Steel Welding Electrodes
Steel Welding Electrodes
Welding is the strongest method for bonding metal to metal. It is a process for joining or fastening two pieces of similar metals (work pieces) by melting and fusing (i) the base metals being joined, and (ii) the filler metal applied. Welding can be done without the use of a filler metal such as by the use of heat energy alone. Work pieces are welded together by applying extreme heat, resulting in melting both pieces and the filler metal (rod / wire) to a molten state which cools to form a strong joint. The energy to form the joint between metal work pieces most frequently comes from a flame (e.g. oxy-acetylene) or an electric arc. Most of the welding involves ferrous-based metals such as steel and stainless steel. Welding covers a temperature range of 815 deg C to 1,650 deg C.
Arc welding consists of striking of a low-voltage, high-current arc between an electrode (also known as filler material or metal) and the work piece (base metal). The intense heat generated with this arc melts the base metal and allows the joining of two components. The characteristic of the metal which is being welded and the joint type (i.e. groove, fillet, etc.) dictates the welding parameters and the procedure that needs to be followed to obtain a sound weld joint. Arc welding electrodes play an important role in most engineering applications such as steel fabrication. They are one of the most commonly used materials in numerous construction, manufacturing, automobiles, and sometimes in even domestic applications. They support the high-temperature welding arcs and form the base material for welding joints or melting activities for metal fusion purposes.
Metal welding electrodes are used for various welding applications including electric arc welding. Electrodes for manual arc welding (sometimes referred to as stick welding) consist of a rod and a coating material. Welding electrodes are installed in the weld head to touch and maintain contact with the work pieces through the full weld schedule. The stick electrodes are consumable, meaning they become part of the weld. Stick welding electrodes vary by size, material, strength, welding position, iron powder in the flux, and soft arc designation. Electrode size (2.5 mm, 3.2 mm, 4.0 mm and 5 mm etc.) indicates the diameter of the rod core. Each electrode has a certain current range. The welding current increases with the electrode size (diameter). The electrodes are normally manufactured in the length of 250 mm to 450 mm.
The welding electrodes play three different roles during welding namely (i) maintaining uniform current density, (ii) concentrating current at welding points, and (iii) maintaining thermal balance during welding. As a rule, the alloy in the rod is similar to the material to be welded. It is made out of materials with a similar composition to the metal being welded. There are a variety of factors which go into choosing the right electrode for an application. In the shielded metal arc welding (SMAW), the stick electrode are consumable, meaning they become part of the weld, while in the tungsten inert gas (TIG), electrodes are non-consumable as they do not melt and become part of the weld, requiring the use of a welding rod. The metal inert gas (MIG) welding electrode is a continuously fed wire referred to as MIG wire. Electrode selection is critical to ease of clean-up, weld strength, bead quality and for minimizing any spatter.
Welding electrodes are used in arc welding process where the electrical current flows between electrode and base metal and electrical arc generate enough heat to melt both electrode and the base metal. In this process electrode with flux coating is used. The electrode holder holds the electrode which slowly melts away. Slag is formed from melting of flux which protects the weld puddle from atmospheric contamination. Some electrodes can be used with only AC (alternating current) or DC (direct current) power sources while other electrodes are compatible with both the types of power sources.
If the melting point of an arc welding electrode is less, it melts and fills the gap in the work piece. Such an electrode is called consumable electrode. In arc welding, to produce deep weld, consumable electrode is connected to the positive terminal of the power supply i.e., it is made as anode, while work piece is connected to the negative terminal of the power supply i.e., it is made cathode. Welding electrode is normally made of steel with the desired metal composition and it is available in different gauges, to suit the base metal thickness and properties.
There are electrodes which are used for joining the steel, alloy steel etc. i.e. for fabrication. These are also used widely for repairs of machines and equipment. Many cast iron parts and worn out steel parts can be rebuild by depositing metal through arc welding process. For special applications, these electrodes are used to provide hard surface layer having good wear resistance for example hard facing of the surface. Low carbon steel covered electrodes, also normally called coated electrodes, consist of only two major elements; the core wire or rod and the flux covering.
SMAW, also known as ‘stick welding’, is the most widely used process. The arc is struck between the metal to be welded and a flux coated consumable electrode. The fluxes cover the hot weld deposit and protect it from the environment. The solidified glassy product, slag is required to be removed by chipping or with a wire brush. This process uses a consumable electrode to support the arc. Shielding is achieved by the melting of the outer flux coating on the electrode. Filler metal is obtained from the electrode core. Stick electrodes are available in a wide range of types, each of which provide different mechanical properties and operate with a specific type of welding power source. There are several factors to consider in the selection of the stick electrodes. These include (i) base metal properties, (ii) tensile strength, (iii) welding current, (iv) base metal thickness, shape and joint fit-up, (v) welding position, (vi) specification and service conditions, and (vii) environmental job conditions. For the prevention of cracking or other weld discontinuities, the minimum tensile strength of the electrode is to be matched with the tensile strength of the base metal.
Electrodes are normally coated with chemical constituents known as flux, which melts along with the core wire during welding, to protect the welded joint from oxidation. In some situations, alloying elements can be introduced into the welding through fluxes to improve some of the properties of the parent metals. The importance of flux in electrode cannot be overemphasized, as it shields the welding pool from atmospheric oxygen or nitrogen, which is detrimental to the strength and chemical / physical properties of the welded joints.
