In this article we will discuss about:- 1. Introduction to Electrodes 2. Ingredients of Electrodes 3. Electrode Covering/Coating 4. Classification 5. Factors to be Considered in Selection 6. Disadvantages 7. Electrode Efficiency.
Introduction to Electrodes:
Bare, fluxed and heavy coated electrodes are types of metal electrodes most commonly used. Bare electrodes have limited applications as during the welding operation, they are exposed to oxygen or nitrogen of the surrounding air which forms non-metallic constituents and they are trapped in the rapidly solidifying weld metal thereby decreasing the strength and ductility of the weld-metal.
Generally this type of electrodes is used for welding wrought iron and mild steel. Improved welds may be obtained by applying a light coating of flux on the rods with a dusting or washing process. The flux coating assists both in eliminating undesirable oxides and preventing their formation, however, the heavy coated electrodes are by far the most important ones and are used in 95% of the commercial manual-welding.
The type of flux coating depends on the weld metal composition. The slag produced by flux should be light than base metal so that it float on the weld surface to avoid any possibility of trapped slag inclusions. Electrode coating facilitates striking the arc and enables it to be stable.
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The electrode coating provides a gaseous shield, preventing oxidation of molten weld pool; slag retards cooling of the weld, thereby refining its structure, stabilizes arc particularly with a.c. supply; flux coating also compensates for elements like carbon, manganese, nickel etc. likely to be lost partially due to combustion.
It may be mentioned that some slags, particularly produced by aluminium fluxes, chemically react with the underlying weld and must therefore be removed by washing and brushing using hot water. Coating on electrode reduces slag fluidity which is the requirement for over-head welding.
Commercially metal electrodes are available in 1.5 to 9.5 mm diameter and 35 to 45 centimeters of length, whereas carbon electrodes are available in 4.5 to 12.5 mm diameter and length about 25 cm. Diameter of electrode is selected depending mainly on the thickness of the parent metal and the welding current to be used. Diameter of electrode controls the penetration.
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In general, the electrodes can be divided into three categories depending upon other aspects as well as their technological properties.
(a) Bare electrodes are most commonly used in automatic and semi-automatic welding.
(b) The electrodes with thin or stabilizing flux coating. This coating usually consists of lime mixed with soluble gals which serves as a binder. The thickness of such coating lies within 0.1 — 2.25 mm. These electrodes are used for welding of less important structures since the welded seams possess low mechanical properties.
(c) The electrodes which thick flux coating ensure both high mechanical properties of welded seam, as well as the required chemical composition of the welding metal. The materials used for flux coating are termed as components.
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The components may be sub-divided into the following categories:
(i) Gas Forming:
The gas forming components are organic matters such as, starch, wood pulp etc. which form gas layer, thus isolating the welding zone from the ambient air.
(ii) Slag Forming:
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The slag forming components are: china clay, felspar, manganese and titanium ores etc. These components produce slag which by covering the molten metal prevents it from coming into contact with the ambient air, and also ensures gradual cooling of molten metal. The merging of welding metal with basic metal in this case, is considerably smooth.
(iii) Reducing:
The reducing components such as ferro- silicon, ferro-titanium, ferro manganese reduce the oxides which are likely to be formed in the liquid bath of molten metal.
(iv) Alloying:
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The alloying components such as ferro- silicon, ferro-manganese, ferro-chromium, ferro molybdenum, chromium oxide are used for rendering this metal heat-proof.
(v) Stabilizing:
The stabilizing components, also form slag. In the presence of arc, these components, ionise the zone between the electrode and the part to be welded and thus ensuring the stable burning of the arc.
(vi) Binding:
The binding components, such as liquid glass or destrine serve for binding other components of the flux coating, in order to render the coating to be more durable. The electrode with flux coating may be made either mechanically or by pressing or by immersing the rod into the bath containing liquid flux.
Ingredients of Electrodes:
The coating on electrodes comprises of following ingredients:
(i) Cellulose:
It provides a reducing gas shield and increases arc voltage.
(ii) Potassium Aluminium Silicate:
It stabilizes the arc and gives strength to the coating.
