MULTI-COATED ELECTRODE FOR WELDING STAINLESS STEEL

Abstract

The invention relates to a coated electrode comprising a central metal core being surrounded at least in part by an outer coating containing rutile and at least one lithium-based compound and being free of sodium feldspar and potassium feldspar. According to the invention, the electrode comprises at least one inner coating arranged between the outer coating and the central metal core, said inner coating containing at least one sodium-based compound and/or at least one potassium based compound. Associated process for welding stainless steel.

Claims

1. A coated electrode comprising: a central metal core; an outer coating at least partly surrounding the central metal core, the outer coating comprising rutile and at least one lithium-based compound, wherein the outer coating is essentially free of sodium (Na) feldspar and potassium (K) feldspar; and at least one inner coating arranged between the outer coating and the central metal core, the inner coating comprising rutile and one or both of at least one Na-based compound and at least one K-based compound.

2. The according to claim 1, wherein the at least one inner coating is a single inner coating, wherein the inner coating covers at least a part of the central metal core, and wherein the outer coating covers at least a part of the single inner coating.

3. The electrode according to claim 1, wherein the outer coating comprises: at least one lithium-based aluminosilicate, wherein the at least one lithium-based aluminosilicate is present in an amount of 5 to 45% by weight on the basis of a total weight of the outer coating or in an amount such that Li from the at least one lithium-based aluminosilicate is present in an amount of 0.2 to 3% by weight on the basis of the total weight of the outer coating; at least one extrusion agent essentially free of one or both of Na and K; lithium silicate as binder; and one or more metal elements in the form of ferroalloys or of individual elements in an amount of 10 to 55% by weight on the basis of the total weight of the outer coating.

4. The electrode according to claim 1, wherein the outer coating is free of sodium-based and potassium-based compounds.

5. The electrode according to claim 1, wherein a total proportion of Na and K in the outer coating is less than or equal to 0.14% by weight on the basis of a total weight of the outer coating.

6. The electrode according to claim 1, wherein Na.sub.2O and K.sub.2O is present in the outer coating at an amount less than or equal to 0.35% by weight on the basis of a total weight of the outer coating.

7. The electrode according to claim 1, wherein the outer coating comprises 1 to 4% by weight of Li.sub.2O on the basis of a total weight of the outer coating.

8. The electrode according to claim 1, wherein the inner coating comprises: at least one lithium-based aluminosilicate or Li from the at least one lithium-based aluminosilicate present in an amount of 0.1 to 1% by weight on the basis of a total weight of the inner coating; lithium silicate; and about 10 to 55% by weight on the basis of the total weight of the inner coating of one or more metal elements in the form of ferroalloys or of individual elements.

9. The electrode according to claim 1, wherein the inner coating comprises one or both of a sodium-based aluminosilicate and a potassium-based aluminosilicate.

10. The electrode according to claim 9, wherein the one or both of the sodium-based aluminosilicate and the potassium-based aluminosilicate comprises a slate powder.

11. The electrode according to claim 1, wherein the inner coating comprises one or both of at least one extrusion agent and/or at least one arc initiating and stabilizing agent comprising Na and/or K.

12. The electrode according to claim 1, wherein one or both of Na.sub.2O and K.sub.2O are present in the inner coating at an amount greater than or equal to 0.4% by weight on the basis of a total weight of the inner coating.

13. The electrode according to claim 1, wherein when present, one of both of Na.sub.2O and K.sub.2O are present in the inner coating at an amount less than or equal to 1% by weight on the basis of a total weight of the inner coating.

14. The electrode according to claim 1, wherein a total proportion of Na and K in the inner coating is greater than or equal to 0.15% by weight on the basis of a total weight of the inner coating.

15. The electrode according to claim 1, wherein a total proportion of Na and K in the inner coating is less than or equal to 0.36% by weight on the basis of a total weight of the inner coating.

16. The electrode according to claim 1, wherein the inner coating comprises up to 2.5% of Li.sub.2O on the basis of a total weight of the inner coating.

17. The electrode according to claim 1, wherein a weight ratio between the outer and the inner coating ranges from 1.5/1 to 2.5/1.

18. The electrode according to claim 1, wherein one or both of the outer coating and the inner coating comprises, on the basis of a total weight of the one or both of the outer coating and the inner coating, a powder comprising: 0.8 to 18.5% by weight of Al.sub.2O.sub.3; 5 to 40% by weight of SiO.sub.2; 15 to 45% by weight of TiO.sub.2; 1.4 to 4.2% by weight of CaO; 1.4 to 4.2% by weight of MgO; and 0.5 to 10% by weight of CaF.sub.2.