Arc welding with a fusible electrode (the most important of the fusion process) is more complex in character and was developed very slowly. During the 1890s, arc welding was accomplished with bare metal electrodes. The welds produced were porous and brittle because the molten weld puddle absorbed large quantities of oxygen and nitrogen from the atmosphere. Operators noticed that a rusty rod produced a better weld than a shiny clean rod. Observations also showed that an improved weld can be made by wrapping the rod in newspaper or by welding adjacent to a pine board placed close to and parallel with the weld being made. In these cases, some degree of shielding the arc form the atmosphere was being accomplished. These early observations led to the development of the coated electrode.
Around 1920, the A.O. Smith Corporation developed an electrode spirally wrapped with paper, soaked in sodium silicate, and then baked. This was the first of the cellulosic type electrodes. It produced an effective gas shield in the area and greatly improved the ductility of the weld metal.
Because of the method used to manufacture these paper covered electrodes, it was difficult to effectively add other ingredients to the coating. In 1924, the A.O. Smith Corporation began work on coatings which could be extruded over the core wire. These coatings consisted of a mixture of minerals, ferro-alloys and in some cases organic materials, bonded with sodium or potassium silicate. This method allowed the addition of other flux ingredients to further improve or modify the weld metal. By 1927, these electrodes were being produced commercially.
Since 1927, several improvements have been made and many different types of electrodes have been developed and produced. Through variations in the formulations of the covering and the amount of covering on the mild steel core wire, many different classifications of electrodes are produced today.
These days, arc welding electrodes are of different types, each with its own covering materials. For examples, titanium based electrode which is also a foreign electrode is composed of 40 % titanium oxide, 10 % calcium carbonate, 10 % ferro alloys, 5 % potassium, 3 % feldspar, 2 % mica and minute amount of nickel, silicon, chromium, sulphur, molybdenum, cellulose, starch and dextrin. Some special electrodes have metallic salts included in the flux to add alloying substances to the weld metal. A good flux covered electrode can produce a weld which has excellent physical and chemical properties.
Classification of electrodes
Electrode consists of a coated metal rod having around the same composition as the base metal. Electrodes are mainly classified into five main groups namely (i) low carbon steel which is used for the majority of welding, (ii) medium and high carbon steel, (iii) low alloy and high alloy steel (including stainless steel), (iv) cast iron, and (v) non ferrous such as aluminum, copper, and brass.
For ensuring that there is a consistency in composition and properties between wires or rods from a variety of manufacturers, specifications have been produced which enable a wire or rod to be easily and uniquely identified by assigning the consumable a ‘classification’, a unique identification which is universally recognized. There are a large number of specifications covering the whole range of ferrous and non-ferrous filler metals. The two most popular schemes are AWS (American Welding Society) scheme and the EN/ISO method.
The identification of the solid wires / rods is relatively simple, as the chemical composition is the major variable although both the AWS and the EN/ISO specifications detail the strength which can be expected from an all-weld deposit carried out using parameters given in the specification. However, it is to be noted that most welds contain some parent metal and that the welding parameters to be used in production can be different from those used in the test. The result is that the mechanical properties of a weld can be significantly different from those quoted by the wire / rod supplier, hence the need to always perform a procedure qualification test when strength is important. In addition, the mechanical properties specified in the full designation include the yield strength (In the EN/ISO specifications, the classification can indicate either yield or ultimate tensile strength).
When selecting a wire/rod, it is necessary to remember which the yield and ultimate tensile strengths are very close together in weld metal but can be widely separated in parent metal. A filler metal which is selected because its yield strength matches that of the parent metal may not, hence, match the parent metal on ultimate tensile strength. This can cause the cross joint tensile samples to fail during procedure qualification testing or perhaps in service.
The classification number series for welding electrodes of AWS has been adopted by the welding industry. The electrode identification system for steel arc welding is set up as follows.
- E– It indicates electrode for arc welding.
- The first two (or three) digits – They indicate tensile strength (the resistance of the material to forces trying to pull it apart) in thousands of pounds per square inch (psi, one thousand of psi is 6895 kPa) of the deposited metal.
- The third (or fourth) digit– It indicates the position of the weld. Number 0 indicates the classification is not used, number 1 is for all positions, number 2 is for flat and horizontal positions only, and number 3 is for flat position only.
- The fourth (or fifth) digit– indicates the type of electrode coating and the type of power supply used, AC or DC power, straight or reverse polarity (DCSP or DCRP).
- The types of coating, welding current, and polarity position designated by the fourth (or fifth) identifying digit of the electrode classification are listed in the Tab 1.
- The number E6010– It indicates an arc welding electrode with minimum stress relieved tensile strength of 60,000 psi (415 (MPa), is used in all positions, and reverse polarity direct current is required.
|Tab 1 Coating, current and polarity types designated by the fourth digit in the electrode classification number
|AC, DCSP, DCRP
|AC, DCSP, DCRP
|Iron powder titania
|AC, DCSP, DCRP
|Low hydrogen sodium
|Low hydrogen potassium
|Iron powder iron oxide
|Iron powder low hydrogen
|AC, DCSP, DCRP
|Note: When the fourth (or last) digit is 0, the type of coating and current to be used is determined by the third digit.
Classification system for submerged arc electrodes
The system for identifying solid bare carbon steel for submerged arc is as follows.
- The prefix letter E is used to indicate an electrode. This is followed by a letter which indicates the level of manganese, i.e., L for low, M for medium, and H for high manganese. This is followed by a number which is the average amount of carbon in points or hundredths of a percent. The composition of some of these wires is almost identical with some of the wires in the gas metal arc welding (GMAW) specification.
