FR2944463A1 - Metal part reinforcing method, involves rolling roller on surface portion by relative displacement between roller and metal part so as to cause cold rolling of material superficial layer that is located under roller - Google Patents

Abstract

The method involves rolling a roller (16) on a surface portion (14A) by relative displacement between the roller and a metal part (14) while pressing and cooling the roller against the surface portion so as to cause cold rolling of a material superficial layer that is located under the roller. Maximum height of a profile is 5 to 250 micrometer. Total height of a primary profile is 8 to 300 micrometer. Another roller is in a shape of a revolution cylinder with a roll band having width higher or equal to width of the surface portion.

Description

The invention relates to a method of reinforcing a metal part having a rough surface portion. This method is intended, in particular, for the reinforcement of a part having a portion of gross surface of manufacture or roughened and, more particularly, a portion of gross surface of foundry or rough forging. In the present application, a part or a material is called "metallic" when it is made of metal, alloy and / or intermetallic compound. In addition, a part, a part surface or a part surface portion is said to be "manufacturing blank" when it has not been machined or subjected to any treatment, after the manufacture of the part. For example, a surface portion of a workpiece made by foundry, which has not been machined or subjected to any treatment, subsequent to demolding of the workpiece, is a portion of gross manufacturing surface. In this case, this surface portion is also called "foundry gross". Likewise, a surface portion of a forging workpiece, which has not been machined or subjected to any treatment, subsequent to forging of the workpiece, is a portion of a gross work surface. In this case, this surface portion is also called "rough forging".

A part, a part surface or a portion of part surface, which has undergone a rough machining, is called "roughed". For example, a surface portion of a fabricated part, which has only been deburred, is a rough surface portion. The raw metal parts of manufacture or blanks, and more particularly the raw parts of foundry or rough of forging, have for common point to present a bad state of surface. Likewise, a metal part made by casting, even when machined on part of its surface, retains portions of rough foundry surfaces which have a poor surface condition.

However, the physical factors associated with a poor surface state, such as the high roughness, the residual stresses and the elastic limit circumscribed to the surface layer of material, located beneath these surface portions, have a significant negative influence on the surface. the general mechanical performance of the part and, in particular, the fatigue strength of the latter.

Thus, it has been demonstrated that the fatigue resistance of a cast-off piece produced by sand-casting is significantly lower than the fatigue strength of the same part which has also undergone, after demoulding of the piece, a machining of its surface. Although being more favorable than sand casting, the shell molding also leads to a surface condition detrimental to the fatigue strength of the molded part. In the end, the disadvantages related to the poor surface condition of a part, reduce the potential for use of this part and this is particularly true for castings.

In this aspect of the role of the surface condition, it must be added that a part is rarely solicited at all points in the same way and that there are critical areas where the stresses to which the part is subjected are maximum. This is the case for certain part geometries (for example a groove, a concave connection radius, a shoulder, etc.) that cause local stress concentrations. In planar flexion, for a part having a reinforcement rib, the zone of the top of the rib is generally a zone of maximum stress. It is, therefore, in these areas of maximum stress that the part should be reinforced if one wants to improve the mechanical strength thereof.