(iii) Metal Carbonates:
These produce a reducing atmosphere and adjust the basic nature of slag.
(iv) Mineral Silicates:
These provide slag forming materials and strengthen the coating.
(v) Ferromanganese and Ferrosilicon:
These are used to deoxidise the weld metal.
(vi) Rutile:
It forms a highly fluid and quick freezing slag and adjusts the basic nature of slag.
(vii) Clays and Gums:
These are used to produce a pasty material for extruding the coating during manufacture of electrodes.
(viii) Iron Powder:
It increases the amount of metal deposited and draws larger current and increases productivity. Deposition efficiency may by more than 100% with high yielding electrodes.
Electrode Covering/Coating:
It plays an important role in performance of welding process of the joint. It provides vapour shield to protect molten metal and does not allow it to react with O2 and N2 in air. It provides flux for cleansing the metal surface as oxides form slag.
It controls the weld profile and provides an ionised path for conducting current from electrode tip to work and maintain the arc, and also controls the penetration properties of the arc, and controls the melt-off rate of the electrode. It adds alloy materials to the weld deposits where particular composition is called for.
De-oxidisers (Silicon, Al, Mn) in coating reduce oxides on work or formed by oxidation of molten metal. Arc stabilisation is another important function of coating and for this purpose potassium titanate is added in coating. Coating on electrode reduces the adhesive force between the molten metal and the end of the electrode.
Types of Electrode Coating are:
(i) Gas Shielded/Cellulosic Electrodes:
These coatings contain 30% cellulose or organic material (wood flour/alpha flock) which breaks down in arc to give voluminous gas shield of H2, CO2 and CO for protection of weld metal. Burning off rate of electrode is low, and slag quantity is small and friable which makes electrode suitable for welding in positional and vertical positions. Direct current in necessary and the strong plasma jet formed gives good penetration.
(ii) Rutile Electrodes:
Half of coating comprises of mineral rutile (titania-TiO2) which imparts good arc stability. Protection against contamination is obtained by H2, CO, CO2, NOx and acidic slag. AC can be readily used. It is ideal for general engineering due to characteristics like easily controllable slag, slow spatter, medium penetration, high deposition rate.
(iii) Iron Oxide/Silicate Electrodes:
Coating for this consists of Fe2O3, MnO and associated silicates. Gas shielding is low but voluminous acid slag enables slag metal reaction. It can be used for both ac and dc supplies. Suitable for fillet and deep groove welded joints. Deposition rates are high, penetration good, and spatter low.
(iv) Basic Electrode (Lime-Ferric or Lime- Fluorsper):
This coating contains high proportion of CaCO3 and CaF2. H2 content is kept very low by minimising clays/ other minerals with combined water. Electrodes are stored in dry condition. Protection against contamination is by CO2 – CO gases. Slag allows deoxidation of the metal.
Deposit has high resistance to hot/cold cracking, an important feature for high strength steels. Arc voltage is high and DC with positive electrode is employed. If AC is to be used, then potassium salts are added in covering. These require good care in starting, stopping and weaving arc as the arc length has to be short.
Nowadays commercially several types of electrodes are available and it is worthwhile to understand their characteristics and select the one best suited for given application.
Various types of electrodes available are:
i. Cellulosic,
ii. Medium rutile,
iii. Heavy rutile,
iv. Rutile acid,
v. Oxidising,
vi. Basic,
vii. Iron oxide acid,
viii. Iron powder electrodes,
ix. Low hydrogen electrodes.
The American classification system for electrode classification consists of a prefix letter E specifying an electrode, a group of 2 or 3 digits specifying weld metal strength, type of covering, weld position and current characteristics.
Classification of Coatings:
Generally coatings on electrodes are classified as:
(i) Cellulose Coatings:
Provide gas shield, deeply penetrating arc and rapid burning rate.
(ii) Rutile Coatings:
(a) Fairly viscous (mineral) for butt and fillet welds or
(b) Fluid type for flat and horizontal position.