- The electrode wires used for submerged arc welding are given in AWS specification, ‘Bare Mild Steel Electrodes and Fluxes for Submerged Arc Welding’. This specification provides both the wire composition and the weld deposit chemistry based on the flux used. The specification does give composition of the electrode wires. When these electrodes are used with specific submerged arc fluxes and welded with proper procedures, the deposited weld metal meets the mechanical properties needed by the specification.
- In the case of the filler rods used for oxyfuel gas welding, the prefix letter is R, followed by a G indicating that the rod is used expressly for gas welding. These letters are followed by two digits which are 45, 60, or 65. These designate the approximate tensile strength in 1000 psi (6895 kPa).
- In the case of non-ferrous filler metals, the prefix E, R, or RB is used, followed by the chemical symbol of the principal metals in the wire. The initials for one or two elements follow. If there is more than one alloy containing the same elements, a suffix letter or number can be added.
- The specifications of AWS are most widely used for specifying bare welding rod and electrode wires.
The most important aspect of solid welding electrode wires and rods is their composition, which is given by the specification. The specifications provide the limits of composition for the different wires and mechanical property requirements. Occasionally, on copper-plated solid wires, the copper can flake off in the feed roll mechanism and create problems. It can plug liners, or contact tips. A light copper coating is desirable. The electrode wire surface is to be reasonably free of dirt and drawing compounds. This can be checked by using a white cleaning tissue and pulling a length of wire through it. Too much dirt clogs the liners, reduces current pickup in the tip, and can create erratic welding operation.
Temper or strength of the wire can be checked in a testing machine. Wire of a higher strength feeds through guns and cables better. The minimum tensile strength recommended by the specification is 140,000 psi (965 MPa).
The continuous electrode wire is available in many different packages. They range from extremely small spools which are used on spool guns, through medium-size spools for fine-wire GMAW. Coils of electrode wire are available which can be placed on reels which are a part of the welding equipment. There are also extremely large reels weighing many hundreds of pounds. The electrode wire is also available in drums or payoff packs where the wire is laid in the round container and pulled from the container by an automatic wire feeder.
The EN/ISO specification for non-alloyed steel solid wires is BS EN ISO 14341. This specification classifies wire electrodes in the as-welded condition and in the post weld heat-treated (PWHT) condition, based on classification system, strength, Charpy-V impact strength, shielding gas and composition. The classification utilizes two systems based either on the yield strength (System A) or the tensile strength (System B).
- System A– It is based on the yield strength and average impact energy of 47 joules of all-weld metal.
- System B– It is based on the tensile strength and the average impact energy of 27 joules of all-weld metal.
In most cases, a given commercial product can be classified to both systems. Then either or both classification designations can be used for the product. The symbolization for mechanical properties is summarized in Tab 2A for classification system A and Tab 2B for classification system B. For classification system B, the ‘X’ can be either ‘A’ or ‘P’, where ‘A’ indicates testing in the as-welded condition and ‘P’ indicates testing in the post weld heat-treated condition. The symbol for the flux type is summarized in Tab 3.
|Tab 2A Symbols for mechanical properties based on classification system A
|Minimum yield strength
|Ultimate tensile strength
|Charpy-V test 47 J at temperature deg C
|440 to 570
|470 to 600
|500 to 640
|530 to 680
|560 to 720
|Table 2B Symbols for mechanical properties based on classification system B
|Minimum yield strength
|Ultimate tensile strength
|Charpy-V test 27 J at temperature deg C
|550 to 740
|570 to 770
|Tab 3 Symbols for electrode core type and position based on classification system A
|Flux Core Type
|Rutile, slow freezing slag
|Rutile, fast freezing slag
|All except V-down
|Flat butt, flat and HV fillet
|Flat butt and fillet
|Rutile or basic/fluoride
|V-down and (3)
|Basic/fluoride, slow freezing slag
|Basic/fluoride, fast freezing slag
Electrode flux coating
The coating material of the electrode is known as flux. The coating consists of metals, minerals, and organic substances. The object of the electrode coating is to provide easy striking and a stable arc. The coating material also contains elements which affect the transport of metal across the arc and provide good mechanical and chemical properties to the alloy formed between the base material and the rod core in the electrode. The coating material has several functions namely (i) ionization, (ii) crater formation, (iii) metal transfer, (iv) compensating for the loss of alloying elements, (v) gas shield, and (vi) slag formation to protect the crater and clean the weld pool.
The formulation of welding electrode flux coatings is based on well-established principles of metallurgy, chemistry, and physics. The coating protects the metal from damage, stabilizes the arc, and improves the weld in other ways, which include (i) smooth weld metal surface with even edges, (ii) minimum spatter adjacent to the weld, (iii) a stable welding arc, (iv) penetration control, (v) a strong, tough coating, (vi) easier slag removal, and (vii) improved deposition rate.
As the electrode burns during the welding process, the flux produces a gaseous shield around the weld. This prevents harmful contaminants from hurting the weld. The main three harmful elements present in the atmosphere are hydrogen, oxygen, and nitrogen.
