EP3494242A1

EP3494242A1 - Method for manufacturing a steel part, including the addition of a molten metal to a supporting part, and part thus obtained

Info

Publication number: EP3494242A1
Application number: EP16757349.2A
Authority: EP
Inventors: Pierre-Olivier Santacreu; Aurélien BUTERI; Jérôme BRIDEL
Original assignee: Aperam SA
Current assignee: Aperam SA
Priority date: 2016-08-03
Filing date: 2016-08-03
Publication date: 2019-06-12
Also published as: US20190160602A1; RU2019102734A3; RU2019102734A; KR20190034222A; CN109563602A; WO2018025063A1; BR112019001807A2; JP2019532816A; MX2019001258A; CA3032268A1

Abstract

The invention relates to a steel part including a supporting part (1) and a portion (17) formed by a solder (2; 7), in the form of molten metal (5; 12), on the supporting part (1), forming a heat affected zone (HAZ) (6) thereof and a molten area (21) between the HAZ (6) and the portion (17) formed by the addition of molten metal (5; 2). The supporting part (1) is made of 70-100 % steel with martensitic microstructure, the composition of which 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 ≤ Mo + W ≤ 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 remainder being iron; and complies with the conditions: A = % Mn + % Ni + % Cu + 30*(% C + % N) - 3*(% Ti + % Nb) ≥ 1.5 %; B= % Cr + % Mo + 5*% V + % W + % Si + % Al ≥ 9 %. The composition of the solder (2; 7) prior to being used 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 ≤ 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 %; traces ≤ Sn + Pb ≤ 0.04 %; the remainder being iron. The hardness of the HAZ (6) is no more than 20 % lower than that of the rest of the supporting part (1), and the martensite content of the HAZ (6) is no less than 70 %. The molten area (21) has a dilution rate of 50 wt % to 95 wt %, preferably of 75 wt % to 85 wt %. The invention also relates to a finished steel part thus produced, at least one of the portions formed by a process for adding molten metal (5; 12) being a reinforcement element (17; 24, 25, 26) for the supporting part (1; 22).

Description

Method of manufacturing a steel part comprising the addition of a molten metal on a support part, and part thus obtained

The present invention relates to metallurgy, and more specifically the manufacture of stainless steel volume parts from sheets, and to have localized additions of material, deposited after the possible shaping of the sheets, such as reinforcing elements.

The manufacture of steel parts by hot or cold forming (forging, molding, stamping, stamping, etc.) can lead to parts of more or less complex shapes. It can happen, and we will see examples in the description that follows, that these parts have, after their formatting, a geometry that make them present areas where their mechanical characteristics would be insufficient for a given intended application. They would need, for this purpose, to be reinforced by the localized presence of overthickness, or ribs, or other types of configurations of similar function.

One could consider introducing such extra thicknesses or reinforcing elements during the shaping of the part itself, so as to achieve it in one piece. However, this is not always possible for parts of relatively complex shapes and very precise dimensions, or at the cost of complications in the manufacturing process (multiplication of the shaping steps, and / or the need to proceed to a important final machining to obtain the desired configuration and precise dimensions) that would make the production cost unbearable for large series parts.

However, it is highly desirable to have such parts that are reinforced only where necessary, because they can thus confer relatively small thicknesses on the bulk of their volume and thus save material, therefore cost and weight, which are advantageous, for example for automotive parts: structural elements, suspension arm ... This may also allow to widen the possibilities of choice of the main material of the part, in that taking into account relatively simple configurations of the initial part (which will be called "support piece" in the rest of the text) not yet reinforced locally that would allow this process, the mechanical properties of the material in use would be the main criterion of this choice, and that the ability of the material to be shaped in a complex manner may no longer be a criterion of imperative choice.

It has therefore already been imagined to make these reinforcement elements, added locally, by a direct deposit of molten metal on a support part initially set form. This deposition may be carried out, typically and in particular, using a laser, an electron beam or an electric arc, which are processes that melt the filler material just before or after moment of its contact with the support metal. This is initially in the form of powder, wire or ribbon. Figures 1 and 2 which will be discussed below show the general principles of two such processes (jet of powder melted by laser and electric arc melted wire). They are similar, in certain points of view, to the realization of a welding by a process with contribution of material by the metallurgical mechanisms which are put into play, in particular for the joining of the support part and the element of reinforcement, and, in other respects, a 3D printing, with supply of metallic material in that it is intended to give the reinforcing elements of precise shapes and dimensions.

