NO124145B - - Google Patents

Description

Graphite or carbon electrode provided with an oxidation-resistant protective coating and method for coating an electrode with such a protective coating.

The present invention relates to a graphite or carbon electrode for electric arc furnaces or electrothermal processes where electric arcs are used, which electrode is provided with at least one oxidation-resistant or oxidation-preventing, electrically conductive coating that protects the surface of the electrode from being oxidized, the coating essentially consists of metals, alloys or mixtures thereof and contains Fe, Al and Si and, possibly, a smaller amount of one or more of the elements Mn, B, Ti, Zr and P, and the total thickness of the coating is at least o.1 mm . The invention also relates to a method for coating an electrode with such a protective coating.

Electrodes of graphite or other carbon-containing material are generally used in electric arc furnaces and in electrothermal processes where electric arcs are used, for example in melting furnaces for steel. When using such electrodes, it has been shown that the electrodes to a large extent are quickly consumed due to oxidation on the surfaces exposed to the hot atmosphere inside the oven. This loss of electrode material is a major disadvantage, as the electrodes' operating time is greatly shortened.

The purpose of the present invention is to provide

an electrode which exhibits good properties at least in terms of anti-oxidation, anti-surface splitting, low specific resistance, low modulus of elasticity and high mechanical strength.

However, it is relatively difficult, but not impossible, to achieve all of the above-mentioned properties. As the two properties, antioxidation and anti surface splitting, are in opposition to each other, a stronger and less voluminous electrode can resist oxidation, but not surface splitting, while a porous and voluminous electrode can resist surface splitting but not oxidation at the same time. Any attempt to produce electrodes with all of the above-mentioned properties therefore entails increased production costs because you have to use extremely clean materials and precisely controlled manufacturing processes.

It has previously been proposed to protect the electrodes against oxidation by coating the surfaces with a layer of heat-resistant material consisting of aluminum together with boron, silicon, titanium, zirconium, chromium or molybdenum. The layer can be applied to the electrodes by melting aluminum on an electrode holder made of copper. However, aluminum has a tendency on its surfaces to oxidize to Al20^ and such an oxidized surface layer constitutes a poor electrical conductor which can hinder the electrothermal process and can be the cause of sparks which in turn can result in defects in the aluminum layer and damage to the electrode holder, which means that the layer has a reduced protective effect. The previous attempts to produce functional electrodes have therefore so far not been successful and these electrodes have therefore not been accepted within the industry.

With the help of the present invention, one has succeeded in eliminating or reducing the above-mentioned disadvantages to a minimum, by using a layer which is based on Fe instead of Al. Iron has the further advantage that it can be applied in a simple and cheaper way.

The electrode according to the present invention is characterized in that the applied protective coating contains 15-75 weight percent Fe, 0.2-15 weight percent Si, 0.2-30 weight percent Al and 4-60 weight percent Cr, possibly 0.2-10 weight percent of one or several of the elements Mn, B, Ti, Zr, Ni, Mo, W, V, Nb, Hf, Y, Co, Cu and Be and 0.005-0.2 weight percent P and/or 0.01-4.5 weight percent

C, in addition possibly 0.01-0.1 weight percent N^, and that the total thickness of the protective coating on the electrode surface is a maximum of 1.0 mm.

The method for coating an electrode according to the invention is characterized by the fact that, with spray coating,

ker a composite wire comprising an outer mantle of Fe or

F alloy or Al and an inner core of powder of metals such as

has poorer machinability than the mantle, which wire contains a total of 15-75% by weight Fe, 4~60% by weight Cr, 0.2-15% by weight Si and 0.2-30% by weight Al, and possibly also at least 0.2% by weight of one or more of the elements Ni, Mn, Mo, W, B, Ti, Zr, V, Nb, Hf, Y, Co, Cu and "Be and optionally at least 0.01 weight percent N?, at least 0.005 weight percent P and/or at least 0, 01 weight percent C and that the protective coating is applied to the electrode to a thickness between 0.1-1.0 mm.

The metallic antioxidation layer can be produced by spraying onto the electrode metal powder which essentially consists of chromium, such as metallic chromium, an alloy of ferrochrome, silicon chrome, manganese chrome, aluminum chrome and/or nickel chrome.