In addition to providing the filler metal to the weld pool, electrode flux serves the several other functions such as (i) it adds alloying ingredients to the weld metal and area in order to change the physical properties, (ii) it establishes the polarity and electrical characteristic of the electrode, (iii) it produces an inert gas, which shields the base metal molten pool and weld bead and protects them from oxygen, nitrogen, and hydrogen in the atmosphere, (iv) it provides fluxing agents, impurity scavengers and deoxidizers to clear the pool, (v) it prevents corrosion from taking place, (vi) It forms slag to protect the cooling metal and allows metal to cool at a slower rate protecting the metal properties and preventing a hard brittle weld, (vii) It provides easier starting arc, stabilizer, reduce splatter, and (viii) it permits better penetration and X-ray quality during the weld inspection.
The flux coating of the electrodes changes to neutral or reducing gases such as carbon monoxide or hydrogen. These gases as they surround the arc proper prevent air from coming in contact with the molten metal. For example, the gases prevent oxygen which can approach the molten metal and prevent it from combining with it. The gases normally do not protect the hot metal after the arc leaves the weld point. The coating also contains special fluxing ingredients which promote fusion and tend to remove impurities from the molten metal. As the electrode flux coating residue cools, it forms a covering of materials over the weld. This residue covering prevents the air from the hot metal. This covering over the molten metal forms slag.
The coatings of welding electrodes for welding mild and low alloy steels can have from 6 to 12 ingredients, which include the following.
- Cellulose– It is to provide a gaseous shield with a reducing agent in which the gas shield surrounding the arc is produced by the disintegration of cellulose.
- Metal carbonates– It is to adjust the basicity of the slag and to provide a reducing atmosphere.
- Titanium dioxide– It is to help form a highly fluid, but quick-freezing slag and to provide ionization for the arc.
- Ferro-manganese and ferro-silicon– It is to help deoxidize the molten weld metal and to supplement the manganese content and silicon content of the deposited weld metal.
- Clays and gums– It is to provide elasticity for extruding the plastic coating material and to help provide strength to the coating.
- Calcium fluoride– It is to provide shielding gas to protect the arc, adjust the basicity of the slag, and provide fluidity and solubility of the metal oxides.
- Mineral silicates– It is to provide slag and give strength to the electrode covering.
- Alloying elements including nickel, molybdenum, and chromium– They provide alloy content to the deposited weld metal.
- Iron or manganese oxides– These are to adjust the fluidity and properties of the slag and to help stabilize the arc.
- Iron powder– It is to increase the productivity by providing extra metal to be deposited in the weld.
The principal types of welding electrode coatings for low carbon steel are described below.
- Cellulose-sodium (EXX10) – Electrodes of this type has cellulosic material in the form of wood flour or reprocessed low alloy electrodes have upto 30 % paper. The gas shield contains carbon dioxide and hydrogen, which are reducing agents. These gases tend to produce a digging arc which provides deep penetration. The weld deposit is somewhat rough, and the spatter is at a higher level than other electrodes. It does provide extremely good mechanical properties, particularly after aging. This is one of the earliest types of electrodes developed, and is widely used for cross country pipe lines using the downhill welding technique. It is normally used with DC power with the electrode positive (reverse polarity).
- Cellulose-potassium (EXX11) – This electrode is very similar to the cellulose-sodium electrode, except more potassium is used than sodium. This provides ionization of the arc which makes the electrode suitable for welding with AC power. The arc action, the penetration, and the weld results are similar. In both E6010 and E6011 electrodes, small amounts of iron powder can be added. This assists in arc stabilization and slightly increases the deposition rate.
- Rutile-sodium (EXX12) – When rutile or titanium dioxide content is relatively high with respect to the other components, the electrode is especially appealing to the welder. Electrodes with this coating have a quiet arc, an easily controlled slag, and a low level of spatter. The weld deposit has a smooth surface and the penetration is less than with the cellulose electrode. The weld metal properties are slightly lower than the cellulosic types. This type of electrode provides a fairly high rate of deposition. It has a relatively low arc voltage, and can be used with AC power or with DC power with electrode negative (straight polarity).
- Rutile-potassium (EXX13) – This coating is very similar to the rutile-sodium type, except that potassium is used to provide for arc ionization. This makes it more suitable for welding with AC power. It can also be used with DC power with either polarity. It produces a very quiet, smooth running arc.
- Rutile-iron powder (EXXX4) – This coating on the electrode is very similar to the rutile coatings mentioned above, except that iron powder is added. If iron content is 25 % to 40 %, the electrode is EXX14. If iron content is 50 % or more, the electrode is EXX24. With the lower percentage of iron powder, the electrode can be used in all positions. With the higher percentage of iron powder, it can only be used in the flat position or for making horizontal fillet welds. In both cases, the deposition rate is increased, based on the amount of iron powder in the coating.
- Low hydrogen-sodium (EXXX5): Coatings which contain a high proportion of calcium carbonate or calcium fluoride are called low hydrogen, lime ferritic, or basic type electrodes. In this class of coating, cellulose, clays, asbestos, and other minerals which contain combined water are not used. This is to ensure the lowest possible hydrogen content in the arc atmosphere. These electrode coatings are baked at a higher temperature. The low hydrogen electrode family has superior weld metal properties. They provide the highest ductility of any of the deposits. These electrodes have a medium arc with medium or moderate penetration. They have a medium speed of deposition, but require special welding techniques for best results. Low hydrogen electrodes are to be stored under controlled conditions. This type is normally used with DC power with electrode positive (reverse polarity).
- Low hydrogen-potassium (EXXX6) – This type of coating is similar to the low hydrogen-sodium, except for the substitution of potassium for sodium to provide arc ionization. This electrode is used with AC power and can be used with DC power, electrode positive (reverse polarity). The arc action is smoother but the penetration of the two electrodes is similar.
