Process of hybrid arc-laser welding of ferritic steels

DESCRIPTION

ARC-LASER HYBRID WELDING PROCESS OF FERRICAL STEELS

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

Welding by arcolaser hybrid welding of ferritic steels, such as C-Μη steels defined by EN 10025, micro-alloyed steels defined by EN 10113, or so-called quenched steels according to EN 10137, has , most often, characteristics of resilience or more often low temperature toughness in the molten metal, as well as hardness values ​​in this same zone much higher than that of the base metals.

These mediocre metallurgical characteristics very sharply limit the extent of arcolaser hybrid welding in certain areas of industry, in particular in the fields of shipbuilding, manufacture and laying of pipelines for the transport of petroleum products, offshore, etc.

This problem results from the fact that these steels have been chemically balanced to give them the intended mechanical properties in view of their fabrication process, that is, rolling and subsequent cooling conditions, or heat treatment during their manufacture. for example in the form of plates or tubes.

In fact, the mechanical properties of a steel result, on the one hand, from its chemical composition and, on the other hand, from its microstructure.

The microstructure of a steel and like welding, that is, of the molten metal constituted by the deposited metal and the molten base metal during welding, develops during cooling from the austenitic state to high temperature. to room temperature.

Therefore, for a given chemical analysis, this microstructure, and therefore the mechanical properties of steel (or welding), is a function of the cooling conditions.

Considering, for example, a steel containing about 0.12% by weight of carbon with a low cooling rate, its structure is essentially composed of ferrite, that is, iron atoms stacked according to a crystallographic structure. and a low percentage, typically on the order of 13%, of perlite, ie alternate ferrite and cementite lamellae, which is Fe iron carbide₃C at 6.66% carbon. Its Vickers hardness is then about 130 and its breaking strength is on the order of 400 to 500 MPa.

By contrast, this same steel will have a martensitic structure, that is, a supersaturated solid 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 has extremely rapid cooling from the austenitic state (high temperature).

For cooling rates between these two extremes, mixed structures will be developed comprising martensite, lower bainite, upper bainite, and ferrite + perlite, which will correspond to intermediate mechanical properties.

Continuous cooling transformation diagrams, commonly called TRC diagrams, which are well known to metallurgists, indicate the various microstructures that develop, as well as the hardnesses corresponding to them according to the cooling rate for a given steel and the standard conditions. austenization for this steel, in particular the temperature (usually 50 ° above the full austenite transformation point) and the austenization duration (usually 30 minutes).

Such diagrams show, on the other hand, 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, by comparing the steel diagrams of various compositions, that the cooling rates that the different microstructures mentioned above create are a function of the set of steel alloy elements.

In fact, all alloying elements have an influence on the hardness, that is, on the ability of a steel to acquire a fully martensitic structure, thus also on the critical quenching speed, which is the minimum cooling rate since the austenitic state. which gives a 100% martensitic structure.

The carbon content, in addition to influencing the temperability, also conditions the mechanical properties of the various structures.

The hybrid laser-arc welding processes, due to the high power density associated with it and the high welding speeds that can be achieved, which are many times higher than laser-only welding, lead to very fast cooling speeds.

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

This phenomenon can be alleviated by the addition of a cold metal inflow metal, ie a welding wire extended at the plane of the plate joint, immediately upstream of the impact of the laser beam and arc used during laser-TIG or plasma hybrid welding, or in the form of consumable electrode wire when the process is a laser / MIG or MAG hybrid.

In so doing, it is intended to adjust the temperability of the molten metal, most of the time reducing its alloying content relative to the metal or the base metals, but also increasing it in the case of very soft steels, ie those with a yield strength of less than 240 or 280 MPa.

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

However, doing so is often insufficient because, with hybrid laser welding processes, the ratio of inflow metal to molten metal in most cases is in the order of 20% by weight and very rarely exceeds 40%. which, even using commercially available low-load alloy wire, means 0.5% by weight manganese wire, It does not allow the temperability of the molten metal to be sufficiently lowered to prevent the formation of hard and brittle structures in the case of steels which, without this addition, already lead to a hard and brittle structure.

On the other hand, in the case of very soft steels, for which it may be desired to increase the temperability to avoid the formation of a coarse and brittle structure, the use of a wire more loaded with alloying elements than the metal to be welded. In order to obtain a thinner structure in the molten metal, it does not appear to be a satisfactory solution either, because this thinning of the structure is accompanied by a large increase in hardness and therefore does not lead to if not a slight decrease in fragility.

DE-A-4 006 167, considered to be the best state of the art, discloses a process for laser welding of steel, allowing the two longitudinal edges of a metal sheet to be welded to obtain a welded tube, and discloses in combination the features of claim 1.

In addition, US-A-5 744 782 proposes a process for fused welding wire arc of high strength steels, whose electrode wire contains iron, carbon, manganese, nickel, molybdenum, silicon, copper. and boron.

The problem then is to improve laser-arc hybrid welding processes so that welds can be obtained whose microstructure is almost free of hard and brittle micro-constituents, that is, having improved resilience and more generally properties. as well as tensile characteristics more closely related to those of the base metals, such as increased elongation, breaking strength and yield strength, remaining at the same time superior to those of the added materials.

