process of hybrid arc-laser welding of ferritic steels

The present invention relates to a hybrid laser-arc welding process of ferritic steels using a filler wire and a weld obtained with such a hybrid process, that is to say a welding process using simultaneously an electric arc and a laser beam combining with each other.

Welds carried out by hybrid laser-arc welding on ferritic steels, such as C-Mn steels defined by standard EN 10025, micro-alloyed steels defined by standard EN 10113 or so-called "hardened-tempered" steels according to the EN 10137 standard, most often have low resilience characteristics or more generally toughness at low temperature in the molten metal, as well as hardness values ​​in this same zone much higher than that of base metals

These mediocre metallurgical characteristics very strongly limit the extension of hybrid laser-arc welding in certain fields of industry, in particular in the fields of shipbuilding, manufacture and laying of tubes for the transport of petroleum products, offshore ...

This problem results from the fact that these steels have been chemically balanced to give them the targeted mechanical properties taking into account their production process, that is to say the rolling and subsequent cooling conditions or else the heat treatment which they undergo during their manufacture, for example in the form of sheets or tubes.

Indeed, the mechanical properties of a steel result, on the one hand, from its chemical composition and, on the other hand, more important, from its microstructure.

The microstructure of a steel and, in the same way, of a weld, i.e. molten metal consisting of deposited metal and base metal molten during the execution of the weld, develops during the cooling from the austenitic state at high temperature to room temperature.

From there, for a given chemical analysis, this microstructure and therefore the mechanical properties of the steel (or of the weld) are a function of the cooling conditions.

If we consider for example a steel containing about 0.12% by weight of carbon, with a low cooling rate, its structure is composed essentially of ferrite, that is to say of iron atoms stacked according to a centered cubic crystallographic structure, and a small percentage, typically of the order of 13%, of perlite, that is to say alternating lamellae of ferrite and cementite which is the iron carbide Fe₃C at 6.66% carbon. Its Vickers hardness is then about 130 and its breaking strength of the order of 400 to 500 MPa.

On the other hand, this same steel will have a martensitic structure, that is to say a solid supersaturated solution of carbon in the centered cubic iron, and a Vickers hardness of the order of 400, while its breaking load will be 1300 to 1400 MPa if it undergoes extremely rapid cooling from the austenitic state (high temperature).

For cooling rates between these two extremes, we will see the development of mixed structures composed of martensite, lower bainite, upper bainite, and ferrite + perlite, to which intermediate mechanical properties will correspond.

Continuous cooling transformation diagrams, commonly called "TRC diagrams", which are well known to metallurgists, indicate the various microstructures which develop as well as the hardnesses which correspond to them according to the cooling rate for a given steel and the standard conditions of austenitization for this steel, namely the temperature (generally 50 ° C above the point of complete transformation into austenite) and the austenitization time (generally 30 minutes).

Such diagrams also show that for a given steel, the difference in mechanical properties between the martensitic structure and the ferrite + perlite structure is all the greater the higher its carbon content. They also show, if we compare the diagrams of steels of various compositions, that the cooling rates which generate the various microstructures mentioned above are a function of all of the alloying elements of the steel.

In fact, all the alloying elements have an influence on the hardenability, that is to say the ability of a steel to acquire a totally martensitic structure, therefore also on the critical quenching speed which is the speed of minimal cooling from the austenitic state to obtain a 100% martensitic structure.

The carbon content, in addition to affecting the hardenability, also conditions the mechanical properties of the various structures

Hybrid laser-arc welding processes, because of the high power density associated with them and the high welding speeds which they allow to achieve, which are often higher than laser welding alone, lead to speeds very fast cooling.

It follows then that with so-called ferritic steels, the microstructure of the weld is very different from that of the base metal, which leads in this zone to much higher hardness and tensile characteristics than those of assembled steels. but also to a ductility and a tenacity of the welded joint too low for many applications.

This phenomenon can be mitigated by the addition of a filler metal in the form of a "cold" wire, that is to say a welding wire unwound at the joint plane of the sheets, immediately upstream of the impact of the laser beam and the arc used during hybrid laser - TIG or plasma welding, or in the form of consumable electrode wire when the process is a Laser / MIG or MAG hybrid.

Indeed, by proceeding in this way, it is sought to adjust the quenchability of the molten metal most often by reducing its content of alloying elements relative to the base metal (s) but also, in the case of steels very soft, that is to say having an elastic limit below 240 or 280 MPa, increasing it.

In both cases, it is sought to adjust the quenchability of the molten metal so that, under the effect of the thermal cycle generated by the hybrid welding, it develops a less fragile microstructure.

However, to do so is often insufficient since, with hybrid laser welding processes, the proportion of filler metal in the molten metal is most often of the order of 20% by weight and very rarely exceeds 40% by weight, which, even using the wires least loaded with commercially available alloying elements, that is to say wires containing 0.5% by weight of manganese for example, does not make it possible to lower the quenchability of the molten metal sufficiently to avoid the formation of hard and fragile structures in the case of steels which, without this addition, already lead to a hard and fragile structure.

Furthermore, in the case of very mild steels for which one may want to increase the hardenability to avoid the formation of a coarse and fragile structure, the contribution of a wire more loaded with alloying elements than metal to be welded, in order to obtain a finer structure in the molten metal, also does not appear to be a satisfactory solution because this refinement of the structure is accompanied by a sharp increase in hardness and therefore only leads to a slight decrease in brittleness.

Document DE-A-4 006 167 considered to be the best state of the prior art teaches a method of laser welding of steel making it possible to weld the two longitudinal edges of a metal sheet so as to obtain a welded tube and discloses in combination the Features of claim 1.

In addition, document US-A-5,744,782 proposes an arc welding process with fusible wire of high strength steels, which electrode wire contains iron, carbon, manganese, nickel, molybdenum, silicon , copper and boron.

The problem which then arises is to improve the hybrid laser-arc welding processes so as to be able to obtain welds whose microstructure is almost free of hard and fragile micro-constituents, that is to say having improved properties. in terms of toughness and more generally of toughness as well as tensile characteristics more in line with those of base metals, in particular increased elongation, lower breaking strength and yield strength while remaining higher than that of assembled materials.

In other words, the invention aims to improve the properties of the weld joints obtained by hybrid laser-arc welding with the addition of fusible wire and assist gas.