19. The electrode according to claim 1, wherein one of both of the outer coating and the inner coating comprises, on the basis of a total weight of the one or both of the outer coating and the inner coating: 0.4 to 10.0% by weight of Al; 2.0 to 19% by weight of Si; 9.0 to 27% by weight of Ti; 1.0 to 3.0% by weight of Ca; and 0.8 to 2.5% by weight of Mg.

20. A method of manufacturing an electrode, the method comprising: providing a central metal core; carrying out a concentric extrusion of an inner coating around at least a part of the central metal core, the inner coating comprising rutile and one or both of at least one sodium-based compound and at least one potassium-based compound; carrying out a concentric extrusion of an outer coating around at least a part of the at least one inner coating, the outer coating comprising rutile and at least one lithium-based compound and being free of sodium feldspar and potassium feldspar; and baking the central metal core coated with the inner coating and the outer coating in a furnace.

21. An assembly of an inner coating and an outer coating for an electrode, the outer coating covering at least a part of the inner coating, the outer coating comprising rutile and at least one lithium-based compound and being free of sodium feldspar and potassium feldspar, and wherein the inner coating comprises one or both of at least one sodium-based compound and at least one potassium-based compound.

22. A process for arc welding one or more stainless steel workpieces, in which an electrode according to claim 1 is used to produce at least one welded joint on the one or more workpieces.

Description

[0081] The present invention will now be better understood thanks to the following detailed explanations.

[0082] Low Fume Emission and Low Cr.sup.VI Content

[0083] In order to considerably reduce the contents of compounds containing the element Cr.sup.VI in the fume, the formulation means employed consist in providing a coated electrode having an inner coating and an outer coating, and adopting the solution of eliminating in the formulations of the outer coating all ingredients containing the alkaline metal elements Na and K and in substituting them with equivalent ingredients based on lithium (Li).

[0084] Thus, the Na-based and K-based compounds (KAISi.sub.3O.sub.8 and NaAlSi3O8) normally present are replaced in the outer coating with equivalent or similar Li-based compounds, such as spodumene (LiAl(Si2O.sub.6)), petalite (LiAlSi.sub.4O.sub.10) or eucryptite (LiAlSiO.sub.4) for example.

[0085] The main function of these compounds used as coating constituents is to control the viscosity of the liquid slag, help to form the slag and therefore to shield the deposited metal, and to help to stabilize the arc during welding.

[0086] But the viscosity of the lithium silicates used within the context of the invention is generally very low, i.e. typically from 15 to 50 centipoise (cp) at room temperature (20 C.), and therefore much less than those of the conventional Na and/or K silicates, the viscosity range of which is typically from 150 to 600 cp. The density of the lithium silicate used within the context of the invention is around 1.2.

[0087] Consequently, owing to the high fluidity and the very specific rheological properties of the lithium silicate recommended within the context of the conventional solution of eliminating in the formulations all ingredients containing the alkaline metal elements Na and K and in substituting them with equivalent ingredients based on lithium (Li), substantial difficulties do arise at various stages in the process for manufacturing the environmentally friendly stainless steel electrodes, in particular: [0088] the low viscosity of the Li silicate results in a lack of tack of the latter and, consequently, results in difficulties, on the one hand, in obtaining good plasticity of the paste used for its preparation during the mixing/wetting steps and, on the other hand, in compacting and extruding the paste and in forming it around the metallic core of the electrode; [0089] the nature of the Li silicate causes an embrittlement effect in the coating, which occurs during the final electrode baking cycle.

[0090] Thus, the electrodes thus obtained have coatings that are less strong from the mechanical standpoint (resistance to impact, dropping, rubbing, bending, etc.) while they are being packaged, transported and subsequently used in an industrial environment, than standard rutile smooth-fusion electrodes. These electrodes also exhibit lower operating weldability than smooth-fusion type electrodes.

[0091] To alleviate the abovementioned difficulties, it is proposed a multi-coated electrode comprising at least one outer coating and one inner coating arranged between the metal core and the outer coating, wherein the outer coating is free of Na and K compounds and the inner coating contains Na and/or K compounds.