- Low hydrogen-potassium (EXXX8) – The coatings in this class of electrodes are similar to the low-hydrogen type mentioned above. However, iron powder is added to the electrode, and if the content is higher than 35 % to 40 %, the electrode is classified as an EXX18.
- Low hydrogen-iron powder (EXX28) – This electrode is similar to the EXX18, but has 50 % or more iron powder in the coating. It is usable only when welding in the flat position or for making horizontal fillet welds. The deposition rate is higher than EXX18. Low hydrogen coatings are used for all of the higher-alloy electrodes. By additions of specific metals in the coatings, these electrodes become the alloy types where suffix letters are used to indicate weld metal compositions. Electrodes for welding stainless steel are also the low-hydrogen type.
- Iron oxide-sodium (EXX20) – Coatings with high iron oxide content produce a weld deposit with a large amount of slag. This can be difficult to control. This coating type produces high-speed deposition, and provides medium penetration with low spatter level. The resulting weld has a very smooth finish. The electrode is usable only with flat position welding and for making horizontal fillet welds. The electrode can be used with AC power or DC power with either polarity.
- Iron-oxide-iron powder (EXX27) – This type of electrode is very similar to the iron oxide-sodium type, except it contains 50 % or more iron power. The increased amount of iron powder greatly increases the deposition rate. It can be used with AC power or DC power of either polarity.
There are many types of coatings other than those mentioned above, most of which are normally combinations of these types. These are for special applications such as hard surfacing, cast iron welding, and for non-ferrous metals.
Types of electrodes
When molten metal is exposed to air, it absorbs oxygen and nitrogen, and becomes brittle or is otherwise adversely affected. A slag cover is needed to protect molten or solidifying weld metal from the atmosphere. This cover can be obtained from the electrode coating. The composition of the welding electrode coating determines its usability, as well as the composition of the deposited weld metal and the electrode specification. The metal-arc electrodes can be grouped and classified as bare or thinly coated electrodes, and shielded arc or heavy coated electrodes. The covered electrode is the most popular type of filler metal used in arc welding. The composition of the electrode covering determines the usability of the electrode, the composition of the deposited weld metal, and the specification of the electrode. The type of electrode used depends on the specific properties required in the weld deposited.
These include corrosion resistance, ductility, high tensile strength, the type of base metal to be welded, the position of the weld (flat, horizontal, vertical, or overhead); and the type of current and polarity required.
When selecting an electrode, the first criteria is to select one which produces a weld metal quality equal to or better than that of the base material and, when necessary, is approved for the material in question. Welding position and type of joint are other factors, which influence the choice of electrode, as different electrodes have different properties in different welding positions and types of joint. The most common types of electrodes are (i) organic type (cellulose). (ii) rutile type, (iii) acid type, and (iv) basic type (low hydrogen).
Organic electrodes contain large quantities of organic substances such as cellulose. The metal transfer is referred to as explosion arc and the electrodes are well suited for vertical down welding.
Rutile electrodes contain large quantities of the mineral rutile (upto 50 %) or components derived from titanium oxide. Rutile electrodes can also contain cellulose. The rutile type of electrode has especially good welding properties both with AC power and DC power. The organic-rutile electrode is normally the cold welding type, characterized by a spray arc globular transfer, which is an advantage when welding in different positions. This type of electrode when alloyed is well suited for re-surfacing and hard surfacing because of its shallow penetrations and high weld build-up. Big opening between plates can easily be bridged using this type of electrode. The rate of welding is not particularly high, but the deposit is of good quality and slag is easily removed. Unalloyed rutile electrodes are not normally recommended for welding steel with nominal tensile strength exceeding 440 Mpa. The impact values are low because of oxygen level in the weld metal. Rutile electrodes are relatively insensitive to moisture.
Acid electrodes produce an Iron oxide / manganese oxide / silica type of slag, the metallurgical character of which is an acid. The coating contains oxides of the low pH value hence the term acid. Acid electrodes provide good fusion, a high rate of welding, and are equally suitable for AC power and DC power. The arc is stable and slag is easily removable, even if it is the first bead in a ‘V’ groove weld. Alloyed acid electrodes are suitable for welding steel with a nominal tensile strength of upto 440 Mpa.
Basic electrodes are frequently referred to as low hydrogen electrodes. After special heat treatment the coating has very low hydrogen content, hence the name. Basic electrodes with low moisture absorption (LMA) have lower initial moisture content and the speed of re-moisturing is much lower than of normal basic electrodes. Unalloyed basic electrodes give moderate welding speed in the flat position but are faster than other types when welding vertically upwards. The reason for this is that basic electrodes can be deposited at a higher current in the vertical position than other types of electrode. In addition, the amount of weld metal deposited per electrode is greater than that of other electrodes, which can be used in this position. This results in a smaller number of electrode changes. The normal result is hence a higher fusion rate and higher arc-time factor when welding vertically upwards with basic electrodes compared with other types.
The slag is normally not quite as easy to remove as the slag from acid or rutile electrodes, but, in spite of this, it can be classed as easily detachable. The slag from basic electrodes has a lower melting point than that from rutile or acid electrodes. The risk of slag inclusions during normal production welding is hence unusually small when basic electrodes are used, even if the slag is not completely removed between beads during multi-run welding. The weld metal from basic electrodes has low hydrogen content and normally has good toughness even at low temperatures. Basic electrodes are less likely to produce either hot cracks or cold cracks compared with other types of electrode.
