ES2498540B2 - MANUFACTURING PROCEDURE OF MECHANICAL ELEMENTS OF HOLLOW GEOMETRY WITH SUBMICROMETRIC OR NANOMETRIC STRUCTURE - Google Patents

Abstract

Method of manufacturing mechanical elements of hollow geometry with submicrometric or nanometric structure. # The present invention relates to a method of manufacturing mechanical elements of submicrometric and / or nanometric grain structure, which have hollow interior geometry, and generically denominated. Like rings Said process preferably comprises the processing of preforms of a starting material by severe plastic deformation (SPD) by angular channel extrusion (ECAE), and the subsequent isothermal forging of said processed preform, by the application of a stamping matrix of Forged configured to apply a hollow geometry to the preform by its forging. The mechanical elements obtained by the process of the invention have improved mechanical properties in terms of greater strength, hardness or toughness, as well as improved functional properties, derived from its submicrometric and / or nanometric structure.

Description

DESCRIPTION

Manufacturing process of hollow geometry mechanical elements with submicrometric or nanometric structure.

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FIELD OF THE INVENTION

The present invention relates to the technical sector of the metalworking industry. More specifically, the invention is framed within the techniques of forming mechanical parts by means of isothermal industrial forging, from starting materials subjected to severe plastic deformation.

STATE OF THE PREVIOUS TECHNIQUE

At present there is a high interest in the development of submicrometric and / or nanometric grain structure materials, 15 since these structures have physical and functional properties (such as mechanical strength, elastic limit and hardness) that result attractive for its industrial application. That is why, in recent years, there has been a growing interest in the development of these materials, which has led to the proposal of various procedures for obtaining them. These procedures include the 20 angular channel extrusion techniques (in English, Equal Channel Angular Extrusion / Presing, ECAE / ECAP). The angular channel extrusion process (ECAE) was initially proposed by V.M. Segal (V.M. Segal et al., Russian Metallurgy, Vol. 1, 1981, pp. 99-105) and has been applied to a large number of materials, in order to obtain processed materials of submicrometric and / or nanometric structure. Some relevant examples of these techniques can be found in the following works: (R.Z Valiev et al., Progress in Materials Science, Vol. 45 (2) 2, 2000, pp. 103-189); (A. Azushima et al., CIRP Annals - Manufacturing Technology, Vol. 57, Issue 2, 2008, pp. 716-735) and (RZ Valiev et al., Progress in Materials Science, Volume 51, Issue 7, pp. 881-981).

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However, despite the large number of scientific works that exist in relation to the improvement of properties of materials that have been processed by severe plastic deformation (in English, "Severe Plastic Deformation", or SPD) the industrial applications of such materials They are much smaller.

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Although there are different works in the literature related to the forging of various metal alloys, there are few attempts included in the literature on pieces or billets of starting materials that have been deformed by SPD processes and subsequently subjected to some kind of process of forged. In this regard, the following should be noted: (PK Chaudhury et al., Materials Science and Engineering: A, 410-411, 2005, pp. 316-318, where a solid part is manufactured from material previously deformed by SPD), (JH Lee et al. CIRP Annals - Manufacturing Technology, Vol. 57 (1), 2008, pp. 261-264, which addresses the manufacture of a solid impeller), (WJ Kim et al., Materials Science and Engineering: A, 487 (1-2), 2008, pp. 360-368, which deals with the manufacture of a micro-gear of approximately one millimeter of radius, also solid, using in this case an extrusion process 45 ).

Unlike all the previous works, which are intended to obtain solid elements, the present invention is specifically aimed at obtaining mechanical elements or parts that have hollow interior geometry (referred to as said elements, in generic form, as "rings "), Preferably with symmetry of revolution with respect to its longitudinal axis, which have improved physical and functional properties compared to other elements with hollow interior geometry of the state of the art. As will be seen below,

Obtaining said elements by means of the process described in the present invention improves, in addition to the properties of the final pieces, also the conditions of the forging process itself compared to other prior art methods.

BRIEF DESCRIPTION OF THE INVENTION 5

The object of the present invention is to obtain improved mechanical elements, by applying materials obtained through SPD by ECAE to industrial techniques for producing mechanical elements, using these materials as starting material for isothermal forging processes that enable the obtaining 10 final elements with hollow interior geometry, and presenting submicrometric and / or nanometric grain structure. These mechanical elements include, among others, the following: rings, inner and outer housings for bearings, plain bearings, guiding rings, rigid couplings, flanges, lifting rings or friction rings, as well as any other mechanical elements which have a hollow interior section 15 and which are intended to have a submicrometric and / or nanometric structure.