The solution of the invention is a hybrid welding process using a laser beam combined with an electric arc with the supply of fusible welding wire and shielding gas, in which said wire is melted by said laser beam and / or said arc. electric so as to produce a solder joint on at least one piece of steel to be welded, characterized in that said solder joint contains from 30 to 1000 ppm by weight of titanium, at least 0.7% by weight of manganese, from 50 to 1000 ppm by weight of oxygen and less than 10% of nickel.

Depending on the case, the method of the invention may include one or more of the following technical characteristics:<ul id="ul0001" list-style="dash" compact="compact"><li>the part or parts are made of ferritic steel.</li><li>the weld joint has an acicular ferrite type microstructure.</li><li>the weld comprises from 30 to 800 ppm of titanium and / or from 100 to 450 ppm of oxygen, preferably from 50 to 500 ppm of titanium and / or from 120 to 350 ppm of oxygen.</li><li>the weld comprises from 0.7 to 2% by weight of manganese and / or less than 1500 ppm by weight of aluminum, preferably from 0.8 to 1.7% of manganese and / or less than 500 ppm of aluminum, more preferably less than 300 ppm aluminum.</li><li>the weld comprises aluminum and oxygen in proportions such that: [Al] / [O] &lt;2.5 where [Al] is the weight proportion of aluminum and [O] is the weight proportion of oxygen, preferably [Al] / [O] &lt;1.5.</li><li>the solder contains less than 0.6% molybdenum, less than 80 ppm of boron, less than 1% of silicon, less than 0.20% of carbon, less than 0.035% of sulfur and less than 0.035% of phosphorus, preferably less than 0.3% molybdenum, less than 50 ppm boron, 0.1 to 0.6% silicon and 0.03 to 0.13% carbon.</li><li>the solder comprises less than 0.07% of niobium, less than 0.07% of vanadium, from 1 to 200 ppm of nitrogen and less than 1% of chromium, preferably less than 100 ppm of nitrogen, less than 0.03% of niobium, less than 0.05% vanadium and less than 0.3% chromium.</li><li>the laser beam assist gas and / or the arc shielding gas is a gas mixture containing oxygen up to 20% by volume and / or CO₂ up to 40% by volume.</li><li>the assist gas of the laser beam is a gas mixture containing, in addition, at least one inert gas, preferably helium, argon or their mixtures.</li><li>the fusible wire is a solid wire or a cored wire containing at least one element chosen from titanium, manganese, iron, nickel and optionally boron, molybdenum, carbon or chromium.</li></ul>

The solid or flux-cored welding wire capable of being used in a hybrid laser / arc welding process with the addition of fusible welding wire and shielding gas according to the invention, comprises from 100 to 10,000 ppm of titanium, from 50 to 5000 ppm of oxygen, up to 1500 ppm of boron, and the rest being essentially iron.

In particular, the wire contains one or more alloying elements of ferritic steels chosen from manganese, silicon, molybdenum, nickel, and carbon.

More generally, the welds carried out by traditional arc welding processes but without the laser can have various microstructures among which there is the structure called acicular ferrite which has excellent toughness properties.

This type of microstr ucture is specific to welds and never occurs in steels, even when they are subjected to cooling conditions similar to those of welds.

On the other hand, in welds, such a micro-structure may appear, in certain cases, for a very wide range of cooling rates from the austenitic state, including those which, in the case of conventional steels, lead to structures. hardening and martensite and / or lower bainite type which are hard and fragile, as recalled by the document Antunes M., Bonnet C., "Application of a hardenability test in search of factors having an influence on the formation of acicular ferrite", Information days on Metallurgy of the molten zone, Société Française de Métallurgie / Société des Ingénieurs Soudeurs - Southeast section, Conference N ° 9, Publication of Autogenic Welding, 1981.

Acicular ferrite only exists in molten metal in the presence of certain inclusions which serve as intragranular germs for ferrite during the transformation of austenite during cooling.

We know that the existence of these inclusions depends on the oxygen content of the molten zone but, for there to be intragranular germination of ferrite on cooling, it seems necessary that these complex inclusions locally have oxide on their surface. titanium TiO or a titanium / manganese oxide MnTi₂O₄, as described by Blondeau R., “Metallurgy and mechanics of welding”, Hermes Science, Lavoisier 2001, page 162.

It is then clear that titanium plays a fundamental role.

However, it is not enough to have a minimum of titanium, typically a few tens of ppm by weight, for it to be found in one or the other of the desired forms.

In fact, it is also essential that the kinetics of the redox reactions which lead to the formation of inclusions make it possible to achieve this result.

Thus, in addition to the titanium content and the oxygen content, the nature and the quantity of all of the deoxidizing elements, that is to say the elements having a strong affinity for oxygen, such as aluminum , silicon, calcium ... possibly present in the molten metal, will also intervene and have a significant impact on the resulting micro-structure, regardless of the origin of these elements: wire, base metal or gas .

In addition, the nitrogen content must also intervene because, if titanium and aluminum are hungry for oxygen, these elements also have a strong affinity for nitrogen so that nitrogen, as well as all the elements which are greedy, such as boron, vanadium, niobium, etc., will interfere in redox reactions and condition the appearance in the molten metal of the inclusions necessary for the transformation of austenite into acicular ferrite.

However, the molten metal resulting from hybrid laser-arc welding with or without filler metal generally does not meet the conditions necessary for the germination of the acicular ferrite so that most often it has a martensitic or martensite / hard bainite microstructure and fragile, incompatible with a large number of applications.

In view of the above, the solution of the invention which makes it possible to overcome these problems consists in providing, during the laser-arc hybrid welding, in the molten metal by means of the wire or preferably of the wire / gas couple, the elements allowing the formation of inclusions favorable to the germination of acicular ferrite.

However, to do this, it must be taken into account that what ultimately matters is the chemical analysis of the molten metal, which results from a mixture between the base metal and the metal deposited by the wire / gas couple used, the proportion of both expressed generally in welding by the dilution rate which is the proportion of base metal in the molten metal; the proportion being that by weight, by volume or by surface evaluated from a macrography since it is a proportion and the densities of the base metal and of the deposited metal are almost the same.