Such local reinforcement of the part is sometimes achieved by providing a greater thickness of material in the areas of maximum stress. However, the piece obtained is heavier, which can be troublesome, especially in the field of automotive or aerospace where the weight of the parts used is a constant concern. The object of the invention is to propose a method which makes it possible to locally reinforce a metal part having a rough surface portion, so as to increase the potential of use of this part. This objective is achieved by a method of reinforcing a metal part having at least one rough portion, in which a roller is rolled on said surface portion by relative displacement between the roller and the workpiece, while pressing the roller against said surface portion so as to cause the work hardening of the surface layer of material located under the roller, these operations (ie exerting pressure on the workpiece by means of the roller and roll it on the surface portion) being made cold, that is to say without heating the room or the roller, which are therefore at room temperature. With this method, the surface state of said surface portion is improved by reducing the roughness thereof. The aforementioned disadvantages related to a poor surface condition are diminished. In addition, the hardening of the surface layer of the metal part, which consists in plastically deforming, cold, this surface layer has the effect of increasing its strength. In the end, the piece has a portion of less rough surface and, under this surface portion, a layer of reinforced surface material, which improves the overall mechanical behavior of the part, including its resistance to fatigue, and therefore its potential for use without the cost of manufacturing this part is increased too much. In addition, such a method can be implemented locally, on the areas of the surface of the part that require it and, more particularly, on the zones of maximum stress. Since the process is cold-rolling and uses relatively simple and small tools, it can easily be used in a workshop, for example a trimming workshop. This process can therefore be implemented in situ, at the place of manufacture of the part. It is therefore possible to dispense with the services of a contract caterer or, more broadly, a subcontractor to implement such a process. This is an advantage of this process compared to other known methods of surface treatment of a metal part, such as nitriding or induction quenching.

In general, the method can be applied to a part made of a material which during its manufacture (in particular by foundry) or its shaping leads to a rough surface. For example, it may be a piece of aluminum, aluminum alloy, magnesium, cast iron, steel, brass, bronzes, zinc alloy, etc. However, this method is not very suitable for materials that are too fragile or have a very high hardness. According to one embodiment, the rough surface portion of the part to be reinforced has a roughness such that: the average roughness difference Ra is between 1 and 40 μm; The average profile height Rz is between 4 and 150 μm; the maximum height of profile Rmax is between 5 and 250 μm; and the total height of the primary profile Pt is between 8 and 300 μm. The definitions and methods of measurement of roughness values Ra, Rz, Rmax and Pt are given in standard NF-EN-ISO 4287.

When the roughness values Ra, Rz, Rmax and Pt are below the aforementioned values, the method is of less interest because, the surface state of said surface portion being relatively satisfactory, the improvement of the general mechanical behavior of the piece after implementation of the process will be more limited.

When the roughness values Ra, Rz, Rmax and Pt are beyond the aforementioned values, the roughness of the surface portion is very important and the hollow profiles of this surface may not be reached by the roller so that the The action of the roller on this surface may be too uneven to be acceptable.

According to one embodiment, the method applies to a portion of a rough surface of manufacture or roughing, more particularly to a portion of a gross casting surface and even more especially to a blank casting piece. This is a preferred application of the process because there is today a real need to increase the potential of use of the castings, which the method of the invention allows to achieve without increasing too much. cost of manufacturing the final part. The type of relative movement between the roller and the workpiece is generally a function of the geometry of the surface portion to be reinforced. Generally, it is the part that is mobile and moves under the roller that does not move or moves slowly, or it is the roller is movable and moves on the piece that does not move or that moves slowly. In all cases, the roller turns around its axis of revolution when it rolls on the part. According to one embodiment, the roller is driven in a reciprocating translation movement, said surface portion is elongate, and the part is held in a fixed position so that the surface portion extends longitudinally in the direction of translation of the roller. According to another embodiment, the part has a symmetry of revolution and the roller is rolled on said surface portion by driving the part in rotation about its axis of revolution.