The object of the present invention is achieved by a method of manufacturing hollow geometry mechanical elements, comprising the following steps:

 - process a preform of a starting material by SPD by ECAE; twenty

 - subjecting said processed preform to an isothermal slab, by applying a slab stamping matrix, configured to apply a hollow geometry to the preform by its slab.

With this method, improved hollow mechanical elements are achieved, as a consequence of its submicrometric and / or nanometric grain structure, which also makes it easier to obtain it, given that the forging of these materials is made more easily, given the improvement in the creep of the processed preforms, which allows forging even at room temperature.

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In a preferred embodiment of the invention, the preform comprises a selection of the following materials or alloys thereof, individually or in combination: aluminum, steel, magnesium, titanium. Said materials are suitable both for processing by ECAE, and for subsequent forging, being also materials of a high industrial application, whose properties improve significantly thanks to the process of the invention. In different embodiments of the invention, the preforms may have a solid or hollow cross section.

Preferably, the processing by ECAE and / or the isothermal forging can be carried out both at room temperature, and at a temperature other than room temperature. For this last case, said ECAE processing and / or said slab is carried out at a temperature preferably between 100 ° C and 600 ° C, this temperature range being suitable for obtaining rings, inner and outer housings for bearings, plain bearings , guide rings, rigid couplings, flanges, lifting rings or friction rings.

 Four. Five

In another preferred embodiment of the invention, ECAE processing is performed by applying two to eight extrusion passes. The application of a plurality of passes is effective for the homogenization of the processed material in the submicrometric or nanometric grain obtained. Said passes can be applied by different ECAE routes depending on the material processed, such as an extrusion route 50 of type B (consisting of rotating 90 ° to the preform in relation to the longitudinal axis thereof after each stage of extrusion in angular channel ), C (which comprises the rotation of the preform 180º on its extrusion axis between each pass) or Bc (comprising four passes, with a

180º turn on its longitudinal axis, after the first pass, followed by another 90º turn on said longitudinal axis, after the second pass, and a last 180º turn on the longitudinal axis, after the third pass). More preferably, the preform is processed at a speed between 10 and 100 mm / min.

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In different embodiments of the invention, the preform or the mechanical element obtained can be subjected to machining, as well as a selection of the following thermal treatments, individually or in combination: stress relief, recrystallization, precipitation hardening. This improves the structural and surface physical properties of the materials obtained in the different steps of the process, which results in the improvement of the properties of the final mechanical element. Said final element can also be subjected to surface treatments, such as anti-corrosion or anti-wear treatments.

On the other hand, the printing dies used in the present procedure can be used either alone or housed in matrix holders, depending on the specific characteristics of the applied process, or the printing press used, which can be, for example, hydraulic or mechanical Also, the stamping dies can comprise one or more ejectors for the extraction of mechanical elements, as well as being configured for the simultaneous forging of two or more mechanical elements in each stage of forging.

For the heating of the printing dies, it is possible to use means such as systems based on electrical resistance or systems based on electrical induction, the preforms can also be heated externally, to be subsequently introduced into the printing matrix.

For the improvement in the step of isothermal forging, it is also possible to apply a lubricant, such as Teflon spray, or graphite to the processed preform.

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Finally, another additional object of the invention is the hollow geometry mechanical elements obtainable from a process according to any of the embodiments described in the present application, which have a submicron or nanometric grain structure, and which preferably have an accumulated plastic deformation ( ε) between 0.5 and 100 and, more preferably, between 1 and 20. 35

DESCRIPTION OF THE DRAWINGS

Figure 1 shows a diagram of the stamping matrix and possible ring geometries to be obtained, from preforms of material that have been previously processed by severe plastic deformation processes, such as, for example, angular channel extrusion .

Figure 2 shows three examples of rings that can be obtained with the geometry shown.

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Figure 3 (a) shows a micrograph with the microstructure of the AA5083 before being processed by ECAE (initial grain size of approximately 200 µm). Figure 3 (b) shows said alloy after being processed twice by angular channel extrusion (ECAE), where the accumulation of deformation that has been imparted to the material can be observed.

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Figure 4 shows a simulation by finite elements of the plastic deformation () accumulated inside the rings, taking into account that part of the material that

it presents an initial deformation of ( = 2), and that corresponds approximately to that which would be obtained after two passes of ECAE (4a, 4b).