It is thus possible to give the greater part of the support part a minimum necessary thickness, combined with a manufacturing process as simple as possible (stamping, for example). This support piece is only completed a posteriorly by reinforcing elements that are attached, themselves formed by a relatively inexpensive deposition process and sized to give the support part only a minimal heave. Typically, the addition of reinforcement elements in the form of ribs or, in general, stiffeners, of the order of 1 mm thick is possible, which would not allow, or not easily, forming processes monoblock of the final piece such as forging or molding.

However, it should be known that like any thermal process, the addition of molten metal on the support part thermally affects a portion of the thickness of the support part in the vicinity of its surface, in the deposition zones of the molten metal. This Thermally Affected Zone (ZAT), as encountered in material-fed welding processes, is modified in two ways:

There is created a diffusion zone of the filler metal in the support part (and vice versa), and it is necessary to control this diffusion so that it does not have negative consequences on the properties of the final part;

In and in the vicinity of this diffusion zone, there is a modification of the microstructure of the support piece, which may also have adverse consequences on the properties of the final piece.

Specifically, in the case where this method of adding molten metal is applied to a support piece made of a steel with high mechanical properties obtained by a strong presence of martensite, a significant degradation of their mechanical characteristics is observed in the ZAT, in the form of mainly a loss of hardness, due to a softening of the microstructure. This softening is linked to a grain enlargement and / or a metallurgical transformation consisting of a transformation of the martensite of the ferrite support part and carbides. This is called the reversion of martensite. In addition, significant residual stresses can be established in the part that has undergone the treatment, because of the different expansion characteristics of the various zones and materials that are involved.

A problem of fragility of the stiffening elements thus added also arises frequently. When the part is stressed in bending or in torsion, it is the parts which undergo the strongest constraints. Minimum toughness for the deposited metal is required, which is not always the case with the solidification structures obtained when using the molten metal addition processes.

The object of the invention is to propose a method for manufacturing a final part comprising a support part and added parts by means of a process for adding molten metal, for example reinforcing elements, which makes it possible to eliminate or at least strongly limit the risks of occurrence of the problems mentioned above.

To this end, the subject of the invention is a process for manufacturing a final steel part comprising a support part and at least one part formed by a process for adding a filler metal, in the form of molten metal. on a portion of the surface of the support member, forming a thermally affected zone (ZAT) on the support member and a fused zone between the ZAT and the portion formed by the addition of molten metal, characterized in that:

the support piece is made of a martensitic microstructure chromium steel at 70-100%, preferably 90-100%, in the quenched or tempered state, the remainder of the microstructure being composed of ferrite, austenite and carbides and / or carbonitrides, the composition of which, 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≤ Mo + W≤ 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 its 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≤ 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%; preferably traces Ti 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 more than 20% lower than that of the remaining parts of the support piece, and that the martensite content of the HAZ is greater than or equal to 70%;

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

The process for adding molten metal may consist of adding molten metal powder by means of a laser beam or an electron beam.

The process for adding molten metal may consist of adding a molten metal from a wire whose fusion is caused by the establishment of an electric arc between the wire and the support piece, or by a laser or by an electron beam.

The invention also relates to a final steel piece characterized in that it was manufactured by the preceding method, and in that at least one of the parts formed by a method of adding molten metal is a reinforcing element for the support piece.

As will be understood, the invention consists in combining the production of the support part in a martensitic steel with a high Cr content (5.0-16.5%, so it is not necessarily a steel stainless) and of determined composition, and the production of the added parts by addition of molten metal with a metal consisting of a stainless steel of initial composition (before its use as powder, wire, tape or other in the method of the invention) also well determined, and which is, surprisingly, very different from that of the metal constituting the support piece.

Indeed, the added molten metal is here, obligatorily, a stainless steel to

15.0-19.0% of Cr, which therefore contains more often more Cr than the metal of the support piece. And it also contains between 6.0 and 13.0% of Ni, so still significantly more than the metal of the support piece.

The levels of other elements that Cr and Ni must have the two steels used are also well defined.

The invention is therefore based above all on a particular choice of the pair of materials used, which will be seen in what is advantageous in the context of the manufacture of a final piece by direct deposition of molten metal on a support piece.

