US3490942A - Coated electrode for the welding of alloys with very low carbon content - Google Patents

Description

United States Patent Int. Cl. B23k 35 /00; C23d 3/00 US. Cl. 117-205 3 Claims ABSTRACT OF THE DISCLOSURE A coated electrode for welding alloys, particularly steels with a carbon content lower than 0.030%, in which the coating contains a binder and a mixture of powders comprising from 0% to 6% of basic carbonates; from 3.5% to 10% of deoxidizing substances; from 0% to 1% of organic substances; from 3% to 10% of free MgO; from 3% to 10% of free A1 0 with a weight ratio of MgO to A1 0 lying between 0.3 and 3.33; from 13% to 35% of a powder of metallic alloying substances which enter into the composition of the deposited alloy, at least one of the oxides of acid afiinity of the group TiO and ZrO fluxing agents and complex silicates.

The present invention relates to a coated electrode for use in welding alloys with a carbon content lower than 0.030%, more especially alloy steels with such a carbon content. The invention relates, more especially, to an electrode the coating of which contains a binder in addition to a mixture constituted by powdered alloy metals, deoxidizers, magnesium oxide and aluminum oxide (these two oxides being in a combined state), titanium oxide and/or zirconium oxide (these two oxides being either free or combined), fluidifiers (fluxes) and silicates of various metals.

In the present specification, the expression coefficient of basicity expresses the ratio B/A in which B expresses the number ofgram-molecules or CaO-l-tMgO -l-BaO-l- K O+Na O+LiO +CaF and -A expresses the number of gram-molecules of SiO +TiO ZrO It is known that if alloy steels, in particular stainless steels containing 18% chromium and 8% of nickel or 18% chromium, 8% nickel and 3% molybdenum, have a carbon content higher than 0.050%, the risk arises that intercrystalline corrosion will occur during Welding or thermal treatments, due to the precipitation of carbides.

Coated electrodes as generally employed for welding these alloy steels are known as low carbon electrodes; if they are of good quality, in most cases they allow the deposition of a metal the carbon content of which is lower than 0.060%. Such electrodes are either of the type having a coefficient of basicity at least equal to 3 and the coating of which contains compounds with an acid affinity, such as titanium oxide and/or zirconium oxide, as well as complex silicates of various metals, in

3,490,942 Patented Jan. 20, 1970 a maximum proportion of 15% in relation to the total weight of the powder which it contains, or of the type having a coefiicient of basicity at most equal to 0.6 and the coating of which contains a far more substantial amount of these compounds which have an acid aflinity, possibly together with amphoteric oxides, for example aluminum oxide. In this latter type of electrode, the mixture of powders added to the binder to constitute the coating generally contains from 15 to 25% of alkaline earth carbonates, from 35 to 45% of titanium oxide and/or zirconium oxide and from 5 to 15% of complex silicates, in addition to alloy metals, deoxidizers and fluidifiers based on compound containing fluorine.

These two types of electrode will deposit metal having a carbon content higher by 0.015% to 0.035% than that of the wire which constitutes their core. Now, goodquality wires in general use contain from 0.015% to 0.030% of carbon. It is this that explains why the carbon content of the deposited metal is almost always lower than, but approximating to, 0.060%.

Provided that wires are chosen the carbon content of which is relatively small, approximately 0.020% at most, for example, and provided that rigorous selection is made of the raw materials for the coating, so that they do not contribute much carbon, it is quite easy to deposit metal for welding alloy steels, the carbon content of which does not exceed 0.050%. Such choice of wire and of raw materials does not, however, in the majority of cases, allow the deposition of Welding metal the carbon content of which is lower than 0.030%. The odds that welding metal will be deposited with a carbon content less than 0.020% are only one to two in a hundred, even in the case when the wire has a very low carbon content. It has been confirmed, in fact, that one and the same coating surrenders a larger proportion of carbon to the deposited metal when it is covering a core with a very low carbon content than when it is covering a core with a slightly higher carbon content. In this way the same coating contributes from 0.010% to 0.025% of carbon to the deposited metal when it is covering a wire containing 0.025% of carbon, and contributes from 0.015 to 0.035% of carbon to the deposited metal when it is covering a wire containing only 0.015% of carbon.