Figure 5 shows the microstructure obtained in the rings forged from the material previously processed by ECAE. As can be seen, two micrographs have been taken at 5 2500 magnifications (Figure 5a) and at 10,000 magnifications (Figure 5b). The rings correspond to the shape shown in Figure 4, once it has been cut, patterned and polished. It can be seen that there are grains of submicron size in the material obtained by isothermal forging.

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DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a process comprising the application of severe plastic deformation and, specifically, of angular channel extrusion, for obtaining highly deformed starting materials, with the aim of being subsequently used in isothermal forging processes. for the production of parts with hollow internal geometry. This procedure uses isothermal forging as a means for forming materials that have been previously processed by SPD by ECAE, in order to obtain mechanical elements of submicrometric and / or nanometric structure that have hollow interior geometry, generically referred to as “rings” . Said 20 mechanical elements present, thanks to their obtaining by means of the described procedure, mechanical properties superior to those obtained by conventional forging methods, such as, for example, greater mechanical resistance, greater elastic limit and greater hardness. Likewise, with the proposed procedure, mechanical elements are obtained that have a significant grain refinement, this being submicrometric and / or nanometric, with the consequent improvement in their functional properties. Likewise, this procedure is especially suitable for obtaining hollow pieces, since the preprocessing by ECAE to which the starting materials are subjected significantly improves their creep, which is especially advantageous for obtaining parts with revolution symmetry , as in most of the hollow parts used in the industry.

Preferably, to carry out the isothermal forging of the materials, a press is used in which stamping dies are housed, such as those shown, by way of example, in Fig. 1, where a plurality of different patterns can be seen in the die , which can give rise to pieces of different geometry (such as those shown in Figure 2). It should be noted, however, that it is possible to use any other type of geometry in the stamping dies for the manufacture of the rings, as required for the specific formation procedure.

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The starting material can come from an extrusion in angular channel in which it has been subjected to one or several passes of ECAE, carried out both at room temperature and at a temperature other than room temperature. Said material may be machined, in order to have preforms that fit into the cavities of the stamping dies for the realization of the isothermal slab. Four. Five

As indicated, when the material used for the manufacture of the rings has not been previously processed by SPD processes, it is based on a material that has a large initial deformation, being very difficult, if not impossible, its use for ring forging at room temperature (25 ° C). The 3 (a) shows the starting material without being deformed by ECAE. On the other hand, in Fig. 3 (b) the large accumulation of deformation bands in the material is observed after being processed twice by channel extrusion

angular to the AA5083 aluminum alloy starting material. Therefore, this isothermal forged patent of said materials is proposed.

Once said starting material is processed by SPD / ECAE, the rings will be stamped by means of isothermal forging, using specially designed matrices, similar to those shown in Fig. 1, and the geometry of said rings may vary. 5 matrices for the manufacture of different ring geometries. The manufacture of the rings may consist of one or more stages of forging, depending on the complexity of the ring that is intended to be manufactured as well as the mechanical properties of the material to be forged.

A simulation by finite elements of the forging of a ring is shown in Fig. 4, using two forging stages. It has been considered in the simulation by finite elements that the material presents an initial starting deformation of  = 2, which is approximately equivalent to the one that occurs after two ECAE passes. As indicated, it is possible to use a greater or lesser number of ECAE passes as well as to use different processing routes in said SPD process. The objective is to have a material that has a large accumulation of plastic deformation which will be used as raw material for the manufacture of the rings.

Once the rings have been manufactured, it may be necessary to perform machining and / or finishing operations on them in order to obtain the desired final geometry.

The temperatures at which the isothermal forging is made may vary, depending on the characteristics of the starting material and the geometry of the mechanical elements. Likewise, thermal treatments may be imparted to the materials that are to be forged in order to facilitate their conformation.

As indicated above, through the use of isothermal forging on material that has been previously processed by SPD by ECAE, the rings obtained have improved mechanical properties, while at a lower temperature than what would be necessary to use to manufacture said rings. with starting materials without having been previously processed by SPD, it may not even be possible to form materials that have not been previously deformed by SPD. Table 1 of this document shows comparative data of the microhardness values obtained on rings made of AA5083 aluminum alloy, previously deformed by ECAE at room temperature. Before the forging of these materials (“Previous tests”, in the table), they have been given two types of heat treatments: the first one consisting of arranging the materials (preforms of the AA5083 alloy, previously processed by ECAE and machined , in a treatment oven and increase the temperature from room temperature to 320 ºC, with a heating ramp of 12 ºC / min, followed by a rapid cooling in water.In the second case the preforms of the AA5083 have been arranged , previously processed by ECAE and mechanized, and the temperature has been increased from room temperature to 340 ºC with the same heating ramp of 12 ºC / min.

