Patent Application
20050189337
The present invention relates to the field of environmentally friendly coated electrodes of the rutile type, with smooth melting, low fume emission and low emission of hexavalent chromium (CrVI) and having a strong coating, that is to say one that is not friable or only slightly friable, these being intended in particular for the welding of stainless steel.
The fume emitted during welding operations, arising from complex processes, namely vaporization/condensation/oxidation or vaporization/oxidation/condensation, counts among the detractions associated with arc welding. Consequently, the welding fume, the nature and the quantity of which constitute an increasing concern in manufacturing plants, necessitates the use of protection systems, such as fume extractors, so as to preserve the health of operators and members of the personnel working nearby.
From a general standpoint, a stainless steel is defined as an iron alloy whose nominal chromium content is at least 11% by weight. Its use is justified when good oxidation resistance and corrosion resistance are required. Among stainless steels there are several sub-categories of steel, namely:
Consequently, the high content of the element chromium in stainless steels means that, when they are being welded, the constituent particles of the welding fume contain a high content of compounds containing the element chromium, namely trivalent chromium (CrIII), namely the least toxic form of the element chromium, and/or hexavalent chromium (CrVI), a form known as being highly toxic for humans since it is considered to be a carcinogen.
In the case of welding stainless steels, the hexavalent chromium element (CrVI), resulting from the welding fume and present in the air breathed, is therefore particularly regulated owing to its potential toxicity.
Thus, knowing that the regulations in force in most countries indicate that the tolerated average exposure value (AEV) is 5 mg/m3 of air for “harmless” fume particles and that that of the element CrVI contained in the fume is equal to 0.05 mg/m3, as reported by P. J. Cunat in “Le chrome dans les fumées de soudage des aciers inoxydables, [Chromium in stainless steel welding fume], Matériaux et Techniques, No. 1-2 2002, the maximum tolerated concentration of CrVI, in order for this not to entail the need for reducing the maximum fume content in the air breathed, must be at most 1%, i.e. (0.05/5)×100. Below 1%, CrVI is therefore not a factor limiting the amount of permissible fume in the air breathed.
In comparison, since the AEV of trivalent chromium (CrIII) is 0.5 mg/m3, its maximum permissible concentration in fume, in order not to entail the need for reducing permissible fume in the air breathed, is 10%.
Beyond this figure, in welding shops, in order to limit the amount of fume and the proportion of CRVI in the air breathed by operators below the maximum permissible values, using conventional welding products for stainless steels the ventilation of the welding shop must be very much better than that needed when using products for conventional steels.
By adjusting the formulation of a conventional coated electrode, it is possible to reduce the welding fume at source. These formulation modifications thus constitute the most effective way of limiting the harmful effects caused in the welder's environment, even before installing often expensive equipment, such as fume extractors.
This is all the more so as the method of welding with a coated electrode, owing to its ease of implementation, is widely used for welds in confined spaces in certain welding shops or worksites where it is sometimes difficult to install really effective fume extraction.
The principle of CRVI generation in fume is illustrated by equations [1] and [2] below and lies in the formation, during welding, of certain noxious compounds containing the element CrVI, such as for example Na2CrVIO4, K2CrVIO4, NaK3(CrVIO4)2 or K2NaCrVIF6 resulting from the reaction of the elements sodium (Na) and potassium (K) present in the electrode composition with chromium (Cr):
2Na+Cr+2O2→Na2CrVIO4 [1]
2K+Cr+2O2→K2CrVIO4 [2]
To reduce the contents of these compounds containing the element CrVI in the fume, the document by S. Kimura, M. Kobayashi, T. Godai and S. Mimato, “Investigations on chromium in stainless steel welding fumes”, Welding Journal, pages 195s-203s, July 1979, proposed the elimination, in electrode coating formulations, of all ingredients containing the elements Na and K and substitution with “equivalent” ingredients based on lithium (Li).
