The invention relates to metallurgy, and more precisely the manufacturing volume stainless steel parts from sheet metal, and to present material additions localized deposited after the possible shaping of sheets, such as reinforcement elements.

The manufacture of steel parts by a hot forming or cold (forging, casting, stamping, pressing ...) can lead to the production of shaped work more or less complex. It can happen, and we will see examples in the following description, these parts have, after shaping, a geometry that make their present areas where their mechanical characteristics would be insufficient for a given application envisaged. They would need, for this purpose, to be strengthened by the localized presence of thickened portions or ribs, or other similar function configurations.

One could consider introducing such extra thickness or reinforcing elements in the shaping of the piece itself, so to make it in one piece. However, this is not always possible for parts of relatively complex shapes and very precise dimensions, or when the price of complications in the manufacturing process (multiplication of formatting steps, and / or need for a important end machining for the configuration and desired precise dimensions) that would make the unbearable cost of production for large series of pieces.

Yet it is highly desirable to have such parts which are reinforced that where necessary, because we can and give them relatively small thickness over most of their volume and thus achieve material savings, so cost and weight, which are advantageous, for example for automotive parts: structural elements, suspension arms ... this can also help broaden the range of options from the main material of the part, in that light relatively simple configurations of the original piece (which will be called "support member" in the following text) not yet locally reinforced that allow this method, the mechanical properties of material use are the main criteria for this choice, and that theability of the material to be shaped in a complex way may not be a criterion for selection imperative.

It has therefore been devised to achieve these reinforcing elements, added locally, by direct deposition of molten metal on a support part initially formatted. This deposition can be carried out, typically in particular, using a laser, an electron beam or an electric arc, which are methods which perform the melting of the filler material just before or at the time of contact with the support metal. It is initially in powder, wire or ribbon. Figures 1 and 2 which will be commented below show the general principles of two such processes (powder jet melted by laser and wire melted by electric arc). They are similar in some respects, to the achievement of

It is thus possible to impart to the greater part of the support piece a required minimum thickness, coupled with a simple manufacturing process as possible (a drawing, for example). this support part reported by reinforcement elements, which are themselves formed by a relatively economical deposition process and dimensioned to only impart to the support piece minimal heavier one does complete a posteriori. Typically, the addition of reinforcing elements in the form of ribs or, in general, stiffeners, of the order of 1 mm in thickness is possible, that would not allow, or not readily, the forming processes piece of the final part, such as forging or molding.

However, one should know that as any thermal process, the molten metal addition on the support piece thermally affects a portion of the thickness of the support part in the vicinity of its surface, in the disposal areas of the molten metal. This Thermally affected zone (HAZ), as occurs in welding processes with filler material is modified in two ways:

There create a broadcast area of ​​the filler metal in the support piece (and vice versa), and you have to control this distribution so that it has no negative impact on the final part properties;

In and near the diffusion zone, there is a change in the microstructure of the support piece, which can also have adverse effects on the properties of the final part.

Specifically, in the case where this is applied molten metal adding method on a support part made of a steel with high mechanical characteristics obtained by a large presence of martensite, significant degradation was observed for their mechanical characteristics in the HAZ, as mainly a loss in hardness due to softening of the microstructure. This softening is linked to a

grain growth and / or a metallurgical transformation consisting of a transformation of the martensite of the support piece into ferrite and carbides. This is called reversion of martensite. In addition it can establish important residual stresses in the part having undergone the treatment, due to the different expansion characteristics of the different areas and materials that are in play.

A problem of fragility stiffening elements and added also arises frequently. When the part is stressed in bending or torsion, it is the parties who suffer the highest stresses. A minimum toughness for the weld metal is required, which is not always the case with the solidification structures obtained during the use of the methods by the addition of molten metal.

The object of the invention is to provide a method of manufacturing a finished part having a support part and parts added through a method of adding molten metal, such as reinforcement elements, which allows to remove or at least greatly reduce the risk of occurrence of the aforementioned problems.

To this end the invention relates to a method for manufacturing a final steel part having a support part and at least a portion formed by a method of adding a filler metal, as molten metal on a portion of the surface of the support part, forming a heat affected zone (HAZ) on the support part and a molten zone between the HAZ and the portion formed by the addition of molten metal, characterized in that:

- the support part is made of a chromium steel with a martensitic microstructure at 70-100%, preferably 90-100%, in the hardened state or back, the remainder of the microstructure is composed of ferrite, austenite and carbides and / or carbonitrides, whose composition, in percentages by weight, consists of:

* 0,01 %≤C≤ 1 .5% ;

* 0,01 %≤N≤0.2% ;

* 0,2%≤ Mn≤ 1 ,2% ;

* 0.2≤ Si≤ 1 ,2% ;

* traces≤ Al≤ 0,1 %

* traces≤ S + P≤ 0,05% ;

* 5,0%≤Cr≤ 16,5% ;

* traces≤ Ni≤ 3,5% ;

* Traces≤ W≤ Mo + 2,0%;

* traces≤ Cu≤ 3,0% ;

* Traces≤ Ti + Nb + Zr + V + Ta≤ 2%;

* traces≤ Co≤ 0,5% ;

* traces≤ Sn + Pb≤ 0.04%

* traces≤ B≤0.01 % ;

* The balance being iron and impurities resulting from preparation;

and meets the conditions:

A =% Mn +% Ni +% Cu + 30 * (% C +% N) - 3 * (% Ti +% Nb) ≥ 1 .5%

B =% Cr +% Mo + 5 * % V +% W +% Si +% AI≥ 9%;

- in that the composition of the filler metal before use consists of:

* 0,01 %≤C≤ 0.1 % ;

* 0,01 %≤N≤0.2% ;

* 0,2%≤ Mn≤ 2,0% ;

* 0.2≤ Si≤ 1 ,2% ;

* 15,0%≤Cr≤ 19,0% ;

* 6,0%≤ Ni≤ 13,0% ;

* Traces≤ W≤ Mo + 3,0%;

* traces≤ Cu≤ 3,0% ;

* traces≤ Co≤ 0,5% ;

* traces≤ B≤ 0,01 % ;

* traces≤ S + P≤ 0,05% ;

* Traces≤ Ti + Zr + Nb + V + Ta≤ 2%; of preference traces≤ Ti + Nb + Zr + V +

Ta≤ 1 ,0% ;

* traces≤ Sn + Pb≤ 0.04% ;

* The balance being iron and impurities resulting from preparation;

- in that the hardness of the HAZ is not lower by more than 20% than that of the remaining portions of the support part, and that the martensite content of the HAZ is greater than or equal to 70%;

- and in that the molten zone has a bypass ratio (% Ni (molten metal) -% Ni (metal carrier)) / (% Ni (filler metal) -% Ni (metal carrier)) 50 to 95 % by weight, preferably 75 to 85% by weight.

The addition of molten metal process may consist in an addition of metal powder melted by means of a laser beam or an electron beam.

The addition of molten metal process may consist in an addition of a molten metal from a wire whose melting point is caused by the establishment of an electric arc between the wire and the support part, or by a laser or by an electron beam.

The invention also relates to a final steel part characterized in that it has been manufactured by the above method, and in that at least one of the parties

formed by a molten metal addition method is a reinforcing member for the support part.

Comme on l'aura compris, l'invention consiste à combiner la réalisation de la pièce support en un acier martensitique à teneur élevée en Cr (5,0-16,5% ; donc il ne s'agit pas forcément d'un acier inoxydable) et de composition déterminée, et la réalisation des parties ajoutées par addition de métal fondu à l'aide d'un métal consistant en un acier inoxydable de composition initiale (avant son utilisation sous forme de poudre, fil, ruban ou autre dans le procédé de l'invention) également bien déterminée, et qui est, de façon surprenante, très différente de celle du métal constituant la pièce support.

En effet, le métal fondu ajouté est ici, obligatoirement, un acier inoxydable à

15,0-19,0% de Cr, qui contient donc le plus souvent plus de Cr que le métal de la pièce support. Et il contient aussi entre 6,0 et 13,0% de Ni, donc toujours nettement plus que le métal de la pièce support.

Les teneurs en autres éléments que Cr et Ni que doivent avoir les deux aciers utilisés sont également bien définies.

L'invention repose donc avant tout sur un choix particulier du couple de matériaux utilisés, dont on verra en quoi il est avantageux dans le contexte de la fabrication d'une pièce finale par dépôt direct de métal fondu sur une pièce support..

L'invention sera mieux comprise à la lecture de la description qui suit, donnée en référence aux figures annexées suivantes :

Figure 1 schematically shows the principle of a method of molten metal intake in the form of a liquid powder made by a laser beam;

Figure 2 schematically shows the principle of a method of molten metal intake in the form of a wire, the melting is effected by a welding torch;

Figure 3 which shows a fixing of a tube clamp, provided with stiffeners arranged on the circular portion of the flange and the collar by the method according to the invention;

Figure 4 which shows in cross-section along IV-IV of these stiffeners and its contact area with the circular portion of the flange;

Figure 5 shows the results of Vickers hardness HV1 measures (NF EN ISO 6507, 2006, 1 denoting the load in kgf) carried on the flange section and one of his stiffeners;

Figure 6 shows a micrograph of the connection zone between the flange and the stiffener;

Figure 7 shows a micrograph of a part of the same connection area, highlighting the HAZ and the molten zone;

Figure 8 shows a micrograph of the connection zone between the flange and the stiffener, on which are plotted results for Vickers hardness HV0.1 measures;

Figure 9 shows a micrograph of the connection zone between the flange and the stiffener, on which is indicated the points where we carried out dilution measurements of the stiffener of the material in the flange material;

Figure 10 represents a suspension arm cut and stamped on which stiffeners were added by the method according to the invention.

Figure 1 shows generally the principle of 3D printing on a metal support part 1 by supplying molten metal, more precisely by melting a metal powder 2 by means of a laser.