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Subsequently, said preforms ("Ring tests", in the table) have been processed by isothermal forging at 200 ° C, 250 ° C and 300 ° C, in two stages of conformation, as shown in Fig. 4. Also, in the
Fig. 5 shows the microstructure of the forged rings at 250 ° C. As can be seen, there is a significant refinement of the grain size, which is less than 1 µm. It should be noted that the starting material (AA5083) has an initial grain size of approximately 200 µm.

To exert the compression force necessary to process the material, a hydraulic press can be used in which the forging dies will be housed, as shown in Fig. 1, although the use of other types of presses would also be possible. The printing dies used can be as shown in Fig. 1, which have several cavities for the manufacture of different types of rings or could have a different number of 5 cavities depending on the capacity of the press and the type of geometry that you want to manufacture. These matrices may or may not be housed in matrix holders. As heating medium, heating resistors or inductors may be used, among other heating systems, so that from a setpoint temperature they are able to increase the temperature to perform the isothermal forging. Once every 10 stage of the process of forging the rings is finished, the extraction of the material will be necessary. To do this, matrices that have ejectors may be used or directly by mechanical means the manufactured parts (rings) can be extracted without the need for ejectors. Likewise, the process object of the present invention can be carried out at room temperature. fifteen

In addition, subsequent machining operations (drilling, turning, milling, etc.) and / or subsequent finishing operations, such as polishing and grinding, can be used, among others. Likewise, it is possible to subject the materials obtained by isothermal forging to subsequent heat treatments, such as stress relief, recrystallization 5 or precipitation, among others.

EXAMPLES OF EMBODIMENT OF THE INVENTION

Example 1: Isothermal forging at 250 ºC of alloy AA5083, previously processed by 10 extrusion in angular channel, for the manufacture of inner and outer bearing housings.

It is based on preforms of the AA5083 aluminum alloy that have been previously deformed by two angular channel extrusion passes (ECAE), according to a route C (which comprises the rotation of the preform 180 ° on its extrusion axis between each pass), 15 at a speed of 20 mm / min and at room temperature, using matrices in angular channels of 20 mm in diameter, with 90º of intersection angle between channels and 3 mm for the radii of agreement between channels. The preforms (or test pieces) are turned until leaving them with a diameter of 19 mm and a length of 43 mm. Subsequently, they are given a heat treatment consisting of keeping the preforms in a heat treatment oven until reaching a temperature of 340 ° C, with a heating ramp of 12 ° C / min. Once the temperature is reached they cool in water. The material thus obtained will be used for the manufacture of housings (inner and outer) of bearings by means of isothermal forging at 250 ºC, using two stages of isothermal forging. In the first one, a preform is obtained that will be re-stamped at the same temperature and in isothermal slab to obtain the 25 outer and inner bearing housings. Teflon spray is used as a lubricant in the operation. Subsequently, after finishing operations on the forged parts (elimination of the burr, drilling, turning and grinding) the inner and outer housings of the bearings that will have a submicrometric and / or nanometric structure will be obtained.

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Example 2: Isothermal forging at 275 ° C of alloy AA5083, previously processed by extrusion in angular channel at 275 ° C, for the manufacture of flanges.

It is based on preforms of the AA5083 aluminum alloy that has been previously deformed by means of four angular channel extrusion passes (ECAE), according to route B (consisting of rotating 90º to the preform in relation to its longitudinal axis after each stage of extrusion in angular channel), at a speed of 30 mm / min and at a temperature of 275 ºC, using matrices in angular channel of 50 mm in diameter and 5 mm for the radii of agreement between channels. The specimens are turned until they are 45 mm in diameter and 20 mm long. The material thus obtained will be used for the manufacture of flanges by isothermal forging at 40 275 ° C, using two stages of forging. In the first one, a preform is made, obtaining a preform that will be re-stamped at the same temperature and isothermal forging to obtain the flanges. Teflon spray is used as a lubricant in the operation. Subsequently, after finishing operations on the forged parts (elimination of the burr, drilling, turning and grinding) the flanges will be obtained that will have a submicrometric and / or nanometric structure.