Although it has already been confirmed that electrodes with a coeflicient of basicity lower than 0.6 contribute less carbon in relation to the Wire than electrodes with a coefficient of basicity higher than 3, the first-mentioned electrodes having a carbonate-content lower than the last-mentioned ones, there is little hope of regularly reaching, for the deposited metal, a carbon content that is at most equal to 0.020% by employing electrodes with a coefficient of basicity lower than 0.6.

The present invention has for its principal object to provide an electrode which will allow the regular deposition of welding metal for welding alloys and alloy steels, the carbon content of which is not more than 0.015% higher than the carbon content of the wire.

The electrode according to the invention contains at most 6% of basic carbonates, from 3.5 to 10% of deoxidizing alloys, at most 1% of organic substances, from 3 3 to 10% of free MgO, from 3 to 10% of free A1 with the weight ratio of MgO to A1 0 lying between 0.3 and 3.33, and from 13 to 35% of metal powders which enter into the composition of the deposited alloy.

The invention will be more fully understood from the following more detailed description in which are set out, by way of example only, various preferred features which may be exhibited by electrodes in accordance with the invention.

Thus, it may be mentioned that a content of basic carbonates that is equal to, at most, 6% of the total weight of the powders which form part of the constitution of the coating and a content of organic substances equal to at most 1% of the total weight of powders have the effect of lowering the carbon content of the deposited metal. If the basic carbonates and the organic substances are dispensed with entirely, it may be that the carbon content of the deposited metal will not exceed the carbon content of the wire by more than 0.005%

The decrease in the content of basic carbonates would have the effect of decreasing too greatly the degree of basicity of the coating, if it were not compensated for by the presence of magnesium oxide in a free state.

The replacement of alkaline or alkaline-earth carbonates by silicates, titanates or aluminates would likewise have the effect of decreasing the degree of basicity too greatly, lowering it to, for example, less than 0.6. In addition, certain of them would harden upon contact with water or free alkalis.

In order to ensure a proper fusion of the coating, however, it is also impossible to replace the carbonates by a mixture of silica and alumina because the silicon content of the deposited metal would then be too high. It would in fact exceed 0.60%, which is the maximum value generally permitted for stainless steels with 18% chromium and 8% nickel, or for those with 18% chromium, 8% nickel and 3% molybdenum. With a silicon content higher than 0.60%, in fact, the ferrite rating exceeds, for instance,

The employment of oxides of alkaline and alkaline earth metals in the free state has been envisaged, but only magnesium oxide is suitable; the other oxides of this kind are much too reactive with water for coating and conservation of the electrodes to be possible with them.

A 10% proportion of magnesium oxide is used in the absence of basic carbonate. If basic carbonate is present, the proportion of magnesium oxide is that much the less, the greater the proportion of basic carbonate, and is only 3% when the mixture contains the maximum of 6% of alkaline carbonate.

The use of free magnesium oxide has the disadvantage that fusion takes place with sputtering, and that removal of the slag is troublesome. The electrode according to the invention, however, does not have these two disadvantages, thanks to the other features it possesses.

In order to facilitate removal of the slag, the electrode must simultaneously satisfy the three following conditions: (1) the proportion of free magnesium oxide should lie between 3 and 10% of the total weight of the powders; (2) the free magnesium oxide should be mixed with a proportion of free aluminum oxide likewise lying between 3 and 10% of the total weight of the powders; (3) the ratio of the amount of free magnesium oxide to the amount of free aluminum oxide should lie between 0.3 and 3.33.

The mixture of powders may accordingly comprise, for example, the same proportion .of each of these two free oxides, for example 6% (the ratio of the two oxides being then equal to 1) or 3% of magnesium oxide and 10% of aluminum oxide (a ratio equal to 0.3) or 10% of magnesium oxide and 3% of aluminum oxide (a ratio equal to 3.3

From tests that have been carried out it emerges that, in general, the higher the tree m gnesium oxide content the lower the aluminum oxide content should be, and inversely.

To prevent fusion accompanied by much sputtering, the electrode should contain an amount of metal powders (contributing the alloy elements) that lie between 13% and 35 of the total weight of the mixture of powders used for the coating. The diameter of the coating can thus be increased without increasing the amount of slag, and by reason of this fact, fusion proceeds more gently and the weldling procedure is carried out more easily.