The invention will be better understood on reading the description which follows, given with reference to the following appended figures:

FIG. 1, which schematically represents the principle of a process for supplying molten metal in the form of a powder rendered liquid by a laser beam;

Figure 2 which shows schematically the principle of a method of supplying molten metal in the form of a wire whose fusion is performed by a welding torch;

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

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

Figure 5 shows the results of Vickers HV1 hardness measurements (standard NF EN ISO 6507 2006, 1 designating the load kgf) made on the section of the flange and one of its stiffeners;

Figure 6 shows a micrograph of the connection area between the flange and the stiffener; Figure 7 which shows a micrograph of a part of this same connection area, highlighting the ZAT and the melted zone;

FIG. 8 which shows a micrograph of the connection zone between the flange and the stiffener, on which are reported the results of Vickers HV0,1 hardness measurements;

FIG. 9 which shows a micrograph of the connection zone between the flange and the stiffener, on which points have been indicated where measurements of dilution of the material of the stiffener in the material of the flange have been made;

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

FIG. 1 generally represents the principle of 3D printing on a metal support piece 1 by adding molten metal, more precisely by melting a metal powder 2 by means of a laser.

The support part 1, that is to say the initial part on which the deposit must take place, is fixed. It is projected on its surface, by conventional means not shown, a metal powder jet 2 which is intended to constitute the filler metal which will form the deposit 3 after its solidification. The supply source of the powder 2 is scrolled relative to the surface of the support part 1, in the figure plane and from left to right in the example shown. A laser beam 4 is also projected on the surface of the support part 1, also moving in order to accompany the scrolling of the powder jet 2, and to achieve a melting of the powder 2 deposited on the support metal in the zone d impact of the laser beam 4, so as to form a liquid well 5. The laser also causes a partial and very superficial melting of the metal 1. The liquid well, solidifying when it is no longer in the field of the laser beam 4 which has moved, forms the deposit 3 whose composition corresponds to that of the powder 2 or drifting closely. This point will be examined in detail later. Under this deposit 3 is, in the vicinity of the surface of the support part 1 and a thickness of the order of 300μηι, a Thermally Affected Area (ZAT) 6 whose microstructure has been influenced by the contact with the laser beam 4 and the liquid well 5, in a manner comparable to that which occurs during a material-fed welding, with a morphology in successive layers also very close, qualitatively, of what is observed during a welding by contribution of material.

FIG. 2 generally represents the principle of 3D printing on a metal support piece 1 by adding molten metal by means of a welding wire 7 or the like (ribbon for example) which is unwound in the direction of the support piece 1 through a welding torch 8, which is itself scrolled relative to the workpiece support 1 in the figure plane and from left to right in the example shown. Conventionally, a power supply 9 is connected on the one hand to the support part and on the other hand to the welding wire 7 via the torch 8, the inner space 10 is supplied by a protective gas flowing towards the support piece 1. This results in the formation of an electric arc between the end of the wire 7 and the support part 1, so that the welding wire is liquefied at its lower end January 1, and the drops of liquid are deposited in layers (Corresponding to drops of liquid coming off the wire 7 on the support part 1 to form a liquid well 12. This solidifies when it leaves the field of action of the electric arc and forms, as in the example previous, a deposit 3 having essentially the composition of the welding wire 7. Here again, the vicinity of the surface of the support part 1 has a ZAT 6 under the deposit 3.

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

It is, of course, very desirable, even indispensable, that all these operations be automated to the maximum, in particular with regard to the speed of movement of moving tools and the mass flow rate of their supply of filler metal, powder, wire , ribbon or whatever, that will determine the shape that will take the reinforcing elements.

In Figures 1 and 2, there is shown a layer of filler metal constant thickness, but this is of course not a generality, as will be seen in other figures.

These methods of adding metal to a support part are known in the prior art, and are only described here as a reminder. In particular, the automation of operations is a usual practice in the implementation of this type of process, and the present invention uses it in a manner similar to that which is usually practiced.

Other methods, for example using an electron beam to obtain the melting of the filler metal, are also known for this purpose or imaginable, and the invention is independent, in principle, of the precise choice of the process used.

The invention is based on a particularly advantageous choice of the pair formed by the compositions of the support part 1 on the one hand, and the filler metal on the other hand, that it is initially in the form of a powder 2, of wire 7 ribbon or whatever.

