WO2011099044A1

WO2011099044A1 - Welding wire and related process of manufacturing

Info

Publication number: WO2011099044A1
Application number: PCT/IT2011/000033
Authority: WO
Inventors: Katia Petitto; Daniele Costantin
Original assignee: Elbor S. R.L.
Priority date: 2010-02-09
Filing date: 2011-02-07
Publication date: 2011-08-18
Also published as: ITRM20100044A1

Abstract

This invention relates to a welding wire, comprising a core (20), made of a material comprising or consisting of iron, externally provided with a surface coating, characterised in that said coating comprises or consists of a surface layer (21) of magnetite. The invention further relates to a process of manufacturing such a welding wire, characterised in that it comprises the following steps: A. cleaning (30) a wire comprising or consisting of iron for removing surface impurities; and B. passivating (31) the wire obtained from step A so as to form a surface layer (21 ) of magnetite.

Description

WELDING WIRE AND

RELATED PROCESS OF MANUFACTURING

The present invention relates to a welding wire that allows to carry out welding in a simple, effective, reliable, safe for operators, and inexpensive, improving the quality of welding, increasing the productivity of welding processes, and avoiding risks for health of operators due to the presence of fumes and/or of harmful substances released by the wire during welding.

The present invention further relates to a process of manufacturing such wire, that is ecological, safe for operators, and inexpensive, increasing uniformity of quality of the manufactured wire.

Presently, metal wires for arc welding, usually of steel, have the surface coated by a copper film, having thickness of about 15 μΐτι, to the end of protecting the wires for a time interval guaranteed by the manufacturer, usually equal to 6 months, from oxidation due to weather. Moreover, the surface coating maintains the wires plain and devoid of surface roughness.

With reference to Figure 1 , it may be observed that the process of manufacturing conventional welding wires begins with a step 1 of unwinding a wire rod, i.e. a metal rod with circular cross-section having diameter larger than 5 mm, usually equal to 5,5 mm, from a skein on which it is wound.

Afterwards, the wire rod is subjected to a step 2 of pickling, for preparing the external surface thereof by eliminating possible chemical substances and/or rolling scale and/or residue of rust.

The wire rod is then subjected to a first step 3 of dry drawing for obtaining an intermediate wire having diameter of 2 mm, that is collected in reels usually having weight of about 2000 Kg.

During the passage in the drawing machine metal hardens, whereby (for many types of starting wire rod) it is necessary a step 4 of annealing for rendering it more ductile, during which the intermediate wire is for instance heated without contact with air at 750°C for 12 hours and subsequently very slowly cooled for other 12 hours until its temperature gets back to room temperature. In this regard, for some particular types of the so-called low alloy steel (also called weathering steel or COR-TEN steel), highly sensitive to hardening, such treatment must be repeated after each rolling passage, whereby drawing cycles are interposed with a plurality, preferably three, of annealing cycles.

Afterwards, the intermediate wire is subjected to a step 5, wherein a wet drawing is carried out for obtaining a wire, having intermediate diameter (slightly larger, usually approximately by 10%, than the final one), followed, without interruptions, by a copper plating bath. During the copper plating bath, the wire is coated through chemical bath by a copper layer having thickness of about 15 μηι. In particular, the chemical bath through which the wire is caused to pass is composed by sulphuric acid (H₂S0₄), copper sulphate (CUS0₄) and water (H₂O), and copper plating occurs due to chemical exchange by etching the wire iron by the acid solution. During copper plating, a part of iron is dissolved in the chemical bath.

Step 5 ends with a final calibration cold drawing, also called skin- pass, wherein the diameter of the wire coated with copper is calibrated through a compression of approximately 10% of its diameter.

The thus obtained wire is then wound in a successive step 6 of winding in commercial reels, usually having weight of 15 Kg, which are packed.

When directed to some special applications, the wire, instead of step 6, is subjected to a step 7 wherein it is straightened and cut in straight pieces, usually having length ranging from 900 to 1000 mm, which are packed.

However, conventional metal wires for arc welding suffer from some drawbacks, related to both the manufacturing process and welding process.

First of all, copper plating of step 5 of the manufacturing process has some problems related to both deposition of a homogeneous coating having constant thickness and environmental impact of the same copper plating.

In fact, when the concentration of sulphuric acid and of copper sulphate progressively decreases, the chemical bath is manually topped up, and this, along with the increase of iron concentration, entails a discontinuity of the characteristics of the same bath which affect the quality of copper deposition rendering it not homogeneous and not much regular. In particular, the speed of deposition of copper on the wire decreases when iron percentage in the chemical bath increases, whereby, since it is a continuous process wherein each part of wire remains in the bath for the same time interval, when the bath is more and more enhanced with iron, the thickness of the deposited layer varies and its surface finishing worsens.