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

The solution of the invention is a hybrid welding process utilizing a laser beam combined with an electric arc using fuse welding wire and shielding gas, wherein said wire is fused by said laser beam and / or Said electric arc for making a welding joint in at least one piece of steel to be welded, characterized in that said welding joint contains from 30 to 1000 ppm by weight of titanium, at least 0.7% by weight of manganese , 50 to 1000 ppm by weight oxygen and less than 10% nickel.

As the case may be, the process according to the invention may comprise one or more of the following technical characteristics:

- the part or parts are ferritic steel,

- the welding joint has an acicular ferrite microstructure,

welding comprises 30 to 800 ppm titanium and / or 100 to 450 ppm oxygen, preferably 50 to 500 ppm titanium and / or 120 to 350 ppm oxygen,

- the welding comprises from 0.7 to 2 wt.% manganese and / or less than 1500 ppm by weight aluminum, preferably 0.8 to 1.7 wt.% manganese and / or less than 500 ppm aluminum, preferably still less than 300 ppm aluminum,

- welding comprises aluminum and oxygen in such proportions that: [A1] / [O] <2.5, where [Al] is the weight ratio of aluminum and [0] is the weight ratio of oxygen, preferably [Al] / [0] <1.5,

- welding comprises less than 0,6% molybdenum, less than 80 ppm boron, less than 1% silicon, less than 0,20% carbon, less than 0,035% sulfur and less than 0,035% carbon. 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,

- the welding comprises less than 0.07% niobium, less than 0.07% vanadium, from 1 to 200 ppm nitrogen and less than 1% chromium, preferably less than 100 ppm nitrogen, less than 0, 03% niobium, less than 0.05% vanadium, and less than 0.3% chromium,

- The laser beam assist gas and / or arc shielding gas is a gaseous mixture containing oxygen up to 20% by volume and / or CO₂ up to 40% by volume,

- the laser beam assist gas is a gaseous mixture additionally containing at least one inert gas, preferably helium, argon or mixtures thereof,

- the fusible wire is a solid wire or coated wire, containing at least one element chosen from titanium, manganese, iron, nickel and possibly boron, molybdenum, carbon or chromium.

The solid or coated welding wire which can be used in a laser / arc hybrid welding process using fused welding wire and shielding gas according to the invention comprises from 100 to 10,000 ppm titanium, from 50 to at 5000 ppm oxygen, up to 1500 ppm boron, the remainder being essentially iron.

In particular, the wire contains one or more ferritic alloys selected from manganese, silicon, molybdenum, nickel and carbon.

More generally, welds made by traditional but non-laser arc welding processes may have several microstructures, among which there is a structure called acicular ferrite, which has excellent toughness properties.

This type of microstructure is specific to welds and is never found in steels even when subjected to similar cooling conditions as welds.

In welds, by contrast, such a microstructure may appear in certain cases for a very wide range of cooling speeds from the austenitic state, including those which, in the case of classic steels, lead to martensite quench structures. and / or inferior bainitis, which are hard and brittle, as reported by Antunes M., Bonnet C., Application of an essay on shifting to the collection of facets while having an influence on the formation of acoustic ferrite, Metallurgical Information Days of the molten area, Societé Française de Métallurgie / Société des Ingénieurs Soudeurs - Section sud-est, Conférence n ° 9, Publication of the Soudure Autogène, 1981.

Acicular ferrite exists in the molten metal only in the presence of some inclusions that serve as intragranular germs for the ferrite when austenite is transformed during cooling.

The existence of these inclusions is known to depend on the oxygen content of the melt zone, but for intragranular germination of ferrite on cooling it seems necessary that these complex inclusions present locally on their surface TiO or a titanium oxide / manganese MnTi₂O₄, as described by Blondeau R., Metallurgie et mechanique du soudage, Hermes Science, Lavoisier 2001, p. 162.

It is therefore evident that titanium plays a key role.

However, having a minimum of titanium, typically a few dozen ppm by weight, is not sufficient for it to be in one or another of the desired forms.

Indeed, it is also essential that the kinetics of the oxidation-reduction reactions leading to the formation of inclusions enable this result to be achieved.

Thus, in addition to the titanium content and the oxygen content, the nature and quantity of the whole of the deoxidizing elements, that is to say, elements which have a high affinity for oxygen, such as aluminum, silicon, calcium, etc., if present in the molten metal, they will also intervene and have a non-negligible impact on the resulting microstructure, regardless of the origin of these elements, the welding wire, the base metal or the gas.

In addition, the nitrogen content must also intervene because, if titanium and aluminum are avid for oxygen, these elements also have a high affinity for nitrogen, although nitrogen, as well as all its avid elements, such as boron, vanadium, niobium, etc., will interfere with oxidation-reduction reactions and condition the appearance in the molten metal of the inclusions necessary to transform austenite into acicular ferrite.

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

In view of the foregoing, the solution of the invention which solves these problems is to introduce, during laser-arc hybrid welding, into the molten metal, through the wire or preferably the wire / gas pair, the elements which allow the formation of inclusions. favorable to the germination of acicular ferrite.

However, to this end, account must be taken of the fact 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 pair used, generally the ratio of welding to each other by the dilution ratio, which is the ratio of the base metal to the molten metal, the ratio being the weight, volume or surface ratio measured from a macrography, since it is a ratio and the densities of the base metal and the deposited metal are almost the same.

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

The foregoing shows that the problem is extremely complex.