Thus, it is known either to substitute Na or K feldspars, such as KAlSi3O8 or NaAlSi3O8, present in conventional non-environmentally friendly formulations for electrode coatings with Li-based aluminosilicate compounds having very similar properties, such as petalite LiAlSi4O10, spodumene LiAl(SiO3)2 or eucryptite LiAlSiO4, or to replace the standard Na and K silicates with Li silicate.
However, this solution has always been difficult to implement and has never been really able to be established as an industrial practice since the use of a lithium-based binder as replacement for sodium- and/or potassium-based binders results in electrodes having a fragile, or even highly friable, coating, making the electrodes thus formulated unusable in an industrial environment where the electrodes are often accidentally knocked or roughly handled, leading to their rapid deterioration when they are not mechanically robust enough.
Moreover, compounds based on Na and K, whether in the form of powders and/or liquid silicates, are conventionally used almost automatically in the coatings of coated electrodes in order to give the products their good arc characteristics, especially arc stability and dynamics. This is the reason why electrodes formulated on the basis of lithium silicate alone, and therefore containing no Na and K, exhibit operating weldability that is very inferior to that of standard electrodes.
The document drawn from the experience of the Boehler Thyssen Welding group and published by V. E. Spiegel-Ciobanu “Entwicklung schadstoffarmer hoch legierter Cr—Ni-Schweisszusatze—Teil 1: Reduktion des CrVI-Gehalts im Schweissrauch [Development of low-pollutant welding filler wires for high Cr—Ni alloys—Part 1: reduction of the CrVI content in welding fume]”, Schweissen und Schneiden, 55(4), pages 198-200, May 2003 describes the difficulty of producing such environmentally friendly stainless steel electrodes containing no Na and K, in particular because of the low strength of their coating, and confirms their significantly inferior operating weldability compared with that of standard stainless steel products.
Finally, although the principle of substituting ingredients containing the elements Na and K with “equivalent” ingredients based on Li has been known for a long time for lowering the fume emission content and the amount of CrVI in the fume, only the document by T. Griffiths and A. C. Stevenson “Development of stainless steel welding electrodes having a low level of toxic chromium in the fume”, The 5th International Symposium of the Japan Welding Society, Advanced Technology in Welding, Materials Processing and Evaluation, 5JWS-IV-3, Tokyo, April 1990 describes the manufacture of stainless steel electrodes formulated from exclusively Li silicate and compounds and having a robust coating, with low fume and CrVI emissions, and having operating properties that are said to be “satisfactory”.
However, it turns out in practice that the operating properties of these electrodes have proved to be very inferior to those rutile-type electrodes said to be “smooth fusion” electrodes so that, since the publication of that document, no electrode of this type has appeared on the stainless steel electrode market.
Moreover, the documents by D. O'Donnell and R. Bishel, “Stable low fume stainless steel welding electrode”, Inco Alloys International Inc., 1991 and U.S. Pat. No. 5,124,530 and the document by Koike Hiroyuki, “Cr-contained coated electrode”, Nippon Steel Corp., 1989 and JP-A-1249297 themselves propose stainless steel electrodes with fume emission reduced simply by the use of mixed silicates based on Na, K and Li.
However, the use of mixed silicates based on Na, K and Li does not lower the CrVI content in the fume sufficiently, owing to the presence of Na and K elements resulting in the inevitable formation of hexavalent chromium according to the mechanisms of formulae (1) and (2) mentioned above.
Moreover, several other publications have dealt with fume emissions during welding, and mention may be made, by way of indication, of the following documents:
Given the state of the art, the problem that arises is how to improve coated electrodes intended for welding stainless steels so as:
In other words, the problem that arises is to provide a range of environmentally friendly coated electrode formulations, with a robust coating, of the smooth-fusion rutile type, intended for welding stainless steels, which result in a deposited metal (after fusion) whose chemical composition is in accordance with the standards relating to the various grades of stainless steel, in particular to the standards EN 1600 and AWS A5.4.
