched surfaces.
A technique used to increase the oxidation resistance of the stainless steel tubing is a surface modification technique that is called shot peening. This method involves inducing compressive stresses in the surface of the tube via impact of peening beads. This treatment causes the grain boundary density close to the surface (˜50 μm) to increase through cold deformation, and sets up favorable diffusion paths for Cr+ ions to the surface. This increases the chromium diffusion to the surface to produce a protective Cr₂O₃oxide layer and an FeCr₂O₄spinel oxide layer that does not exfoliate as easily as clues a magnetite oxide layer formed in chromium poor tube surfaces.
Shot peening of interior surfaces of stainless steel superheater tubing of a steam generator can be used to delay the onset of exfoliation deterioration that is often experienced in utility boiler applications. Shot peening not only increases the oxidation resistance, but it also increases the fatigue life, hardness/durability, closing of porosity, and stress corrosion cracking resistance.
Conventional shot peening of the interior surface of tubing is accomplished by delivering a shot medium, such as small steel balls or cut wire, by a high velocity pneumatic lance with a diverter nozzle on the end. In such a conventional shot peening method, as shown, tor example, in U.S. Patent Application Publication No. 2009/0071214, the shot travels parallel to the tube interior surface through the lance that is inserted into the tube, and is ejected at the end of the lance. Usually, a diverter plate deflects the shot at an incident angle towards the interior surface of the tube. To cover the whole inner surface of the tube, the lance is rotated and moved along the tube length. When using such a lance delivery system, only straight lengths of the tubing can be shot peened. Another weakness of the method is that the efficiency of the cold working imparted to the tube surface is decreased due to a relatively low incident angle of shot impact. A portion of the kinetic energy of the shot is also lost by friction along the lance, and by impact with the diverter plate. During the peening operation, the diverter plate experiences high wearing, which again, affects the efficiency of the cold working.
An object of the present invention is to provide a new method of and an apparatus tor shot peening stainless steel superheater tubing.