A chromium alloy's antioxidant property is improved

when the chromium content exceeds 4 $ ° and it becomes very marked when the chromium content exceeds 20 "?<>.

By adding aluminum and silicon to the above

alloys, the antioxidant property is significantly increased.

There are two methods of metallization, namely wire spraying and powder spraying. However, the wire spraying method is very

expensive due to poor malleability of the thread. An alloy such as contains 30-40 # chromium, 4-8% aluminum, I-3 $ silicon and the rest iron, thus exhibits such poor formability that the wire-shaped product cannot be produced at low cost in a simple manner. It is therefore economically motivated to apply the coating metals to the electrode by powder spraying, whereby it is also possible to determine the metal composition solely taking into account d-e antioxidant properties, without any consideration having to be given to the formability in wire form. The invention can therefore be used to improve the graphite electrode's antioxidant properties by metallizing its surface with metal powder containing at least 4 f° Cr and/or o.2 - 3° f° Al and/or at least o.2 fo Si and possibly at least o.2 fo of one or more of the elements Ni, Mn, Mo, W, B, Ti, Zr, V, Nb, Hf, Ta, Y, Co, Cu and Be, the remaining amount of Fe containing at least o.ol % N^ and/or at least 0.005$ P and/or at least o.ol$ C.

In the following, the invention will be explained in more detail using the examples below.

The constituents are mixed, melted in a melting furnace and cooled, after which the metal mixture is ground into powder in a coal mill. The obtained metal powder is then sprayed onto the peripheral surface of an electrode using a spray gun (type Metco 2P) in the following way: After the surface of a 400 mm thick graphite electrode had first been cleaned to thereby remove oil, grease, paint and foreign substances, was the electrode placed in a rotating device, e.g. a lathe. The produced metal powder was then sprayed onto the surface of the rotating electrode by means of the above-mentioned spray gun which was arranged at a distance of about 150 mm from the electrode. The electrode with the coating thus produced, which exhibits a thickness of 0.25 mm, was used in a 30 tonne furnace and it turned out that 2.6 kg of electrode was consumed per tonne of casting. As a comparison with this, it turns out that an electrode without any coating was consumed with 5.4 kg per tonne of casting compound. A comparison between these two results shows that a remarkable reduction in electrode consumption has been achieved with the electrode according to the invention.

The above-mentioned metal powders were mixed well in a mixing apparatus and then sprayed onto the surface of a graphite electrode in accordance with Example 1. The film placed on the electrode surface had a thickness of 0.25 mm. The metal-over drawn electrode was introduced into a 30 tonne furnace and it turned out that the electrode consumption was 2.4 kg per tonne cast.

Example 3.

Weight percentage Ferrochrome (Cr 62.7 1°> Si 6.7 fo, rest Fe) 100

The ferrochromium alloy was ground in a ball mill and sprayed onto the surface of an electrode to a film of approx. o.2 mm thickness. In addition, a 3>2 mm thick aluminum wire (33.% Al) was sprayed onto the surface by means of a spray gun, the resulting coating having a thickness of 0.2 mm. The double metal film supplied electrode, thus having an inner film of ferrochromium and an outer film of aluminium, was used in a 30 ton furnace. It turned out that 2.9 kg of electrode was consumed per tons of molding compound.

With regard to the fact that a chrome-based alloy has superior antioxidant properties, but a small formability in wire form and as an electric spray process is more efficient than a gas process, according to a preferred embodiment of the invention, one can spray coat the surface of the electrode by means of a metal wire of more economical way, which is achieved by enclosing a powder of an alloy essentially consisting of chromium inside an envelope of a metal sheet, consisting of steel or aluminum.

By placing around the powdered alloy an envelope of a metal sheet consisting of steel or aluminum, which alloy exhibits good anti-oxidation properties, but little formability in wire form, a powdered alloy enclosed in the special wire form can be used more effectively for spraying by means of electricity as a heat source than-.: compared to powder-

spr#fcing.

Example 4.