Moreover, when iron percentage in the chemical bath reaches the value of approximately 30 g/litre, copper deposition becomes do much defective that the bath must be considered as exhausted and hence it must be replaced. The exhausted bath cannot be directly discharged because it is highly polluting. Therefore, it must be collected in special airtight containers and stored until it is given to companies specialised in treatment of hazardous special waste. In this regard, it should be noted that the European Union has established severe rules for disposal of waste and sediment containing salts of copper and iron, which require elimination of copper employed in all hot processes.

When afterwards, during execution of an arc welding, metal wires enter the electric arc in order to be melted and deposited on the material to weld, a significant portion of copper of the surface coating vaporises, since arc temperature is very high, and another portion melts together with the weld metal material.

As a consequence of copper vaporisation, during welding gas emissions, i.e. fumes, are generated which are very toxic to humans. Such fumes must be hence sucked with suitable hoods, moved away from the work environment, caused to pass in filtering deposits, and only after their neutralisation they may be discharged in the air.

With reference to the portion of copper that, instead of vaporising, melts together with the weld metal material, it is an element hazardous for the welding efficiency. In fact, the presence of copper, even in minimal amounts, in the weld metal material causes spatters of melted material, it produces porosities in weld bead, and it alters the characteristics of the base material (i.e. the material on which welding is carried put) in proximity to the weld, up to production of a fragility of the welded zone and/or a decrease of the resistance to tensile stress and of the impact strength. In this regard, it should be noted that these negative effects of melting of copper with the weld material are much dependent on the residual percentage of not vaporised copper, that varies at each instant of the welding process, causing a remarkable decrease of reliability and repeatability of the obtained welds.

A further drawback of conventional welding wires is due to the frequent detachment of the copper film or small parts thereof during movement of the wire, in particular during the welding process. This causes formation of a small dust that clogs the nozzle of the coated electrode consequently turning off the electric arc.

Another drawback of conventional welding wires is due to the fact 5 that, generally, at high values of voltage and current of the electric welding process, the weld presents irregularities of deposition of the weld metal material and spatters of melted material occurs in every direction. In case of welding wires coated with copper, these negative phenomena are remarkable and hence it is necessary to limit the current and,0 consequently, the energy of the arc and the welding progress speed.

Therefore, the conventional surface protection through copper plating of the welding wire significantly penalises reliability, repeatability and quality of welding and causes many problems in relation to operator safety and environmental impact in both industrial processes for5 manufacturing the wire and welding processes employing the copper coated wire.

It is an object of the present invention, therefore, to obtain more reliable welds also having better mechanical characteristics of fragility, resistance to tensile stress and impact strength of the welded zone with o respect to the ones of the prior art.

It is still an object of the present invention to eliminate the risks for the operator health to drastically reduce the problems of environmental impact presently existing in welding processes.

It is a further object of the present invention to drastically reduce the 5 problems of environmental impact presently existing in the process of manufacturing conventional welding wires.

It is specific subject-matter of the present invention a welding wire, comprising a core, made of a material comprising or consisting of iron, externally provided with a surface coating, characterised in that said o coating comprises or consists of a surface layer of magnetite.

Always according to the invention, the surface layer of magnetite may be formed starting from said material of the core.

Still according to the invention, the surface layer of magnetite may have thickness ranging from 0,2 μιτι to 4 μιη, preferably from 0,5 μιη to 25 μΐη, more preferably from 0,5 μηπ to 1 ,5 μΐη, still more preferably equal to 1 μΐη.

Furthermore according to the invention, the welding wire may have circular shape.

The advantages offered by the welding wire according to the invention are numerous and significant.

First of all, the welding wire according to the invention has the surface layer harder and with a higher melting point with respect to the rest of the metal of the wire. Consequently, at the moment of melting the wire in the arc, the external layer of magnetite remains solid for a longer time and constitutes a sort of guide for the arc and for the melted metal, with a stabilising effect for the same arc. In other words, during welding, the wire core melts whereas the external layer of magnetite, not yet decomposed, exerts a reduction of the arc diameter with consequent concentration of the energy of the same arc. This allows to significantly improve performance, obtaining an optimal regularity of welding, enabling, during welding cycles, the use of higher power and the achievement of higher progress speeds, improving welding quality and increasing productivity.

Moreover, the higher hardness of the surface layer with respect to the metal of the core of the wire according to the invention protects metal from rust and avoid the onset of the problems present in a conventional copper coating, such as the formation of scale and dust which constitutes one of the most important causes of clogging of the nozzle of the coated electrode and of interruption of the wire flow, which consequently turn off the electric arc and reduce productivity.