It should be understood that this composition of the filler metal, as defined in the invention, is that which exists before it is deposited and melted on the support part 1, and therefore does not take account of the modifications at least the composition could be subjected to during the operation, such as oxygen uptake, resulting in oxidized inclusions and possibly a decarburization, and a recovery of nitrogen. These modifications can occur in particular if the operation does not take place in an atmosphere perfectly inert with respect to the deposited liquid metal.

Regarding the metal constituting the support part 1, it must have a high proportion of martensite in its structure at the time of implementation of the method. This proportion is at least 70%, and preferably between 90 and 100%. Indeed, this strongly or very predominantly martensitic structure provides the support part 1 with high mechanical characteristics, which means that most of the part can be made of a relatively thin material, and that it is only locally that its reinforcement by stiffeners is necessary. The rest of the microstructure, if it is not 100% martensitic, is composed of ferrite, austenite and carbides and / or carbonitrides.

In addition, its martensitic transformation start temperature Ms must be less than or equal to 500 ° C. and the increase in the volume of the metal of the support part 1 during this transformation, at a speed of 30 ° C./s or more, must be between 2 and 6%. This temperature Ms and the associated volume change are insensitive to the cooling rate up to 2 ° C / s and the metal 1 is therefore described as self-tempering.

This characteristic is original in that such relatively high expansions occurring during the martensitic transformation are far from being a generality for the steels that would have been likely to be used for producing a part with high mechanical characteristics. This large expansion is made necessary, in the context of the invention, to compensate for the contraction that will undergo the liquid well 5, 12 of filler metal during its solidification, so as to ensure the good continuity of the material forming the deposit 3. The temperature Ms and the associated volume change are preferably determined experimentally, for example by dilatometric measurements as is well known and described in the Precise Metallurgy J.Barralis and G.Maeder, AFNOR Nathan ISBN 2-09-194017-8.

The steel forming the support part 1 must also have a high resistance to softening, resulting in a low diffusion of carburigenic elements and carbon. Concretely, the hardness of the ZAT 6 will not be more than 20% lower than that of the remaining parts of the support part 1 which have not been influenced by the molten metal input. This gives a satisfactory homogeneity of the mechanical properties, which are not too degraded in the ZAT 6 with respect to the nominal properties of the support part 1. FIG. 3 represents a flange 13 for fixing a tube made by the method according to the invention. It consists of a sheet preformed by stamping to give it a generally circular shape 14, and is provided with a collar 15 surrounding a central orifice 16. The circular portion 14 and the collar 15 are made in one piece during Formatting.

The collar 15 is, as is known, reinforced by stiffeners 17 (also called "reinforcing elements") approximately in the shape of a right-angled 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 hypotenuses 18 of the right triangles forming the stiffeners 17 may have, in fact, a concave shape, having a constant or variable curvature. Here again, this characteristic is conventional for such flanges 13 and does not fall within the scope of the invention.

By way of nonlimiting example, the flange 13 has a thickness of 3 mm, its circular portion 14 has a diameter of 145 mm, the orifice 16 has an outside diameter of 62 mm, the collar 5 has a thickness of 15 mm the 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.

FIG. 4 shows, in cross-section along the line IV-IV of FIG. 3, one of the stiffeners 17 and its zone of contact with the circular portion 14 of the flange 13. As a result of the molten metal feed which has led to the formation of the stiffener 17, found on said circular portion 14, going from the upper surface 19 where the stiffener 17 is to the lower surface 20 and the right of the stiffener 17:

A "melted zone" 21 which results from the dilution of a portion of the molten metal 5, 12 in the metal 1 of the circular portion 14 of the flange 13, and therefore has on average a composition intermediate between those of these metals; A ZAT 6 whose nominal composition is that of the metal of the support piece, but within which there may possibly be localized changes related to the possible privileged diffusion of certain elements inside the support piece due to heating during the deposition of molten metal, or, in its upper part, residual diffusion of the molten metal; also, the metallurgical structure is modified more or less substantially compared to what it was before the molten metal deposition, due to warming related to this deposit;

And an area 22 corresponding to the remainder of the circular portion 14 of the flange 13, which has not been significantly affected thermally and chemically by the molten metal deposition operation, and has retained its original composition and metallurgical structure. The inventors have found that according to the invention, the steel of the support part 1 must have the following composition, expressed in percentages by weight, coupled with a microstructure at least strongly martensitic (from 70 to 100% of martensite, better still, 90% by weight). at 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≤ Mo + W≤ 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 remainder being iron and impurities resulting from smelting.