SUMMARY OF THE INVENTION

According to one aspect, the present invention provides a method of shot peening an inner surface of a stainless steel tube, the method comprising the steps of arranging a set of at least two electromagnets around the stainless steel tube, the electromagnets having an iron core with an air gap adjacent to an outer periphery of the stainless steel tube, providing at least one magnetic shot particle inside the stainless steel tube, projecting the at least one magnetic shot particle repeatedly against the inner surface so as to shot peen the inner surface, and performing the step of projecting by activating the electromagnets according to a predetermined sequence, so as to cause the at least one magnetic shot particle to repeatedly hit the inner surface.
According to another aspect, the present invention provides an apparatus for shot peening an inner surface of a stainless steel tube, the apparatus comprising a set of at least two electromagnets arranged around the stainless steel tube and having an iron core with an air gap adjacent to an outer periphery of the stainless steel tube, at least one magnetic shot panicle provided inside the stainless steel tube, and a controller for controlling the activation of the electromagnets according to a predetermined sequence so as to cause the at least one magnetic shot particle to repeatedly hit the inner surface so as to shot peen the inner surface.
A general principle of the present invention is that shot peening is actualized by a rapidly varying magnetic field in a cross section of the tube to be peened, so as to cause at least one magnetic shot particle inserted in the stainless steel tube to repeatedly impact the inner surface of the cross section of the tube at different positions. The magnetic shot particles are advantageously steel balls, but, generally, they can be any particles that are strongly attracted by magnets and have a suitable size, shape, hardness, and strength to be used as shot particles for shot peening the inner surface of stainless steel tubing. For simplicity, the magnetic shot particles are in the following examples, usually called steel balls, but generally, they do not have to be spherical, and the material does not have to be steel.
The varying magnetic field is brought about by leading suitable electrical currents through the solenoids of a set of electromagnets, so as to activate the electromagnets according to a predetermined pattern or sequence. Thereby, a system of electromagnets surrounding the stainless steel tube is used to effect a shot peened or a metallurgical cold worked finish to the inside surface of the tube.
In order to generate a suitable magnetic field within the stainless steel tube, the electromagnets advantageously have an iron core forming a magnetic circuit with an air gap. The ends of the iron core facing the air gap, which are simply said to be below, ends of the iron core, are arranged adjacent to the stainless steel tube, and are designed so as to give rise to a sufficiently large magnetic field inside the tube.
The electromagnets are activated according to a suitable sequence to maximize the acceleration of the steel bail or steel balls from one side of the stainless steel tube to the other, in order to cold work the interior wall of the tube by the impacts of the balls. By the electromagnets, it is possible to obtain nearly a direct impact of the steel balls towards the tube wall. Thereby, a maximum amount of kinetic energy is carried to the tube surface, resulting in a high shot peening efficiency.
Generally, a magnetic shot peening apparatus comprises at least two electromagnets. When using two electromagnets that are arranged on two sides of the tube, it is possible, in turn, to activate the electromagnets, so as to cause a steel ball to alternately hit the inner wall close to an end of the iron core of each of the electromagnets. Thereby, the inner surface will be shot peened locally near the ends of the iron cores. Due to varying paths of the steel ball or steel bails, the shot peened areas are not point-like, but they have an extent that depends, e.g., on the magnetic field profile. The most simple activation sequence of two electromagnets Includes activating the magnets in turns at a predefined frequency. The frequency, which does not have to be constant, van be optimized in a straightforward way. Ideally, the active magnet is switched each time the ball hits the surface of the tube. Another possibility is to utilize the elasticity of the collisions of the steel bails on the tube walls and to let the steel balls bounce a few times against a wall section before switching the magnetic field. Naturally, because of self induction in the coils, it is not possible to immediately change the magnetic field, but there is some slowness in the switching.
The magnetic field must be powerful enough to attract the at least one steel ball across the inside diameter of the tube, and accelerate the steel ball towards the inner wall of the tube. In order to generate an efficient magnetic field within the tube, the tube has to be of a non-magnetic material, such as of stainless steel, so as not to disturb the penetration of the magnetic field inside the tube. Naturally, the shot particles have to be of a magnetic or a magnetizable material, such as steel, to be attracted by the magnetic field. Such shot particles will then be attracted in a non-uniform magnetic field along the strongest magnetic field gradient towards the nearest end of the iron core.
In order to create a sufficiently high magnetic field strength and magnetic field gradients within the stainless steel tube, each of the electromagnets has to be a part in a carefully designed magnetic circuit. As mentioned above, the electromagnets advantageously comprise an iron core arranged through the solenoid and have an air gap adjacent to the stainless steel tube. Because the magnetic force accelerating a steel ball increases very strongly when the ball is close to an end of the iron core, it is important that the electromagnets are arranged around the stainless steel tube, so that, the ends of the iron cores are as close to the outer surface of the tube as possible. The ends of the iron core are advantageously converging, so as to locally increase the magnetic field and the magnetic field gradient, as well as the impact energy of the steel ball against the wall.
Generally, the shot peening is to be imparted uniformly, i.e., throughout the inner surface of the stainless steel tube. Therefore, the electromagnets have to be designed and operated so as to accomplish cold working of the whole inner surface of the stainless steel tube. Complete three hundred sixty degree coverage of an inside periphery of the tube can be obtained either by rotating a small number of electromagnets relative to the stainless steel tube or by arranging a sufficiently large number of electromagnets around the tube. To cover the length of the tube, the electromagnets can be moved at a suitable velocity along the tube. Another possibility is that the electromagnets are fixed, and the tube is moved through them at a suitable velocity.