Thus, if for example a weld is made, the dilution rate of which is 80% by weight, the content of each of the elements in the molten metal will be equal to 80% of the content of this element in the base metal to which 20% of the content of this same element is added in the metal deposited by the wire or the wire / gas pair.

The above shows that the problem is extremely complex.

However, the tests carried out within the framework of the present invention and given below by way of illustration, show that it is possible to propose solutions which work in most cases, that is to say which lead to the formation of non-fragile microstructures in the deposited metal, by adjusting the proportions of certain particular elements within the weld, therefore also by controlling their relative proportions in the filler materials, in particular via the wire / gas pair.

Adjusting the oxygen content of the molten metal

Current steels have a very low oxygen content, generally less than 30 ppm by weight, and most often contain a residual aluminum content, typically of the order of 100 to 500 ppm by weight, because this element is used as an element. calming, during the development of these steels.

As a result, all the oxygen present in these steels is found in the form of inclusions of aluminates, which are not capable of serving as a germ for acicular ferrite.

Even adding titanium via the filler wire, it cannot be found in the form of the oxides necessary for the germination of the acicular ferrite because aluminum is more reactive than titanium vis-à-vis oxygen and by the very development of steels, it is always in excess compared to the residual oxygen of the steel.

If one wishes to be able to form titanium oxides in the molten metal, it is therefore imperative to increase its oxygen content relative to that of the base metal so that, even after reaction with aluminum, there is excess oxygen so that it can react with the titanium.

Several means can be used independently or jointly to supply oxygen to the molten metal:<ul id="ul0002" list-style="dash" compact="compact"><li>assist gas used for laser welding may contain oxygen or CO₂, the latter decomposing by releasing oxygen due to the high temperatures existing in the vicinity of the liquid metal during the execution of the weld, and / or</li><li>solid or filled type filler wire, the oxygen content of which can be much greater than that of the base metal; thus, a solid wire can contain several hundred ppm of oxygen and a cored wire can contain several thousand, and / or</li><li>the shielding gas used for the "arc" part, in the case of hybrid arc / laser welding which, depending on the design of the hybrid welding equipment, may or may not be different from the shielding gas assisting the laser beam.</li></ul>

Figure 1 shows the evolution in hybrid arc-laser welding, more specifically in hybrid MAG / CO laser welding₂, the oxygen content in the molten metal as a function of the oxygen content in helium as well as the oxygen content in the molten metal for the He + 8% CO mixture₂ (% by volume) using in all cases a power of either 6 kW or 8 kW, and the same filler metal consisting of a solid wire of diameter 1.2 mm and of type G2Si according to the EN standard 440.

The tests were carried out on a welded thickness of 6 mm, at a welding speed of 1 m / min at 6 kW and 1.7 m / min at 8 kW, for a gas flow rate of 30 l / min, for a wire speed of 14 m / min, an intensity of 370-390 A and a voltage of 39-42V.

As can be seen in FIG. 1, the proportion of oxygen in the molten metal of a weld obtained by hybrid arc-laser welding increases when the oxygen content in the assist gas increases.

We also see that as in the case of laser welding only or arc welding alone, the replacement of oxygen with CO₂ in equivalent content, leads to a smaller increase in the oxygen content in the molten metal

Additional tests conducted in parallel show that the amount of oxygen to be observed in the welds must be between approximately 50 and 1000 ppm, but we will see later that the lower limit is in fact a function of the aluminum content of the molten metal. The upper limit results from the fact that the increase in the oxygen content results in an increase in the density of inclusions in the weld which generates a reduction in the breaking energy at the ductile level (see R. Blondeau: " Metallurgy and mechanics of welding ”, Hermès Science, Lavoisier 2001). It is therefore unnecessary to introduce more oxygen than the amount necessary to obtain the inclusions essential for the germination of acicular ferrite

Adjusting the titanium content of the molten metal

Titanium being essential for the inclusions to effectively play the role of germs for the transformation of austenite into acicular ferrite during the cooling of the weld, it turns out to be necessary to bring it via the solid or cored wire if the base metal, that is to say the part or parts to be welded, does not contain sufficient quantities thereof.

Thus, it has been observed that, whatever the welding process, no needle-shaped ferrite appears if the titanium content of the molten metal is less than 30 ppm by weight and that beyond a certain value which can vary between about 800 and 1000 ppm, the transformation into acicular ferrite is suppressed or greatly deteriorated.

It is therefore necessary that the molten metal, that is to say the mixture of the base metal and the metal deposited in proportion to the dilution ratio, contains between 30 and 1000 ppm of titanium, preferably between 50 and 800 ppm by weight.

Ideally, the chemical composition of the deposited metal, which results from the chemical composition of the wire used and from the reactions with the laser assist and / or arc protection gas (see Figure 1 and Table 5), must contain a sufficient amount of titanium so that, when the deposited metal resulting from the melting of the wire mixes with the base metal in a proportion corresponding to the dilution rate, the mixture thus obtained contains a titanium content of between approximately 30 and 1000 ppm.

In practice, since the range of acceptable titanium content is relatively wide, achieving this objective is relatively simple since most base metals do not contain titanium and for the others, their content is generally less than 200 or 250 ppm. .

Under these conditions, it can be seen that if a chemically balanced wire is used to deposit, with the gas used, a metal which contains between 150 and 1000 ppm by weight of titanium, the mixture with the base metal will have a content of titanium in the area required for transformation into acicular ferrite for a dilution rate of between 80% and 0%, if a steel not containing titanium is welded, and whatever the dilution rate if one welds a steel which typically contains less than 800 ppm.

The analysis of the deposited metal is never identical to the analysis of the wire and the transfer coefficient of the various elements, i.e. the ratio between the content of this element in the deposited metal to its content in the wire, is a function of all the elements present in the wire but also, for certain elements, of the nature of the shielding gas.

Thus, if we want to have 200 ppm of titanium in the deposited metal, the content in the wire will not be the same according to the other elements present, such as C, Mn ...

In other words, for the titanium content of the molten metal to be between 30 and 1000 ppm, preferably between 50 and 800 ppm, it is then necessary, if the base metal diluted in the molten metal does not make it possible to reach the value minimum, add the necessary titanium using the filler wire, taking into account the dilution rate, as explained above, but also the titanium transfer coefficient between the wire and the metal deposited, the transfer coefficient being the ratio between the titanium content of the deposited metal and that of the wire used. This coefficient is always less than 1 and it is lower the greater the oxidation power of the gas used, that is to say the arc protection gas in the case of hybrid arc welding. -laser contains all the more oxygen and or CO₂.