The solution of the invention is a coated electrode formed from a central metal core at least partly covered with a robust coating forming a covering over the said core, the said coating containing at least one lithium compound, preferably in general a feldspar, and being free of sodium feldspar and potassium feldspar, characterized in that the coating comprises (the percent by weight of each compound in question being expressed relative to the total weight of coating of the electrode):
Within the context of the invention, the term “free of” a given compound is understood to mean that the said compound has not been intentionally included in the coating and, ideally, that the said coating does not contain any of it at all. However, the possible presence of this compound in trace form as unavoidable impurities is not excluded, although not desirable. Electrodes whose coating therefore contains such compound traces would be considered as being included within the field of protection provided by the present invention.
Depending on the case, the electrode of the invention may include one or more of the following technical features:
The invention also relates to a stainless steel arc welding process in which an electrode according to the invention is used to produce at least one welded joint on one or more workpieces to be welded, and to the coating of such an electrode. The operation of coated-electrode arc welding starts when the operator initiates the welding arc by touching/rubbing the tip of his electrode on the workpiece, the said electrode and the said workpiece forming an integral part of the electrical installation, in the same way as the welding generator, these being connected to one another via the combination of cables of the installation and the earth connection. The intense heat thus produced causes the tip of the electrode and the base metal to melt at the point of impact of the arc. Metal is then transferred through the arc to the workpiece. The metal is thus deposited on the workpiece progressively as the electrode is consumed by being melted. The operator must then ensure that the arc is maintained by keeping the tip of the electrode at a certain height above the workpiece and by moving it at a uniform speed along the workpiece. While the weld is being deposited, a sufficient quantity of heat is maintained in order to melt the tip of the electrode and the zone subjacent to the arc on the workpiece to be welded.
In general, a coated electrode for arc welding is an electrically conducting rod, called a core, surrounded by an adherent covering, usually called a coating, from the tip of which the welding arc emanates. The energy of the arc is thus used as a means of heating the workpieces to be joined together.
The invention also relates to a coating for an electrode, characterized in that it comprises, the percent (%) by weight of each compound being expressed relative to the total weight of coating of the electrode:
During development of the coated electrode, the metal core is generally chosen, as far as possible, in such a way that its chemical composition corresponds to the grade of the base metal to be welded. However, it may also be made of mild steel, that is to say containing practically no alloying element with the exception of a small amount of manganese, the alloying elements essential for depositing the desired grade then being provided by the coating—this then being called a “synthetic” electrode. Whatever the case may be, the content of alloying elements of the coating is never zero, as this non-zero content makes it possible to improve the mechanical properties of the weld and to compensate for the losses due to volatilization of the metal elements during melting of the electrode when an alloyed core, whose composition is close to that of the metal to be deposited, is used, or to provide the alloying elements necessary for synthesizing the composition of the metal to be deposited when a mild steel core is used.
The coating has paramount influence on the welding characteristics and the resulting properties of the deposited metal. Its major roles are not only electrical and mechanical, but also metallurgical.
The main functions that the ingredients in the coating composition must provide are numerous. Most of the constituents may have more than one function and the combination of several constituents depending on the precise contents may allow a particular function to be achieved.
The various coating constituents may thus be classified in various families, namely the constituents in powder form and the constituents in liquid form.
The constituents in powder form are in particular:
Moreover, the constituents in liquid form are especially the binders, which most often are liquid silicates used for agglomerating the dry powders making up the coating before paste that allows extrusion to take place is formed.
The blend making up the coating composition for manufacture of a coated electrode is prepared in an operating method comprising the following steps.
The ingredients in dry form that have to make up the coating composition are firstly weighed and blended so as to obtain a uniform blend. A binder (or several binders) is(are) then added in order to wet the dry blend within a mixer.
After the rheological properties of the coating paste have been assessed, the latter is formed and then a concentric extrusion of the coating around the metal cores, precut to the required length, is carried out by means of an electrode press.
This therefore results in electrode concentricity or centring of the coating extruded around the cores. Good centring is necessary for the quality of the final product. The tips of the electrodes must then be prepared by brushing the coating. The initiating tip of the electrodes is usually prepared by graphitizing or aluminizing, depending on the nature of the product.