According to a preferred, embodiment of the present invention, the shot peening apparatus comprises a circular frame with a set of more than two electromagnets arranged symmetrically at an outer periphery of the stainless steel tube. Preferably, the set of electromagnets comprises at least six, even more preferably, at least eight electromagnets arranged symmetrically around the tube. With such a set of multiple electromagnets, it is possible to reach a relatively uniform shot peening of a periphery of the inner surface of the tube by repeatedly activating the electromagnets according to a predetermined sequence. The surface of the periphery of the tube is shot peened by causing the steel bail or steel balls to have predetermined paths across the stainless steel tube, which give rise to impacts uniformly covering the whole periphery. In some cases, it may also be possible to activate the magnets in a random order. Preferably, a large number of electromagnets, such as eight electromagnets having sixteen iron core ends, are arranged around the tube, so that the iron core ends are distributed evenly over the periphery of the tube. The whole inner surface can then be shot peened by moving the frame with the set of magnets at a suitable speed along the tube.
Increasing the number of the electromagnets usually leads to decreasing of their size, which may have a drawback that the magnetic field strength is not high enough and the magnetic field does not sufficiently extend to the center portion of the cross section of the tube. Therefore, the number of electromagnets arranged on a single ring, or perimeter, around the tube, is usually, at most, ten. It is also possible, however, to arrange the electromagnets on two rings, or even more than two rings, which are arranged close to each other in a common circular frame. Thereby, it is possible to increase the number of the electromagnets to twelve or even higher, and still maintain the size of the magnets, and the distance of the ends of their iron cores, large enough that a sufficiently large magnetic field extends to the center portion of the stainless steel tube.
According to another preferred embodiment of the present invention, uniform shot peening is not based on increasing the number of the electromagnets, but rather, on rotating the stainless steel tube around its axis while moving the set of electromagnets relative to the tube in the direction of the axis of die tube. Thereby, it is possible to keep a rotating tube in a fixed position, and to move the set of electromagnets along the stainless steel tube, i.e., in the direction of the axis of the tube. Another possibility is to keep the electromagnets in a fixed position, and to move a rotating tube to be peened in the longitudinal direction, too.
According to a further preferred embodiment of the present invention, the stainless steel tube to be shot peened is completely fixed, and the shot peening is based on moving the set comprising a relatively small number of electromagnets, such as from two to four electromagnets, attached to a circular frame along the stainless steel tube, while the frame is rotated around the axis of the tube. The rotation of the frame can be continuous, but according to a preferred embodiment, the rotation is limited to a back-and-forth movement over an angle determined by the number of electromagnets. For example, when using four electromagnets, the angle is advantageously about ninety degrees. Naturally, if the number of electromagnets is large enough, such as eight, there is no need to rotate the frame.
By this method, it is also possible to shot peen curved tubes, i.e., tubes that are not straight. This is an important advantage, because the shot peening can thereby be performed when the stainless steel tube is already in its final shape. Because the cross section of a tube with, bends is generally not exactly constant, it may be advantageous to use a segmented frame of the electromagnets, by which the radial locations of the electromagnets can be varied. By using such a frame, it is possible to keep the ends of the iron cores close enough to the outer surface of the tube to be peened, even when the cross section of the tube is not constant.
The shot peening is advantageously performed when the stainless steel tube to be peened is in a horizontal direction, because, then, the steel balls do not tend to fall from the section of the magnetic field in the tube. On the other hand, it is also possible to perform the shot peening in a tube which is, instead of horizontal, for example, vertical. When shot peening non-horizontal tubes, the magnetic field has to be continuously high enough to prevent the balls from falling from the magnetic field.
The area peened by a single hit of the steel ball depends on the size of the ball. Very small shot particles, typically, of a size less than one millimeter, are commonly used in conventional shot peening methods. According to the present invention, the shot particles, steel bails, preferably, have a diameter larger than one millimeter, typically, from one millimeter to one centimeter. Preferably, the diameter of the steel balls is from two percent to twenty percent of the inner diameter of the stainless steel tube to be shot peened.
According to a preferred embodiment of the present invention, only one steel ball is used as a shot particle inside the stainless steel tube. When using only one steel ball, the diameter of the ball is advantageously from five percent to twenty percent of the internal diameter of the stainless steel tube. According to another preferred embodiment of the present invention, more than one, preferably, from two to five steel balls are used at the same time as shot particles. When using from two to five steel balls, the diameter of the balls is advantageously from two percent to ten percent of the internal diameter of the stainless steel tube.
An advantage of electromagnetic shot peening according to the present invention is that a high incident angle of impact of the steel shot is obtained and, thereby, ail kinetic energy developed in the acceleration of the steel shot by magnetic attraction is delivered during the impact to the tube wall. Also, there is no diverter plate that would require regular maintenance.
The above brief description, as well as further objects, features, and advantages of the present invention will be more fully appreciated by reference to the following detailed description of the currently preferred, but nonetheless, illustrative, embodiments in accordance with the present invention, when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OP THE DRAWINGS

