Welding Electrodes

The electrodes are made ​​of electrically conductive material and are designed to supply electric current to the welding point. Types of electrodes:

metal - steel, cast iron, copper, brass, tungsten, bronze, etc.;
melting metal electrodes - coated and combined electrodes, welding plates and strips solid wire;
nonconsumable metal electrodes - electrode rods of tungsten electrodes for welding;
non-metal (consumable) - Carbon and graphite electrodes.

Covered electrodes for manual arc welding
Covered electrodes for manual welding rods are long, typically from 250 to 700 mm, made of wire coated with a layer of coating. One end of the electrode length of 20-30 mm has no coverage for its attachment to the electrode holder.

The length of the electrode depends on its diameter and chemical composition of the rod. For example, small diameter rods, which consist of high-alloy steels are made shorter to reduce the electrical resistance (and heating) during welding, and small diameter rods of low carbon steels have a high electrical conductivity and, therefore, can be long.

welding electrode

Welding electrodes should provide:

• steady burning of the arc, even melting of metal and stable transfer it to the weld pool;
• adequate protection of molten electrode metal and metal weld pool from air;
• obtain the desired weld metal chemical composition and mechanical properties;
• good formation of the joint, minimal loss through burning and spraying;
• possible performance of the welding process;
• good separability and easy udalyaemost slag from the surface of the seam;
• sufficient resistance of coatings against mechanical damage (crumbling, chipping with a relatively light impact, in the process of heating of the electrode during welding, etc.) and the inadmissibility of the sharp deterioration during storage;
• minimal toxicity of gases released during welding, the observance of sanitary norms.

These requirements are ensured through the selection of components of the coating of the electrode. The substances that make up the coating can be divided into the following groups.

Blowing components provide protection for gas welding zone from the air. When heated, they decompose with the release of gas, displacing the air. As the gas-forming components usually appear in the input surface minerals (marble, magnesite) or organic materials (flour, starch, dextrin).

Slag-forming components provide protection for the molten slag and metal crystallizing from the air. When melting, they form a slag which floats on the surface of the weld pool. Slag is also covered with a drop of the electrode metal. Slag-forming components (acidic oxides SiO 2 , TiO 2 , Al 2 O 3 , basic oxides, CaO, MnO, MgO; halogens CaF 2 ) are contained in marble, granite, hematite, quartz sand, ores, ilmenite and rutile concentrate.

Deoxidizing components are used to recover part of the metal in weld pool in the form of oxides. These include iron compounds - carbon ferromanganese, ferrosilicon, and ferrotitanium.

Stabilizing components provide arc stability due to the presence of elements with low ionization potential - sodium, potassium, calcium, etc. The latter are in marble, chalk, feldspar, soda ash, potash and other substances.

Alloying components the weld metal attach additional properties such as high strength, corrosion resistance, etc. are added to the coating in the form of ferrous alloys - ferrochrome, ferrotitanium, ferrovanadium. The main method of alloying the weld metal is doped through the rod electrode, extra - through the coating.

Binders bind powder coating materials into a homogeneous mass. Most often used as binders sodium (Na 2 Si0 2 ) or potassium (K 2 Si0 2 ) water glass. After drying, it cements the coating. To improve the molding properties of the coating composition are introduced into his plasticizers - Bentonite, Kaolin, dextrin, mica.

Adding to the surface of iron powder (up to 60% by weight of coating) can increase the productivity of welding.

Some materials cover several functions. For example, marble is blowing, slag-forming and stabilizing mineral.

Coating of welding electrodes
Electrode coatings can be created in different ways. In one possible dominance of gas-forming components in the other - slag. As the gas-forming components may be used minerals or organic compounds. Derivation of the weld metal hydrogen can be carried out with fluorine or oxygen. To varying degrees, can be carried out cleaning of the weld metal from unwanted impurities including the phosphorus and sulfur.

Depending on the approach, there are four basic types of coverage.

Sour coating (indicated by GOST 9466-75 by the letter "A" ) is created on the basis of the origin of the ore. As the slag-forming components are used oxides, blowing - organic compounds. With the melting of the coating and the molten metal in the combustion zone of the arc, a large amount of oxygen. So add a lot to cover deoxidizers - manganese and silicon.