However, the tests conducted within the scope of the present invention and given below by way of illustration show that it is possible to propose solutions that work in most cases, i.e. leading to the formation of non-brittle microstructures in the deposited metal by adjusting the proportions of certain particular elements within the weld, thus also controlling their relative proportions in the inflow materials, notably via the wire / gas pair.

Adjustment of oxygen content of molten metal

Actual steels have a very low oxygen content, generally less than 30 ppm by weight, and in most cases contain a residual aluminum content, typically of the order of 100 to 500 ppm by weight, because this element is used as calmage during the preparation of these steels.

Therefore, any oxygen present in these steels is in the form of aluminate inclusions, which are unlikely to serve as a germ for acicular ferrite.

Even if titanium is added by means of the welding wire, it cannot be in the form of oxides necessary for the germination of the acicular ferrite, because aluminum is more reactive than titanium in relation to oxygen and the steel itself is always in excess of the residual oxygen of the steel.

If it is desired 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 oxygen remains. in excess so that it can react with titanium.

Various means may be used independently or in combination to provide oxygen to the molten metal:

- The assist gas used for laser welding may contain oxygen or CO₂by decomposing the latter and releasing oxygen due to the high temperatures in the vicinity of the liquid metal during welding, and / or

- solid or coated welding wire, the oxygen content of which may be significantly higher than that of the base metal; thus, a solid wire may contain several hundred ppm oxygen and a coated wire may contain several thousand ppm oxygen, and / or

- the shielding gas used for the arc part in the case of hybrid arc / laser welding which, according to the design of the hybrid welding equipment, may or may not differ from the shielding gas assisting the laser beam.

Figure 1 shows the evolution in hybrid arc-laser welding, more precisely in hybrid MAG / C0 laser welding.₂, the oxygen content of the molten metal as a function of the oxygen content of helium as well as the oxygen content of the molten metal for the He + 8% C0 mixture₂ (% by volume) using in all cases a power output of either 6 kW or 8 kW and the same inflow metal consisting of solid wire of diameter 1,2 mm and type G2Si in accordance with EN 440 .

The tests were carried out at 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 of 30 1 / min at a welding speed. 14 m / min, an intensity of 370-390 A and a voltage of 39-42 V.

As shown in Figure 1, the oxygen ratio of the molten metal of a laser arc welding weld increases as the oxygen content in the assist gas increases.

It is also seen that, as in the case of laser-only or arc-only welding, the replacement of oxygen with C0₂ with an equivalent content leads to a lower increase in the oxygen content of the molten metal.

Complementary tests conducted in parallel show that the amount of oxygen to be respected in welds should 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 oxygen content translates into an increase in the density of inclusions in the weld, which results in a decrease in the breaking energy at the ductile level (see R. Blondeau: Métallurgie et mechanique du soudage, Hermès Science, introduce more to obtaining

Lavoisier 2001). It is therefore useless oxygen that the amount required inclusions indispensable for the germination of acicular ferrite.

Adjustment of titanium content of molten metal

Since titanium is indispensable for inclusions to effectively play the role of germs for the transformation of austenite into acicular ferrite, during cooling of the weld, it must be supplied by solid or coated wire, if the base metal, that is, the part or parts to be welded do not contain enough of it.

Thus, it has been observed that, whatever the welding process, acicular ferrite does not appear if the titanium content of the molten metal is less than 30 ppm by weight, and beyond a certain value, which may range from about 800 and at 1000 ppm, the transformation into acicular ferrite is suppressed or markedly deteriorated.

It is therefore necessary that the molten metal, i.e. the mixture of base metal and deposited metal in proportion to the dilution rate, contains between 30 and 1000 ppm 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 reactions with the laser assist and / or arc shield gas (see Figure 1 and Table 5), must contain a quantity of sufficient titanium so that when the deposited metal from the wire melt mixes with the base metal in a proportion corresponding to the dilution rate, The mixture thus obtained contains a titanium content of between about 30 and 1000 ppm.

In practice, and since the range of acceptable titanium contents is relatively large, achieving this objective is relatively simple because most base metals contain no titanium and for others their content is generally less than 200 or less. 250 ppm.

Under these conditions, it is seen that if a chemically balanced wire is used to deposit, with the gas used, a metal containing between 150 and 1000 ppm by weight of titanium, the mixture with the base metal will have a titanium content in the domain. required for conversion to acicular ferrite at a dilution rate of 80 to 0% if a steel containing no titanium is welded, and whatever the dilution rate is when welding a steel typically containing less than 800 ppm of that metal.

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

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

In other words, so that the titanium content of the molten metal is between 30 and 1000 ppm, preferably between 50 and 800 ppm, then it is necessary if the base metal diluted in the molten metal does not allow it to reach the minimum value, provide the required titanium by means of the welding wire, taking into account the dilution rate as explained above, but also the titanium transfer coefficient between the wire and the deposited metal, The transfer coefficient being the ratio between the titanium content of the deposited metal and the wire used. This coefficient is always less than 1 and the lower the oxidizing power of the gases used, that is, the more oxygen and or CO₂contain arc shielding in the case of hybrid laser arc welding.

Cast metal temperability adjustment

Temperability is a well-known notion of metalurists, which translates a steel's ability to acquire a 100% martensitic structure.

Temperability may be characterized, in particular, by the critical quenching speed, which is the slowest cooling rate since the austenitic state (high temperature: above 900 ° C for welding steels) which allows the steel to be considered. a 100% martensitic structure.