In addition, this composition must satisfy the following two relations 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 accomplishment of the martensitic transformation, and the satisfaction of the condition B, in particular the influence of Si and Mo, is favorable to a 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 austenitization when the metal is brought to a temperature above 700 ° C and high mechanical properties for martensite. Above 1, 5%, the implementation by conventional methods would be limited, and especially the resilience of the support would become insufficient.

Its Mn content is between 0.2 and 1.2%. A minimum of 0.2% is required to achieve austenitization. Above 1, 2% of the oxidation problems are to be feared during the deposit if it is not carried out under a neutral or reducing atmosphere.

Its Si content is between 0.2% and 1.2%.

If can be used as a deoxidizer during the elaboration, just like Al, to which it can be added or substituted. A minimum quantity of 0.2% is necessary because the silicon is a limiting element softening the support 1 when it is affected thermally. Beyond 1, 2%, it is considered that it excessively promotes the formation of ferrite and thus makes it more difficult to austenitize and obtain a steel of predominantly martensitic structure. In quantities greater than 1, 2%, it also weakens the support.

Its S + P content is between traces and 0.05%, in order to guarantee a low contamination of the melt zone 5, 12 and thus to avoid a fragility of the melt zone 5, 12.

Its Cr content is between 5.0 and 16.5%. The minimum content of 5.0% is justified to ensure a self-tempering character for the metal of the support 1. A content greater than 16.5% would make it difficult to austenitize and obtain a predominantly martensitic structure.

Its Ni content is between traces and 3.5%.

An addition of Ni is not essential to the invention. The presence of Ni within the prescribed maximum limit of 3.5% may, however, be advantageous for promoting austenitization. Exceeding the 3.5% limit would, however, lead to an excessive presence of residual austenite and an insufficient presence of martensite in the microstructure after cooling. It would also pose cost problems.

Its Mo + W content is between traces and 2.0%.

The presence of Mo or W is not essential and Mo may be present only in the form of traces resulting from the elaboration. However Mo limits the softening of the martensite of the ZAT during the deposit. Mo and W are favorable for good corrosion resistance. Above 2.0%, austenitization would be hampered and the cost of steel unnecessarily increased.

Its Cu content is between traces and 3.0%, preferably between traces and 0.5%.

These requirements on Cu are conventional for this type of steel. In practice, this means that an addition of Cu is not essential and that the presence of this element may be due only to the raw materials used. A content greater than 0.5%, which would be a voluntary addition, but can help with austenitization. More than 3% of the problems of cracking in the melted zone can occur.

Its Ti + Nb + Zr + V + Ta content is between traces and 2%.

Ti is a deoxidizer, like Al and Si, but its cost and its lower efficiency than that of Al, with equal added quantity, makes its use in general not very interesting from this point of view. It may be of interest in that the formation of Ti nitrides and carbonitrides can limit grain growth and favorably influence certain mechanical properties and weldability. However, this formation may be a disadvantage in the case of the process according to the invention, since Ti tends to hinder the austenitization due to the formation of carbides, and the TiN degrade the resilience. A maximum content of 0.5% is therefore not to be exceeded.

V and Zr are also elements capable of forming nitrides degrading the resilience. Zr, like Ti, hinders austenitization and is also a reason to limit its presence.

Nb and Ta are important elements for obtaining good resilience, and

It improves resistance to pitting corrosion. But since they can interfere with austenitization, they must not be present in quantities exceeding what has just been prescribed.

The condition Ti + Nb + Zr + V + Ta between traces and 2% is the result of all these considerations.

Its Al content is between traces and 0.1%.

Al is used as a deoxidizer during processing. It is not necessary that after the deoxidation it remains in the steel an amount exceeding 0.1%, because there would be a risk of having difficulties in obtaining the martensitic microstructure.

Its Co content is between traces and 0.5%. This element is, as

Cu, likely to help with austenitization. But it is useless to put more than 0.5%, because austenitization can be assisted by less expensive means.

Its Sn + Pb content is between traces and 0.04%. These elements are not desired because they are harmful for the solidification of the melted zone.