Adjustment of the hardenability of the molten metal

Hardenability is a concept well known to metallurgists which translates the ability of a steel to acquire a 100% martensitic structure.

The quenchability can be characterized in particular by the critical quenching rate which is the cooling rate from the austenitic state (high temperature: generally greater than 900 ° C. for the steels encountered in welding) the slowest which makes it possible to confer on the steel considered a 100% martensitic structure.

It can be evaluated from the diagrams of transformations in continuous cooling (TRC) which translate in graphical form the various structural transformations that a steel undergoes as a function of the rate of cooling from the austenitic state.

The more the TRC diagram is shifted to the right (long time) in the temperature / time axes, the lower the critical quenching speed and the more the steel exhibits high quenchability.

If the conditions are met so that the inclusions can be active as seeds for the transformation of austenite into acicular ferrite in a weld, which depends in particular on balancing in titanium, aluminum, oxygen, ... as explained above, it is also necessary, for this microstructure to appear, that the austenite has not previously transformed into a decomposition product forming at a temperature higher than that of acicular ferrite, knowing that acicular ferrite is formed cooling between 550 and 450 ° C.

Indeed, certain other microstructural constituents, such as widmanstatten ferrite, perlite or granular bainite, can appear at higher temperatures.

It is therefore necessary that the molten metal has sufficient quenchability to prevent the transformation of the austenite at a temperature above 550 ° C. under the cooling conditions specific to laser or hybrid arc / laser welds.

In other words, it must be ensured that the quenchability of the molten metal, resulting from the mixing of the base metal and the deposited metal as a function of the dilution rate, is not too weak to prevent the austenite from turning into coarse and therefore not very resilient constituents before reaching the temperature range allowing the transformation into acicular ferrite (below 550 ° C) nor too large to avoid that austenite does not transform into martensite even in the presence of inclusions favorable to the germination of acicular ferrite.

The chemical composition of the filler wire, which obviously influences the hardenability of the molten metal, must therefore be balanced taking into account the analysis of the base metal, the dilution rate and the transfer coefficients of the various chemical elements which, like for titanium, the oxidation power of the gaseous mixture in which the metal drops transit from the wire to the molten bath, during welding, is a function.

Although the thermal cycles generated by hybrid arc-laser welding are very fast in comparison with the thermal cycles generated by more conventional welding processes of the MIG / MAG type, submerged arc, plasma, etc., within the framework of the invention , no acicular ferrite formation has ever been observed in cords obtained by hybrid arc-laser process, the manganese content of which was less than 0.7%.

A minimum manganese content of 0.7%, preferably at least 1%, should therefore be observed in the molten metal.

In addition, it has been observed that the presence of complementary alloying elements, such as molybdenum, nickel, chromium or boron, generally increases the proportion of needle-like ferrite in the molten metal, this being particularly pronounced for boron especially when it is associated with molybdenum but too large additions can be detrimental because too high quenchability of the molten metal will lead to a hard and fragile martensitic structure even if the inclusions necessary for the germination of ferrite acicular are present.

The distinction made between boron and molybdenum compared to the other alloying elements is explained by the fact that the action of these elements on the quenchability is more important on the transformations occurring at high temperature, i.e. - say at more than 550 ° C, which corresponds to the upper part of the TRC diagrams, than on those occurring at lower temperature, that is to say at less than 550 ° C.

Thus, in addition to the minimum manganese content of 0.7%, preferably 1%, it may be advantageous to introduce boron and / or molybdenum into the molten metal in order to increase the proportion of acicular ferrite in the weld and at the same time the resilience at low temperature,

Adjusting the oxygen content of the molten metal

The experiments carried out within the framework of the present invention have shown that it is also necessary, in order to obtain a structure rich in acicular ferrite in the weld and by the same token good toughness at low temperature, that the Al / O ratio is less than 2.5, preferably less than 1.5.

In fact, to obtain good toughness values ​​at low temperature in a weld obtained by a hybrid arc / laser process and to avoid having too high hardnesses in the molten metal, it is necessary to form in the molten metal inclusions capable of serving as germs so that the transformation of the austenite on cooling produces acicular ferrite and, as indicated previously, these inclusions of titanium-rich oxides can only form if all the oxygen present in the molten metal is not fully bound to aluminum, the reactivity of which towards oxygen is greater than that of titanium. Experience shows that this is only the case if the Al / O weight ratio in the molten metal is less than 2.5.

To satisfy this condition, the aluminum essentially coming from the dilution of the base metal in the weld, it is advisable to enrich the molten metal with oxygen via the wire, the welding gas or the combination wire / gas put used during laser-arc welding.

Tests carried out within the framework of the present invention (see below) show that excellent ductility, toughness and resilience results, without excessive hardness, are obtained in welds produced by implementing a hybrid arc process. laser, when the analysis of the molten metal, that is to say of the weld, contains the elements given in the following Table 1 (contents expressed in proportion by weight).<tables id="tabl0001" num="0001"><img file="EP1559498B1_D0001.tif" /></tables>

However, the welds have better characteristics when they contain the elements of Table 1 in the preferred proportions of Table 2 below (contents expressed in proportion by weight).<tables id="tabl0002" num="0002"><img file="EP1559498B1_D0002.tif" /></tables>

The analytical ranges of Tables 1 and 2 correspond to the molten metal which results from the analysis of the base metal or base metals if two parts are made of different steels by welding, and that of the metal deposited depending on the wire. , gas or wire / gas pairing, and taking into account the dilution rate.

In practice, to satisfy these forks and obtain a weld according to the invention, it is advisable to proceed as follows.

The composition of the base metal constituting the parts to be assembled (or the part if it is a question of welding the edges of a tube) is determined or an average composition corresponding to those of the two base metals if the parts are in different steels.

The dilution rate, that is to say the proportion of the base metal in the weld to be produced, is evaluated.

The composition of the metal to be deposited is then determined (wire / gas pair) so that the base metal + deposited metal mixture leads to a composition of the molten metal, that is to say of the weld, in the ranges of tables 1 or 2 above, taking into account the dilution rate evaluated.