[0092] In an unexpected way, Na and K compounds, even contained in small amounts in the overall coating material of the electrode, that is to say when considering the inner and the outer coatings globally, are still able to provide good welding operative performance and robustness when said compounds are not uniformly dispersed in the overall coating but rather located in major proportion, even exclusively, near the central metal core of the electrode. In this way, the Na and K compounds are placed as close as possible to the area of the welding arc and are able to ensure good operative performances.

[0093] Thanks to the invention, it is thus possible to formulate an electrode having, in its overall coating, a spatial distribution of Na and K compounds that is controlled so as to reduce fume and Cr.sup.VI emissions with respect to standard rutile smooth fusion electrodes, while having welding and strength performances similar to that obtained with those standard rutile smooth fusion electrodes.

[0094] Table 1 below illustrates, for an electrode of 308L grade with a central core made of stainless steel of the 308L type and having a diameter of 3.2 mm, the formulation basis of an outer coating and an inner coating according to the invention (ranges of % values).

[0095] According to the invention, the electrode may also have a diameter of 2.5 mm, 3.2 mm, 4.0 mm or 5.0 mm.

[0096] The presence of the Na-based and K-based compounds in the outer coating comes from residual traces of these elements. Despite the precautions taken, the formulation of the outer coating may therefore not be completely free of the elements Na and K, which are in the form of impurities that are unavoidable but not intentionally added.

TABLE-US-00001 TABLE 1 Composition (% by weight in the Inner Outer considered coating) coating coating Li.sub.2O 1-2 2-3 K.sub.2O + Na.sub.2O 0.6-0.8 0-0.2 TiO.sub.2 15-40 15-40 SiO.sub.2 10-30 10-30 Al.sub.2O.sub.3 3-10 3-10 Carbonates 5-15 5-15 CaF.sub.2 1-8 1-8 Metallic materials (Cr, Ni, Mn, Fe) balance Balance

[0097] Table 2 below illustrates, for two electrodes (A and B) of the 308L type, with a 3.2 mm diameter central core made of stainless steel of the 308L type, these being formulated on the same formulation basis and from the same lithium silicate introduced in liquid form in a fixed amount for wetting, the influence of the choice of feldspar type on the amount of hexavalent chromium in the welding fume generated by these electrodes.

[0098] The electrode of Formula A was formulated from a blend of dry powders according to the prior art, whereas the electrode of Formula B consisted of dry powders according to the invention, both formulations being manufactured by means of a lithium silicate according to the invention.

[0099] The percentages (%) are expressed as % by weight in the constituent in question.

TABLE-US-00002 TABLE 2 B (Invention) A Outer Electrode type (Prior art) Inner coating coating Raw materials Various metal 22.5% 21% 21% (powder + elements binder) of the Oxides, 48.9% 55% 49% coating carbonates, composition (% fluorides and by weight in the other extrusion coating agents Type of Na and K Specific low K Spodumene = aluminosilicate feldspar: alumino-silicate (Li 22% (preferably slate compound): powder) = 9% and 25% Spodumene = 11% Composed Li silicate Li silicate K + Na silicate Silicate SiO.sub.2 4.6% 3.46% 4.32% (dry Li.sub.2O 0.06% 0.48% 0.6% part) K.sub.2O 1.8 0 0 Na.sub.2O 0.2% 0.06% 0.08% Total 100% Rate of Cr.sup.VI emission 9.9 mg/min 2.35 mg/min Rate of fume emission 0.30 g/min 0.21 g/min Resultant Cr.sup.VI in the fume 3.24% 1.08%

[0100] It may be noted that, in Table 2, the Li.sub.2O content of 0.48% is the Li.sub.2O which is sourced exclusively by the silicate (dry part), whereas in Table 1, the Li.sub.2O content of between 1 and 2% represents the total Li.sub.2O (sourced by the silicate plus the spodumene.

[0101] As Table 2 shows, Electrode B according to the invention results in fume containing 3 times less Cr.sup.VI than with Electrode A. When considering the rate of fume emission expressed in mg/min, the electrode B according to the invention releases 4 times less Cr.sup.VI.

[0102] Likewise, the rate of fume emission from Electrode B according to the invention is greatly reduced compared to that from Electrode A.

[0103] Indeed, Electrode B has an outer coating formulated from spodumene as substitute for the Na and K feldspars used in Electrode A, while the inner coating is formulated from spodumene and slate powder as a specific silico-aluminate having a low K content.