A core of ferrochrome powder (62.7 f° Cr, 6.7% Si, rest Fe) with a particle size smaller than 5° mesh (ca.o.3 ram) which powder was produced in a ball mill, was placed inside a mantle of a cylindrical bent strip steel (of 15 mm width and o.2o mm thickness) of mild carbon steel (o.l% C and 0.04 $ Si). From the core and the strip steel combined with the core, a 3>2 mm thick wire was produced which was then metallized on the surface of a graphite electrode using a spray gun (Arcos S.A., type SPRAYOMATIC) in the following way: The surface of the graphite electrode was cleaned first, in order to remove grease, oil, paint and foreign substances, and was then placed in a rotating device, e.g. a lathe, after which the above-mentioned metal wire was metallized by means of a spray gun which was arranged at a distance of approx. 250 mm from the electrode. The protective cover or film thus formed had a thickness of 0.3 mm. The coated electrode was used in a 3° ton furnace, and it turned out that the electrode consumption went up to 2.9 kg per tonne of casting compound.

An uncoated electrode was consumed with 5.4 kg per tonne of casting compound. The consumption of the coated electrode according to the invention was thus significantly less.

Example 5.

A core of metallic chromium powder (99 fo) was arranged

in a jacket of stainless steel (18 fo Cr, 8 fo Ni, rest Fe), and the jacket was shaped as a cylindrical bent strip steel frnd 15 mm width and o.2 mm thickness).The core and the with this combined steel jacket were shaped to a 3|2 mm thick wire which was then sprayed onto the surface of an electrode with a 45° mm diameter. The metal layer thus obtained had a thickness of 0.3 mm. The consumption of the coated electrode went up to 2.6 kg per tons of molding compound.

Example 6.

A core of ferrochromium powder (62.7 f° Cr> 6.7 f> Si, residue Fe) was arranged into a jacket of stainless steel (18 fo Cr, 8 fo Ni, residue which was formed from a strip steel of a width of 15 mm and 0.2 mm thickness The core and with this combined sheath was formed into a 3.2 mm thick wire which was then sprayed onto the surface of an electrode to form a coating of about 0.25 mm thickness. An additional layer was then placed on the electrode by spraying a 3>2~ mm thick metal wire of aluminum ($99) onto the surface to a thickness of about 0.15 mm. 2.4 kg of this electrode was consumed per .ton of molding compound.

The percentages stated in the above descriptions and subsequent patent claims mean percentage by weight.

Claims (2)

1. Graphite or carbon electrode for electric arc furnaces or electrothermal processes, where electric arcs are used, which electrode is provided with an oxidation-resistant or oxidation-preventing, electrically conductive coating having a thickness of at least 0.1 mm and consists of one or more layers which essentially consist of metals, alloys, or mixtures thereof and contain Fe, Al and Si and possibly smaller amounts of one or more of the elements Mn, B, Ti, Zr and P, characterized by that the applied protective coating contains 15-75 weight percent Fe, 0.2 - 15 weight percent Si, 0.2 - 30 weight percent Al and 4-60 weight percent Cr, possibly 0.2 - 10 weight percent of one or more of the elements Mn, B , Tri, Zr, Ni, Mo, W, V, Nb, Hf, Y, Co, Cu and Be, and 0.005 - 0.2 weight percent P and/or 0.01 - 4>5 weight percent C, in addition optionally 0, 01 - 0.1 weight percent Ng, and that the total thickness of the protective coating on the electrode surface is a maximum of 1.0 mm. 2. Process for coating graphite or carbon electrodes by spraying with an oxidation-protective, electrically conductive and essentially metal-based coating for the production of the electrode provided with the protective coating according to claim 1, characterized in that during spray coating, a wire composite is used which comprises an outer sheath of Fe or Fe alloy or Al and an inner core of powder of metals having poorer machinability than the sheath, which wire contains 15-75 wt.% Fe, 4-60 wt.% Cr, "0.2-15 wt.% Si and 0 .2-30 weight percent Al, as well as optionally 0.2-10 weight percent of one or more of the elements Ni, Mn, Mo, W, B, Ti, Zr, V, Nb, Hf, Y, Co, Cu and Be and optionally 0.01-0.1 weight percent Ng at least 0.005-0.2 weight percent P and/or at least 0.01-4.5 weight percent C, and that the protective coating is applied to the electrode to a thickness of between 0.1 and 1.0 mm.