Thus, for example, if the manganese (Mn) content of the base metal is 1% and the dilution rate is estimated at 80%, the preferred manganese range (see Table 2) for the molten metal is between 0 , 8 and 1.5% is respected if the manganese content of the deposited metal (wire / gas pair) is between 0 and 3.5% since, for an Mn content of 1% in the base metal (ie in the steel of the parts to be welded) and a dilution rate of 80%, a weld will be obtained, that is to say a molten metal, containing 80% of Mn from the base metal and therefore 20% of Mn from the deposited metal (wire / gas pair).

Therefore, to obtain a weld containing 0.8% Mn (low value of the range in Table 2), then it is necessary to use a wire (deposited metal) free of Mn, that is to say without Mn. In other words, in this case, all of the manganese which will be found in the weld is solely derived from the steel of the parts to be welded.

On the other hand, to obtain a weld containing 1.5% of Mn (high value of the range of Table 2), it is advisable to use a wire / gas couple leading to a deposited metal containing approximately 3.5% of Mn, since, in this case , the manganese that will be found in the weld will come from 80% of the steel of the parts to be welded (i.e. 0.8% of the 1.5% desired) and 20% of the wire / gas couple (i.e. 0.7% on the 1.5% desired).

A similar calculation can be made for each of the chemical elements to be introduced into the weld, which makes it possible to precisely define the composition of the metal to be deposited, that is to say of the wire / gas couple, as a function of the parts to be welded.

It should be noted that since the dilution rate, that is to say that the proportion of base metal in the molten metal of laser welds with wire ratio, is generally of the order of 60 to 80% , the metal deposited by the wire / gas pair, therefore the filler wire, must contain very little or, on the contrary, a high amount of manganese, depending on the manganese content of the base metal or metals, and most often high titanium and boron contents compared to the ranges targeted for molten metal, so that the small percentage of metal deposited in the weld makes it possible to reach the midpoints of the ranges of values ​​recommended for these elements in the weld.

Furthermore, concerning the elements Cr, N, V ​​and Nb, in practice, there is only a very small advantage in having these elements in the weld.

However, their presence is almost inevitable due to dilution with the base metal which often contains several of them or because they are present as unavoidable residual impurities in the metals to be joined or in the filler product. (case of nitrogen for example).

It should therefore be ensured that their contents are as low as possible and never exceed the maximum values ​​given in Tables 1 and 2 above.

In addition, concerning the Ni element, the maximum content of 10% given in Table 1 corresponds to the very specific case of steels with 9% nickel.

When welding such steels with a very high nickel content, it is ensured beforehand that the oxygen content is not too high and, if necessary, this oxygen content is adjusted to make it compatible with welding of this type of nickel steel, that is to say that preferably one works with oxygen contents close to the low value of the oxygen range of Table 1.

Apart from these steels with a high nickel content, the most conventional steels can be welded effectively while respecting maximum nickel contents in the weld of the order of 2 to 3% by weight approximately.

In view of the above, it has been demonstrated ranges of compositions of the metal to be deposited by the wire / gas pair which make it possible to obtain the compositions of the molten metal from the welding of Tables 1 or 2. These ranges are given in Table 3.

The deposited metal should preferably have a metallurgical analysis corresponding to the upper part of the composition ranges, when the dilution rate of the base metal in the weld is high (&gt; 85%). On the contrary, its analysis should rather correspond to the lower part of the ranges indicated when the dilution rate is significantly lower, for example 75% or less.<tables id="tabl0003" num="0003"><img file="EP1559498B1_D0003.tif" /></tables>

In addition, as mentioned above, the analysis of the metal deposited by the wire / gas pair used during hybrid arc-laser welding depends on the analysis of the wire and on the oxidation power of the gas mixture because this conditions the transfer coefficients various alloying elements contained in the wire.

In the case of hybrid arc-laser welding, whether of the TIG or plasma type, the filler metal is supplied in the form of a cold fusible wire.

In this case, the chemical exchanges are relatively low and there are only significant differences with regard to the oxygen content which can be increased compared to that of the wire when the latter contains only a small amount thereof ( solid wires, the oxygen content of which is less than 150 ppm) or reduced when the wire contains a lot of them, as is the case for example with certain metal cored wires. On the other hand, the contents of manganese, silicon and titanium are always lower in the deposit than in the wire, the difference being all the greater the greater the oxidation potential of the laser assistance gas, namely the oxygen and / or CO content₂.

It should be noted that in TIG and plasma welding, it is not possible to use oxidizing protective gases because otherwise the tungsten electrode would be destroyed.

However, in the case where the hybrid welding equipment makes it possible to have two separate gas supply lines, one for assisting the laser beam and the other for the TIG or plasma arc, the laser assistance gas can be different and therefore contain oxygen because, in this case, this oxidizing gas is not in direct contact with the tungsten electrode.

Likewise, it is also possible to use a double-flow plasma torch or double-flow TIG, that is to say torches with two gas circuits, in which the central gas or plasma gas does not contain oxygen, while the annular gas may contain it because it is not in direct contact with the electrode.

In hybrid GMAW / laser welding, a drop of liquid metal is transferred from the end of the fusible wire to the liquid bath through the arc, which involves much more intense chemical exchange and loss of elements. much more pronounced.