The support piece 1, that is to say the initial workpiece on which deposition is to occur, is fixed. Is projected on its surface by conventional means not shown, a stream of metal powder 2, which is intended to form the filler metal that will form the deposit 3 after solidification. 2 the powder supply source is scrolling relative to the surface of the workpiece support 1, in the plane of figure and from left to right in the example shown. It also projects onto the surface of the workpiece support 1 a laser beam 4, also in scroll so as to accompany the travel of the powder jet 2, and to achieve a melting of the powder 2 deposited on the substrate metal in the zone impact of the laser beam 4, so as to form a molten pool 5. The laser also causes partial and very superficial melting of the metal 1. The molten pool, and solidifies when it is no longer in the laser beam of the field 4 which is moved form the deposit 3 whose composition corresponds to that of the powder 2 or drift closely. This will be discussed in detail later. Under this deposit 3 is located in the vicinity of the surface of the support piece 1 and a thickness of the order of 300μηι a Thermally-affected zone (HAZ) 6 whose microstructure has been influenced by the contact with the laser beam 4 and the molten pool 5, so similar to what occurs during a welding filler material, with a morphology in layers also very close qualitatively to that observed when welding by supplying material. it is no longer in the laser beam 4 of the field that is moved form the deposit 3 whose composition corresponds to that of two or drift powder closely. This will be discussed in detail later. Under this deposit 3 is located in the vicinity of the surface of the support piece 1 and a thickness of the order of 300μηι a Thermally-affected zone (HAZ) 6 whose microstructure has been influenced by the contact with the laser beam 4 and the molten pool 5, so similar to what occurs during a welding filler material, with a morphology in layers also very close qualitatively to that observed when welding by supplying material. it is no longer in the laser beam 4 of the field that is moved form the deposit 3 whose composition corresponds to that of two or drift powder closely. This will be discussed in detail later. Under this deposit 3 is located in the vicinity of the surface of the support piece 1 and a thickness of the order of 300μηι a Thermally-affected zone (HAZ) 6 whose microstructure has been influenced by the contact with the laser beam 4 and the molten pool 5, so similar to what occurs during a welding filler material, with a morphology in layers also very close qualitatively to that observed when welding by supplying material. form the deposit 3 whose composition corresponds to that of two or drift powder closely. This will be discussed in detail later. Under this deposit 3 is located in the vicinity of the surface of the support piece 1 and a thickness of the order of 300μηι a Thermally-affected zone (HAZ) 6 whose microstructure has been influenced by the contact with the laser beam 4 and the molten pool 5, so similar to what occurs during a welding filler material, with a morphology in layers also very close qualitatively to that observed when welding by supplying material. form the deposit 3 whose composition corresponds to that of two or drift powder closely. This will be discussed in detail later. Under this deposit 3 is located in the vicinity of the surface of the support piece 1 and a thickness of the order of 300μηι a Thermally-affected zone (HAZ) 6 whose microstructure has been influenced by the contact with the laser beam 4 and the molten pool 5, so similar to what occurs during a welding filler material, with a morphology in layers also very close qualitatively to that observed when welding by supplying material.

2 shows generally the principle of 3D printing on a metal support part 1 using filler metal melted by means of a welding wire 7 or the like (e.g. tape) which is unwound in the direction of the supporting part 1 through a welding torch 8, which is itself carried by scroll relative to the workpiece

support 1 in the plane of figure and from left to right in the example shown. Conventionally, a power supply 9 is connected firstly to the support piece and secondly to the welding wire 7 by means of the torch 8, the interior space 10 is supplied with a protective gas flowing in the direction of the support part 1. This results in the formation of an electric arc between the end of the wire 7 and the support piece 1, so that the welding wire liquefies at its lower end 1 1 and the liquid droplets are deposited in layers (corresponding to liquid drops coming off the wire 7 of the support part 1 to form a molten pool 12. the latter solidifies as it exits the scope of the electric arc and form,

Alternatively, it would also be possible to ensure the fusion of the welding wire 7 or a tape of the same composition, by a laser beam or an electron beam.

It is of course highly desirable, if not essential, that all these operations are automated to the maximum, especially with regard to the frame rate of moving tools and mass flow of their supply filler metal powder wire , ribbon or other, which will determine the form that the reinforcing elements.

In Figures 1 and 2, there is shown a metal layer for providing uniform thickness, but it is of course not a generality, as will be seen in other figures.

These metal addition methods on a support piece are known in the prior art, and are disclosed herein as a reminder. In particular, the automation of operations is a usual practice in the implementation of this type of processes, and the present invention uses a similar manner to what is usually practiced.

Other processes, for example making use of an electron beam for melting the solder, are also known for this purpose or imaginable and the invention is independent, in principle, the specific choice process used.

The invention relies on a particularly advantageous choice of the couple formed by the compositions of the supporting part 1 on the one hand, and the second filler metal, the latter is initially in powder form 2, wire 7 tape or the like.

It should be understood that the composition of the filler metal, as it defines in the invention is one that exists before filing its merger and the workpiece support 1, and does not take into account the changes at least local that this composition may suffer during the operation, such a recovery of oxygen, resulting in formation of oxide inclusions and optionally decarburization, and a nitrogen recovery. These modifications may occur in particular if the operation does not take place in a completely inert atmosphere vis-à-vis the deposited molten metal.

Regarding the metal constituting the support piece 1, it must have a high proportion of martensite in the structure at the time of implementation of the method. This proportion is at least 70%, and preferably between 90 and 100%. In fact, this structure strongly or very predominantly martensitic, provides the support piece 1 high mechanical characteristics, which make the bulk of the part may be made of a relatively thin material, and that it is only locally its reinforcement stiffener is required. The remainder of the microstructure, if it is not 100% martensitic, consists of ferrite, austenite and carbides and / or carbonitrides.

De plus, sa température Ms de début de transformation martensitique doit être inférieure ou égale à 500°C et l'augmentation de volume du métal de la pièce support 1 durant cette transformation, à une vitesse de 30°C/s ou davantage, doit être comprise entre 2 et 6%. Cette température Ms et le changement de volume associé sont peu sensibles à la vitesse de refroidissement jusqu'à 2°C/s et le métal 1 est donc qualifié d'autotrempant.

Cette caractéristique est originale en ce que de telles dilatations relativement élevées se produisant lors de la transformation martensitique sont loin d'être une généralité pour les aciers qui auraient été susceptibles d'être utilisés pour la réalisation d'une pièce à hautes caractéristiques mécaniques. Cette forte dilatation est rendue nécessaire, dans le cadre de l'invention, pour compenser la contraction que subira le puits liquide 5, 12 de métal d'apport lors de sa solidification, de façon à assurer la bonne continuité de la matière formant le dépôt 3. La température Ms et la variation de volume associée sont déterminées, de préférence, de façon expérimentale, par exemple par des mesures dilatométriques comme cela est bien connu et décrit dans le Précis de métallurgie de J.Barralis et G.Maeder, AFNOR Nathan ISBN 2-09-194017-8.