The advantages of acid coating of the electrodes:

• low tendency to form pores in the lengthening of the arc welding and metal dross and rusty edges;
• high productivity of welding by heat release in oxidation reactions;
• stable burning arc welding with direct and alternating current.

The disadvantages of this coating are reduced ductility and toughness of the weld metal, which is associated with the inability to weld alloy due to oxidation of the alloying additives. In the absence of calcium in the coating in the weld metal contains sulfur and phosphorus, which increase the probability of formation of crystallization cracks. One of the main disadvantages of this coating is to allocate a large amount of harmful substances due to high content of manganese compounds in aerosols and silicon. Therefore, welding electrodes coated with the acid used in recent years is rare.

Scope of electrodes coated with acid - welding structures made of non-critical low-carbon steels.

Basic coverage (denoted by the letter in accordance with GOST 9466-75 "B" ) is created on the basis of fluoride (fluorspar CaF 2 ), as well as calcium and magnesium carbonates (marble CaCO 3 , magnesite MgCO 3 and dolomite CaMg (CO 3 ) 2 ). Gas protection is provided by carbon dioxide that is released during the decomposition of carbonates:

CaCO 3 → CaO + CO 2

With the help of calcium weld metal is well cleaned of sulfur and phosphorus. Fluorine is introduced in limited quantities (to maintain the stability of the arc) and binds the hydrogen and water vapor in the thermal resistant compounds:

CaF 2 + H 2 O → CaO + 2HF

2CaF 2 + 3SiO 2 → 2CaSiO 3 + SiF 4

SiF 4 + 3H → SiF + 3HF

Due to the low hydrogen content in weld metal welding electrodes with basic coating is also called "low hydrogen."

The advantages of the basic coverage of the electrodes:

• low probability of formation of crystallization cracks, high ductility and toughness of the weld metal due to the low content in the weld metal oxygen and hydrogen, as well as its good refining;
• high resistance to cold brittleness - the appearance or increase in brittleness with decreasing temperature;
• opportunities because of the low doping oxidizing ability of coatings;
• lower toxicity compared with acidic coatings;
• the deposition rate increased with the introduction of iron powder.

Disadvantages of the basic coverage:

• tendency to form pores with increasing length of the arc, increasing humidity coating, the presence of rust and scale on the welded edges, which requires more skill welder, as well as unnecessary to remove the edges and calcination of the electrodes before welding;
• lower stability of the arc due to the fluorine, which has a high ionization potential, and therefore the welding electrodes with basic coating is usually performed on the short arc DC reverse polarity.

Scope of the electrodes with basic coating:

• Welding of critical constructions made from carbon steel operating at alternating loads or low temperatures to -70 ° C;
• welding of structural steel, heat resistant, corrosion-resistant, Scaling, and other special steels and alloys;
• welding alloy steels.

Due to the presence of fluoride in aerosols at welding in an enclosed space is necessary to ensure quality of ventilation air, and welders are encouraged to work with the means of individual protection of respiratory organs, or with clean air in the breathing zone.

Well-known brands of welding electrodes with basic coating: ESAB OK 48.00 (left) and SSSI 13/55 Russian producers (right)

Rutile (indicated by GOST 9466-75 the letter "P" ) is created based on the rutile concentrate TiO 2 , which provides protection for the slag and aluminum silicates (feldspar, mica, kaolin) and carbonate (marble, magnesite). Gas protection is provided by carbonates and organic matter (cellulose). As an alloying element and deoxidizer ferromanganese used in some coatings injected iron powder (GOST 9466-75 marked the letters "AJ" ). With the help of calcium carbonate present in the CaCO 3 , the weld metal are removed from the sulfur and phosphorus.

Advantages of welding electrodes with rutile:

• higher deposition rate with the introduction of iron powder;
• low toxicity;
• compared with electrodes with basic coating - the stability of the arc welding with direct and alternating current, a higher resistance to pore formation, the best formation of the joint with a smooth transition to the base metal, less sensitive to an increase in arc length less than the coefficient of the metal spray is more convenient Welding in the vertical and overhead positions (in the absence of iron powder in them or their content less than 20%).