Examples of chemical transfers in the arc as a function of the nature and the proportion of the oxidizing constituents of the shielding gas with different types of wires are given in Table 4 which gives the analyzes of wires and of the deposited metal obtained with these wires. using shielding gases having different oxidizing compounds in hybrid MAG / Laser welding.<tables id="tabl0004" num="0004"><table frame="all"><title><u style="single">Table 4</u></title><tgroup cols="11"><colspec colnum="1" colname="col1" colwidth="26mm" /><colspec colnum="2" colname="col2" colwidth="25mm" /><colspec colnum="3" colname="col3" colwidth="13mm" /><colspec colnum="4" colname="col4" colwidth="12mm" /><colspec colnum="5" colname="col5" colwidth="12mm" /><colspec colnum="6" colname="col6" colwidth="12mm" /><colspec colnum="7" colname="col7" colwidth="12mm" /><colspec colnum="8" colname="col8" colwidth="13mm" /><colspec colnum="9" colname="col9" colwidth="15mm" /><colspec colnum="10" colname="col10" colwidth="15mm" /><colspec colnum="11" colname="col11" colwidth="15mm" /><thead><row><entry namest="col1" nameend="col2" align="center" valign="top">Oxidizing components of the shielding gas</entry><entry align="center" valign="top">VS</entry><entry align="center" valign="top">Mn</entry><entry align="center" valign="top">Yes</entry><entry align="center" valign="top">Mo</entry><entry align="center" valign="top">Or</entry><entry align="center" valign="top">Ti</entry><entry align="center" valign="top">B</entry><entry align="center" valign="top">NOT</entry><entry align="center" valign="top">O</entry></row></thead><tbody><row><entry align="center" valign="middle">Thread: MC1</entry><entry align="center">-</entry><entry align="center">0,006</entry><entry align="center">1,65</entry><entry align="center">1,06</entry><entry align="center">-</entry><entry align="center">-</entry><entry align="center">0,11</entry><entry align="center">0,0050</entry><entry align="center">0,0033</entry><entry align="center">0,2350</entry></row><row><entry morerows="1" align="center" valign="middle">Deposited metal</entry><entry align="center">18% CO2</entry><entry align="center">0,011</entry><entry align="center">1,33</entry><entry align="center">0,88</entry><entry align="center">-</entry><entry align="center">-</entry><entry align="center">0,041</entry><entry align="center">0,0050</entry><entry align="center">0,0045</entry><entry align="center">0,0548</entry></row><row><entry align="center">3% CO2 + 1% O2</entry><entry align="center">0,010</entry><entry align="center">1,45</entry><entry align="center">1,02</entry><entry align="center">-</entry><entry align="center">-</entry><entry align="center">0,058</entry><entry align="center">0,0050</entry><entry align="center">0,0046</entry><entry align="center">0,0420</entry></row><row><entry align="center" valign="middle">Thread: MC2</entry><entry align="center">-</entry><entry align="center">0,15</entry><entry align="center">1,74</entry><entry align="center">0,57</entry><entry align="center">0,32</entry><entry align="center">0,93</entry><entry align="center">0,08</entry><entry align="center">0,0009</entry><entry align="center">0,0050</entry><entry align="center">0,2120</entry></row><row><entry align="center" valign="middle">Deposited metal</entry><entry align="center">10% CO₂</entry><entry align="center">0,085</entry><entry align="center">1,44</entry><entry align="center">0,41</entry><entry align="center">0,31</entry><entry align="center">0,92</entry><entry align="center">0,020</entry><entry align="center">0,0001</entry><entry align="center">0,0055</entry><entry align="center">0,0454</entry></row><row><entry align="center" valign="middle">Thread: F1</entry><entry align="center">-</entry><entry align="center">0,105</entry><entry align="center">1,70</entry><entry align="center">0,59</entry><entry align="center">0,31</entry><entry align="center">0,95</entry><entry align="center">0,067</entry><entry align="center">0,0009</entry><entry align="center">0,0066</entry><entry align="center">0,0150</entry></row><row><entry morerows="2" align="center" valign="middle">Deposited metal</entry><entry align="center">18% CO₂</entry><entry align="center">0,089</entry><entry align="center">1,37</entry><entry align="center">0,47</entry><entry align="center">0,30</entry><entry align="center">0,91</entry><entry align="center">0,025</entry><entry align="center">0,0004</entry><entry align="center">0,0073</entry><entry align="center">0,0400</entry></row><row><entry align="center">10% CO₂</entry><entry align="center">0,083</entry><entry align="center">1,57</entry><entry align="center">0,55</entry><entry align="center">0,30</entry><entry align="center">0,94</entry><entry align="center">0,032</entry><entry align="center">0,0004</entry><entry align="center">0,0082</entry><entry align="center">0,0276</entry></row><row><entry align="center">3% O₂</entry><entry align="center">0,077</entry><entry align="center">1,54</entry><entry align="center">0,54</entry><entry align="center">0,30</entry><entry align="center">0,93</entry><entry align="center">0,028</entry><entry align="center">0,0004</entry><entry align="center">0,0074</entry><entry align="center">0,0229</entry></row><row><entry align="center" valign="middle">Thread: F2</entry><entry align="center">-</entry><entry align="center">0,047</entry><entry align="center">1,69</entry><entry align="center">0,79</entry><entry align="center">-</entry><entry align="center">-</entry><entry align="center">0,14</entry><entry align="center">0,0010</entry><entry align="center">0,0025</entry><entry align="center">0,0043</entry></row><row><entry morerows="1" align="center" valign="middle">Deposited metal</entry><entry align="center">9% CO₂</entry><entry align="center">0.067</entry><entry align="center">1,35</entry><entry align="center">0,62</entry><entry align="center">-</entry><entry align="center">-</entry><entry align="center">0,069</entry><entry align="center">0,0009</entry><entry align="center">0,0047</entry><entry align="center">0,0271</entry></row><row><entry align="center">1.5% O₂</entry><entry align="center">0,040</entry><entry align="center">1,54</entry><entry align="center">0,68</entry><entry align="center">-</entry><entry align="center">-</entry><entry align="center">0,080</entry><entry align="center">0,0009</entry><entry align="center">0,0048</entry><entry align="center">0,0214</entry></row></tbody></tgroup></table></tables>

The contents of residual impurities have not been reported in Table 4. Furthermore, the wires marked MCx are filled wires without slag and the wires marked Fx are solid wires.

The results recorded in Table 4 very clearly show that the analysis of the deposited metal is always significantly different from the analysis of the wire used, the amplitude of the differences being a function of the nature and the quantity of the oxidizing gases present in the mixture. gas in which the arc shoots out.

The more the gas mixture is oxidizing, the more the losses of elements having a strong affinity for oxygen such as manganese, silicon, titanium, ... are important while the oxygen content in the deposited metal can increase or decrease compared to that of the wire used according to the oxygen content of this wire.

With regard to carbon, the result is different depending on the nature of the oxidizing gas and the carbon concentration of the wire used.

When the oxidizing element is oxygen, there is always a decrease in the carbon content of the deposited metal compared to that of the wire, a decrease which is all the more significant the greater the oxygen content of the shielding gas.

When the oxidizing element is CO₂, the deposited metal is enriched in carbon with respect to the wire if the latter has a very low carbon content. On the other hand, a decrease in the carbon content in the deposit is observed if the carbon content of the wire is high.