Its B content is between traces and 0.01%.

B is not obligatory, but its presence is advantageous for hardenability. Its addition above 0.01% does not bring significant additional improvement.

Its N content is between 0.01% and 0.2%. It is an element that helps austenitize from 0.01%, but beyond 0.2% it would limit the quenchability. And, as we have seen and for the reasons that have been said, relations A and B must also be satisfied.

The satisfaction of the conditions on MS, the dilation during the martensitic transformation and the hardness of the ZAT 6, which we have seen to be important elements for the success of the process according to the invention, result automatically from the coupling between the composition and the microstructure as defined.

According to the invention, the composition of the filler metal 2, 7, called to constitute the molten metal 5, 12 and then the deposits 3 forming the reinforcing element or elements 17, must have the following composition:

* 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%; preferably, trace Ti Ti + Nb + Zr + V + Ta 1 1.0%;

^* traces≤ Sn + Pb≤0.04%;

^* The remainder being iron and impurities resulting from smelting.

As has been said, it is the composition of the filler metal 2, 7 in solid form (wire, ribbon ...) or pulverulent before its melting and its deposition on the support part 1.

It is a structural stainless steel at least predominantly austenitic. The preferred condition on the sum Ti + Nb + Zr + V + Ta helps to ensure that this structure will be predominantly austenitic.

This composition must first lead the reinforcing element 17 to fulfill its role correctly when using the final part. It must, for this, have a good ductility resulting in an elongation at break of at least 15%, preferably between 30 and 40%, and a fine metallurgical structure consisting essentially of austenite (at least 80%), the rest being ferrite and / or carbonitrides, with a grain size of less than 300 μηι, a good fatigue strength greater than 200 MPa and a good resistance K1 c> 50 MPa.m ^1/2 to crack propagation between -40 ° C and + 80 ° C (according to ISO 12135).

For uses at temperatures higher or lower than these limits, the composition just mentioned would also be suitable, but it is preferable that the C content be between 0.01 and 0.05% for uses at low temperatures, in order to have a stable austenite and a good ductility of the martensite hardening possibly present. For high temperature applications, C levels are preferred from 0.04 to 0.1% to improve heat resistance. For high temperatures, AISI 321 steel and AISI 304H steel may be recommended, and for low temperatures AISI 316L steel, AISI 305 steel and AISI 304L steel, at least steels falling within these ranges. classes of nuances and which have, moreover, their precise compositions within the limits mentioned above.

In addition, this composition must guarantee, in conjunction with the choice of support metal 1, that the dilution of the filler metal 2 or 7 in the support metal 1 can take place 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.

Molten zone 21, as has been said, is an area where both metals have been subjected to a dilution process. The filler metal 2 or 7 must represent from 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 (solder 2 or 7) -% Ni (carrier metal 1))

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

The choice of the composition of the filler metal 2, 7 and the percentage of dilution thereof in the metal of the support part 1, which can be controlled in particular by the conditions under which the deposition of metal takes place fused with the particular characteristics of the installation used and determined by means of simple models and experiments, ensures that the solidification takes place under good conditions and with a good result, namely a mostly austenitic microstructure but may contain ferrite and / or martensite in less (<20%), thus resilient, which ensures that the stiffener 17 (generally, the deposit 3) can be effective and will not present excessive fragility at the level its junction with the circular portion 14 of the flange 13 (and, in general, with the support part 1). We will thus not find large ferritic grains fragile, no cracking hot, no sigma phase, and the hardness at this junction is actually less than or equal to 350 HV1. Under this melted zone 21, there is the ZAT 6 of a depth of 300μηι of which we spoke previously. Its composition is nominally that of the support part 1 with the reservations that 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 part 1, by a maximum of 20%, better by a maximum of 10%. These limits would also be found if other methods of measuring hardness were used.

This lower hardness of the ZAT 6 relative to the remainder of the support part 1 is due to warming undergone by the ZAT 6 during the deposition of molten metal in contact with the liquid well 5, 12. When the temperature in the ZAT 6 exceeds 800 ° C about, part of the martensite can turn into austenite, and so we have a softening of the microstructure. It would be very detrimental to the mechanical properties of the ZAT 6 that this softening persists, and it is therefore necessary that during the cooling of the ZAT 6, a predominantly martensitic structure is restored (at least 70% of martensite), this percentage of martensite being, preferably, higher in the ZAT 6 than in the remainder of the support part 1 to obtain a relatively high compressive residual stress state in the ZAT 6. This can be done if the martensitic transformation start temperature Ms of the metal of the support part 1 is less than or equal to 500 ° C and greater than 100 ° C. The result is that the ZAT 6 actually has a compressive residual stress state, which is more favorable to the mechanical properties of the flange 13-stiffener 17.