FIG. 1 shows a schematic side view of a shot peening apparatus according to a first embodiment of the present invention.

FIG. 2 shows a schematic cross-sectional view (taken along section line 2-2 of FIG. 1) of the shot peening apparatus according to the first embodiment of the present invention.

FIG. 3 shows a schematic side view of a shot peening apparatus according to a second embodiment of the present invention.

FIG. 4 shows a schematic cross-sectional view (taken along section line 4-4 of FIG. 3) of the shot peening apparatus according to the second embodiment of the present invention.

FIG. 5 shows a schematic cross-sectional view (also taken along section line 4-4 of FIG. 3) of a shot peening apparatus according to a third embodiment of the present invention.

FIG. 6 shows a schematic side view of a shot peening apparatus according to a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The schematic diagrams of FIGS. 1 and 2 show a shot peening apparatus 10 arranged on a stainless steel tube 12, such as a superheater tube. FIG. 1 shows the apparatus 10 as a side view and FIG. 2 shows the same apparatus as a cross section across the tube taken along line 2-2 of FIG. 1. The shot peening apparatus 10 comprises two electromagnets 14, each having a solenoid 16 wound around an iron core 18. The magnetic circuit of the electromagnets includes a wide air gap 20 (FIG. 2) between the ends 22, 22′ of the iron core 18 adjacent to the outer surface 24 of the stainless steel tube 12.
By means of the electromagnets 14, a varying magnetic held 26 is created in the interior of the stainless steel tube 12 by using a controller 38 for leading a varying electrical current through the solenoids 16. By alternately activating the two electromagnets 14, a steel ball 28 arranged in the stainless steel tube 12 is caused to accelerate from one side of the tube 12 to the other, and to hit the tube 12 so as to shot peen the inner surface 30 of the tube.
The magnetic circuit of the electromagnets 14 is advantageously designed so as to obtain a sufficient magnetic field within the stainless steel tube 12. For that purpose, as is well-known to persons skilled in art, the number of turns in the solenoids 16 has to be high enough, and the iron core 18 has to be of a suitable soft ferromagnetic material. A special feature of the electromagnets 14 is that the shape of the iron cores 18 is designed so as to provide a suitable magnetic field within the tube 12. Therefore, the ends 22 and 22′ of the iron core 18 are shaped suitably to be positioned as close as possible to the outer surface of the tube 12, The tips 32 of the iron cores 18 facing the air gap 20 advantageously have a converging shape so as to concentrate, as much as possible, the magnetic flow lines within the tube 12.
According to a preferred embodiment of the present invention, the apparatus 10 comprises a rotating device 34 for rotating the stainless steel tube 12, in order to evenly shot peen a cross section of the tube 12. The apparatus 10 also comprises a transfer device 36 for moving the tube 12 longitudinally through the electromagnets 14 so as to cover the whole length of the tube 12, Alternatively, the electromagnets 14 can be moved by a transfer device (not shown in FIGS. 1 and 2), along the tube 12, to shot peen the whole length of the tube 12, or at least, a longitudinally extended portion of the tube 12. For that purpose, the electromagnets 14 are advantageously attached to a frame (not shown in FIGS. 1 and 2), which can be moved along the tube 12.
The schematic diagrams of FIGS. 3 and 4 show another apparatus 10′ for shot peening a stainless steel tube 12. FIG. 3 shows she apparatus 10′ as a side view and FIG. 4 shows the same apparatus as a cross section across the tube taken along line 4-4 of FIG. 3. The apparatus 10′ comprises eight electromagnets 14 that are arranged symmetrically around an outer periphery of the tube 12. Here, again, each of the electromagnets 14 comprises a solenoid 16 wound around an iron core 18. The magnetic circuits of each of the electromagnets 14 include an air gap adjacent to the stainless steel tube 12. Each of the electromagnets 14 can be activated by a controller (not shown) according to a predefined sequence so as to shot peen the inner surface of the tube 12 by one or more steel balls 28, 28′. Due to the large number of the electromagnets 14, it is possible to sufficiently evenly shot peen a cross section of the tube 12 without rotating the tube 12. However, in order to shot peen the whole length of the tube 12, naturally, the tube 12 has to be moved through the electromagnets 14, or the electromagnets 14 have to be moved along the length of the tube 12.