The equilibrium point, i.e. the carbon content of the wire which leads to an identical content in the deposit is an increasing function of the CO content₂ shielding gas and is in the vicinity of 0.08% by weight of the wire for a CO content₂ 20% by volume.

The following examples illustrate all of the rules set out above which improve the properties of welded joints by the hybrid MAG / Laser process.

The 3 tests were carried out with 8 mm thick sheets from the same batch, the chemical analysis of which is given in Table 6.

These tests are distinguished on the one hand by the nature of the shielding gas and on the other hand by the nature of the filler wire used; In the 3 cases, the firing power of the CO2 laser used was 8 kW, the welding speed of 2.1 m / min and the parameters of the electric arc were adjusted to obtain the same deposition rate whatever either the wire used. Indeed, as indicated above, two wires were used for these tests, which had a diameter of 1.2 mm but one was a solid wire and the other a wire filled with metal powders (type "Metal cored") . These wires do not have the same density, which requires implementing a higher wire speed for the cored wire than for the solid wire if one wants to have the same deposition rate with the two wires.

This difference between cored wire and solid wire also implies that the intensities which make it possible to obtain these same deposition rates are not identical: a cored wire having a greater electrical resistance than a solid wire of the same diameter, its melting speed for a given intensity is greater than that of a solid wire (the current passes through the metallic envelope of the cored wire which has heard a smaller section than that of solid wire of the same diameter and therefore greater resistance).

Therefore, the intensity of the current necessary to melt the same mass of wire per unit of time (deposition rate) is lower for the cored wire than for the solid wire. The electrical parameters have also been adjusted to obtain the same arc length regardless of the shielding gas used; this implies using a slightly higher welding voltage when the shielding gas does not contain an active compound, i.e. oxygen in this case.

All the welding conditions for joints J27, J29 and J34 are given in Table 5.<tables id="tabl0005" num="0005"><table frame="all"><title><u style="single">Table 5</u></title><tgroup cols="8"><colspec colnum="1" colname="col1" colwidth="21mm" /><colspec colnum="2" colname="col2" colwidth="29mm" /><colspec colnum="3" colname="col3" colwidth="14mm" /><colspec colnum="4" colname="col4" colwidth="18mm" /><colspec colnum="5" colname="col5" colwidth="26mm" /><colspec colnum="6" colname="col6" colwidth="19mm" /><colspec colnum="7" colname="col7" colwidth="21mm" /><colspec colnum="8" colname="col8" colwidth="20mm" /><thead><row><entry align="center" valign="top">Sample</entry><entry align="center" valign="top">Gas</entry><entry align="center" valign="top">Wire</entry><entry align="center" valign="top">P_laser kW</entry><entry align="center" valign="top">V_welding m / min</entry><entry align="center" valign="top">V_wire m / min</entry><entry align="center" valign="top">Intensity A</entry><entry align="center" valign="top">Voltage V</entry></row></thead><tbody><row><entry align="center">D27</entry><entry align="center">(70% He + 30% Ar) + 3% O₂</entry><entry align="center">FA7</entry><entry align="center">8</entry><entry align="char" char="." charoff="40">2.1</entry><entry align="char" char="." charoff="37">9.2</entry><entry align="center">321</entry><entry align="char" char="." charoff="55">34,2</entry></row><row><entry align="center">D29</entry><entry align="center">(70% He + 30% Ar) + 3% O₂</entry><entry align="center">MC20</entry><entry align="center">8</entry><entry align="char" char="." charoff="40">2.1</entry><entry align="char" char="." charoff="37">9.8</entry><entry align="center">292</entry><entry align="char" char="." charoff="55">34,2</entry></row><row><entry align="center">D34</entry><entry align="center">70% He + 30% Ar</entry><entry align="center">FA7</entry><entry align="center">8</entry><entry align="char" char="." charoff="40">2.1</entry><entry align="char" char="." charoff="37">9.2</entry><entry align="center">321</entry><entry align="char" char="." charoff="55">35,4</entry></row><row><entry align="center" /><entry align="center" /><entry align="center" /><entry align="center" /><entry align="center" /><entry align="center" /><entry align="center" /><entry align="center" /></row></tbody></tgroup></table></tables>