It can be assessed from continuous cooling transformation (CRT) diagrams that translate, in graphical form, the various structural transformations that a steel undergoes as a function of the cooling rate since the austenitic state.

The farther to the right is the TRC (long time) diagram on the temperature / time axes, the lower the critical quenching speed and the more temperability displayed by the steel.

high

If the conditions are met so that as germs for the

inclusions can transform acicular ferrite to be active austenite in welding, which depends notably on the balances of titanium, aluminum, oxygen, etc., as explained above, it is also necessary, for this microstructure to appear, that the austenite does not have previously transformed into a decomposition product which forms at a higher temperature than the acicular ferrite, in which the acicular ferrite is known to cool between 550 and 450 ° C.

Indeed, some others form some microstructural constituents, such as widmanstatten ferrite, perlite or granular bainite, may appear at higher temperatures.

It is therefore necessary for the molten metal to have sufficient temperability to prevent the transformation of austenite at a temperature above 550 ° C under the proper cooling conditions of laser or hybrid laser arc welding.

In other words, care must be taken to ensure that the temperability of the molten metal resulting from the mixture of the base metal and the deposited metal as a function of the dilution rate is not too low to prevent austenite from becoming coarse constituents and, therefore, poorly resilient before reaching the temperature range that allows the transformation into acicular ferrite (below 550 ° C) or too large to prevent austenite from turning into martensite, including the presence of inclusions favorable to the germination of acicular ferrite.

The chemical composition of the welding wire, which obviously influences the temperability of the molten metal, must therefore be balanced by taking into account the analysis of the base metal, the dilution rate and transfer coefficients of the various chemical elements, which as such. As for titanium, they are a function of the oxidizing power of the gas mixture in which metal droplets pass from the wire to the fusion bath during welding.

Although the thermal cycles produced by hybrid laser arc welding are very fast compared to the thermal cycles produced by the most classic welding processes such as MIG / MAG, submerged arc, plasma, etc., within the scope of the invention, never The formation of acicular ferrite was observed in the strands obtained by the hybrid laser-arc process whose manganese content was less than 0.7%.

It is therefore appropriate to respect a minimum manganese content of 0,7% in the molten metal, preferably at least 10%.

Furthermore, it has been observed that the presence of complementary alloying elements such as molybdenum, nickel, chromium or boron generally increases the proportion of acicular ferrite in the molten metal, this being particularly pronounced for boron, especially when associated with molybdenum, but higher additions can be detrimental, because too high a temperability of the molten metal will lead to a hard and brittle martensitic structure, even if the inclusions necessary to germinate the acicular ferrite are present.

The distinction made between boron and molybdenum in relation to other alloying elements is explained by the fact that the action of these elements on temperability is more pronounced in transformations taking place at high temperatures, ie above 550 ° C, which corresponds to the upper part of the TRC diagrams, which at those produced at lower temperatures, ie below 550 ° C.

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

Adjustment of oxygen content of molten metal

Experiments conducted within the scope of the present invention have shown that it is also necessary to obtain a weld-rich acicular ferrite structure and therefore good low temperature toughness that the Al / O ratio is less than 2.5, preferably less than

1.5.

Indeed, in order to obtain good low temperature toughness values ​​in a welding obtained by the hybrid laser-arc process and to avoid having very high hardnesses in the molten metal, it is necessary to form inclusions in the molten metal which can be germs for the transformation of austenite. on cooling produce acicular ferrite and, as indicated above, These inclusions of titanium-rich oxides can only form if all the oxygen present in the molten metal is not fully bonded to aluminum, whose reactivity to oxygen is higher than that of titanium. Experience shows that this is not the case if the weight ratio Al / O in the molten metal is less than

2.5.

To satisfy this condition, and since aluminum is essentially derived from the dilution of the base metal in welding, it is desirable to enrich the molten metal in oxygen by means of the wire, the welding gas or the wire / gas combination used during arcolaser welding.

Tests carried out within the scope of the present invention (see below) show that excellent ductility, toughness and resilience results are obtained, without excessive hardness, in welding performed using a hybrid laser-arc process when the analysis of the molten metal, either that is to say, welding contains the elements given in Table 1 below (contents in weight ratio).

Table 1

<td>ç</td><td>Si</td><td>Mn</td><td>s</td><td>P</td><td> 0</td><td>You</td><td>Ni</td>

<td> &lt; 0,2%</td><td> &lt;</td><td>0.7 to</td><td> &lt; 0,035%</td><td> &lt; 0,035%</td><td>50 to</td><td>30 to 1000</td><td> &lt;</td>

<td></td><td> 1%</td><td> 1, 8%</td><td></td><td></td><td>650 ppm</td><td>ppm</td><td> 10%</td>

<td>Mo</td><td>Cr</td><td>N</td><td>B</td><td>Nb</td><td>V</td><td>Hello</td>

<td> &lt; 0,6%</td><td> &lt; 1%</td><td>10 to 200 PPm</td><td>&lt;80 ppm</td><td> &lt; 0,07%</td><td> &lt; 0,07%</td><td> &lt; 2,5%</td>

However, welds have better characteristics when they contain the elements of Table 1 in the preferred proportions of Table 2 below (contents expressed as weight ratio).