L'acier formant la pièce support 1 doit également présenter une forte résistance à l'adoucissement, se traduisant par une faible diffusion des éléments carburigènes et du carbone. Concrètement, la dureté de la ZAT 6 ne sera pas inférieure de plus de 20% à celle des parties restantes de la pièce support 1 qui n'ont pas été influencées par l'apport de métal fondu. On obtient ainsi une homogénéité satisfaisante des propriétés mécaniques, qui ne sont pas trop dégradées dans la ZAT 6 par rapport aux propriétés nominales de la pièce support 1 .

3 shows a flange 13 for fixing a tube made by the method according to the invention. It consists of a preformed sheet metal by stamping to impart a generally circular shape 14, and is provided with a collar 15 surrounding a central orifice 16. The circular part 14 and the collar 15 are made in one piece when Formatting.

The collar 15 is, as is known, reinforced by stiffeners 17 (also called "strengthening elements") approximately rectangle-shaped triangle, which bear on the outer wall of the collar and on the upper face of the circular portion 14 of the flange 13. as in the example shown, the hypotenuse 18 of right triangles forming the stiffeners 17 may have, in fact, a concave shape, having a constant or variable curvature. Again, this feature is standard for such flanges 13 and not within itself of the invention.

By way of nonlimiting example, the flange 13 has a thickness of 3 mm, the circular portion 14 has a diameter of 145 mm, the orifice 16 has an outer diameter of 62 mm, the flange 5 has a thickness of 15 mm , stiffeners 17 have a length of 22 mm and a thickness of 0.7 to 1 mm, and the radius of curvature of their hypotenuses is 150 mm.

In Figure 4 shows, in cross section according to line IV-IV of Figure 3, a stiffener 17 and its contact area with the circular portion 14 of the flange 13. After the molten metal intake which led to the formation of the stiffener 17, there is on said circular portion 14, ranging from the upper surface 19 where the stiffener 17 to the bottom surface 20 and the right of the stiffener 17:

A "melt zone" 21 resulting from the dilution of a portion of the molten metal 5, the metal 12 in one of the circular portion 14 of the flange 13, and thus has on average a composition intermediate between those of these metals; - A HAZ 6 whose nominal composition is that of the metal of the support part, but within which there may optionally be noted localized changes related to possible privileged diffusion of certain elements inside the support part due to warming experienced from the molten metal supply, or, in its upper part, to a residual scattering of the molten metal; also, the metallurgical structure is changed more or less significantly compared to what it was prior to the deposition of molten metal, due to warming due to the deposit;

And an area 22 corresponding to the remainder of the circular portion 14 of the flange 13, which has not been substantially thermally and chemically affected by the operation of depositing the molten metal, and has retained its initial composition and metallurgical structure.

The inventors have found that according to the invention, the workpiece support 1 of steel should have the following composition, expressed in percentages by weight, coupled with a microstructure at least greatly martensitic (70 to 100% martensite, more preferably 90 100% martensite):

* 0,01 %≤C≤ 1 .5%

* 0,01 %≤N≤0.2%

* 0,2%≤ Mn≤ 1 ,2% ;

* 0.2≤ Si≤ 1 ,2% ;

* traces≤ Al≤ 0,1 %

* traces≤ S + P≤ 0,05% ;

* 5,0%≤ Cr≤ 16,5% ;

* traces≤ Ni≤ 3,5% ;

* Traces≤ W≤ Mo + 2,0%;

* Traces≤ with ≤ 3.0%;

* Traces≤ Ti + Nb + Zr + V + Ta≤ 2%;

* traces≤ Co≤ 0,5% ;

* traces≤ Sn + Pb≤0.04% ;

* traces≤ B≤ 0.01 % ;

* The remainder being iron and impurities resulting from smelting.

In addition, this composition must meet the following two equations A and B:

A =% Mn +% Ni +% Cu + 30 * (% C +% N) - 3 * (% Ti +% Nb) ≥1.5%

B =% Cr +% Mo + 5 * % V +% W +% Si +% AI≥ 9%.

Indeed, the satisfaction of the relation A is favorable to the fulfillment of the martensitic transformation, and the satisfaction of the condition B, by including the influence of Si and Mo, is favorable to good resistance to softening.

The composition of the martensitic steel used for the support 1 according to the invention is justified as follows.

Its C content is between 0.01% and 1, 5%.

The minimum content of 0.01% is justified by the need to obtain an austenitizing when the metal is heated to a temperature beyond 700 ° C and high mechanical properties to martensite. Over 1, 5%, the implementation by conventional methods is limited, especially resilience of the support becomes insufficient.

Its Mn content is between 0.2 and 1, 2%.

A minimum of 0.2% is required for austenitization. Above 1, 2% of oxidation problems are to be expected during the deposition if it is not carried out in a neutral or reducing atmosphere.

If its content is between 0.2% and 1, 2%.

If can be used as a deoxidizer during the development, as Al, which it can supplement or replace. A minimum amount of 0.2% is necessary because silicon is an element limiting the softening of the carrier 1 as it is affected thermally. Beyond 1, 2%, it is considered excessively promotes the formation of ferrite and therefore makes it more difficult austenitizing and obtaining a predominantly martensitic structure of steel. In excess of 1, 2%, it also weakens the support.

Sa teneur en S + P est comprise entre des traces et 0,05%, afin de garantir une faible contamination de la zone fondue 5, 12 et donc éviter une fragilité de la zone fondue 5, 12.