Finally, after the electrodes have been predried in the ambient atmosphere, they are baked in a furnace. This baking operation may be carried out, optionally in steps, up to a temperature of around 350-500° C.
The present invention will now be better understood thanks to the following detailed explanations and the appended figures.
Low Fume Emission and Low CrVI Content
In order to considerably reduce the contents of compounds containing the element CrVI in the fume, the formulation means employed consist in adopting 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).
Thus, the Na-based and K-based compounds (KAlSi3O8 and NaAlSi3O8) normally present are replaced with equivalent or similar Li-based compounds, such as spodumene (LiAl(Si2O6)), petalite (LiAlSi4O10) or eucryptite (LiAlSiO4) for example.
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.
Table 1 below illustrates, for two electrodes (A and B) of the 316L type, with a 2.5 mm diameter central core made of stainless steel of the 304L 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.
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.
As Table 1 shows, Formulation B according to the invention, formulated from spodumene as substitute for the Na and K feldspars used in Formulation A, results in a CrVI concentration in the fume of 0.6% instead of 2.7%, i.e. about 4 times lower.
Likewise, the rate of fume emission from Formulation B according to the invention is greatly reduced compared to that from Formulation A.
Moreover, within the context of the present invention, it was also necessary to consider extrusion agents for formulating the coated electrodes. 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.
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.
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 with other compounds containing neither of these elements. Thus, it is recommended within the context of the present invention to completely proscribe the extrusion agents frequently employed, such as Na or K alginates, 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.4SiO2.H2O) or else clay (with a typical formula of Al2O3.2SiO2.2H2O).
This is illustrated by the difference between the electrode C, which is in accordance with the invention except for the extrusion agents containing Na and K according to the prior art, and electrode D, which in all points is in accordance with the invention. Table 2 shows the effect of replacing the Na-based and K-based extrusion agents (in electrode of formulation C) with extrusion agents free of Na and K (in electrode of formulation D) on the amount of CrVI produced and the rate of fume emission, for electrodes of the smooth-fusion type and of the same diameter of 3.2 mm, manufactured from Li silicate (on the basis of 3.62% SiO2+0.52% Li2O+0.06% Na2O dry form by weight in the coating) for the two stainless steel grades, 308L and 316L.
As may be seen, electrode D according to the invention results in about a 20% lower rate of fume emission than electrode C and a chromiumVI content reduced by more than 10% compared with electrode C.
Moreover, Table 3 specifies, for the combinations of extrusion agents used in formulations C and D of Table 2, the corresponding contents of elements Ca, Na and K.
The percentages (%) are expressed as % by weight in the constituent in question.
As may be seen in Table 3, the coated electrode D according to the invention contains approximately the same proportion of calcium as the electrode C, but contains, however, about 12 times less Na and 8 times less K.
The presence of the elements Na and K in the combination of extrusion agents used for formulation D comes from residual traces of these elements. Despite the precautions taken, formulation D is therefore not completely free of the elements Na and K, which are in these formulations in the form of impurities that are unavoidable but not intentionally desired.
Furthermore, to produce environmentally friendly stainless steel electrode formulations according to the invention, it is also essential to replace the Na-based and/or K-based binders normally used with purely Li-based binders.
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.
Essentially all the water contained in these silicates is removed from the coating during the final baking of the electrodes, in order to leave in the coating only the dry part of the silicates introduced, that is to say the alkaline part composed of the compounds Na2O, K2O and Li2O.
The optimum amount of binder to be used depends mainly on its viscosity, on the coupling established between binder and extrusion agents, and on the particle size distribution of the powders used in the formulation.
One of the constraints imposed on the manufacture of environmentally friendly stainless steel electrodes according to the invention is that lithium silicate has to be used instead of the conventional Na and/or K silicates.
Moreover, in order for the CrVI content in the fume to remain below the 1% objective set, the amount of lithium silicate must not exceed a certain maximum amount since above this permissible maximum amount the element CrVI again becomes the limiting factor in the welding fume.