Still, during welding, the surface layer magnetite, instead of melting, decomposes in iron and free oxygen due to the high temperatures produced by the electric arc. The presence of the oxygen coming from the magnetite decomposition will have no negative effect in case of welding in an atmosphere of C0₂ or of inert gas, because the oxygen is released only within the arc and does not immediately combine with the slag, without affecting in any way the weld metal nor the one in proximity to the weld bead. This permits not to alter the characteristics of the base material in proximity to the weld, especially with reference to the resistance to tensile stress and to the impact strength, significantly improving the welding quality.

In other words, the welding wire according to the invention permits to obtain: unchanged characteristics of the base material in proximity to the weld, thanks to the complete absence of the material of the surface layer, i.e. the magnetite, in the weld metal; optimal regularity of welding due to the stabilising effect of the magnetite surface layer on the arc; higher productivity of the welding process, thanks to the achievement of higher progress speeds and to the reduction of interruptions.

Furthermore, the welding wire according to the invention drastically reduces the problems of environmental impact, eliminating fumes and hazardous substances produced by the coating of the prior art wire in proximity to the electric arc during the welding process.

Finally, the welding wire according to the invention is effectively protected from the oxidation due to weather for a time interval longer than 6 months.

It also is specific subject-matter of the present invention a process of manufacturing a welding wire as previously described, characterised in that it comprises the following steps:

A. cleaning a wire comprising or consisting of iron for removing surface impurities; and

B. passivating the wire obtained from step A so as to form a surface layer of magnetite.

Always according to the invention, step B may comprise a thermal and/or electrical treatment of the wire obtained from step A.

Still according to the invention, step B may comprise the following sub-steps:

B.1 immersing the wire obtained from step A into an atmosphere with oxygen content ranging from 21 % VA up to 100%;

B.2 heating the wire obtained from step A up to a temperature no lower than 300°C and no higher than 710°C, more preferably no lower than 450°C and no higher than 550°C, still more preferably equal to 500°C, preferably for a time interval no shorter than 0,5 seconds and no longer than 100 seconds, more preferably no longer than 50 seconds, still more preferably no longer than 20 seconds, even more preferably no longer than 2 seconds, the sub-step B.2 being preferably carried out by means of current flow through the wire.

Furthermore according to the invention, step B may comprise, after sub-step B.2, the following sub-step:

B.3 cooling the wire obtained from sub-step B.2, preferably through rapid cooling, more preferably through immersion into a liquid bath and/or per passage under cooling liquid jets and subsequent drying.

Always according to the invention, step A may comprise cleaning the wire comprising or consisting of iron in a bath of water (H₂0) and caustic soda (NaOH).

Still according to the invention, the process may further comprise, after step A and before step B, the following step:

C. subjecting the wire obtained from step B to a calibration cold drawing, or skin-pass, wherein the wire obtained from step A is preferably compressed by no more than 10% of its diameter.

The manufacturing process according to the invention allows to obtain a better constancy of the quality of the manufactured wire.

Moreover, the process uses substances not hazardous to the operator health, such as oxygen.

Still, the process eliminates the problems of environmental impact due to acid baths, exhausted sediment and toxic substances to dispose.

Finally, the manufacturing process according to the invention has a higher productivity.

The present invention will be now described by way of illustration, and not by way of limitation, according to its preferred embodiments, by particularly referring to the Figures of the annexed drawings, in which:

Figure 1 schematically shows a process of manufacturing a welding wire according to prior art;

Figure 2 shows a cross-section view of a preferred embodiment of the welding wire according to the invention; and

Figure 3 schematically shows a preferred embodiment of the process of manufacturing the welding wire according to the invention.

In the following, the same reference numerals will be used for similar elements in the Figures.

With reference to Figure 2, it may be observed that the preferred embodiment of the welding wire according to the invention comprises a core 20, made of a material comprising or consisting of iron (included, for instance, steel and cast iron), having preferably circular shape, externally provided with a surface layer 21 of magnetite (F₃0₄, also known as ferrous-ferric oxide), preferably having thickness ranging from 0,5 μιη to 2 μιη, perfectly adherent to the wire core 20 since it is formed starting from the material of the wire core 20 and, hence, it is a structural part of the same wire.

With reference to Figure 3, the steps of the preferred embodiment of the process of manufacturing the welding wire according to the invention may be observed.

Similarly to the process of manufacturing the welding wire according to the prior art shown in Figure 1 , the process of Figure 3 comprises a step 1 of unwinding a wire rod, a step 2 of pickling, a first step 3 of dry drawing, possibly followed by a step 4 of annealing (and steps 3 and 4 can be also repeated many times), and a final step that can be a step 6 of winding and packing or a step 7 of straightening and cutting in straight pieces and subsequent packing.