[0104] Slate powder, also called sericite, is a multi-mineral, metamorphic argilaceous rock made of an aggregate of minerals and colloidal substances. Its essentially mineral composition includes quartz, mica, chlorite, sericite & oxides of iron with occasional spots/knots of minerals like garnet, pyrite, andulasite.

[0105] The use of slate powder as a silico-aluminate material according to the invention has the particular advantage of ensuring simultaneously good extrusion behavior and good weldability, while containing a low K content.

[0106] Moreover, within the context of the present invention, it is also advantageous to consider extrusion agents for formulating the coated electrodes.

[0107] In general, these are organic materials which, in combination with the binders and powders used, make it possible to obtain good consistency of the paste and acquisition by the latter of its rheological properties so that it can be extruded around the metal core of the electrode.

[0108] In addition, good paste consistency makes it possible to achieve good coating strength after baking. Moreover, the extrusion agents have to be chosen judiciously, since drying the electrodes results, within the coating, in them decomposing into ash, the hydroscopic nature of which is deleterious to the electrodes.

[0109] While taking all this into account, within the context of the present invention, certain constituent extrusion agents of conventional electrode coatings, which traditionally contain the elements Na or K, were replaced in the outer coating with other compounds containing neither of these elements.

[0110] Thus, it is recommended within the context of the present invention to proscribe the extrusion agents frequently employed, such as Na or K alginates, from the inner and outer coating and to replace them with suitable extrusion agents according to the invention, such as carboxymethylcellulose (CMC), hydroxyethylcellulose or any other water-soluble organic substance or resin, calcium alginate, plant-based polymers, such as guar gum, talc (with a typical formula of 3MgO.4SiO.sub.2.H.sub.2O) or else clay (with a typical formula of Al.sub.2O.sub.3.2SiO.sub.2.2H.sub.2O).

[0111] Advantageously, the inner coating may contain at least one extrusion agent containing Na and/or K, so as to maintain operative welding performance and resistance performance similar to that of smooth-fusion type electrode.

[0112] This is illustrated in Table 3, by the difference between the smooth-fusion type electrode C having a single coating wherein extrusion agents contain Na and K according to the prior art, and electrode D, which is in accordance with the invention and where the Na-based and K-based extrusion agents were replaced with extrusion agents free of Na and K in the outer coating. The electrodes are of the same diameter of 3.2 mm and manufactured from Li silicate.

[0113] Furthermore, to produce environmentally friendly stainless steel electrode formulations according to the invention, it is advantageous to replace, in the outer coating, the Na-based and/or K-based binders normally used with purely Li-based binders.

[0114] The binders are generally aqueous silicates used in liquid form for agglomerating the dry powders making up the coating before the paste used for the extrusion is formed. The amount of silicate used must be such that a thin film is created between the powder particles, the silicate or silicates acting as a bridging agent between the powder particles.

[0115] Coating Robustness of the Coated Electrodes

[0116] By complying with the formulation rules according to the invention, it is possible for environmentally friendly stainless steel electrodes to have a robust coating after they have been baked, and to be manufactured on an industrial scale under satisfactory conditions.

[0117] In order to quantitatively assess the coating robustness of the electrodes manufactured in the course of the development, several types of tests were carried out: [0118] bending test: the electrodes are bent on a cylinder having a diameter which depends on the electrode diameter. For a 2.5-mm diameter electrode, the bending diameter is 230 mm. For a 3.2-mm diameter electrode, the bending diameter is 300 mm. For a 4.0-mm or 5.0-mm diameter electrode, the bending diameter is 540 mm. The evaluated features are coating adherence to the central core and coating cohesion; [0119] falling test: this test consists in successively dropping ten electrodes, obtained from the same manufacturing run, from a height of 75 cm onto a metallic surface, and in expressing the robustness of their coatings with a fraction by weight of the coating lost after one fall. The evaluated features are coating adherence to the central core and shock resistance; [0120] vibration test: a plastic box is partly filled with electrodes (40% of the internal volume of the box remains free) and is subjected to vibration conditions during 2 minutes on a mechanized industrial sieve equipment. The evaluated feature is the resistance to abrasion.

[0121] For each electrode, the results are expressed in terms of weight loss, following the relation:

Weight loss (%)=100(Initial electrode weight Final electrode weight)/Initial electrode weight.