Table 5 illustrates, for two 316L grade electrodes with a diameter of 2.5 mm, formulated on the same formulation basis and using spodumene, the influence of choice of silicate type, that is to say binder type, on the amount of CrVI in the welding fume and the rate of fume emission.
Formulation E according to the invention, formulated from Li silicate as a substitute for Na/K silicates in Formulation F according to the prior art gives a CrVI concentration in the fume of 0.6% instead of 3.0%, i.e. a reduction of 80%.
Likewise, the rate of fume emission from Formulation E is greatly reduced, being almost 50% of that of Formulation F.
Coating Robustness of the Coated Electrodes
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.
Consequently, owing to the high fluidity and the very specific rheological properties of the lithium silicate recommended within the context of the invention, substantial difficulties do arise at various stages in the process for manufacturing the environmentally friendly stainless steel electrodes, in particular:
Thus, without taking precaution in the formulation, the electrodes thus obtained have very fragile coatings and cannot thus claim to be sufficiently 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.
To alleviate the abovementioned difficulties, it is necessary not only to judiciously choose the extrusion agents (e.g. CMC, guar gum, alginates), in particular their nature, amount and combination, which are compatible with the requirements necessary for formulating environmentally friendly electrodes according to the invention, but also to control the particle size distribution of the dry blend.
By complying with these formulation rules and using exclusively lithium silicate as binder, it is possible for environmentally friendly stainless steel electrodes of the smooth-fusion type, having a robust coating after they have been baked, to be manufactured on an industrial scale under satisfactory conditions.
In order to quantitatively assess the coating robustness of the electrodes manufactured in the course of the development, a drop test was carried out.
This test consists in successively dropping ten electrodes, obtained from the same manufacturing run, from a height of 1 m onto a hard horizontal surface, for example a concrete floor, and in expressing the robustness of their coatings with a fraction by weight of the coating lost after one fall, then after two falls.
The result expressed for each series corresponds to the mean calculated for the ten electrodes of the series in question.
These results show that the parameters described have a considerable influence on the strength of the coating on the corresponding electrodes.
Moreover, 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 7% of the coating being lost after one drop in respect of electrodes having a core diameter of 3.2 mm or less, and less than about 15% in respect of electrodes whose core diameter is greater than 3.2 mm. It is also important to note that, during welding, no excessive or abnormal 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.
Another test aimed at assessing the coating strength of the said electrodes, consisting in bending them around a compressed-gas cylinder having a diameter of 230 mm, was used to confirm the good robustness of the coating on the environmentally friendly electrodes formulated from lithium silicate according to the present invention.
Moreover, another series of tests consisted in manufacturing a number of prototype electrodes of the 316L type, by varying the particle size distribution of the dry blend, while also varying the type of rutile (TiO2) and calcite (CaCO3) powders used, these being the predominant non-metallic powders in the formulation of the coating for the smooth-fusion environmentally friendly electrodes.
Crossed tests were carried out by using feldspars and Na/K-based silicates for manufacturing conventional stainless steel electrodes, and spodumene and Li silicate free of Na/K, these being necessary in order to manufacture the environmentally friendly stainless steel electrodes of the invention.
For these tests Na/K-free extrusion agents were used.
The particle size distribution of these powders is given in Table 6.
(fractions in % by weight used on screens, obtained by screening the powder in question in order of decreasing size of screen)
The data given in Tables 7a and 7b show the importance of having a good distribution of the dry blend for obtaining electrodes with a robust coating, each table providing a matrix of tests for demonstrating the influence of the nature of the coating powders and the silicate used on the coating robustness of the electrodes and their environmental friendliness.
(1): loss by weight in the drop test as a % in accordance with the test described with regard to the results of
(2): for assessing the robustness of the electrode coatings, “x” means that the coating is too friable to allow the electrode to be packaged, transported and used under industrial conditions; “yes” means that the coating robustness of the manufactured electrodes conforms to the requirements and allows them to be packaged, transported and used satisfactorily;
(3): “x” means not conforming and “yes” means conforming to the requirements.