Although the lower limit of 13% is not a critical limit, in practice sputtering is acceptable when the mixture contains this proportion of metal powders. The upper limit of 35% is also not a critical limit. But if the mixture contains more than 35% of metal powders, the amount of slag that may form from a coating having a given diameter such as is commonly employed in practice may become too small. The increase in the coating diameter that would then perforce be adopted in order to form a suitable amount of slag would make manipulation of the electrode during welding more difficult.

To reduce the carbon content in the deposited metal, it is equally important to limit the content of ingredients, other than carbonates, that contribute carbon during the welding procedure. From this point of view, it is preferable not to incorporate organic substances, and in the case when such substances are present, not to incorporate them to a greater extent than 1% of the total weight of the powders.

Ferro-alloys often contribute carbon to the deposited metal. Accordingly it is also preferable to limit the content of ferro-alloys whenever it i possible to replace deoxidizing ferro-alloys by the deoxidizing metal elements that they contain.

The mixture of dry powders entering into the composition of the coating .of the electrode should, as in the case of known electrodes, be associated with one or more aqueous alkaline silicates, the proportion of which is from 15 to 30% of its weight.

Carbon contents in the deposited metal lower than 0.030% and even lower than 0.020% are obtained, according to whether the wire constituting the core of the electrode contains at most 0.020% of carbon or at most 0.010% of carbon, together with the following mixture of dry powders:

um feldspar, nepheline-syenite, asbestos, mica, clay,

kaolin, bentonite; 13 to 35 of metal powders which enter into the constitution of the deposited alloy.

This mixture is characterized by the absence of carbonates and of organic substances, by a maximum content of titanium oxide and/or zirconium oxide of 35% and by a minimum content of these oxides of 20%.

The metal powders present in the coating depend on the particular composition of the alloy (and in particular alloy steel) that is to be deposited. The alloy elements in powder form most used nowadays are chromium, nickel, molybdenum, titanium and niobium, these elements being present either separately almost free from foreign elements, or as alloys with one another or in the form of ferro-alloys; in the latter case they are present in a proportion lying between 3.5 and 10% of the total weight of the dry powders.

In the coating of an electrode according to the invention, for use in welding stainless steel with 18% chromium and 8% nickel there will be, for instance, a mixture of powders of chromium, nickel and/or ferro-chromium and/or ferro-nickel in addition to iron powder, these powders being in proportions such that, bearing in mind the composition of the wire, the compoistion of the metal deposited corresponds substantially to the composition of the alloy steel parts to be welded.

To deposit steel containing about 19% chromium, 8% nickel and 73% iron, use is made for example of an electrode the core of which, having a weight of 23 grams and a composition corresponding to 18% chromium, 8% nickel and 74% iron, is covered with a coating layer of 16 grams containing 30% of a mixture of metal powders intended to enter into the deposited metal and consisting of 53% chromium, 5% nickel and 42% iron.

It is known that when a mixture of powders of this type is contained in a coating, the proportions of chromium, nickel and iron that pass into the deposit are of the order of 50%, 100% and 90% respectively. The fusion of the said mixture of additive metals accordingly causes 26% chromium, 5% nickel and 37% iron to pass into the deposited metal, these percentages being expressed with respect to the weight of the mixture of powders of the additive metals.

The fusion of the coating accordingly brings about the passage into the deposited metal of:

It is also known that during the fusion of the wire a loss of chromium of approximately 2% occurs.

The fusion of the wire accordingly brings about the passage into the deposited metal of:

(18-2) 23 --3.68 g. of chromium 8 23 1.84 g. of nickel 100 17.02 g. of iron The deposited metal accordingly contains:

1.248 g.+3.68 g.=4.928 gfof chromium 0.24 g.-|-l.84 g.=2.08 g. of'nickel 1.77 g.+17.02 g.=l8.796 g. of iron out of a total of 25.804 g. of deposited metal.