When the part has a symmetry of revolution (for example, when it is a tree, a circular plate, a rotor, a rim, etc.), the relative movement between the roller and the piece is preferably obtained by rotating the piece about its axis of revolution relative to the roller which remains fixed, at least during one or more complete turns of the piece on itself. When the piece has an elongated portion to be reinforced such as a rib, a groove, a connection or a veil (for example, when it is a suspension arm, a support, a lever, etc.) the relative movement between the roller and the workpiece is a translation movement and back and forth along said elongate portion. Furthermore, if we consider that said surface portion extends in a mean plane, denoted XY plane and defined by the intersecting orthogonal axes X and Y, and if we consider the height axis, denoted Z axis, as being a axis perpendicular to the XY plane, it will be noted that the method allows the reinforcement of an elongate part having different heights (Z axis) or inclined parts whose inclination remains below 40 ° for most materials according to their coefficient of friction and their rate of deformation. To a certain extent, the part to be reinforced may also include a change of curvature in its main plane (XY plane) but in this case the tooling is adapted, for example, by the addition of a pivot bearing device and a tray with ball rails located under the workpiece holding system. According to one embodiment, one or more of the following operations are carried out before rolling the roller on the surface to be reinforced: the roughness of the initial surface is measured based on the values of Ra, Rz, Rmax and Pt to ensure that the application of the method is of sufficient interest; the profile, the section and the surface area of the surface portion to be reinforced are determined as a function of the location of the maximum stresses that the part is caused to undergo under conditions of use; the trajectory of the roller is determined according to the aforementioned profile; the shape and the radius of the roller (or the radii of the roller when the latter is not in the form of a cylinder of revolution) are chosen, depending on the profile, the section and the area mentioned above. For the radius (s) of the roller two criteria are, if necessary, to be considered: the radius of curvature "r" possibly present on the path of the roller and the rate of acceptable deformation of the material during the penetration of the roller . It is generally ensured that, with respect to said radius of curvature "r", the radius "R" of the roller is in a ratio R / r at most equal to 0.8; the plasticity stress of the material is evaluated by a hardness or compression test; - a support is provided on which the piece will rest and be maintained; and when the roller or the workpiece is cycled, the number of passes required is determined. Furthermore, it will be noted that the method makes it possible to take account of the shape tolerances relative to the raw parts. Similarly, in the case of a rough surface, the method makes it possible to take account of the shape tolerances. In particular, for revolution symmetry pieces having a concentricity defect, the mounting of the roller is flexible so as to accompany such a geometric deviation. In this case, it is nevertheless preferable to lower the speed of rotation of the part. For example, the application of the method to a workpiece having a 0.6 mm concentricity defect is not a problem. For a casting, the passage on the joint plane is possible. For example, the application of the method to a part having a mating offset of 0.2 mm, is not a problem. According to one embodiment, said surface portion 25 is cleaned before application of the roller by removing any grain of sand or foreign bodies from this surface portion. An operation of grit removal or shot blasting is possible before application of the roller. In a more general manner, all cleaning processes such as tribofinishing, washing, brushing are applicable upstream of the passage of the roller, without contraindication. In the case where the cleanliness of said surface portion can not be guaranteed, the roller is associated with a brushing tool which precedes it during the relative movement between the workpiece and the roller. It will be noted that the passage of the roller causes a small reduction in the dimension of the part (i.e. a depression of the surface portion of the part). This