Sa teneur en Cr est comprise entre 5,0 et 16,5%. La teneur minimale de 5,0% se justifie pour assurer un caractère autotrempant pour le métal du support 1 . Une teneur supérieure à 16,5% rendrait difficile l'austénitisation et l'obtention d'une structure majoritairement martensitique.

Sa teneur en Ni est comprise entre des traces et 3,5%.

Un ajout de Ni n'est pas indispensable à l'invention. La présence de Ni dans la limite prescrite de 3,5% au maximum peut, cependant, être avantageuse pour favoriser l'austénitisation. Un dépassement de la limite de 3,5% conduirait cependant à une présence trop importante d'austénite résiduelle et à une présence insuffisante de martensite dans la microstructure après le refroidissement. Elle poserait aussi des problèmes de coût.

Sa teneur en Mo + W est comprise entre des traces et 2,0%.

La présence de Mo ou W n'est pas indispensable et Mo peut n'être présent que sous forme de traces résultant de l'élaboration. Cependant Mo limite l'adoucissement de la martensite de la ZAT lors du dépôt. Mo et W sont favorables à une bonne tenue à la corrosion. Au-dessus de 2,0%, l'austénitisation serait gênée et le coût de l'acier inutilement augmenté.

Sa teneur en Cu est comprise entre des traces et 3,0%, de préférence entre des traces et 0,5%.

Ces exigences sur Cu sont classiques pour ce type d'aciers. Dans la pratique, cela veut dire qu'un ajout de Cu n'est pas indispensable et que la présence de cet élément peut n'être due qu'aux matières premières utilisées. Une teneur supérieure à 0,5%, qui

correspondrait à un ajout volontaire, peut cependant aider à l'austénitisation. Au delà de 3% des problèmes de fissuration dans la zone fondue peuvent survenir.

Sa teneur en Ti + Nb + Zr + V + Ta est comprise entre des traces et 2%.

Ti est un désoxydant, comme Al et Si, mais son coût et sa moindre efficacité que celle de Al, à quantité ajoutée égale, rend son emploi en général peu intéressant de ce point de vue. Il peut avoir un intérêt en ce que la formation de nitrures et carbonitrures de Ti peut limiter la croissance des grains et influer favorablement sur certaines propriétés mécaniques et la soudabilité. Toutefois, cette formation peut être un inconvénient dans le cas du procédé selon l'invention, car Ti tend à gêner l'austénitisation du fait de la formation de carbures, et les TiN dégradent la résilience. Une teneur maximale de 0,5% est donc à ne pas dépasser.

V et Zr sont aussi des éléments susceptibles de former des nitrures dégradant la résilience. Zr, comme Ti, gêne l'austénitisation et c'est aussi une raison pour limiter sa présence.

Nb et Ta sont des éléments importants pour l'obtention d'une bonne résilience, et

Ta améliore la résistance à la corrosion par piqûre. Mais comme ils peuvent gêner l'austénitisation, ils ne doivent pas être présents dans des quantités dépassant ce que l'on vient de prescrire.

La condition Ti + Nb + Zr + V + Ta comprise entre des traces et 2% est la résultante de toutes ces considérations.

Sa teneur en Al est comprise entre des traces et 0,1 %.

Al est utilisé comme désoxydant lors de l'élaboration. Il ne faut pas qu'après la désoxydation il en subsiste dans l'acier une quantité dépassant 0,1 %, car il y aurait un risque d'avoir des difficultés à obtenir la microstructure martensitique.

Sa teneur en Co est comprise entre des traces et 0,5%. Cet élément est, comme

Cu, susceptible d'aider à l'austénitisation. Mais il est inutile d'en mettre davantage que 0,5%, car l'austénitisation peut être assistée par des moyens moins coûteux.

Sa teneur en Sn + Pb est comprise entre des traces et 0,04%. Ces éléments ne sont pas désirés car ils sont néfastes pour la solidification de la zone fondue.

Sa teneur en B est comprise entre des traces et 0,01 %.

B n'est pas obligatoire, mais sa présence est avantageuse pour la trempabilité. Son addition au-dessus de 0,01 % n'apporte pas d'amélioration supplémentaire significative.

Sa teneur en N est comprise entre 0,01 % et 0,2%. C'est un élément qui aide à l'austénitisation à partir de 0,01 %, mais au-delà de 0,2 % il limiterait la trempabilité.

Et, comme on l'a vu et pour les raisons qui ont été dites, les relations A et B doivent aussi être satisfaites.

La satisfaction des conditions sur Ms, la dilatation pendant la transformation martensitique et la dureté de la ZAT 6, dont on a vu qu'elles étaient des éléments importants pour la réussite du procédé selon l'invention, résultent automatiquement du couplage entre la composition et la microstructure telles qu'on les a définies.

Selon l'invention, la composition du métal d'apport 2, 7, appelé à constituer le métal fondu 5, 12 puis les dépôts 3 formant le ou les éléments de renforcement 17, doit répondre à la composition suivante :

* 0,01 %≤C≤ 0.1 % ;

* 0,01 %≤N≤0.2%

* 0,2%≤ Mn≤ 2,0% ;

* 0.2≤ Si≤ 1 ,2% ;

* 15,0%≤Cr≤ 19,0% ;

* 6,0%≤ Ni≤ 13,0% ;

* traces≤ Mo +W≤ 3,0% ;

* traces≤ Cu ≤ 3,0% ;

* traces≤ Co≤ 0,5% ;

* traces≤ B≤ 0,01 % ;

* traces≤ S + P≤ 0,05% ;

* traces≤ Ti + Nb + Zr + V + Ta≤ 2% ; de préférence traces≤ Ti + Nb + Zr + V + Ta≤ 1 ,0% ;

* traces≤ Sn + Pb≤ 0,04% ;

* le reste étant du fer et des impuretés résultant de l'élaboration.