Disadvantages of the electrodes with rutile:

• reduced plastichnoct and toughness of weld metal inclusions from SiO 2 ;
• not be used for welding of structures operating at high temperatures;
• compared with electrodes with basic coating - less resistance to weld metal hydrogen sulfide stress cracking, which leads to the destruction of welded pipes in the fields of hydrogen sulfide compounds; lower resistance to crystallization cracks, more alloying elements are oxidized and iron and is therefore not used for welding medium-and high-alloy steels; high phosphorus content in the weld metal and the tendency to cold brittleness.

Scope of welding electrodes with rutile:

• welding and welding critical structures of the low carbon and low alloy steels of certain types, except for structures operating at high temperatures;
• in some cases for welding steels of medium, if the coating contains a large amount of iron powder.

Cellulose surface (indicated by GOST 9466-75 by the letter "C" ) is created based on organic compounds (50%) - cellulose, flour, starch, providing gas protection. For the protection of the slag in a small amount applied rutile concentrate, marble, carbonates, aluminosilicates and other substances. The welded joint is formed a thin layer of slag. Alloying of the deposited metal doped rod is performed, as well as by the addition of ferro-alloys in the coating and metal powders. As the use of ferro manganese deoxidizers. The weld metal chemical composition corresponds to poluspokoynoy or killed steel.

The benefits of welding with cellulosic electrodes:

• the quality of the root penetration weld;
• the possibility of welding in confined spaces due to the thinness of the coating;
• welding in all positions.

Disadvantages of cellulosic coatings:

• increased spray (15%) due to the small amount of slag-forming components and the high surface tension of molten metal;
• increased amount of hydrogen in the weld metal.

Scope of cellulosic electrodes - Welding of the first (hard-to-) layer butt pipelines.

It is also used and mixed coating : kislorutilovoe (indicated by GOST 9466-75 the letters "AR" ), Rutile-basic (indicated by GOST 9466-75 letters "RB" ), rutile-cellulosic (indicated by GOST 9466-75 the letters "RC" ) and others (indicated by GOST 9466-75 by the letter "P" ).

TABLE. The designation covers welding electrodes

Cover type	Designation according to GOST 9466-75	The international designation of ISO	The old designation in accordance with GOST 9467-60
sour	А	A	P - rudnokisloe
the main	B	B	F - ftoristokaltsievoe
rutile	Р	R	T - rutile (titanium)
cellulose	C	C	O - organic
mixed coating
kislorutilovoe	АР	AR
Rutile-basic	RB	RB
Rutile-cellulosic	RC	RC
Other (mixed)	P	S
rutile with an iron powder	RJ	RR

Type of welding electrode characterizes the properties of the weld metal. For structural steels - is the mechanical properties (Tensile strength, impact strength, elongation, bend angle) for alloy steels with special properties (heat-resistant, heat resistant, corrosion-resistant, etc.) - the chemical composition (carbon, silicon, chromium , manganese, nickel and other elements). Type designation of the electrode (regulated by GOST 9467-75 and GOST 10052-75) contains the letter "E" , after which put a temporary resistance at the gap δ in (kg / mm 2 ). For example, "E46A" means that the metal deposited by these electrodes has a strength of 46 kg / mm 2 (460 MPa) and improved plastic properties. For welding high-alloy structural steels and high strength type of electrode can be E70, E85, E100, E125, E150.

Examples of designations such as an electrode for welding steels with special properties:

• "E09H2M" - in the weld metal contains about 0.09% carbon, 2% chromium, 1% molybdenum;
• "E10H25N13G2B" - in the weld metal contains about 0.1% carbon, 25% chromium, 13% nickel, 2% manganese, 1% niobium.

See also:
welding transformer
welder
welding rectifier
welding inverter
manual arc welding

Welding materials:

welding electrodes welding wire welding flux for arc welding carbon dioxide helium

argon protective gas mixture acetylene propane-butane IAF