(1): loss by weight in the drop test as a % in accordance with the test described with regard to the results of
(2): for assessing the robustness of the electrode coatings, “x” means that the coating is too friable to allow the electrode to be packaged, transported and used under industrial conditions; “yes” means that the coating robustness of the manufactured electrodes conforms to the requirements and allows them to be packaged, transported and used satisfactorily;
(3): “x” means not conforming and “yes” means conforming to the requirements.
These Tables 7a and 7b show that the simultaneous/combined use of the “environmentally friendly” ingredients, namely Li silicate, spodumene and Na/K-free extrusion agents, makes the task even more tricky if it is desired to manufacture environmentally friendly stainless steel electrodes with a strong coating.
To formulate stainless steel products of the smooth-fusion and low fume emission type requires the use of a number of powders having a specific nature depending on the precise contents.
Obtaining stainless steel electrodes of the strong, environmentally friendly, smooth-fusion type is therefore determined by the use of a particle size distribution not limited to only small screen sizes, when Li-based ingredients are used exclusively.
This may be obtained by the use of a rutile powder, a compound in a predominant amount in the coating flux, the particle size distribution of which lies predominantly above 100 μm.
In general, this is obtained by the combined use of a major portion of the blend, the mean particle size of which is greater than or equal to 100 μm, with a secondary portion of fine powders, that is to say of size <40 μm.
Operating Performance of Coated Electrodes, in Particular Smooth Fusion and Slag Detachment
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.
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.
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.
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.
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.
For a “smooth fusion” electrode, the slag line is generally very close to the tip of the electrode behind the base of the arc.
The formulation of a smooth-fusion electrode must therefore be such that the slage 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.
In general, the formulation of a smooth-fusion electrode must allow stable fusion and a stable slag line to be obtained.
Apart from the operating aspect during welding, a smooth-fusion stainless steel electrode is characterized by:
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.
Tables 8a and 8b show that obtaining an environmentally friendly stainless steel electrode according to the invention, of the smooth-fusion type and with a robust coating, is dependent on the use of a number of raw materials whose proportions must be judiciously controlled.
More precisely, Tables 8a and 8b indicate test matrices for demonstrating the influence of the nature of the coating powders used on the operating performance of electrodes and the robustness of their coating.
“x” means not conforming and “yes” means conforming to the requirements.
“x” means not conforming and “yes” means conforming to the requirements.
Characteristics of the Final Product
Table 9 below summarizes all the fundamental principles involved in the formulation/manufacture of environmentally friendly stainless steel electrodes according to the invention that must be met in order to optimize their main properties, namely reduced fume emission, low hexavalent chromium content, less than 1% in the fume, possible manufacture, robust coating, good operating weldability (i.e. 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, as shown in the photographs in
(*): i.e. 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.
Table 10 below shows the unique character of the environmentally friendly stainless steel electrodes of the invention manufactured using lithium silicate and Li-based substitution powders compared with non-environmentally friendly “conventional” stainless steel electrodes formulated from Na and/or K silicates and other ingredients containing the elements Na and K.
The environmentally friendly stainless steel electrodes of the invention make it possible to reduce the fume emission by between 25% and 98% compared with standard stainless steel electrodes and result in a CrVI content (expressed as a % in the fume) 4 to 5 times lower than those of standard electrodes.
Only the environmentally friendly electrodes of the invention give a low hexavalent chromium content, which in addition is less than 1%, which means that CrVI is no longer the factor that determines the toxicity of the fume, and consequently the amount of CrVI emitted (expressed in g/min) is from 5 to 9 times less than those from standard stainless steel electrodes.
The values indicated above correspond to electrodes formulated on a 3.2 mm diameter core made of 316L grade.
Comparison of the compositions of the fume emitted by these various electrodes illustrates the difference in their formulations (see Table 11).
The values indicated above correspond to electrodes of 316L grade with diameters of 2.5 and 3.2 mm.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0450398 | Mar 2004 | FR | national |