dimension reduction can, if necessary, be taken into account during the design of the part. By way of illustration, the method can lead to a T6-treated alloy AISi7MgO6 alloy, to a decrease of 0.15 mm, and to EN-GJS-400-15 cast iron, with a decrease of 0.10 mm; The penetration of the roller into the material is a function of the degree of ductility of this material. In general, this penetration is controlled in force or in position. When the penetration of the roller is controlled in force, the control can be done using one or more strain gauges mounted on the head of the device carrying the roller. For example, a strain gauge measures the stress exerted by the roller along the axis of penetration thereof and another strain gauge measures the stress along an axis parallel to the rolling direction of the roller. When the penetration of the roller is controlled in position, the machine is provided with a displacement sensor and a programming box such as those which traditionally equip the machine tools. According to an embodiment, the force applied by the roller on the workpiece is controlled as a function of the compressive strength of the material constituting the workpiece and the stresses that one seeks to exert on the workpiece. The compressive strength of a material is generally known data, but if it is not, surface hardness tests can be performed to establish a good estimate thereof. According to one embodiment, the roller is mounted on a jack (for example, a hydraulic or electric cylinder) and thanks to a force sensor, via an electronic card, the force that is delivered by the jack is regulated and 25 finally framed between two setpoints (high and low). To determine these setpoint values, some preliminary tests on prototype parts can be carried out to characterize the portion of surface to be reinforced: measurement of the degree of deformation, the depth of penetration of the roller, the hardness, control of the appearance of the surface or possible measurement of the residual stresses by X-ray diffraction. The deformation rate and the hardness are two very practical parameters to be measured in the workshop and can be easily connected to the force delivered by the jack. According to an embodiment, in the case where the profile of the surface portion to be hardened comprises concave or convex shapes having a certain radius of curvature (for example a groove), the stress exerted by the roller and the depth penetration of the roller are dependent on the ratio between the radius of the roller and this radius of curvature. Then, one can use a resistive measuring system of the contact surface between the roller and the workpiece, to measure this contact surface and, for example, to follow its evolution as a function of the number of passes. In general, the effect of the passage of the roller on said surface portion is perceptible at several levels. In particular, a surface hardening is characteristic of the hardening phenomenon and the surface portion becomes smooth, its metallic luster increasing significantly. Since cold working is performed cold, the deformation rate of the material is generally low. However, the process can lead to significant deformations of the material. By way of illustration, in a T6-treated AlSi7MgO6 alloy, a deformation of 0.3 mm can be obtained. In an untreated AlSiMgO6 alloy, a deformation of 0.5 mm can be achieved. In an EN-GJS-500-7 cast iron, a deformation of 0.3 mm can be achieved. The passage time and the number of passes (or number of passes) of the roller on the same surface element must be framed otherwise the surface layer may be damaged. This damage can be a shear of the surface layers of material (resulting in the formation of metal flakes) or, more seriously, an appearance of cracks in the material. According to one embodiment, the reinforcement method 25 is associated with a bleed control step, carried out after the work-hardening, in order to guarantee the production of parts without defects. Other features, advantages and applications of the method of the invention will appear better on reading the detailed description which follows. This description refers to the appended figures. Figure 1 is a perspective view of an exemplary device for carrying out the method of the invention. Figures 2 and 3 show, seen from the front and seen in profile, an example of roller. Figure 4 is a schematic representation of another example of a device for carrying out the method of the invention.