Example 3: Isothermal forging at 100 ° C of alloy AA6063, previously processed by extrusion in angular channel at a temperature of 100 ° C, for the manufacture of a plain bearing. fifty

It is based on preforms of the AA6063 aluminum alloy that has been previously deformed by means of eight passes of angular channel extrusion (ECAE), according to route B, at a speed of 50

mm / min and at a temperature of 100 ºC, using matrices in angular channel of 22 mm in diameter, with 90º of intersection angle between channels and 2.5 mm for the radii of agreement between channels. The specimens are turned until leaving them with a diameter of 20 mm and a length of 80 mm. The material thus obtained will be used for the manufacture of bearings by means of isothermal forging at 100 ° C, using two stages. In the first one, a preform is made, obtaining a preform that will be re-stamped at the same temperature and isothermal forging to obtain the bearing. Teflon spray is used as a lubricant in the operation. Subsequently, after finishing operations on the forged parts (burr removal, drilling, turning and grinding), the bearings that will have a submicron and / or nanometer structure will be obtained. 10

Example 4: Isothermal forging at 400 ° C of a low carbon steel, previously processed by extrusion in an angular channel at 425 ° C, for the manufacture of a keyway.

It is based on preforms of a low carbon steel that has previously been deformed by means of two angular channel extrusion passes (ECAE), according to route C, at a speed of 30 mm / min and at a temperature of 425 ºC, using matrices in Angular channel 40 mm in diameter and 6 mm for the radius of agreement between channels. The specimens are turned until they are 36 mm in diameter and 40 mm long. The material thus obtained will be used for the manufacture of keyways by means of isothermal forging at 400 ° C, using three three stages. In the first two stages, an approach preform is made, obtaining a preform that will be re-stamped at the same temperature and isothermal forging to obtain the keyway. Graphite is used as a lubricant. Subsequently, after finishing operations on the forged parts, the keyways will be obtained which will have a submicrometric and / or nanometric structure. Finally, a heat treatment consisting of keeping the keyways for 3 hours at a temperature of 300 ° C will be applied.

Example 5: Isothermal ring forging at 550 ° C of a stainless steel (Fe / Cr18 / Ni8), previously processed by extrusion in an angular channel at 600 ° C, for the manufacture of rigid couplings. 30

It is based on preforms of a stainless steel (Fe / Cr18 / Ni8) that has been previously deformed by means of two angular channel extrusion passes (ECAE), according to route C, at a speed of 10 mm / min and at a temperature of 600 ° C, using Angular channel matrices with a diameter of 20 mm and 2.5 mm for the radius of agreement between channels, with 115 ° angle of 35 intersection between channels. The specimens are turned until leaving them with a diameter of 18 mm and a length of 40 mm. The material thus obtained will be used for the manufacture of rigid couplings by means of isothermal forging at 550 ° C, using three stages. In the first two stages, an approximation to the final form of the rigid coupling is obtained, which will be re-stamped at the same temperature and isothermal forging to obtain the same. It will be used as a graphite lubricant. Subsequently, after finishing operations on the forged parts, rigid couplings will be obtained which will have a submicrometric and / or nanometric structure.

Example 6: Isothermal forging at 150 ºC of the AA6026, previously processed by means of extrusion in angular channel at 150 ºC, for the manufacture of a lifting ring.

It is based on preforms of the AA6026 aluminum alloy that has been previously deformed by means of two angular channel extrusion passes (ECAE), according to route C, at a speed of 100 mm / min and at a temperature of 150 ° C, using matrices in the angular channel of 20 mm of 50 diameter, with 85º of intersection angle between channels and 3 mm and 4 mm for the radii of interior and exterior agreement between channels, respectively. The specimens are turned until leaving them with a diameter of 18 mm and a length of 20 mm. The material thus obtained is

used for the manufacture of rings by isothermal forging at 150 ° C, in a single stage of forging. Teflon spray is used as a lubricant in the operation. Subsequently, after finishing operations on the forged parts (elimination of the burr, drilling, turning and grinding), an aging heat treatment will be applied, consisting of maintaining the ring for 8 hours at a temperature of 125 ° C. With this, rings of elevation will be obtained that will have a submicrometric and / or nanometric structure.