[0122] For the falling and bending tests, the result expressed for each type of electrodes corresponds to the mean calculated for the ten electrodes of the type in question.

[0123] For the vibration tests, whatever the electrode diameter, 40% of the plastic box remains empty.

[0124] The results given in Table 3 show that the simultaneous/combined use of the environmentally friendly ingredients, namely Li silicate, spodumene and Na/K-free compound in the outer coating and Li silicate, spodumene and a low K-content alumino-silicate in the inner coating, lead to stainless steel electrodes according to the invention (B) having a coating whose mechanical resistance is similar to that of standard electrodes.

[0125] These results show that, by properly controlling the lithium silicate used, and also the formulation/manufacturing parameters, it is possible to achieve levels of coating robustness that are equivalent to those of standard, non-environmentally friendly, stainless steel electrodes, that is to say less than about 2%, even less than about 1.5% of the coating being lost after one drop in respect of electrodes having a core diameter of 3.2 mm or less. It is also important to note that, during welding, no sign of embrittlement of the coating is observed when exposed to the heat of the arc that propagates along the electrode. Thus, the melting of the coating during welding meets the requirements for such smooth-fusion electrodes.

[0126] The bending tests also confirmed the good robustness of the coating on the environmentally friendly electrodes formulated from lithium silicate according to the present invention.

[0127] Crossed tests were carried out with conventional stainless steel electrodes (type A) manufactured from feldspars and Na/K-based silicates, and electrode of type B, whose outer coating is manufactured from spodumene and Li silicate free of Na/K and whose inner coating is manufactured from spodumene and low K silico-aluminate, in particular slate powder and Li silicate according to the invention. The results given in Table 4 demonstrate a similar resistance for types A and B.

[0128] Table 3 show results obtained with electrodes of types A and B, of the grade 308L and having a diameter of 3.2 mm.

[0129] Table 4 show results of similar comparative tests carried out for other grades and diameters of electrodes having coating compositions according to prior art or to the invention.

[0130] The results demonstrate that the negative effect on coating resistance given by the various Li compounds (mainly the binder) can be compensated through the particular coating formulation according to the invention, which is robust, practically insensitive when abrasion or bending stresses are applied. Coating adherence on the rod wire is also good; even slightly inferior with respect to prior art electrode. The coating according the invention is robust enough when subjected to shock (falling).

TABLE-US-00003 TABLE 3 B A (Invention) Electrode type (Prior art) Inner coating Outer coating Raw materials Various metal 22.5% 21% 21% (powder + elements binder) of the Oxides, 48.9% 55% 49% coating carbonates, composition fluorides and (% by weight other extrusion in the coating agents Type of Na and K Na and K (Li aluminosilicate feldspar: feldspar =: 0% Spodumene (Li 22% Spodumene compound) = 25% compound) = 11% Specific low K alumino-silicate (slate powder) = 9% Composed Li silicate Li silicate K + Na silicate Silicate SiO.sub.2 4.6% 3.46% 4.32% (dry Li.sub.2O 0.06% 0.48% 0.6% part) K.sub.2O 1.8 0 0 Na.sub.2O 0.2% 0.06% 0.08% Total 100% Results (loss by weight as a %) Bending 0 0 Falling 0.7 0.8 Vibration 0 0.1

TABLE-US-00004 TABLE 4 Type Mass loss (%) Grade-Diameter Type Bending Falling Vibration 308L-2.5 mm A 0 0.4 0 B 0 0.7 0 308L-3.2 mm A 0 0.6 0 B 0 0.6 0.1 308L-4.0 mm A 0 0.5 0.1 B 0 0.8 0.1 308L-5.0 mm A 0 0.4 0.5 B 0 0.6 0.5 309L-2.5 mm A 0 0.8 0.2 B 0 1.6 0.3 309L-3.2 mm A 0 0.6 0.1 B 0 1.1 0.1 309L-4.0 mm A 0 1.4 0.4 B 0 2.0 0.4 316L-2.5 mm A 0 0 0.2 B 0 1.1 0.3 316L-3.2 mm A 0 1.1 0.4 B 0 1.1 0.6 309L-4.0 mm A 0 0.7 0.1 B 0 1.5 0

[0131] Operating Performance of Coated Electrodes, in Particular Smooth Fusion and Slag Detachment

[0132] Fusion reflects the manner in which the electrode melts during welding. It characterizes the transfer of molten coating and metal droplets that takes place between the electrode, which is consumed, and the weld pool on the workpiece to be welded.