Figures 5 and 6 show, seen from the front and in profile, an example of a roller. Figures 7 and 8 show, seen from the front and seen in profile, another example of a roller.

Figure 9 shows an example of application of the method of the invention. Figures 10 and 11 show another example of application of the method of the invention. The device 10 of Figure 1 is a machine tool comprising: - a frame 20; a holding system 12 of a part to be reinforced 14; a reciprocating drive system (not shown) of the holding system 12; and a jack 18 at the lower end of which is mounted a roller 16.

The holding system 12 is integral with the frame 20 and located in the lower part thereof. The part to be reinforced 14 is disposed on the upper face of the system 12 and is held in a fixed position relative to the system 12 by means of jaws 12A. The holding system 12 is such as to move the workpiece 14 in the horizontal plane XY, in the direct orthonormal coordinate system (X; Y; Z) shown in Figure 1. The displacement of the workpiece 14 parallel to the vector Y makes it possible to bring the reinforcing surface portion 14B of the piece 14 in front of the roller 16. A reciprocating drive system (not shown) is associated with the holding system 12 and makes it possible to drive the system 12 and the piece 14 in a translation movement back and forth parallel to the vector X. The cylinder 18 is integral with the frame 20 and located in the upper part thereof. The roller 16 is rotatably mounted about an axis A, parallel to the vector Y, this axis being carried by the lower end of the jack 18. The jack 18 moves the roller 16 along an axis parallel to the vector Z In the example, the part to be reinforced 14 extends longitudinally along a main direction B and has on its upper face a rib 14A. The piece 14 is a raw casting piece and the upper surface of the rib 14A is a rough surface portion 14B to reinforce. The part 14 is fixed to the system 12 so that the surface portion 14B extends parallel to the vector X. The reinforcement of the surface portion 14B is effected, cold (ie the part 14 and the roller 16 are at ambient temperature), by actuating the cylinder 18 to move the roller 16 downwards, parallel to the vector Z, and pressing against the surface portion 14B while the holding system 12 and the part 14 move under the next roller 16 a movement back and forth parallel to the vector X. The roller 16 therefore passes several times at the same point of the surface portion 14B. The roller 16 thus rolls on said surface portion 14B causing the work hardening of the surface layer of material located at the top of the rib 14A. Figures 2 and 3 show in detail the roller 16 of Figure 1. This roller 16 has on its tread a circumferential groove 16A of a profile complementary to that of the top of the rib 14A of the part 14. Figure 4 shows schematically another example of device 110 for implementing the method of the invention. This device 110 is used with a part 114 having a symmetry of revolution about an axis of revolution R. It comprises a system for driving the workpiece 114 in rotation about its axis R as symbolized by the arrow 130. at the orthonormal coordinate system (X, Y, Z) of FIG. 4, the axis R is parallel to the vector X. The device 100 comprises a jack 118. The roller 116 is mounted free to rotate about an axis A, oriented in parallel to the vector X, this axis A being carried by the free end of the jack 118. The jack 118 moves the roller 116 parallel to the vector Z, as symbolized by the double arrow 140. In addition, another system allows to move the cylinder 118 and the roller 116 parallel to the vector X, as symbolized by the double arrow 150.

In this example, the piece 114 has on its lateral surface a groove 114A. The surface of this groove constitutes a rough surface portion 114B to be reinforced. Reinforcement of the surface portion 114E is effected, cold, by actuating the jack 118 to move the roller 116 to the left in FIG. 4, parallel to the vector Z, and pressing it against the surface portion 114B 2944463 II

while the workpiece 114 is rotated about its axis R. The roller 116 is progressively moved parallel to the vector X while the workpiece 114 rotates about its axis R. The speed of movement of the roller 116 parallel to the vector X is however sufficiently low relative to the rotational speed of the workpiece that the position of the roller is substantially the same for several turns of the workpiece 114 on itself. Thus, the roller 116 passes several times at the same point of the surface portion 114B. The roller 116 rolls over the surface portion 114B causing the work hardening of the surface layer of material of the groove 114A. Figures 5 and 6 show in greater detail the roller 116 of Figure 4. As shown, the roller 116 has a convex tread 16B. The width of the roller 116 along the axis A and the radius of curvature of its tread are adapted according to the shape of the groove 114A. FIGS. 7 and 8 show another example of roller 216 having the shape of a cylinder of revolution, of axis of revolution A. This roller 216 is, for example, used to reinforce surface portions having a substantially plane profile. parallel to the axis A. It should be noted that the examples of roller 16, 116, 216, shown in FIGS. 2, 3, 5, 6, 7 and 8, can be used with any device 20 making it possible to implement the method of the invention. Thus, the rollers 16, 116, 216 may be mounted on a device of the type of FIG. 1 or of the type shown in FIG. 4. In order to illustrate the advantages of the method of the invention, it has been applied ci on two test pieces. The first test specimen (Test Piece 1) was made by casting, in shell, T6-treated AISi7MgO6. The second test specimen (specimen 2) was made in shell, by sand casting, of type EN-G.3S 500-7 cast iron. The specimens and the assembly made were similar to that of FIG. 4. In particular, each specimen had a circumferential groove on its lateral surface, the method having been used on the surface of this groove. The roller used was a rectified roll of hardened, nitrided steel. The roughness of the surface of the groove, more precisely the parameters Ra, Rz, Rmax and Pt, were measured before and after reinforcement. The values of these parameters in micrometers (μm) before reinforcement are given in the table below: Ra Rmax Pt Test specimen 1 0.8 4.6 6.8 8.1 Test specimen 2 12 35 36.5 46.5 Values of these parameters in micrometers (μm) after reinforcement are given in the table below: Ra Rmax Pt Test specimen 1 0.1 0.9 1.1 1.8 Specimen 2 0.2 1.7 1.9 2.7 There is therefore a clear decrease in the roughness of the treated surface portion.