The choice of an austenitic shade to constitute the stiffener or stiffeners 17, in connection with the martensitic shade of the support part 1, is motivated by the presence of the molten zone 21 where a diffusion of a metal takes place in the other . The fact that the supply of a solid metal solidifying austenite is performed on a solid support martensitic structure limits the possibilities of diffusion and ensures that the melted zone 21 will be neither too extensive nor too fragile.

The process of forming stiffeners 17 (or any other form of reinforcing elements) by molten metal deposition generally makes it possible, advantageously, to leave these reinforcing elements substantially free from solidification, without a machining operation or subsequent surfacing is necessary. The good satisfaction of this characteristic depends largely on the precision with which the depositing operation is controlled by the control members. But the known devices for depositing molten metal which have been described above are already quite capable of obtaining this precision, and the implementation of the invention does not pose more problems than those already encountered and solved by the skilled person in the prior art.

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

For the flange 13, the composition of the metal is as follows: With A = 3.958 and B = 12.923, the remainder being iron and impurities resulting from the elaboration. The microstructure is 100% martensitic.

For the stiffener 17, the composition of the metal is the following, the rest being iron and impurities resulting from the preparation:

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

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

Measurements of the hardness Hv1 were made at different locations, 0.2 mm apart and distributed along the axis of the longitudinal section of the stiffener 17, over 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, including one in the melted zone 21 and three in the ZAT 6. It has also been carried out measurements on the thickness of the flange, in the vicinity of its periphery. Figure 5 shows the places of measurement of the hardness Hv1 and hardnesses that 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 present a certain dispersion, as it is usual. The hardness measured in the HAZ 6 is only slightly below this average.

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

The hardness measured in the melted zone 21 is 208 Hv1, thus slightly greater than the hardness of the stiffener 17, which tends to confirm that the melted zone 21 results from the diffusion of the filler metal into the metal of the support piece , and that the proportion of the filler metal is predominant, in this case even very large as is preferred for a good connection of the stiffener 17 and the circular portion 14 of the flange 13.

Figures 6 and 7 (the latter being an enlarged portion of Figure 6) show a micrograph of the lower portion of the stiffener 17 and its connection area with the circular portion 14 of the flange 13, after a chemical attack.

It can be seen that the stiffener 17 is composed of a superposition of initially molten metal layers, approximately 300-400 μηι thick, which interpenetrate over approximately 50% of their thickness. This strong interpenetration ensures that the stiffener 17 will not be particularly subject to breakage at an interface between layers. Note that in the case where one would not use a formation of the stiffener 17 by deposition of powder melted by laser, but by a method of supplying molten metal with a wire 7 or a ribbon and a torch 8, there would be found such superposition of layers, but on a thickness that may be larger, of the order of 1 mm.

There is also a good distinction between the melted zone 21 and the ZAT 6, the thickness of which is approximately 350 μηι, which surrounds the melted zone 21 over the entire periphery thereof, including up to the surface of the circular portion 14 of FIG. the flange 13.

FIG. 8 is another enlargement of a portion of FIG. 6, and shows the results of measurements of the hardness Hv 0.1 (and not of the hardness Hv1 as in FIG. 5, as the measurement points are, here , closer together and that in this case, in accordance with the ISO 6705 standard, the imposed load is reduced), performed on the longitudinal axis of the stiffener 17 in its extreme lower part, and, in the extension of this axis, on the zone melted 21, the ZAT 6 of the circular portion 14 of the flange 13 and a portion of the circular portion 14 of the flange 13 not affected by the heat generated during the deposition of the metal of the stiffener 17. The measuring points are distant 100 μηι. There are results which, qualitatively, confirm those of figure 5 by refining them. It can be seen that at the low end of the melted zone, there is a hardness of 250 Hv0.1, as compared with about 200 Hv0.1 in the stiffener 17 and the upper part of the melted zone, following the dilution of the metal. contribution in the support piece. Then, when one passes through the ZAT where there has been no significant dilution of the filler metal in the metal of the flange 13, the hardness increases gradually, but it turns out that the hardness of the ZAT n is not more than 20% less than the highest hardness measured in the circular portion 14 of the flange 13, at depths outside the TAZ.