FIG. 5 schematically shows a cross-sectional view corresponding to FIGS. 2 and 4 of the third embodiment of the present invention for shot peening a cross section of a stainless steel tube 12. (FIG. 5 also shows a cross section across the tube taken along line 4-4 of FIG. 3.) The shot peening apparatus 10″ of FIG. 5 differs from that of FIGS. 1 and 2 in comprising a rotating device 54, here, symbolically, represented by wheels 54, to rotate a frame 40 to which the electromagnets 14 are attached. The number of electromagnets 14 is, here, as an example, four. A controller (not shown in FIG. 5) rotates the frame 40 either continuously or, preferably, back and forth by about ninety degrees, i.e., the angle between adjacent electromagnets. In the apparatus of FIG. 5, the number of electromagnets can be relatively small, and yet, it is not necessary to rotate the tube 12 to evenly shot peen a cross section of the tube 12. This apparatus 10″ is especially useful in applications in which shot peening is performed, to tubes 12 that are manufactured to a state, for example, bent to a shape, in which they can no longer be rotated.
To shot peen the whole length of a stainless steel tube 12, the frame 40, and the electromagnets 14 attached to the frame 40, are advantageously moved by a transfer mechanism (not shown in FIG, 5) along the tube 32. Because the cross section of a bent tube 12 is generally not exactly constant, the frame 40 preferably comprises, in a radial direction, movable segments 42 for movably attaching the electromagnets 14 to the frame 40. Thereby, the radial location of die electromagnets 14 can be adjusted to keep the electromagnets 14 as close as possible to the surface of the tube 12, while moving the frame 40 along the tube 12.
FIG. 6 schematically shows a side view of a fourth embodiment of the present invention for shot peening a stainless steel tube 12. This embodiment differs from the previous ones in that the frame for the electromagnets 36′ comprises multiple rings 44.1, 44.2, and 44.3, to each of which is attached multiple electromagnets 36′ (not shown in FIG. 6). FIG. 6 shows three rings, each of which has, for example, four electromagnets 36′. The number of rings can alternatively be two, or it can be more than three, such as four. Also, the number of electromagnets 36′ in each ring can be, for example, from three to six, or even more. The idea of the embodiment of FIG. 6 is that, instead of arranging a very large number of electromagnets 36 on a single ring, such as eight electromagnets 36 in the embodiment of FIG. 4, the electromagnets 36′ are divided between multiple rings. Thereby, space is provided to more freely optimize the size and shape of the electromagnets 36′, and, thereby, an especially good magnetic field profile can be obtained. The incident angles of the steel balls (not shown) hitting the inner surface of the tube 12 can be maintained in a suitable range, as long as the rings are sufficiently close to each other, and the activation sequence of the electromagnets 36′ is designed so that no tracks of the balls in the tube 12 are from one electromagnet 36′ to another that is at a too long distance along the tube 12 from the one electromagnet 36′.
On the basis of measurements made on test samples that were shot peened by the magnetic shot peening method of the present invention, the magnetic shot peening is at least as effective as conventional shot peening. In tests for stainless steel samples, an equivalent Vickers Micro-hardness of more than 450 Hv, down to a distance of about 50 μm from the surface, was obtained. This is clearly more than the corresponding reference Vickers Micro-hardness values, ranging from 330 to 380 Hv, obtained for the same material by conventional shot peening methods.
While the invention has been described herein by way of examples in connection with what are at present considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features, and several other applications that are included within the scope of the invention as defined in the appended claims.