Example 7: Isothermal forging at 230 ºC of the AA7075, previously processed by extrusion in angular channel at 270 ºC, for the fabrication of the outer casing of guide rings. 10

It is based on preforms of the AA7075 aluminum alloy that has been previously deformed by four angular channel extrusion passes (ECAE), according to BC route, at a speed of 60 mm / min and at a temperature of 270 ° C, using matrices in angular channel 40 mm in diameter, with 100º of intersection angle between channels and 4 mm and 3mm for the radii of interior and exterior agreement between channels, respectively. The specimens are turned until leaving them with a diameter of 18 mm and a length of 20 mm. The material thus obtained will be used for the manufacture of rings by means of isothermal forging at 230 ° C, using two stages. In the first one, a preform is made, obtaining a preform that will be re-stamped at the same temperature and in an isothermal floor for obtaining the guided ring. Teflon spray is used as a lubricant in the operation. Subsequently, after finishing operations on the forged parts (elimination of the burr, drilling, turning and grinding), the guiding rings will be obtained that will have a submicrometric and / or nanometric structure. Subsequently, a heat treatment consisting of keeping the rings for 12 hours at a temperature of 150 ° C will be applied. 25

Claims (26)

1.- Manufacturing process of a hollow geometry mechanical element, which comprises the following steps:  5  - process a preform of a starting material by severe plastic deformation (SPD) by angular channel extrusion (ECAE);  - subjecting said processed preform to an isothermal slab, by applying a slab stamping matrix, configured to apply a hollow geometry to the preform by its slab. 10 2. Method according to claim 1, wherein the preform comprises a selection of the following materials or alloys thereof, individually or in combination: aluminum, steel, magnesium, titanium.  fifteen 3. Method according to any of claims 1-2, wherein the processing by ECAE and / or isothermal forging is carried out at room temperature. 4. Method according to any of claims 1-2, wherein the processing by ECAE and / or isothermal forging is carried out at a temperature other than room temperature. twenty 5. Method according to claim 4, wherein the ECAE processing is carried out at a temperature between 100 ° C and 600 ° C. 6. Method according to claim 4, wherein the isothermal forging is carried out at a temperature between 100 ° C and 600 ° C. 7. Method according to any of claims 1-6, wherein the processing by ECAE is performed by applying between two and eight extrusion passes.  30 8. Method according to any of claims 1-7, wherein the processing by ECAE is carried out by means of at least one extrusion path of type B, type C or type Bc. 9. Method according to any of claims 1-8, wherein the ECAE processing of the preform is carried out at a speed between 10 and 100 mm / min. 35 10. Method according to any of claims 1-9, wherein the mechanical element with hollow geometry has symmetry of revolution with respect to a longitudinal axis. 11. Method according to any of claims 1-10, wherein the preform has a solid cross section. 12. Method according to any of claims 1-10, wherein the preform has a hollow cross section.  Four. Five 13. Method according to any of claims 1-12, wherein the mechanical element obtained is subjected to a selection of the following thermal treatments, individually or in combination: stress relief, recrystallization, precipitation hardening. 14. Method according to any of claims 1-13, wherein the preform or the mechanical element is subjected to machining. 15. Method according to any of claims 1-14, wherein the mechanical element is subjected to a surface treatment to prevent corrosion or an anti-wear coating treatment. 16. Method according to any of claims 1-15, wherein the stamping matrix is housed in a matrix holder. 17. Method according to any of claims 1-16, wherein the stamping die comprises one or more ejectors for the extraction of mechanical elements.  10 18. Method according to any of claims 1-17, wherein the isothermal forging is performed by a hydraulic press or by a mechanical press. 19. Method according to any of claims 1-18, wherein the isothermal slab comprises heating the stamping die, using a system based on electrical resistance or a system based on electrical induction. 20. Method according to any of claims 1-18, wherein the isothermal slab comprises the heating of the preformed processed externally, and its subsequent introduction into the stamping die. twenty 21. Method according to any of claims 1-20, wherein the isothermal slab comprises the application of a lubricant to the processed preform. 22. Method according to any of claims 1-21, wherein the stamping die is configured for the simultaneous forging of two or more mechanical elements at each stage of forging. 23.- Mechanical element of hollow geometry obtainable by means of a method according to any of claims 1-22, which has a submicrometer 30 or nanometric grain structure. 24.- Mechanical element according to claim 23, wherein the accumulated plastic deformation (ε) in said element is in the range 0.5 <ε <100.  35 25. A mechanical element according to claim 24, wherein the accumulated plastic deformation (ε) in said element is in the range 1 <ε <20. 26.- Mechanical element according to any one of claims 23-25, said element being a ring, an inner or outer bearing housing, a plain bearing, a guide ring, a rigid coupling, a flange, a lifting ring or a friction ring