[0133] Fusion that takes place with the transfer of predominantly fine droplets is termed smooth fusion. It is characterized in this case by a regular noise, of low sound intensity, on which a slight crackling is superposed, and is a sign of obvious operating comfort for the welder.

[0134] Smooth fusion is accompanied by a very low amount of spatter during welding. These spatter particles, when they exist, are very fine and represent the amounts of metal that are ejected from the arc during welding or that result from the splashing of the liquid metal droplets in the weld pool.

[0135] In flat welding, the slag line is the line that defines the boundary between the weld pool, that is to say the liquid metal, at the tip of the electrode and the liquid slag floating on the surface.

[0136] Since it defines the size of the weld pool, the shape and the stability of the slag line determines the shape and the regularity of the subjacent weld bead and, in particular, the fineness and the regularity of the striations on the surface of the weld bead after solidification.

[0137] For a smooth fusion electrode, the slag line is generally very close to the tip of the electrode behind the base of the arc.

[0138] The formulation of a smooth-fusion electrode must therefore be such that the slag line appears calm and stable, as otherwise it may constitute an impediment for the welder and generate surface defects in the bead (relatively pronounced and irregularly spaced striations, etc.) or even inclusions of slag in the deposit.

[0139] In general, the formulation of a smooth-fusion electrode must allow stable fusion and a stable slag line to be obtained.

[0140] Apart from the operating aspect during welding, a smooth-fusion stainless steel electrode is characterized by: [0141] in horizontal fillet welding, a generally flat, or even concave, bead appearance; [0142] fine striations regularly spaced apart; [0143] a stable and regular weld bead; [0144] of course, a bead free of defects, such as channels, slag adhesion, cracks or pitting; and [0145] easy slag detachment, or even self-detachable slag, over its entire length or over certain parts.

[0146] In the smooth-fusion rutile formulations, surfactant elements, such as Sb, Bi, Se, Te and S, must be judiciously controlled in the coatings in order to obtain good slag detachment without affecting the operating performance and/or the strength of the product's coating.

[0147] The weld pool visibility is also used to qualify the operative conditions. Indeed, if the weld pool can be easily seen, this means that it does not interfere with the slag, which makes the electrode easy controllable by the welder.

[0148] Advantageously, a good bead aspect means a bead that is concave, regular in shape, having fine and regular ripples, and silver-coloured.

[0149] Good arc striking means that, when the electrode is touched by the workpiece, the arc ignition instantly occurs. Tests of arc re-striking (also called cold re-striking) were also carried out. After starting the welding process, the arc is stopped a few seconds, typically between 7 and 10 seconds, so that the electrode becomes colder. Then the electrode is touched by the workpiece. Good arc re-striking means that the arc ignition occurs instantly, by hitting the electrode on the workpiece only once. This is an advantageous feature since hitting the electrode on the workpiece several times can destroy locally the coating.

[0150] Comparative tests were carried out with electrodes of the 308L grades formulated on a 3.2-mm diameter core made of 308L grade, in flat and horizontal fillet welding positions. Using a 110 A current intensity. The evaluation was done by ranging the results on a scale spanned between 1 to 10, the number 10 corresponding to the highest achievable performance, namely smooth fusion, stable arc, little or no spatter and a weld bead that is attractive, sound, clean, uniform, shiny and finely striated, with good wetting. It can be seen on Table 4 that electrodes of type B exhibit superior operative welding performances than electrodes of type A. Testing conditions are indicated below:

[0151] Welding source: FLEX 4000CEL, OCV=80V

[0152] Base material of the workpiece to be welded: 304L

[0153] Welding positions: flat (PA) and horizontal fillet (PB)

[0154] Polarity: DC+

TABLE-US-00005 TABLE 5 B (Invention) Electrode type A (Prior art) Inner coating Outer coating Raw Various metal 22.5% 21% 21% materials elements (powder + Oxides, 48.9% 55% 49% binder) of the carbonates, coating fluorides and composition other extrusion (% by weight agents in the coating Type of Na and K Na and K feldspar = Spodumen (Li aluminosilicate feldspar: 0% compound) = 25% 22% Spodumene (Li compound) = 11% Specific low K- based alumino- silicate (slate powder) = 9% Composed Li silicate Li silicate K + Na silicate Silicate SiO.sub.2 4.6% 3.52% 4.4% (dry Li.sub.2O 0.06% 0.48% 0.6% part) K.sub.2O 1.8 0 0 Na.sub.2O 0.2% 0% 0% Total 100% Operative performances Strike 10 10 Cold re-strike 10 10 Arc stability 9 10 Weld pool visibility 10 10 Spatters 10 10 Weld bead aspect 9 10 Slag detachability 10 10 Wettability 10 10 Operative performances 9 10 Visible fume 8 10 TOTAL 45 50