The chemical analyzes of the sheets used, the wires and the three welds are given in Table 6, as well as the hardness values ​​in the base metal and the welds.<tables id="tabl0006" num="0006"><table frame="all"><title><u style="single">Table 6</u></title><tgroup cols="7"><colspec colnum="1" colname="col1" colwidth="36mm" /><colspec colnum="2" colname="col2" colwidth="33mm" /><colspec colnum="3" colname="col3" colwidth="15mm" /><colspec colnum="4" colname="col4" colwidth="15mm" /><colspec colnum="5" colname="col5" colwidth="15mm" /><colspec colnum="6" colname="col6" colwidth="24mm" /><colspec colnum="7" colname="col7" colwidth="26mm" /><thead><row valign="middle"><entry align="center">Elements (% by weight)</entry><entry align="center">8 mm thick sheet</entry><entry align="center">ZF J27</entry><entry align="center">ZF J29</entry><entry align="center">ZF J34</entry><entry align="center">FA7 solid wire</entry><entry align="center">Cored wire MC20</entry></row></thead><tbody><row><entry align="center">VS</entry><entry align="center">0.11</entry><entry align="center">0.10</entry><entry align="center">0.10</entry><entry align="center">0.11</entry><entry align="center">0.057</entry><entry align="center">0.092</entry></row><row><entry align="center">Yes</entry><entry align="center">0.006</entry><entry align="center">0.12</entry><entry align="center">0.13</entry><entry align="center">0.24</entry><entry align="center">0.72</entry><entry align="center">0.75</entry></row><row><entry align="center">Mn</entry><entry align="center">1.45</entry><entry align="center">1.44</entry><entry align="center">1.46</entry><entry align="center">1.45</entry><entry align="center">1.33</entry><entry align="center">2.08</entry></row><row><entry align="center">P</entry><entry align="center">0.011</entry><entry align="center">0.013</entry><entry align="center">0.014</entry><entry align="center">0.018</entry><entry align="center">0.021</entry><entry align="center">0.011</entry></row><row><entry align="center">S</entry><entry align="center">0.008</entry><entry align="center">0.009</entry><entry align="center">0.012</entry><entry align="center">0.016</entry><entry align="center">0.025</entry><entry align="center">0.008</entry></row><row><entry align="center">Cr</entry><entry align="center">0.028</entry><entry align="center">0.031</entry><entry align="center">0.027</entry><entry align="center">0.039</entry><entry align="center">0.057</entry><entry align="center">0.048</entry></row><row><entry align="center">Mo</entry><entry align="center">&lt; 0.001</entry><entry align="center">&lt; 0.001</entry><entry align="center">&lt; 0.001</entry><entry align="center">0.005</entry><entry align="center">0.015</entry><entry align="center">&lt;0.001</entry></row><row><entry align="center">Or</entry><entry align="center">0.036</entry><entry align="center">0.035</entry><entry align="center">0.031</entry><entry align="center">0.036</entry><entry align="center">0.040</entry><entry align="center">0.027</entry></row><row><entry align="center">Al</entry><entry align="center">0.031</entry><entry align="center">0.025</entry><entry align="center">0.021</entry><entry align="center">0.023</entry><entry align="center">---</entry><entry align="center">0.007</entry></row><row><entry align="center">Co</entry><entry align="center">0.010</entry><entry align="center">0.009</entry><entry align="center">0.009</entry><entry align="center">0.007</entry><entry align="center">0.005</entry><entry align="center">0.008</entry></row><row><entry align="center">Cu</entry><entry align="center">0.009</entry><entry align="center">0.024</entry><entry align="center">0.023</entry><entry align="center">0.043</entry><entry align="center">0.13</entry><entry align="center">0.09</entry></row><row><entry align="center">Nb</entry><entry align="center">0.038</entry><entry align="center">0.026</entry><entry align="center">0.023</entry><entry align="center">0.029</entry><entry align="center">0.004</entry><entry align="center">&lt;0.001</entry></row><row><entry align="center">Ti</entry><entry align="center">0.002</entry><entry align="center">0.001</entry><entry align="center">0.007</entry><entry align="center">0.003</entry><entry align="center">0.002</entry><entry align="center">0.053</entry></row><row><entry align="center">V</entry><entry align="center">0.001</entry><entry align="center">&lt; 0.001</entry><entry align="center">&lt; 0.001</entry><entry align="center">0.004</entry><entry align="center">0.002</entry><entry align="center">&lt;0.001</entry></row><row><entry align="center">B</entry><entry align="center">0.0003</entry><entry align="center">0.0001</entry><entry align="center">0.0016</entry><entry align="center">0.0007</entry><entry align="center">0.0008</entry><entry align="center">0.0055</entry></row><row><entry align="center">NOT</entry><entry align="center">0.0040</entry><entry align="center">0.0098</entry><entry align="center">0.0062</entry><entry align="center">0.0057</entry><entry align="center">0.0076</entry><entry align="center">0.0042</entry></row><row><entry align="center">O ppm</entry><entry align="center">0.0022</entry><entry align="center">0.0310</entry><entry align="center">0.0250</entry><entry align="center">0.0030</entry><entry align="center">0.0280</entry><entry align="center">0.2420</entry></row><row><entry align="center">Al / 0</entry><entry align="center">14.1</entry><entry align="center">0.81</entry><entry align="center">0.84</entry><entry align="center">7.7</entry><entry align="center">-</entry><entry align="center">0.03</entry></row><row rowsep="0"><entry>Hardness</entry><entry morerows="1" rowsep="1" align="center" valign="middle">175</entry><entry morerows="1" rowsep="1" align="center" valign="middle">253</entry><entry morerows="1" rowsep="1" align="center" valign="middle">250</entry><entry morerows="1" rowsep="1" align="center" valign="middle">302</entry><entry morerows="1" rowsep="1" align="center" valign="middle">-</entry><entry morerows="1" rowsep="1" align="center" valign="middle">-</entry></row><row><entry>Hv0.5</entry></row></tbody></tgroup></table></tables>

In this table 6, the comparison of welds J34 and J27 shows that the addition of an oxidizing component in the shielding gas leads to an increase in the oxygen content in the molten metal, which results in a very marked reduction its hardness which thus approximates that of the base metal and gives the welded joint more homogeneous properties.

We also note by comparing the joint J29 which was obtained by combining the oxidizing gas with the cored wire MC20, to the joint J27 obtained using the solid wire FA7 associated with the same oxidizing gas, that although the cored wire MC20 is much more loaded in manganese than solid wire FA7, the manganese contents of welds J27 and J 29 are quite similar. This results from the fact that the cored wire contains much more oxygen than the solid wire and that this oxygen significantly reduces the transfer of manganese from the wire to the deposited metal.

We also see that, compared to solid wire F7A, the cored wire contains titanium and boron which we find in part in joint J29, the difference between the wire and the joint coming not only from the transfer coefficient of these elements which is much less than the unit but also of the dilution with the base metal, the welded joints being composed, for one part, of the base metal and, for another part, metal deposited in proportion to the dilution rate as explained above.

Finally, we see that, despite this additional presence of titanium and boron, elements which normally have the effect of increasing the hardenability of the steel, the hardness of the welded joint J29 is slightly lower than that of the joint J27 which does not contain it. not.

All this is perfectly consistent with the above and translates, in fact, the evolution of the microstructure of the welded joint by the presence of oxygen which increases the rate of inclusions and thus decreases the hardenability by trapping a part of the alloying elements (Joint J34 and J27) and by the presence of titanium which allows these inclusions to act as germs for the transformation of austenite into acicular ferrite during cooling when the Al / O ratio is less than 2.5 (joint J27 and J29), what we can see on Figure 2 which presents the macrography, the microstructures as well as the hardnesses of these 3 joints.

In FIG. 2, the resilience values ​​at -40 ° C. measured with reduced Charpy-V test pieces of 5 × 10 mm have also been reported, the thickness of the assembled sheets not allowing the use of standard test pieces of 10 × 10 mm.

These resilience values ​​at -40 ° C like the transition curves shown in FIG. 3 illustrate the improvement in the toughness of the welded joints, when the oxygen content of the molten metal is increased and the transformation is allowed. austenite in acicular ferrite by supplying titanium and oxygen and respecting the Al / O ratio indicated above, these 3 conditions are essential for the germination of this acicular ferrite, the fineness of which is the source of good toughness properties.