Comme on l'a dit, il s'agit de la composition du métal d'apport 2, 7 sous forme solide (fil, ruban...) ou pulvérulente avant sa fusion et son dépôt sur la pièce support 1 .

Il s'agit d'un acier inoxydable de structure au moins majoritairement austénitique. La condition préférée sur la somme Ti + Nb + Zr + V + Ta aide à garantir qu'on aura cette structure majoritairement austénitique.

Cette composition doit d'abord conduire l'élément de renforcement 17 à remplir correctement son rôle lors de l'utilisation de la pièce finale. Il doit, pour cela, présenter une bonne ductilité se traduisant par un allongement à la rupture d'au moins 15%, de préférence entre 30 et 40%, et une structure métallurgique fine composée essentiellement d'austénite (au moins 80%), le reste étant de la ferrite et/ou des carbonitrures, avec une taille des grains inférieure à 300 μηι, une bonne tenue à la fatigue supérieure à 200 MPa et une bonne résistance K1 c > 50 MPa.m1/2 à la propagation des fissures entre -40°C et +80°C (selon la norme ISO 12135).

Pour des utilisations à des températures plus hautes ou plus basses que ces limites, la composition que l'on vient de citer serait aussi convenable, mais il est préférable que la teneur en C soit comprise entre 0,01 et 0,05% pour les utilisations à basses températures, afin d'avoir une austénite stable et une bonne ductilité de la martensite d'écrouissage éventuellement présente. Pour les utilisations à hautes températures, on privilégie les teneurs en C de 0,04 à 0,1 % pour améliorer la tenue à chaud. Pour les hautes températures, on peut recommander l'acier AISI 321 et l'acier AISI 304H, et pour les basses températures l'acier AISI 316L, l'acier AISI 305 et l'acier AISI 304L, du moins les aciers relevant de ces classes de nuances et qui ont, de plus, leurs compositions précises dans les limites précédemment citées.

In addition, this composition must guarantee, in conjunction with the choice of metal support 1, the dilution of contributed 2 or 7 metal in the metal support 1 can be carried out under conditions giving access to the results targeted by the invention in combination with the predominantly austenitic structure (at least 80%) of the molten zone 21, the composition according to the invention meets these criteria.

The molten zone 21, as we said, is an area where the two metals were subjected to a process of dilution. The filler metal 2 or 7 must represent 50 to 95% by weight, preferably 75 to 85% by weight.

The dilution ratio is calculated by the following formula:

% Dilution = (% Ni (molten metal 21) -% Ni (Metal support 1)) / (% Ni (filler metal 2 or 7) -% Ni (metal substrate 1))

Typically, this molten zone 21 extends to a depth of about 200 μηι to the right of the stiffener 17 in the example shown.

depositing 3) may be effective and do not present excessive fragility at the level of its junction with the circular portion 14 of the flange 13 (and, in general, with the support part 1). It is not thus found large fragile ferritic grains, no hot cracking, no sigma phase, and the hardness at this junction is in fact less than or equal to 350 HV1. Under this molten zone 21, there is the HAZ 6 with a depth of 300μηι that was mentioned previously. Its composition is nominally that of the support part 1 with the reservations which have been said about a possible diffusion of certain elements such as carbon or nitrogen, which can lead to small local variations in composition. Its hardness Hv1 is however generally decreased relative to the hardness Hv1 of the remainder of the support piece 1, a maximum of 20%, more preferably up to 10%. These limits also find themselves if they used other methods of hardness measurement.

This lesser hardness of the HAZ 6 relative to the remainder of the support piece 1 is due to warming suffered by the HAZ 6 when the molten metal deposition in contact with the molten pool 5, 12. When the temperature in the HAZ 6 exceeds 800 ° C, part of martensite can transform into austenite, and thus a softening of the microstructure. It would be very detrimental to the mechanical properties of the HAZ 6 that this alleviation persists, and it is necessary that during the cooling of the HAZ 6, a mainly martensitic structure is restored (at least 70% martensite), the percentage of martensite being, preferably higher in the HAZ 6 in that the support part 1 of the remainder to obtain a relatively high compressive residual stress state in the HAZ 6. This can be done if the Ms temperature of the martensitic transformation begins of the workpiece support 1 of metal is less than or equal to 500 ° C and above 100 ° C. The result is that the HAZ 6 actually shows a state of compressive residual stress, which is more favorable to the mechanical properties of the flange assembly 13, stiffener 17.

The choice of a austenitic grade to constitute one or more stiffeners 17, in connection with the martensitic shade of the support piece 1, is motivated by the presence of the molten zone 21 which occurs a diffusion of metal in the other . The fact that the provision of a liquid metal solidifying austenite is performed on a solid support with a martensitic structure limits the possibilities for diffusion and ensures that the molten zone 21 will be neither too large nor too fragile.

The method for forming of the stiffeners 17 (or any other form of reinforcing elements) by deposition of molten metal generally allows, advantageously, to let these raw reinforcement elements of solidification, without a machining operation or surfacing higher is necessary. The good appreciation of this feature depends largely on the accuracy with which the deposition operation is controlled by the control elements. But the devices known to deposit molten metal that were described above are already quite adept to achieve this accuracy, and the implementation of the invention does not cause more problems than those already encountered and solved by man skilled in the art.