The superiority of basic electrodes from this point of view appears when welding manganese-alloyed structural steels, pressure-vessel steels and ship quality plate with a nominal tensile strength of 490 MPa to 530 MPa. The higher the hardenability of the steel to be welded, the greater is the necessity to use basic electrodes and the greater the need for low moisture content in the coating.
Double coated electrodes consist of a thin layer of rutile coating extruded onto the core rod followed by a thicker basic layer. In this way the electrode gets the fine droplet transfer of rutile electrodes and the deposit strength of the basic electrode.
All welding electrodes (Fig 1) available are not the same. There are a lot of variations in them. The type of electrode used depends upon the material to be welded, the procedure, the infrastructure to be utilized, and the end output desired. The types of the electrodes which are normally used are given below.
Bare electrodes – Bare welding electrodes are made of wire compositions required for specific applications. These electrodes have no coatings other than those required in wire drawing. These wire drawing coatings have some slight stabilizing effect on the arc but are otherwise of no consequence. Bare electrodes are used for welding manganese steel and other purposes where a coated electrode is not required or is undesirable. Fig 1 shows the transfer of metal across the arc of a bare electrode.
Light coated electrodes – Light coated electrodes have a definite composition. A light coating has been applied on the surface by washing, dipping, brushing, spraying, tumbling. or wiping. The coatings improve the characteristics of the arc stream. The coating normally serves several functions namely (i) it dissolves or reduces impurities such as oxides, sulphur, and phosphorus, (ii) it changes the surface tension of the molten metal so that the globules of metal leaving the end of the electrode are smaller and more frequent and this helps make flow of molten metal more uniform, (iii) it increases the arc stability by introducing materials readily ionized (that is, changed into small particles with an electric charge) into the arc stream, and (iv) some of the light coatings can produce a slag. However, the slag is quite thin and does not act in the same manners the shielded arc electrode type slag. Fig 1 shows arc action obtained while welding with a light coated electrode.
Shielded arc or heavy coated electrodes – Shielded arc or heavy coated welding electrodes have a definite composition on which a coating has been applied by dipping or extrusion. The electrodes are produced normally with three types of coatings namely (i) with cellulose coatings, (ii) with mineral coatings, and (iii) combination of mineral and cellulose coatings. The cellulose coatings are composed of soluble cotton or other forms of cellulose with small amounts of potassium, sodium, or titanium, and in some cases added minerals. The mineral coatings consist of sodium silicate, metallic oxides clay, and other inorganic substances or combinations thereof. Cellulose coated electrodes protect the molten metal with a gaseous zone around the arc as well as the weld zone. The mineral coated electrode forms a slag deposit. The shielded arc or heavy coated electrodes are used for welding steels, cast iron, and hard surfacing.
These shielded welding electrodes produce a reducing gas shield around the arc. This prevents atmospheric oxygen or nitrogen from contaminating the weld metal. The oxygen readily combines with the molten metal, removing alloying elements and causing porosity. Nitrogen causes brittleness, low ductility, and in some cases low strength and poor resistance to corrosion. These electrodes reduce impurities such as oxides, sulphur, and phosphorus so that these impurities do not impair the weld deposit. They also provide substances to the arc which increases its stability. This eliminates wide fluctuations in the voltage so that the arc can be maintained without excessive spattering. By reducing the attractive force between the molten metal and the end of the electrodes, or by reducing the surface tension of the molten metal, the vapourized and melted coating causes the molten metal at the end of the electrode to break up into fine, small particles.
The coatings contain silicates which form a slag over the molten weld and base metal. Since the slag solidifies at a relatively slow rate, it holds the heat and allows the underlying metal to cool and solidify slowly. This slow solidification of the metal eliminates the entrapment of gases within the weld and permits solid impurities to float to the surface. Slow cooling also has an annealing effect on the weld deposit. The physical characteristics of the weld deposit are modified by incorporating alloying materials in the electrode coating. The fluxing action of the slag also produces weld metal of better quality and permit welding at higher speeds. Fig 1 shows arc action obtained while welding with a shielded coated electrode.
Fig 1 Types of electrodes
Stick (SMAW) welding electrodes vary by the following characteristics.
- Size – Common sizes are 1.6 mm, 2 mm, 2.4 mm (most common), 3.2 mm, 4.8 mm, 5.6 mm, 6.4 mm, and 8 mm. Core wire used with electrodes needs to be narrower than the materials which are welded.
- Material – Stick welding electrodes come in cast iron, high carbon steel, low carbon steel, iron-free (non-ferrous) and special alloys.
- Strength – It is referred to as tensile strength. Each weld needs to be stronger than the metal being welded. This means that the materials in the electrode need to be stronger as well.
- Welding position – It can be horizontal and flat etc. Different electrodes are used for each welding position.
- Iron powder mix – It can be upto 60 % in flux. Iron powder in the flux increases the amount of molten metal available for the weld and the weld heat turns powder into steel.
- Soft arc designation – It is used for thinner metals or for metals which do not have a perfect fit or gap.
There are several kinds of electrodes. The most popular stick welding electrodes are given below.
- E6013 and E6012 – These electrodes are for thin metals and joints which do not easily fit together.
- E6011 – These electrodes are good for working on surfaces which are oily, rusted or have dirt. These are versatile in that they work both with DC or AC polarity. These electrodes create little slag which is another big advantage. These electrodes are to not be placed into an electrode oven.
- E6010 – These electrodes are similar to the E6011 but only works with DC power. These electrodes are to not be placed into an electrode oven.