After steps 3 and 4, the process of Figure 3 has a step 5' of wet drawing for obtaining a wire having an intermediate diameter that is larger, preferably by no more than 10%, than the final one obtained after a final calibration.

Afterwards, the manufacturing process according to the invention has a step 30 of cleaning the wire in a bath of water (H₂0) and caustic soda (NaOH), in order to remove any surface impurity of the wire. Step 30 ends with a final calibration cold drawing, also called skin-pass, wherein the diameter of the wire coated by copper is calibrated through a compression (preferably by no more than 10% of its diameter) by means of a passage of the wire into an apparatus called drawing machine. In particular, the final diameter of the wire preferably ranges from 0,6 mm to 1 ,6 mm, more preferably equal to 0,6 mm or 0,8 mm or 0,9 mm or 1 ,0 mm or 1 ,2 mm or 1 ,6 mm.

Afterwards, the cleaned wire is subjected to a step 31 of passivation, wherein the surface layer of magnetite is formed, preferably having thickness ranging from 0,2 μιη to 4 μητι, more preferably from 0,5 μΐτι to 2 μιη, still more preferably from 0,5 μηι to 1 ,5 μητι, even more preferably equal to 1 μΐτι; the surface layer of magnetite is preferably formed through a thermal and/or electrical treatment.

Preferably, step 31 is carried out through a thermal treatment including immersion of the wire in an atmosphere with oxygen content ranging from the environmental natural one (21 % V/V) up to 100% (atmosphere of pure oxygen). The wire in such atmosphere is heated up to a temperature no lower than 300°C and no higher than 710°C, more preferably no lower than 450°C and no higher than 550°C, still more preferably equal to 500°C, preferably for a time interval no shorter than 0,5 seconds and no longer than 100 seconds, more preferably no longer than 50 seconds, still more preferably no longer than 20 seconds, even more preferably no longer than 2 seconds. Preferably, the thermal treatment occurs by means of flow of current, more preferably equal to 70 Ampere, into the wire that is transmitted by the pulleys which convey the wire within the oxygen content atmosphere.

Afterwards, the wire provided with the surface layer of magnetite is subjected to a rapid cooling through immersion into a liquid bath and/or through passage under cooling liquid jets, with a subsequent drying. Alternatively, the wire provided with the surface layer of magnetite could be cooled in a different way, or, in the case where it must then be subjected to a hot treatment, it could be also not being cooled.

The preferred embodiments of this invention have been described and a number of variations have been suggested hereinbefore, but it should expressly be understood that those skilled in the art can make other variations and changes, without so departing from the scope of protection thereof, as defined by the enclosed claims.

Claims

1. Welding wire, comprising a core (20), made of a material comprising or consisting of iron, externally provided with a surface coating, characterised in that said coating comprises or consists of a surface layer (21) of magnetite.

2. Welding wire according to claim 1 , characterised in that the surface layer (21) of magnetite is formed starting from said material of the core (20).

3. Welding wire according to claim 1 or 2, characterised in that the surface layer (21) of magnetite has thickness ranging from 0,2 μιη to 4 μητι, preferably from 0,5 μιη to 2 μιη, more preferably from 0,5 μιη to 1 ,5 μηι, still more preferably equal to 1 μηι.

4. Welding wire according to any one of the preceding claims, characterised in that it has circular shape.

5. Process of manufacturing a welding wire according to any one of claims 1 to 4, characterised in that it comprises the following steps:

A. cleaning (30) a wire comprising or consisting of iron for removing surface impurities; and

B. passivating (31) the wire obtained from step A so as to form a surface layer (21 ) of magnetite.

6. Process according to claim 5, characterised in that step B comprises a thermal and/or electrical treatment of the wire obtained from step A.

7. Process according to claim 6, characterised in that step B comprises the following sub-steps:

B.1 immersing the wire obtained from step A into an atmosphere with oxygen content ranging from 21 % V/V up to 100%;

8. Process according to claim 7, characterised in that step B comprises, after sub-step B.2, the following sub-step:

9. Process according to any one of claims 5 to 8, characterised in that step A comprises cleaning the wire comprising or consisting of iron in a bath of water (H₂O) and caustic soda (NaOH).

10. Process according to any one of claims 5 to 9, characterised in that it further comprises, after step A and before step B, the following step:

C. subjecting the wire obtained from step B to a calibration cold drawing, or skin-pass, wherein the wire obtained from step A is preferably compressed by no more than 0% of its diameter.