[0155] The results of these tests can be extended to current intensity values of 80 A, 115 A, 150 A, and 200 A, used for welding with electrodes having diameters of 2.5, 3.2, 4.0, and 5.0 mm, respectively. Other tests with electrodes of the 309L or 316L grade gave similar results. The operating conditions for each electrode diameter are given in Table 6.

TABLE-US-00006 TABLE 6 Electrode diameter (mm) Plate thickness (mm) Current intensity (A) 2.5 3 75-80 3.2 5 110-115 4.0 10 150 5.0 10 200

[0156] During the tests, the electrodes according to the invention (type B) exhibited a very stable and smooth, spatter-free, arc metal transfer. The strike and cold re-strike were good. The weld bead is almost flat, having a nice silver color aspect, with fine and regular ripples. The weld pool is very clear and visible during welding. The slag removal is good, sometimes even self releasing. In addition, a lower quantity of fumes was noticed. The result of the welding behavior evaluation is summarized in table 5.

[0157] Sensitivity to Cracking Phenomena

[0158] The cracking phenomena were investigated by addressing the potentially most sensitive diameter value, that is to say 5.0 mm. The tests were focused on cracks occurring after the baking cycle, as well on the drying conditions, especially on the water loss dynamic.

[0159] Previous investigations demonstrated that the cracking sensitivity is strongly linked by the moisture amount existing in the coating at the moment of baking. The higher the moisture level, the higher the probability of cracks occurrence. Hence, the bigger the amount of moisture released by the coating during the period of air drying (time between the extrusion and baking phases), the easier the elimination of cracks.

[0160] Since the environmental conditions play also a crucial role on dehydration rate, therefore on the cracks appearance, electrodes according to prior art (type A) and according to the invention (type B) were exposed in different locations, characterized by different temperature and relative humidity, as shown in Table 8.

[0161] The results given in Table 7 show that type B according to the invention allows practically a rate of water releasing during air drying phase that is twice faster than the type A according to prior art. In addition, the type A series need a significantly longer air drying period (24 hours vs. 8 hours for type B) and/or more favorable exposure conditions, that is to say dry spaces. Even in these conditions, the risk of cracking remains high.

[0162] For type B, in normal conditions of 48 hours air drying time, a very low cracking risk is anticipated. Moreover, the achievable shorter air drying time lead to a Work In Progress (WIP) reduction. Indeed, the electrodes are extruded but they still wait to be baked in ovens; the shorter the time period between the extrusion and baking phases, the higher the efficiency, with a relevant positive effect on production efficiency.

TABLE-US-00007 TABLE 7 Weight Weight Weight Air Drying after before after drying Baking Total time extrusion baking baking loss loss loss Cracks Type Drying place (hours) (g/piece) (g/piece) (g/piece) (%) (%) (%) (%) A Laboratory 8 86.70 85.15 82.05 1.72 3.64 5.36 80 A Production 8 86.85 84.55 82.20 2.57 2.78 5.35 70 A Conservation 8 86.75 84.70 82.15 2.29 3.01 5.30 80 chamber A Laboratory 24 86.60 83.37 82.00 3.67 1.64 5.31 0 A Production 24 86.60 83.25 81.95 3.81 1.56 5.37 0 A Conservation 24 86.60 83.25 81.95 3.81 1.56 5.37 0 chamber B Laboratory 8 87.00 84.20 82.00 3.13 2.61 5.75 0 B Production 8 87.10 83.65 82.15 3.89 1.79 5.68 0 B Conservation 8 86.90 83.85 82.00 3.43 2.21 5.64 0 chamber

TABLE-US-00008 TABLE 8 Conservation Production chamber* environnement Laboratory T ( C.) 30 30 25 RH (%) 17 17 20 *The differences in environmental conditions between conservation chamber and production environment are the following: no air currents, stable conditions and no risk of humidity pick-up in the conservation chamber.