- E6018 and E7018 – These electrodes are manufactured with iron powder in the flux. E6018 electrodes possess operating and mechanical property characteristics similar to E7018 except at a lower strength level. The electrode covering and low hydrogen characteristics are also similar.
Some of the most commonly used electrodes are given below.
Mild steel electrodes – These widely used, flexibly applied electrodes primarily consist of two segments consisting of the core material (wire, rod, metal, etc) and the coating on the same. Mild steel electrodes are normally coated with high quality but low carbon steel deposits. These electrodes have considerable tensile and yield strength and can sustain a long-standing arc. They are mostly applied in the welding processes of mild steels, galvanized and low alloy steels.
Mild steel electrodes offer numerous benefits such as from easy weldability to the ability to bend and mould the material. These benefits make them well-suited for a wide range of welding applications. These electrodes are a cost-effective option for many welding applications. Mild steel electrodes are typically designed to outperform the base materials, with higher tensile and yield strengths. Standard mild steel electrodes are known to offer good weldability. These electrodes are available in solid, metal-cored and flux-cored wire options.
Low hydrogen carbon steel electrodes – These high-quality welding electrodes are coated with low hydrogen iron powders and are used primarily for welding carbon and low alloy steels. The general tensile strength which welding can have using these electrodes is under 480 MPa. These electrodes are for versatile applications and can be used for welding through all directions. It results in advanced, long-lasting, and non-cracking weld deposits on steel materials. Relatively high-stress welding can also be done using these electrodes.
Stainless steel electrodes – Another commonly used electrode for welding purposes, especially in very high-temperature applications are stainless steel electrodes. These are made up of different qualities of stainless steels and provide good creep resistance when compared to other categories. Applied on a wide range of metals, these electrodes are one of the best options for welding differing materials—stainless steel to mild and low alloy steels, joining metals with varying and unknown compositions, rough steels, etc.
DC arc welding electrodes – In case of DC arc welding electrodes, the manufacturer’s recommendations are to be followed when a specific type of welding electrode is being used. In general, DC shielded arc electrodes are designed either for reverse polarity (electrode positive) or for straight polarity (electrode negative), or both. Many, but not all, of the DC electrodes can be used with AC power. DC power is preferred for many types of covered, non-ferrous, bare and alloy steel electrodes. Recommendations from the manufacturer also include the type of base metal for which given electrodes are suited, corrections for poor fit-ups, and other specific conditions. In most cases, straight polarity electrodes provide less penetration than reverse polarity electrodes, and for this reason permit greater welding speed. Good penetration can be obtained from either type with proper welding conditions and arc adjustment.
AC arc welding electrodes – Coated electrodes which can be used with either DC power or AC power are available. AC arc welding electrodes are more desirable while welding in restricted areas or when using the high currents required for thick sections since it reduces the arc blow. Arc blow causes blowholes, slag inclusions, and lack of fusion in the weld. AC power is used in atomic hydrogen welding and in those carbon arc processes which need the use of two carbon electrodes. It permits a uniform rate of welding and electrode consumption. In carbon-arc processes where one carbon electrode is used, DC straight polarity is recommended for use since the electrode is consumed at a lower rate.
Non consumable welding electrodes
There are two types of non consumable welding electrodes namely (i) the carbon electrode which is a non-filler metal electrode used in arc welding or cutting, consisting of a carbon graphite rod which may or may not be coated with copper or other coatings, and (ii) the tungsten electrode which is a non-filler metal electrode used in arc welding or cutting, made principally of tungsten.
Carbon electrodes – The carbon electrodes are of three grades namely (i) plain, (ii) uncoated, and (iii) copper coated. The standard provides diameter information, length information, and requirements for size tolerances, quality assurance, sampling, and various tests. Applications include carbon arc welding, twin carbon arc welding, carbon cutting, and air carbon arc cutting and gouging.
Tungsten electrodes – These are non consumable welding electrodes used for gas tungsten-arc (TIG) welding. They are of three types namely (i) pure tungsten, (ii) tungsten containing 1 % or 2 % thorium, and tungsten containing 0.3 % to 0.5 % zirconium. Type of tungsten electrodes can be identified by paint end marks which are green for pure tungsten, yellow for with 1 % thorium, red with 2 % thorium, and brown with 0.3 % to 0.5 % zirconium.
Pure tungsten (99. 5 % tungsten) electrodes are normally used on less critical welding operations than the tungsten electrodes which are alloyed. This type of electrode has a relatively low current-carrying capacity and a low resistance to contamination. Tungsten electrodes with thorium (1 % or 2 % thorium) are superior to pure tungsten electrodes because of their higher electron output, better arc-starting and arc stability, high current-carrying capacity, longer life, and greater resistance to contamination. Tungsten welding electrodes containing 0.3 % to 0.5 % zirconium normally fall between pure tungsten electrodes and tungsten with thorium electrodes in terms of performance. There is, however, some indication of better performance in certain types of welding using AC power.
Finer arc control can be obtained if the tungsten alloyed electrode is ground to a point. When electrodes are not grounded, they are to be operated at maximum current density to achieve reasonable arc stability. Tungsten electrode points are difficult to maintain if standard DC power equipment is used as a power source and touch-starting of the arc is standard practice. Maintenance of electrode shape and the reduction of tungsten inclusions in the weld can best be accomplished by super-imposing a high-frequency current on the regular welding current. Tungsten electrodes alloyed with thorium and zirconium retain their shape longer when touch-starting is used.