Table 2

<td>ç</td><td>Si</td><td>Mn</td><td>s</td><td>P</td><td> 0</td><td>You</td><td>Ni</td>

<td>0.03 a</td><td>0.1a</td><td>0.8 to</td><td> &lt; 0,035%</td><td> &lt; 0,035%</td><td>70 a</td><td>50 to</td><td> &lt;</td>

<td> 0,13%</td><td> 0, 6%</td><td> 1,5%</td><td></td><td></td><td>300 ppm</td><td>500 ppm</td><td> 3, 5%</td>

<td>Mo</td><td>Cr</td><td>N</td><td>B</td><td>Nb</td><td>V</td><td>Hello</td>

<td> &lt; 0, 3%</td><td> &lt; 0, 3%</td><td>10 to 100</td><td>&lt;50 ppm</td><td> &lt; 0,03%</td><td> &lt; 0,05%</td><td> &lt; 1,5%</td>

<td></td><td>ppm</td><td></td><td></td><td></td>

The analytical ranges in Tables 1 and 2 correspond to the molten metal resulting from the analysis of the base metal or base metals by welding two pieces of different steel together, and that of the deposited metal depends on the wire, gas or wire / gas pair, taking into account the dilution rate.

In practice, in order to satisfy these ranges and obtain welding according to the invention, it is desirable to proceed as follows.

The composition of the base metal constituting the joining parts (or the piece if welding of edges of a pipe) or an average composition corresponding to that of the base metals if the pieces are of different steels shall be determined.

The dilution rate, ie the proportion of the base metal in the welding to be carried out, is evaluated.

The composition of the depositing metal (wire / gas pair) is then determined so that the base metal + deposited metal mixture leads to a molten metal composition, i.e. welding, in the ranges of Tables 1 or 2 above, taking into account the assessed dilution rate.

Thus, for example, if the base metal manganese content (Mn) is 1% and the dilution rate is estimated at 80%, the preferred molten metal range (see Table 2) is 0 , 8 and 1.5% is met if the manganese content of the deposited metal (wire / gas pair) is between 0 and 3.5%, because for a 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, ie a molten metal, will be obtained, containing 80% Mn from the base metal and therefore 20% Mn from the deposited metal (wire / gas pair).

Therefore, to obtain a weld containing 0.8% Mn (low range of Table 2), it is therefore appropriate to use a Mn-free (deposited metal) wire, ie without Mn. In other words, in this case, all the manganese found in the welding comes solely from the steel of the parts to be welded.

In contrast, to obtain a weld containing 1.5% Mn (high range of Table 2), it is convenient to use a wire / gas pair leading to a deposited metal containing about 3.5% Mn, because In this case, the manganese found in welding will come from 80% of the steel of the parts to be welded (ie 0.8% of the desired 1.5%) and 20% of the wire / gas pair (ie , 0.7% of the desired 1.5%).

An analogous calculation can be made for each of the chemical elements to be introduced in welding, which allows to precisely define the composition of the metal to be deposited, that is, of the wire / gas pair, depending on the parts to be welded.

It should be noted that since the dilution rate, that is, that the ratio of the base metal to the molten metal of inflow laser welds, is generally on the order of 60 to 80%, the metal deposited by the wire / Therefore, the welding wire must contain very little or, on the contrary, a high amount of manganese, according to the manganese content of the base metal (s), and in most cases high levels of titanium and boron. target ranges for molten metal, so that the low percentage of metal deposited in the weld allows the means of the recommended ranges to be reached

<td colspan="3">for these elements</td><td colspan="4">in welding.</td>

<td></td><td>Per</td><td>other side,</td><td>at the</td><td>which refers to</td><td>elements</td><td>Cr, N,</td>

<td>V e</td><td>Nb,</td><td>in practice,</td><td>there is</td><td>only a weak</td><td>interest</td><td>in having</td>

these elements in welding.

However, their presence is almost inevitable by dilution with the base metal, which often contains several of them, or because they are present as unavoidable residual impurities in the joining metals or in the inflow product (eg nitrogen). .

It is therefore important to ensure that their contents are as low as possible and never exceed the maximum values ​​given in Tables 1 and 2 above.

In addition, with reference to the Ni element, the maximum 10% content given in Table 1 corresponds to the very particular case of 9% nickel steel.

When welding said steels with a very high nickel content, it is firstly ensured that the oxygen content is not very high and, if appropriate, this oxygen content is adjusted to make it compatible with welding of this type. nickel steel, that is, preferably working with oxygen contents close to the low oxygen range in Table 1.

Except for these high nickel steels, the most classic steels can be welded effectively while respecting maximum nickel contents in welding of the order of about 2 to 3% by weight.

In view of the above, ranges of metal compositions to be deposited by the wire / gas pair which give the weldment of the molten metal compositions of Tables 1 or 2 are shown. 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 welding is high (&gt; 85%). On the contrary, its analysis should correspond rather to the lower part of the ranges indicated when the dilution rate is significantly lower, eg 75% or less.