In addition, the hardness of the surface (Vickers hardness test under reduced load, HVO006 or HV0.3) was also measured before and after reinforcement, as well as the depth of the surface layer of hardened material after reinforcement. The results of these measurements are given in the table below: Hardness before Hardness after Depth of treatment hardened layer treatment Specimen 1 90-100 120-140 Up to 1000 pm Specimen 2 200 320-340 Up to 1800 pm On therefore, there is a marked increase in the hardness of the treated surface portion by carrying out the process. Finally, simple tensile tests were carried out and it was found for both specimens that the 0.2% yield strength, denoted Rpo.2%, was significantly improved. Thus, for the Specimen 1, the Rpo.2% improvement was 90 MPa and for the Specimen 2, the Rpo.2% improvement was about 50 MPa. Fatigue resistance tests were also performed. For Specimen 1, the improvement in the endurance limit at 5,106 cycles was 42% and for the Specimen 2, the improvement in the endurance limit at 5,106 cycles was 75%.

It is therefore found that the method of the invention, by being implemented locally on a surface portion of the part, provides a general improvement in the mechanical behavior thereof. Figures 9 to 11 show two particularly interesting application examples of the method of the invention.

In the mechanical sector, it is common to have to use a metal part comprising an elongated arm having a specific section "H" or "I" with two opposite side faces each having two ribs located respectively along the two longitudinal edges of each side face. According to the method of the invention, this piece is reinforced by rolling said roller on at least one of these lateral faces; this roller having a tread with two circumferential grooves spaced from one another, the spacing between these two grooves corresponding to the spacing between said ribs, these grooves having a profile complementary to that of the ribs so that the The top of the ribs is pressed against the bottom of the grooves when the roller rolls on the side face of the arm. A piece 314 of the aforementioned type is shown in FIG. 9. It comprises a flat elongated arm 315 having an I-specific section with opposite lateral faces 315A (only the upper face appears in FIG. 8). These main faces 315A each have, along their longitudinal edges, two ribs 315B. These ribs 315B are separated by a substantially planar surface 315C. To reinforce this piece 314, use is made of a roller 316 having, on its tread 316B, two circumferential grooves 316A spaced apart from one another, the spacing between these two grooves 316A corresponding to the spacing between the ribs 315B of the arm 315 and these grooves 316A having a profile complementary to that of the ribs 315B, so that the bottom of the grooves 316A is pressed against the top of the ribs 315B when the roller 316 rolls on the main faces 315A of the arm 315. In a single operation, the vertices of the two ribs 315B are thus simultaneously reinforced by straining the surface layer of material of these peaks and reducing the roughness of the surface of these peaks. Since these vertices are stress concentration zones on this type of part, the overall mechanical strength of the part 314 is improved at a lower cost. In the mechanical sector, it is also common to have to use a part having a hole for receiving the threaded rod of a screw, this part being intended to be assembled to another part or to a frame by tightening said screw. In this case, the surface portion through which the hole serves as a support zone to the head of the screw, to a nut or to any other element cooperating with said rod, during tightening of the screw. The most frequently encountered problem with this type of part, especially when the piece is light alloy, is the matting of the material in said support zone. More specifically, in operation, the bearing zone eventually sinks under the effect of the pressures exerted on it and there then appears a clearance in the assembly. Such a game is detrimental to the life of the assembly and may, for example, lead to loosening of the screw. To solve this problem, the method of the invention is used by rolling the roller on said surface portion which serves as a support zone so that the surface layer of material located in this bearing zone is reinforced by work hardening. This makes it possible to harden and calibrate prior to plasticizing the support useful for assembly and, in operation, this bearing zone has no or less tendency to sink. In addition, the surface condition of this support zone is improved. This results in an improvement in the life of the assembly. According to one embodiment of the invention, use is made of a roller having a general shape of a cylinder of revolution, having a tread whose width is greater than or equal to that of said bearing zone. This makes it possible to increase the hardness and to improve the flatness of the bearing zone and, thus, to improve the contact between the assembled parts. FIGS. 10 and 11 show a piece 400 of the aforementioned type, having a boss 414. This boss 414 is intended to be pierced, the hole 419 thus formed being intended to receive the rod 452 of a screw 450. The piece 410 is intended to to be fixed to another part (not shown) by tightening the screw 450. During tightening, the head 454 of the screw 450 bears on the upper surface portion 4146 of the boss, as shown in FIG. to the invention and as shown in FIG. 10, this upper surface portion 4146 is reinforced and flattened before assembly by means of the roller 416 which is pressed and rolled on this upper surface portion 414B . The upper surface portion 414B may be reinforced before or after the drilling of the hole 419. It is preferably reinforced prior to the drilling of the hole 419. The roller 416 has a strip of