The tungsten welding electrode extension beyond the gas cup is determined by the type of joint being welded. For example, an extension beyond the gas cup of 3 mm can be used for butt joints in light gauge material, while an extension of around 6.5 mm to 12.5 mm can be necessary on some fillet welds. The tungsten electrode torch is to be inclined slightly and the filler metal added carefully to avoid contact with the tungsten. This prevents contamination of the electrode. If contamination occurs, the electrode is removed, reground, and replaced in the torch.
Manufacturing of covered electrodes
The welding electrodes and fluxes covered by the standards falls normally in three categories namely (i) the mechanical properties of the weld metal obtained with a combination of a particular flux and a particular classification of electrode, (ii) the condition of heat treatment in which those properties are obtained, and (iii) the chemical composition of the electrode (for solid electrodes) as specified in the standards, or the weld metal produced with a particular flux (for composite electrodes). The coatings are used to cater for different types of welding applications. However, in all cases, the coating is formulated to satisfy three major objectives namely (i) to form fusible slags, (ii) to stabilize the arc, and (iii) to produce an inert gas shielding during welding.
These electrodes consist of only two major elements namely the core wire or rod and the flux covering. The core wire is normally low carbon steel. It contains only small amounts of aluminum and copper, and the sulphur and phosphorous levels are kept very low since they can cause undesirable brittleness in the weld metal. The raw material for the core wire is hot-rolled wire rod. It is received in large coils. The wire rod is cleaned, drawn down to the proper electrode diameter. The drawn electrode quality wire of required size is first straightened in a straightening machine and cut to size and stored. Medium carbon, high carbon, and high alloy steel (including stainless steel) wire rods are used for special types of electrodes. Some of the materials for the flux coatings are rutile sands, cellulose, lime stone, magnesium carbonate, metal oxides and metal powder of nickel, manganese, iron, etc.
The process of manufacturing of the electrode rods start with the preparation of core wire where the drawn wire is straightened and cut into required lengths by the straightening and cutting machine. Parallelly the dry flux mix as per the formulation is prepared. The dry flux mix consists of several chemical powders. Each of these chemicals is weighed accurately as per the technology for the type of electrodes and are mixed in a dry mixer to get homogeneous mix.
Binding agent such as potassium silicate is then added to the dry flux mix in a correct proportion to obtain a wet mix in a mixer. The wet mix is then pressed to form briquettes in a hydraulically operated press. These briquettes are made for loading of the flux in the flux cylinder of the extruder. The coating of flux is done by the extrusion press in which the flux is fed through a cylinder under pressure. While the wire is fed from the wire magazine of the electrode press, the briquettes are introduced into the extrusion cylinder of the press. During extrusion the core wire is fed one by one from wire feeder and coated with the flux by way of nozzle / die box system incorporated in the extrusion press. The electrodes coming out from the press are tested in an eccentricity tester. The rejected electrodes are taken into the flux stripping machine where the flux is stripped off. The core wire and flux can be re-used. The electrode rod coming out from the press is passed through a conveyor to the brushing machine for brushing of holding end and cleaning the same on tip end side for easy striking. After that the electrodes are spread on the collecting tray for air drying and after certain period they are fed into the oven.
After air drying of the coated electrodes, they are baked in oven as per the baking cycle. The moisture content in the electrode rod is not to normally exceed 4 %. The electrodes undergo regular sampling / batch testing for mechanical, metallurgical, and chemical testing. The finished electrodes are stored and wrapped in polythene or waxed paper and packed in tubes or boxes. Electrodes need to be stored in a moisture-free environment and carefully removed from any package (the directions are to be followed to avoid damage). Typical flow sheet of the manufacturing process for the mild steel covered electrodes is shown in Fig 2.
Fig 2 Flow sheet for covered electrode production
Electrodes are to be kept dry. Moisture destroys the desirable characteristics of the coating and can cause excessive spattering and lead to porosity and cracks in the formation of the welded area. Electrodes exposed to damp air for more than two or three hours are to be dried by heating in a suitable oven for around two hours at around 250 deg C. After they have dried, the electrodes are to be stored in a moisture-proof container. Bending the electrode can cause the coating to break loose from the core wire. Electrodes are not to be used if the core wire is exposed. Electrodes which have ‘R’ suffix in the AWS classification have a higher resistance to moisture.
Electrode defects and their effects
If certain elements or oxides are present in electrode coatings, the arc stability is affected. In bare electrodes, the composition and uniformity of the wire is an important factor in the control of arc stability. Thin or heavy coatings on the electrodes do not completely remove the effects of defective wire.
Aluminum or aluminum oxide (even when present in 0.01 %), silicon, silicon dioxide, and iron sulphate makes the arc unstable. Iron oxide, manganese oxide, calcium oxide stabilize the arc. When phosphorus or sulphur is present in the electrode in excess of 0.04 %, they deteriorate the weld metal since they are transferred from the electrode to the molten metal with very little loss. Phosphorus causes grain growth, brittleness, and ‘cold shortness’ (brittle when below red heat) in the weld. These defects increase in magnitude as the carbon content of the steel increases. Sulphur acts as a slag, breaks up the soundness of the weld metal, and causes ‘hot shortness’ (brittle when above red heat). Sulphur is particularly harmful to bare, low-carbon steel electrodes with low manganese content. Manganese promotes the formation of sound welds. If the heat treatment given to the wire core of an electrode is not uniform, the electrode produces welds inferior to those produced with an electrode of the same composition which has been properly heat treated.