Table 3

<td>ç</td><td>Si</td><td>Mn</td><td>s</td><td>P</td><td> 0</td><td>You</td>

<td> &lt; 0,15%</td><td> &lt; 0, 6%</td><td>0.5 to</td><td> &lt; 0,035%</td><td> &lt; 0,035%</td><td>50 to 1500</td><td>150 to</td>

<td>(pref.</td><td>pref.</td><td> 6%</td><td>(pref. &lt;</td><td>(pref. &lt;</td><td>ppm</td><td>1000 ppm</td>

<td> &lt; 0,1%)</td><td>0.1a</td><td>(pref.</td><td> 0,025%)</td><td> 0,025%)</td><td>(pref. 100</td><td>(pref.</td>

<td></td><td> 0,5%</td><td>2nd to</td><td></td><td></td><td>at 1000</td><td>300 to 800</td>

<td></td><td></td><td> 3%)</td><td></td><td></td><td>ppm)</td><td>ppm)</td>

<td>Ni</td><td>Mo</td><td>Cr</td><td>N</td><td>B</td><td>Nb</td><td>V</td><td>Hello</td>

<td> &lt; 10%</td><td> &lt; 0,6%</td><td> &lt; 1%</td><td>10 to 200</td><td> &lt; 80</td><td> &lt; 0,07%</td><td> &lt; 0,07%</td><td> &lt; 0,02%</td>

<td></td><td></td><td></td><td>PPm</td><td>PPm</td><td></td><td></td><td></td>

Furthermore, as mentioned above, the analysis of the metal deposited by the wire / gas pair used during hybrid laser arc welding depends on the analysis of the wire and the oxidizing power of the gas mixture, as it conditions the transfer coefficients of the various elements. alloys contained in the wire.

In the case of hybrid laser arc welding, either TIG or plasma, the inflow metal is supplied as a cold fuse wire.

In this case, chemical exchanges are relatively weak and only significant differences in oxygen content are noted, which can be increased from that of the wire when it contains only a low amount of oxygen (solid wires whose oxygen content is less than 150 ppm) or decreased when the wire contains a lot of oxygen, as is the case, for example, with certain metal cored coated wires. In contrast, the manganese, silicon and titanium contents are always weaker in the deposit than in the wire, the greater the difference the greater the oxidation potential of the laser assist gas, namely oxygen and / or CO content.₂.

Note that in TIG and plasma welding, oxidizing shielding gases cannot be used, otherwise the tungsten electrode would be destroyed.

However, where hybrid welding equipment allows two distinct gas inlets, one for laser beam assist and one for TIG arc or plasma, the laser assist gas may be different and therefore contain oxygen, because in this case this oxidizing gas is not in direct contact with the tungsten electrode.

Similarly, a double-flow plasma torch or double-flow TIG can be used, that is, two-gas torches in which the central gas or plasma gas does not contain oxygen, whereas the ring gas can contain it because it is not in direct contact with the electrode.

In GMAW / laser hybrid welding, a transfer of liquid metal droplets from the end of the fusible wire to the liquid bath through the arc takes place, which implies much more intense chemical exchange and much more pronounced element losses.

Examples of chemical arc transfer as a function of the nature and proportion of oxidizing constituents of the shielding gas with different types of wires are given in Table 4. The analysis of the wires and deposited metal obtained from these wires using gases is shown in the table. have different oxidizing compounds in MAG / Laser hybrid welding.

Table 4

<td colspan="2">components gas oxidants from protection</td><td>ç</td><td>Mn</td><td>Si</td><td>Mo</td><td>Ni</td><td>You</td><td>B</td><td>N</td><td> 0</td>

<td>wire: MCI</td><td> -</td><td> 0,006</td><td> 1,65</td><td> 1,06</td><td> -</td><td> -</td><td> 0,11</td><td> 0, 0050</td><td> 0,0033</td><td> 0,2350</td>

<td>metal</td><td>18% C0₂</td><td> 0,011</td><td> 1,33</td><td> 0,88</td><td> -</td><td> -</td><td> 0,041</td><td> 0, 0050</td><td> 0,0045</td><td> 0,0548</td>

<td rowspan="2">deposited</td><td colspan="2">3% C0₂ +</td><td rowspan="2"> 0,010</td><td rowspan="2"> 1, 45</td><td rowspan="2"> 1,02</td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"> 0,058</td><td rowspan="2"> 0, 0050</td><td rowspan="2"> 0,0046</td><td rowspan="2"> 0,0420</td>

<td> 1%</td><td>O₂</td>

<td>wire: MC2</td><td></td><td></td><td> 0, 15</td><td> 1,74</td><td> 0,57</td><td> 0,32</td><td> 0,93</td><td> 0,08</td><td> 0, 0009</td><td> 0,0050</td><td> 0,2120</td>

<td>metal deposited</td><td> 10%</td><td>co₂</td><td> 0,085</td><td> 1,44</td><td> 0, 41</td><td> 0,31</td><td> 0,92</td><td> 0,020</td><td> 0, 0001</td><td> 0,0055</td><td> 0,0454</td>

<td>wire: Fl</td><td></td><td></td><td> 0,105</td><td> 1,70</td><td> 0,59</td><td> 0,31</td><td> 0,95</td><td> 0,067</td><td> 0,0009</td><td> 0,0066</td><td> 0,0150</td>

<td>metal</td><td> 18%</td><td>co₂</td><td> 0,089</td><td> 1,37</td><td> 0, 47</td><td> 0,30</td><td> 0,91</td><td> 0,025</td><td> 0,0004</td><td> 0,0073</td><td> 0,0400</td>