Claims

We Claim:

1. A method of shot peening an inner surface of a stainless steel tube, the method comprising the steps of:

arranging a set of at least two electromagnets around the stainless steel tube, the electromagnets having an iron core with an air gap adjacent to the stainless steel tube;

providing at least one magnetic shot particle inside the stainless steel tube;

projecting the at least one magnetic shot particle repeatedly against the inner surface of the tube so as to shot peen the inner surface; and

performing the projecting step by activating the electromagnets according to a predetermined sequence so as to cause the at least one magnetic shot particle to repeatedly hit the inner surface of the tube.

2. A method of shot peening an inner surface of a stainless steel tube according to claim 1, further comprising the steps of rotating the stainless steel tube around its axis and moving the set of electromagnets relative to the stainless steel tube in the direction of the axis of the stainless steel tube.

3. A method of shot peening an inner surface of a stainless steel tube according to claim 2, further comprising the steps of attaching the set of at least two electromagnets in a frame and moving the frame in the direction of the axis of the stainless steel tube.

4. A method of shot peening an inner surface of a stainless steel tube according to claim 1, further comprising arranging a set of at least six electromagnets symmetrically outside an outer periphery of the stainless steel tube, wherein the stainless steel tube is not rotated.

5. A method of shot peening an inner surface of a stainless steel tube according to claim 4, wherein the set of multiple electromagnets comprises at least eight electromagnets.

6. A method of shot peening an inner surface of a stainless steel tube according to claim 1, further comprising the steps of attaching the set of at least two electromagnets in a frame, rotating the frame around the stainless steel tube, and moving the frame along the stainless steel tube.

7. A method of shot peening an inner surface of a stainless steel tube according to claim 6, further comprising rotating the frame back and form around the stainless steel tube.

8. A method of shot peening an inner surface of a stainless steel tube according to claim 1, further comprising arranging a set of multiple electromagnets on at least two adjacent rings outside the stainless steel tube, wherein the stainless steel tube is not rotated.

9. A method of shot peening an inner surface of a stainless steel tube according to claim 1, further comprising the steps of attaching the set of at least two electromagnets in a frame, and moving the frame along die stainless steel tube, wherein the stainless steel tube is not rotated.

10. A method of shot peening an inner surface of a stainless steel tube according to claim 9, wherein the frame comprises, in a radial direction, movable segments, and the method further comprises the steps of attaching the electromagnets to the movable segments, and moving at least a portion of the segments radially, while moving the frame along a curved portion of the stainless steel tube.

11. A method of shot peening an inner surface of a stainless steel tube according to claim 1, further comprising the step of providing at least two magnetic shot particles inside the stainless steel tube.

12. An apparatus tor shot peening an inner surface of a stainless steel tube, the apparatus comprising:

a set of at least two electromagnets arranged around the stainless steel tube and having an iron core with an air gap adjacent to the stainless steel tube;

at least one magnetic shot particle provided inside the stainless steel tube, and

a controller for controlling the activation of the electromagnets according to a predetermined sequence, so as to cause the at least one magnetic shot particle to repeatedly hit the inner surface of the tube so as to shot peen the inner surface.

13. An apparatus for shot peening an inner surface of a stainless steel tube according to claim 12, further comprising a rotating device for rotating the stainless steel tube around its axis, and a transfer device tor moving the set of electromagnets relative to the stainless steel tube in the direction of the axis of the stainless steel tube.

14. An apparatus for shot peening an inner surface of a stainless steel tube according to claim 13, further comprising a frame, in which frame, the set of at least two electromagnets is attached, and which frame is movable in the direction of the axis of the stainless steel tube.

15. An apparatus for shot peening an inner surface of a stainless steel tube according to claim 12, further comprising a set of at least six electromagnets arranged symmetrically outside an outer periphery of the stainless steel tube, which stainless steel tube is fixed.

16. An apparatus for shot peening an inner surface of a stainless steel tube according to claim 15, wherein the set of multiple electromagnets comprises at least eight electromagnets.

17. An apparatus for shot peening an inner surface of a stainless steel tube according to claim 12, further comprising (i) a frame, in which frame, the set of at least two electromagnets is attached, (ii) a rotating device for rotating the frame around the stainless steel tube, and (iii) a transfer device for moving the frame along the stainless steel tube.

18. An apparatus for shot peening an inner surface of a stainless steel tube according to claim 17, wherein the rotating device is arranged to rotate the frame back and forth around the stainless steel tube.

19. An apparatus for shot peening an inner surface of a stainless steel tube according to claim 12, further comprising a set of multiple electromagnets arranged on at least two adjacent rings outside the stainless steel tube, which stainless steel tube is fixed.

20. An apparatus for shot peening an inner surface of a stainless steel tube according to claim 12, further comprising (i) a frame, in which frame, the set of at least two electromagnets is attached, and (ii) a transfer device for moving the frame along the stainless steel tube, which stainless steel tube is fixed.

21. An apparatus tor shot peening an inner surface of a stainless steel tube according to claim 20, wherein the frame comprises, in a radial direction, movable segments for movable attaching the electromagnets to the frame.

22. An apparatus for shot peening an inner surface of a stainless steel tube according to claim 12, further comprising at least two magnetic shot particles inside the stainless steel tube.