The composition of this metal, expressed in percentages by weight is therefore:

100 4.928X 19.1% of chromium =8.06% of nickel By means of analogous calculations it may be verified that a stainless steel can be deposited which contains approximately 17.5% chromium, nickel, 2.5% molybdenum and 70% iron, from an electrode the coating of which contains 30% of the following mixture of powders: 36% chromium, 18.65% nickel, 16% molybdenum and 29.35% iron, the said coating being applied to a wire having the following composition: 18% chromium, 8% nickel, 0% molybdenum and 74% iron and the ratio of the weights being 16 g. of coating to 23 g. of wire.

As regards the deoxidizers, these may be chosen from among the ferro-alloys such as Fe-Mn, Fe-Ti, Fe-Si-Ti and/ or from among metallic elements such as Mg, Al, Ti, Si and Ca, taken individually or in association with each other.

6 A coating composition that gave particularly favourable results in the case of the welding of stainless steel containing 18% chromium and 8% nickel is as follows:

This mixture of dry powders was mixed with 290 parts by weight of aqueous alkaline silicate.

A steel wire having 18.59% chromium and 10.7% nickel and containing 0.010% carbon, 1.28% manganese and 0.24% silicon, with a diameter of 3.25 millimetres, was coated with this paste in such a manner that, after drying for 1% hours at 375 C. the diameter of the coating was 5.5 millimetres.

This electrode was connected to the positive pole of a source of direct current delivering a current of amperes during the welding of two chamfered plates placed edge to edge, either with both plates horizontal or with one vertical and the other horizontal. Fusion was fairly gentle and projections were few in number. Slag well covered the weld metal behind the pool of such metal and its removal was effected easily by a process of light chipping. The weld bead had the appearance of a series of fine and regular striations. In the case of welding horizontal sheets placed edge to edge this bead was slightly concave, whereas in the case of welding a vertical sheet to a horizontal one the head was very slightly convex. The weight of deposited metal was of the weight of the wire. Chemical analysis of this metal yielded the following results:

C:0.014%, Mn:1.35%; Si:0.526%; Cr:19.59%; Ni:10.l0%. v

If the steel to be welded comprises 18% chromium, 8% nickel and 3% molybdenum, all that is required is to replace the above wire by a wire having the same composition as the steel to be welded and to introduce 10% of molybdenum powder in place of 10% of iron powder in the mixture of powders.

The same type of steel, containing chromium, nickel and molybdenum, may also be welded using a steel wire having 18% chromium and 8% nickel, provided that sufiicient molybdenum powder be incorporated in the mix: ture of metal powders for the deposited metal to have the desired composition.

By way of example, successful use has been made of the following mixture to provide the 375 parts of metal powders involved:

What I claim is:

1. A coated electrode for use in welding alloys with a carbon content lower than 0.030%, in particular alloy steels having such a carbon content, having an electrode wire whose carbon content is not in excess of 0.025% by weight, the coating containing a binder and a mixture of powders comprising from 0% to 6% of basic carbonates, from 3.5% to 10% of deoxidizing alloys, from 0% to 1% of organic substances, from 3% to 10% of free MgO, from 3% to 10% of free A1 0 with the weight ratio of MgO to A1 0 lying between 0.3 and 3.33, from 13% to 35% of a powder of metallic alloying substances which enter into the composition of the deposited alloy, at least one of the oxides of acid aifinity of the group TiO and ZrO fluxing agents, and complex silicates.

7 8 2. A coated electrode as claimed in claim 1, wherein Free A1 0 10 the said mixture of powders comprises: Fluxing agents in the form of CaF 7.48 from 20 to 35% of the group TiO and ZrO Complex silicates and organic substances 9.05 from 4 to 8% of fluxing agents of the group alkaline Powders of metallic alloying substances 29.52

fluorides, alkaline-earth fluorides and cryolite 5 from 6 to 12% of complex silicates of the group po- References Cited tassium and sodium feldspar, nepheline-syenite, as- UNITED STATES PATENTS bestos, mica clay, kaolin and bentonite. 3. A coated electrode as claimed in claim 1, wherein 3,211,582 10/1965 Wasserman et 117-205 the sa1d mixture of powders comprises. Percent 10 WILLIAM L. JARVIS Primary Examiner Deoxidizing substances of the group manganese and ferromanganese 6.70 TiO 29.92 117206, 207; 148-24, 26

Free MgO 7- 15