<td>deposited</td><td> 10%</td><td>co₂</td><td> 0,083</td><td> 1,57</td><td> 0,55</td><td> 0,30</td><td> 0,94</td><td> 0,032</td><td> 0, 0004</td><td> 0,0082</td><td> 0,0276</td>

<td></td><td> 3%</td><td> 0₂</td><td> 0,077</td><td> 1,54</td><td> 0,54</td><td> 0,30</td><td> 0,93</td><td> 0,028</td><td> 0, 0004</td><td> 0,0074</td><td> 0,0229</td>

<td>wire: F2</td><td colspan="2"></td><td> 0,047</td><td> 1,69</td><td> 0, 79</td><td> -</td><td> -</td><td> 0,14</td><td> 0,0010</td><td> 0,0025</td><td> 0,0043</td>

<td>metal</td><td> 9%</td><td>co₂</td><td> 0,067</td><td> 1,35</td><td> 0,62</td><td> -</td><td> -</td><td> 0,069</td><td> 0, 0009</td><td> 0,0047</td><td> 0,0271</td>

<td>deposited</td><td> 1,5</td><td> % 0₂</td><td> 0,040</td><td> 1,54</td><td> 0, 668</td><td> -</td><td> -</td><td> 0,080</td><td> 0, 0009</td><td> 0,0048</td><td> 0,0214</td>

Residual impurities were not listed in Table 4. On the other hand, the wires referred to by MCx are non-slag coated wires and the wires referred to by Fx are solid wires.

The results presented in Table 4 show very clearly that the analysis of the deposited metal is always significantly different from the analysis of the wire used, the magnitude of the differences being a function of the nature and amount of oxidizing gases present in the gas mixture into which the arc erupts.

The more oxidizing the gas mixture, the higher the losses of high oxygen affinity elements such as manganese, silicon, titanium, etc., while the oxygen content of the deposited metal may increase or decrease by relation to the wire used, according to the oxygen content of this wire.

As regards carbon, the result is different according to 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 relative to that of the wire, the greater the higher the oxygen content of the shielding gas.

When the oxidizing element is C0₂, the deposited metal enriches itself with respect to the wire if it has a very low carbon content. In contrast, the carbon content of the deposit is lowered if the carbon content of the wire is high.

equilibrium, that is, the carbon content of the wire leading to an identical content in the deposit, is an increasing function of the C0 content₂ shielding gas and is in the vicinity of 0,08% by weight of the wire for a CO₂ 20% by volume.

The following examples illustrate the set of rules listed above, which allow you to improve the properties of joints welded by the MAG / Laser hybrid process.

The 3 tests were carried out with 8 mm

<td>thickness in the board</td><td>in 6</td><td>one same</td><td>lot, whose analysis</td><td colspan="2">chemistry is referred to</td>

<td colspan="2">These</td><td>essay</td><td>distinguished by</td><td>one</td><td>side by</td>

<td>nature</td><td>of</td><td>gas from</td><td>protection and</td><td>other</td><td>side by</td>

<td>nature</td><td>of</td><td colspan="2">soldering wire used.</td><td>, In</td><td>3 cases, the</td>

<td>potency</td><td>in</td><td>shot of</td><td colspan="2">CO laser₂ used was</td><td>8 kW, the</td>

welding speed of 2.1 m / min and the electric arc parameters were adjusted to obtain the same deposit rate regardless of 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 coated with metal powders (type Metal cored). These wires do not have the same density, which requires the use of a higher wire speed for the coated wire than for the solid wire if one wants to have the same deposit content with both wires.

This difference between coated wire and solid wire also implies that the intensities for obtaining the same deposit rates are not identical: having a coated wire with an electrical resistance greater than a solid wire of the same diameter, its melting rate at a given intensity is greater than that of a solid wire (current passes through the metallic envelope of the coated wire, which, of course, smaller section than solid wire of the same diameter and therefore greater strength).

As a result, the current intensity required to fuse the same wire mass per unit time (deposit rate) is lower for coated wire than for solid wire. Electrical parameters have also been adjusted to obtain the same arc length regardless of the shielding gas employed; This means using a slightly higher welding voltage when the shielding gas does not contain an active compound, ie oxygen in this case.

The joint welding conditions of joints J27, J29 and J34 are the subject of Table 5.

Table 5

<td>Sample</td><td>Gas</td><td>Wire</td><td>Plasma kW</td><td>Vsold. m / min</td><td>air love m / min</td><td>Intensities THE</td><td>Tension V</td>

<td>J27</td><td>(70% He + 30% Ar) + 3% O2</td><td>FA7</td><td> 8</td><td> 2,1</td><td> 9,2</td><td> 321</td><td> 34, 2</td>

<td>J29</td><td>(70% He + 30% Ar) + 3% 0₂</td><td>MC20</td><td> 8</td><td> 2,1</td><td> 9,8</td><td> 292</td><td> 34, 2</td>

<td>J34</td><td>70% He + 30% Ar</td><td>FA7</td><td> 8</td><td> 2,1</td><td> 9,2</td><td> 321</td><td> 35, 4</td>

The chemical analyzes of the plates used, the wires and the three welds are presented in Table 6, as well as the hardness values ​​in the base metal and in the welds.