The dilution of the materials in one another has also been measured in this same embodiment of the invention. Figure 6 shows the locations where quantitative analyzes of chemical composition by scanning electron microscopy were performed. The points called "spectrum 9, 10, 1 1" are located on the stiffener 17 and are representative of its nominal composition. The so-called spectrum points 15, 16, 17 are located in the circular portion 14 of the flange 13, and in an area not chemically and thermally affected by the addition of molten metal, and are representative of the nominal composition of the 13. The points "spectrum 12, 13, 14" are located at the lower end of the melted zone 21, and it is possible to deduce the dilution of the materials in one another by comparing the measurements therein. made with the nominal compositions of the flange 13 and the stiffener 17 determined by the 0 other measurements, by applying for this purpose the formula seen above.

The results of these analyzes are reported in the following Table 1. Only the main element contents of the Cr, Ni and Mo elements have been reported in order to assess the variation of the chemical composition in the different zones of the material of the flange 13 up to the material of the stiffener 17.

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

0

Stiffener Melted Zone Flange

Spectrum Spectrum Medium Spectrum Spectrum Medium Spectrum Spectrum Medium 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%

Neither% 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: metal compositions measured on the stiffener and the flange according to FIG. 9, and dilution of the flange in the stiffener

We will now describe a reference test, in which was used for c the circular portion 14 of the flange 13 a steel of the following composition, not according to the invention: The metal has a martensitic structure at 100% hardness 475 HV1 but does not comply with condition B since A = 8.1% and B = 0.5%.

For the stiffener 17: the composition and structure of the powder are identical and the conditions of deposition similar to that described for the test according to the invention.

The hardness in the ZAT 6 drops from 32% to 325Hv1, ie a drop greater than the maximum of 20% which is typical of the invention, the microstructure is no longer sufficiently martensitic (60%) and has softened by formation of bainite / ferrite / pearlite. The martensitic transformation did not compensate for the withdrawal of the melted zone. The melted zone is predominantly austenitic with a little martensite and shows a dilution of 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 representative results of the invention. The ZAT thus has a mechanical resistance that is too low due to insufficient compression of the base of the stiffener 17. Moreover, the martensite of the melted zone is of a fragile type because it is rich in C and there is the possibility of solidification in the primary austenitic phase of the melted zone, hence the risk of hot cracking.

The example of implementation of the invention that has been presented, relating to a flange for fixing a tube is only a simple and non-limiting example. FIG. 10 shows a suspension arm 22 produced from a cut and stamped preform 23, and to which stiffeners 24, 25, 26 (and others not referenced in FIG. 10) have been added by the method according to FIG. 'invention.

In general, the invention can find application in the field of the manufacture of structural parts, in particular in land vehicles and aircraft, because it is easily possible, thanks to it, to produce parts of different strength properties and optimized by weight from the same support piece, only by modulating the morphology of reinforcement elements added by the method according to the invention.

Claims

1 .- A method of manufacturing a final steel part comprising a support steel part (1) and at least one part (17) formed by a process for adding a filler metal (2; ), in the form of molten metal (5; 12), on a portion of the surface of the support member (1), forming a heat-affected zone (ZAT) (6) on the support steel (1) and a zone melting (21) between the ZAT (6) and the portion (17) formed by the addition of molten metal (5; 12), characterized in that:

the support piece (1) is made of a martensitic microstructure chromium steel at 70-100%, preferably at 90-100%, in the quenched or tempered state, the remainder of the microstructure being 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≤ Mo + W≤ 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 (2; 7) before its 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≤ Mo + W≤ 3.0%;

^* traces≤ Cu≤ 3.0%;

* traces≤ Co≤ 0.5%;

^* traces≤B≤0.01%;

^* traces≤ S + P≤ 0.05%;

* traces≤ Sn + Pb≤ 0.04%;

^* The balance being iron and impurities resulting from preparation;

in that the hardness of the ZAT (6) is not more than 20% less than that of the remaining parts of the support piece (1), and that the martensite content of the ZAT (6) is greater than or equal to equal to 70%;

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

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

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

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