5 shows the results of hardness measurements made on the flange section 13 and a stiffener 17 of Figures 3 and 4, as represented in Figure 4. The materials used are as follows.

To the flange 13, the metal composition is:

A = 3.958 and B = 12.923, the remainder being iron and impurities resulting from the smelting. The microstructure is 100% martensitic.

For the stiffener 17, the metal composition is as follows, the remainder being iron and impurities resulting from the smelting:

Its structure is austenitic more than 90%, typically 98%, the remainder being of delta ferrite. The particle size of the starting powder is between 45 and 90 μηι.

The method of forming the stiffener 17, which has been used is the deposition of molten powder by laser beam. A YAG laser of 600 W with a gas protection by argon was used. The deposition rate was 500 mm / min.

We performed measurements of the hardness Hv1 at various locations remote of 0.2 mm distributed over the axis of the longitudinal section of the stiffener 17, the height of the circular portion 14 of the flange 13 in the extension of the axis the stiffener 17, and up to 2 mm below the surface of the circular portion 14 of the flange 13 on either side of the stiffener 17, which in a molten zone 21 and three in the HAZ 6. was also performed measurements on the thickness of the flange in the vicinity of its periphery. Figure 5 shows the locations of measuring Hv1 hardness and hardness which were measured there.

It turns out that the metal constituting the circular portion 14 of the flange 13 has an average hardness of 386 Hv1. This hardness can have a certain dispersion, as is usual. The hardness measured in the HAZ 6 is only slightly below the average.

The metal constituting the stiffener 17 has a relatively uniform hardness between 158 and 192 Hv1, the highest value being measured at the base of the stiffener.

La dureté mesurée dans la zone fondue 21 est de 208 Hv1 , donc un peu supérieure à la dureté du raidisseur 17, ce qui tend à confirmer que la zone fondue 21 résulte de la diffusion du métal d'apport dans le métal de la pièce support, et que la proportion du métal d'apport y est majoritaire, dans le cas présent même très majoritaire comme cela est préféré pour une bonne solidarisation du raidisseur 17 et de la partie circulaire 14 de la bride 13.

Les figures 6 et 7 (celle-ci étant une partie grossie de la figure 6) présentent une micrographie de la partie inférieure du raidisseur 17 et de sa zone de raccordement avec la partie circulaire 14 de la bride 13, après une attaque chimique.

One distinguishes the stiffener 17 is composed of a superposition of layers of metal initially melted, a thickness of approximately 300-400 μηι each, and interpenetrating about 50% of their thickness. This strong interpenetration ensures that the stiffener 17 will not be particularly prone to breakage at an interface between layers. Note that in the case where not utilize a formation of the stiffener 17 by melted by laser powder deposition, but by a molten metal intake process by a wire 7 or a ribbon and a torch 8, one would find such superposition of layers, but a thickness may be greater, of the order of 1 mm.

It also segregates the molten zone 21 and the HAZ 6 whose thickness is 350 μηι about and surrounding the molten zone 21 over the entire periphery thereof, including to the surface of the circular portion 14 of the flange 13.

Figure 8 is a further enlargement of a portion of Figure 6, and shows the measurement results of the hardness Hv 0.1 (not the hardness Hv1 as in Figure 5, as the measurement points are here , closer together and in this case, in accordance with ISO 6705, is reduced the burden imposed) test on the longitudinal axis of the stiffener 17 in its extreme lower part, and, in the extension of this axis, the area melt 21, the HAZ 6 of the circular portion 14 of the flange 13 and a portion of the circular portion 14 of the flange 13 is not affected by the heat generated during the deposition of the metal stiffener 17. the measurement points are spaced 100 μηι.

It is observed that the results qualitatively confirm those of Figure 5 in refining. It is seen that the lower end of the molten zone, it has a hardness of 250 HV0.1, against about 200 HV0.1 in the stiffener 17 and the upper part of the molten zone, following the dilution of the metal contribution in the support part. Then, when you cross the HAZ 5 where there is no significant dilution of the filler metal in the metal flange 13, the hardness gradually increases, but it turns out that the hardness of the HAZ n is not lower by more than 20% at the highest hardness measured in the circular portion 14 of the flange 13, at depths out of the HAZ.

The dilution of the materials in each other was also measured in the 0 epitome of implementation of the invention. Figure 6 shows the places where quantitative analysis of the chemical composition by scanning electron microscopy were performed. The said points "spectrum 9, 10, 1 1 'are located on the stiffener 17 and are representative of its nominal composition. The said points "spectrum 15, 16, 17 are located in the circular portion 14 of the flange 13, and in a non-zone 5 chemically and thermally affected by the addition of molten metal, and are representative of the nominal composition of the flange 13. the said points "spectrum 12, 13, 14 'are located at the lower end of the molten zone 21, and one can infer the dilution of one material in the

The results of these analyzes are reported in Table 1 below. It is reported that the contents of main elements of Cr, Ni and Mo to assess the variation of the chemical composition in the different areas of the flange material 13 to the stiffener 17 of the material.

5 dilution Ni as defined above is taken as a reference because the Ni content is always frankly different in the two metals involved, is 78%. The dilutions of the other elements are also, in fact, not very different from that of Ni, which appears as quite representative of the overall dilution phenomenon.