Table 6

<td>elements (% in Weight)</td><td>thickness sheet metal 8 mm</td><td>ZF J27</td><td>ZF J29</td><td>ZF J34</td><td>wire massive FA7</td><td>wire coated MC2 0</td>

<td>Ç</td><td> 0,11</td><td> 0,10</td><td> 0,10</td><td> 0, 11</td><td> 0,057</td><td> 0,092</td>

<td>Si</td><td> 0, 006</td><td> 0,12</td><td> 0,13</td><td> 0, 24</td><td> 0, 72</td><td> 0,75</td>

<td>Mn</td><td> 1,45</td><td> 1,44</td><td> 1, 46</td><td> 1, 45</td><td> 1, 33</td><td> 2,08</td>

<td>P</td><td> 0, 011</td><td> 0,013</td><td> 0,014</td><td> 0,018</td><td> 0,021</td><td> 0,011</td>

<td>s</td><td> 0, 008</td><td> 0,009</td><td> 0,012</td><td> 0,016</td><td> 0,025</td><td> 0,008</td>

<td>Cr</td><td> 0, 028</td><td> 0,031</td><td> 0,027</td><td> 0,039</td><td> 0,057</td><td> 0,048</td>

<td>Mo</td><td> &lt; 0,001</td><td> &lt; 0,001</td><td> &lt; 0,001</td><td> 0,005</td><td> 0,015</td><td> &lt; 0,001</td>

<td>Ni</td><td> 0, 036</td><td> 0,035</td><td> 0,031</td><td> 0,036</td><td> 0,040</td><td> 0,027</td>

<td>Al</td><td> 0, 031</td><td> 0,025</td><td> 0,021</td><td> 0,023</td><td> —</td><td> 0,007</td>

<td>Co</td><td> 0, 010</td><td> 0,009</td><td> 0,009</td><td> 0,007</td><td> 0,005</td><td> 0,008</td>

<td>Ass</td><td> 0, 009</td><td> 0,024</td><td> 0,023</td><td> 0,043</td><td> 0, 13</td><td> 0,09</td>

<td>Nb</td><td> 0, 038</td><td> 0,026</td><td> 0,023</td><td> 0,029</td><td> 0,004</td><td> &lt; 0,001</td>

<td>You</td><td> 0, 002</td><td> 0,001</td><td> 0,007</td><td> 0,003</td><td> 0,002</td><td> 0,053</td>

<td>V</td><td> 0, 001</td><td> &lt; 0,001</td><td> &lt; 0,001</td><td> 0,004</td><td> 0,002</td><td> &lt; 0,001</td>

<td>B</td><td> 0,0003</td><td> 0,0001</td><td> 0,0016</td><td> 0,0007</td><td> 0,0008</td><td> 0,0055</td>

<td>N</td><td> 0,0040</td><td> 0,0098</td><td> 0,0062</td><td> 0,0057</td><td> 0,0076</td><td> 0,0042</td>

<td>0 ppm</td><td> 0,0022</td><td> 0,0310</td><td> 0,0250</td><td> 0,0030</td><td> 0,0280</td><td> 0,2420</td>

<td>Hello</td><td> 14, 1</td><td> 0,81</td><td> 0,84</td><td> 7,7</td><td> -</td><td> 0,03</td>

<td>toughness HvO, 5</td><td> 175</td><td> 253</td><td> 250</td><td> 302</td><td></td><td></td>

In this table 6, 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 of the molten metal, which translates into a very sharp decrease in its hardness, which It thus approaches that of the base metal and gives the welded joint more homogeneous properties.

It is also verified by comparing joint J29, which was obtained by combining the oxidizing gas with the coated wire MC20, with the joint J27, obtained using the solid wire FA7 associated with the same oxidizing gas, that although the coated wire MC20 is Much more manganese-laden than FA7 solid wire, the manganese contents of welds J27 and J29 are entirely similar. This results from the fact that coated wire containing much more oxygen than solid wire and that this oxygen significantly decreases the transfer of manganese from the wire to the deposited metal.

On the other hand, with respect to the solid wire FA7, the coated wire contains titanium and boron, which is partly found in joint J29, with the difference between the wire and the joint not only due to the transfer coefficient. of these elements, which is much lower than the unit but also of the dilution with the base metal, the welded joints being composed, on the one hand, of the base metal and, on the other hand, of the deposited metal in proportion to the dilution rate, as explained above.

Finally, despite this complementary presence of titanium and boron, elements which normally have the effect of increasing the temperability of steel, the hardness of welded joint J29 is slightly lower than that of joint J27, which does not contain them.

All of this is perfectly consistent with the above and in fact reflects the evolution of the welded joint microstructure due to the presence of oxygen, which increases the inclusions rate and thus decreases the temperability by capturing a part of the alloy elements (joint J34 and J27), and due to the presence of titanium, which allows these inclusions to play the role of 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), which can be seen in figure 2, which shows the macrography, the microstructures as well as the hardness of these 3 joints.

Figure 2 also shows the resilience values ​​at -40 ° C measured with 5 χ 10 mm reduced Charpy-V specimens, not allowing the thickness of the joined sheets to use standard 10 χ 10 mm specimens.

These resilience values ​​at -40 ° C, like the transition curves shown in Figure 3, illustrate the improved toughness of welded joints by increasing the oxygen content of the molten metal and allowing austenite to be transformed into acicular ferrite. by supplying titanium and oxygen and respecting the Al / O ratio indicated above, These 3 conditions are indispensable for the germination of this acicular ferrite whose fineness is the origin of the good toughness properties.