Claims (9)

REVENDICATIONS1. Method for reinforcing a metal part having a rough surface portion, characterized in that a roller (16) is rolled on said surface portion (14A) by relative movement between the roller (16) and the part (14). ), while pressing, cold, the roller (16) against said surface portion so as to cause the work hardening of the surface layer of material located under the roller. 10 2. Reinforcing method according to claim 1, wherein said surface portion (14A) has a roughness such that: the average roughness difference Ra is between 1 and 40 μm; the average profile height Rz is between 4 and 150 μm; The maximum profile height Rmax is between 5 and 250 μm; and the total height of the primary profile Pt is between 8 and 300 μm. 3. Reinforcement method according to claim 1 or 2, wherein said surface portion (14A) is rough manufacturing or rough. 4. Reinforcement method according to any one of claims 1 to 3, wherein said surface portion (14A) is cast. 25 5. Reinforcing method according to any one of claims 1 to 4, wherein the roller is driven in a reciprocating movement, wherein said surface portion (14A) is elongated, and wherein the piece (14) is held in a fixed position so that the surface portion (14A) extends longitudinally in a direction (B) parallel to the direction of translation of the roller. 6. Reinforcement method according to any one of claims 1 to 4, wherein the piece (114) has a symmetry of revolution and wherein the roller (116) is rolled on said surface portion by driving the piece ( 114) in rotation about its axis of revolution (R). The reinforcing method according to any one of claims 1 to 5, wherein said metal piece (314) comprises an elongate arm (315) having a specific "H" or "I" section with two side faces (315A). ) opposing each having two ribs (315B) located respectively along the two longitudinal edges of each side face (315A); wherein this piece (314) is reinforced by rolling said roller (316) on at least one of these side faces; and wherein said roller (316) has a tread (316B) with two circumferential grooves (316A) distant from each other, the spacing between these two grooves (316A) corresponding to the spacing between said ribs (315B), these grooves (316A) having a profile complementary to that of the ribs (315B) so that the top of the ribs is pressed against the bottom of the grooves when the roller (316) rolls on the side face (315A) of the arm . 8. A method of reinforcement according to any one of claims 1 to 6, wherein said metal part (400) has a surface portion (414B) to be traversed by a hole (419) itself intended to be traversed by the rod (452) of a screw (450), and in which said roller (416) is rolled over said surface portion (414B) which serves as a support zone at the head of the screw (454), a nut or any other element cooperating with said rod, during the tightening of the screw (450). 9. A reinforcing method according to claim 8, wherein a roller (416) having a general shape of a cylinder of revolution and having a tread whose width (L2) is greater than or equal to the width (Li) of said surface portion (414B).