0

Area fondue stiffener flange

Average spectrum spectrum Spectrum Spectrum Spectrum Spectrum Spectrum Spectrum Spectrum Average Average Dilution 9 10 11 12 13 14 15 16 17

Cr% 16,77 1 ,54 17,90 17,40 16,94 17,09 15,71 16,58 11,65 11 ,23 11 ,66 11 ,51 86%

Ni% 12,34 12,72 13,64 12,90 10,21 10,08 10,32 10,20 0,11 0,56 0,60 0,42 78%

Mo% 2,55 2,67 2,37 2,53 1 ,88 2,13 1 ,96 1 ,99 0,14 0,62 traces 0,25 76%

Table 1: compositions of metal measured in the stiffener and the flange as shown in FIG 9, and dilution of the flange in the stiffener

We will now describe a reference test, in which was used for the circular portion 14 c of the flange 13 a steel of the following composition, not according to the invention:

The metal is martensitic structure to 100% of a hardness 475 HV1 but do not meet condition B since A = 8.1%, and B = 0.5%.

For the stiffener 17: the composition and structure of the powder are identical and conditions similar deposits to what has been described for the assay of the invention.

The hardness in the HAZ 6 fall from 32% to 325Hv1 an maximum upper chute 20% which is typical of the invention, the microstructure is not sufficiently martensitic (60%) and is softened by formation of bainite / ferrite / pearlite. The martensitic transformation has not offset the withdrawal of the molten zone. The molten zone is predominantly austenitic with some martensite and shows a dilution Ni very close to 80%, which proves that the condition of a dilution of Ni of 50 to 95% is not a sufficient condition for the obtaining good results representative of the invention. HAZ therefore has too low mechanical strength due to insufficient compression of the base of the stiffener 17. Moreover, the martensite of the melted zone is of

The exemplary implementation of the invention that has been presented on a mounting flange of a tube is a mere example and not limitation. 10 shows a suspension arm 22 made from a cut and stamped preform 23, and to which was jousted stiffeners 24, 25, 26 (and others not referenced in Figure 10) by the method according to 'invention.

In general, the invention may find application in the field of manufacture of structural parts, including ground vehicles and aircraft, because it is easily possible thanks to it, to produce parts of different strength properties and optimized by weight from a single support part, merely by modulating the morphology of the reinforcing elements added by the method according to the invention.

CLAIMS

1 .- A method for manufacturing a final steel part having a base steel piece (1) and at least a portion (17) formed by a method of adding a filler metal (2; 7 ) in the form of molten metal (5; 12) on a portion of the surface of the support piece (1), forming a heat-affected zone (HAZ) (6) on the supporting steel (1) and a region melt (21) between the HAZ (6) and the portion (17) formed by the addition of molten metal (5; 12), characterized in that:

- the support part (1) consists of a chrome steel with a martensitic microstructure at 70-100%, preferably 90-100%, in the hardened state or back, the remainder of the microstructure is composed of ferrite, of austenite and carbides and / or carbonitrides, whose composition, in percentages by weight, consists of:

* 0,01 %≤C≤ 1 .5% ;

* 0,01 %≤N≤0.2% ;

* 0,2%≤ Mn≤ 1 ,2% ;

* 0.2≤ Si≤ 1 ,2% ;

* traces≤ Al≤ 0,1 % ;

* traces≤ S + P≤ 0,05% ;

* 5,0%≤Cr≤ 16,5% ;

* traces≤ Ni≤ 3,5% ;

* Traces≤ W≤ Mo + 2,0%;

* traces≤ Cu≤ 3,0% ;

* Traces≤ Ti + Nb + Zr + V + Ta≤ 2%;

* traces≤ Co≤ 0,5% ;

* traces≤ Sn + Pb≤ 0.04% ;

* traces≤ B≤0.01 % ;

* The balance being iron and impurities resulting from preparation;

and meets the conditions:

A =% Mn +% Ni +% Cu + 30 * (% C +% N) - 3 * (% Ti +% Nb) ≥ 1 .5%

B =% Cr +% Mo + 5 * % V +% W +% Si +% AI≥ 9%;

- in that the filler metal composition (2; 7) before use consists of:

* 0,01 %≤C≤ 0.1 % ;

* 0,01 %≤N≤0.2% ;

* 0,2%≤ Mn≤ 2,0% ;

* 0.2≤ Si≤ 1 ,2% ;

* 15,0%≤Cr≤ 19,0% ;

* 6,0%≤ Ni≤ 13,0% ;

* Traces≤ W≤ Mo + 3,0%;

* traces≤ Cu≤ 3,0% ;

* traces≤ Co≤ 0,5% ;

* traces≤B≤0,01 % ;

* traces≤ S + P≤ 0,05% ;

* Traces≤ Ti + Zr + Nb + V + Ta≤ 2%; of preference traces≤ Ti + Nb + Zr + V + Ta≤ 1, 0%;

* traces≤ Sn + Pb≤ 0.04% ;

* The balance being iron and impurities resulting from preparation;

- in that the hardness of the HAZ (6) is not lower by more than 20% than that of the remaining portions of the support piece (1), and that the martensite content of the HAZ (6) is greater than or equal to 70%;

- and in that the molten zone (21) has a dilution rate (% Ni (molten metal 21) -% Ni (Metal support 1)) / (% Ni (filler metal 2 or 7) -% Ni ( metal support 1)) 50 to 95% by weight, preferably from 75 to 85% by weight.

2. - Method according to claim 1, characterized in that the molten metal addition method (5) consists of an addition of metallic powder (2) melted by means of a laser beam (4) or a beam electron.

3. - Method according to claim 1, characterized in that the molten metal addition process consists of adding a molten metal (12) from a wire (7) whose melting is caused by the establishment an electric arc between the wire (7) and the support piece (1), or by a laser or an electron beam.

4.- final steel part characterized in that it has been manufactured by the method according to one of claims 1 to 3, and in that at least one of the parts formed by an addition of molten metal method (5; 12) is a reinforcing member (17; 24, 25, 26) for the support part (1; 22).