(iii) Iron Oxide Coatings:
(a) Inflated type used for deep groove in flat position. Weld profile is smooth and concave.
(b) Solid type forms a thick covering and is used for single run fillet weld for smooth contour.
(iv) Lime Flour Spar:
Low H2 or basic electrodes. Slag is fluid and weld deposits convex to flat. Used for heavy sections, restrained joints.
Classification of Electrodes:
IS 814 classifies covered electrode for manual metal arc welding (MMAW) of carbon and C-Mn steels as under.
It is given by two letters followed by numerical as given below:
ER 42 11
First letter E indicates covered electrode for MM/AW, manufactured by extrusion process.
Second letter stands for electrode covering
R—Rutile
A—Acid
B—Basic
C—Cellulosic
RR—Rutile, Heavy Coated
S—Any other
Numeral 4 indicates UTS—(410-510 MPa) and yield strength of weld metal (330 MPa)
Second numeral—% elongation, 2 means (22%)
Third digit is for welding position.
It is 1 for all positions, 2 for all except vertical, 3 flat butt weld horizontal/vertical fillet, 4—flat butt and fillet weld, 5 vertical down and flat butt, 6-any other position.
4th digit indicates the current condition in which the electrode is to be used.
Factors to be Considered in Selection of Electrodes:
Following factors need to be considered in selection of appropriate electrode:
i. Specific properties
ii. Type of base metal
iii. Position of the weld
iv. Type of power supply available
v. Current polarity available
vi. Dimensions of the section to be welded
vii. Type of fit permitted by the work.
Disadvantages of Bare Electrodes:
(a) These form oxides and nitrides from atmospheric air.
(b) The striking of the arc is difficult with this type of electrode specially with A.C. supply.
(c) Rapid melting of electrode than parent metal takes place and therefore reasonable depth of penetration of weld is not obtained.
The choice of a correct electrode among hundreds available, to suit a particular job is a vital part of producing a sound weld. The selection of an electrode must be suitable for a particular purpose or requirement; and its proper application thereon results in a low cost fabrication and efficient performance. The British classification of electrodes is discussed in BS—1719, American classification by American Welding Society, and Indian classification is discussed in IS— 815.
In these standards, the electrode is codified by a number consisting of a prefix letter, a number of digits and is some cases, a suffix letter.
These code numbers will indicate the following characteristics of electrodes:
(1) Method of manufacture.
(2) Type of flux covering.
(3) The welding position for which the electrode is suitable.
(4) Welding current condition required.
(5) Characteristics of deposited metal, i.e., ultimate tensile strength, percentage elongation, tensile test values of deposited metal and also impact test values of deposited metal.
(6) Suffix letter is used to indicate that deep penetration is possible.
Electrode Efficiency:
It refers to the metal recovery and deposition coefficient. It is defined as the ratio of the weight of metal actually deposited compared with the weight of metal of the electrode consumed. It varies from 75 to 95%. With iron powder electrodes, efficiency can as high as 200%.
Deep Penetrating Electrode:
It is the covered electrode in which the covering aids the production of a penetrating arc to give a deeper than normal fusion in the root of a joint.
Storage:
Electrodes should be stored in dry place as moisture can seep into coating and hydrogen may diffuse into parent metal leading to porosity and cracking problems.
Shielded Metal Arc Welding (SMAW) Electrodes:
Mild steel electrodes are classified as:
(i) Fast freeze,
(ii) Fast fill,
(iii) Fill freeze, and
(iv) Low-Hydrogen-Types.
Fast freeze electrodes are cellulose coating electrodes. Due to their fast freezing action, puddle solidifies rapidly after being deposited from the electrode. Their shielding atmosphere is rich in hydrogen and thus they produce intense heat and deep biting action.
It is well suited for maintenance work because these burn through rust, grease, or used steel. These are also well suited for out-of-position welding due to their excellent puddle control and rapid solidification. These electrodes call for whipping motion (whip-and-pause technique) for good results.
Fast-fill electrodes contain iron powder in the flux coating which helps create the high deposition rate. Because of their highly fluid puddle, these are used for flat and horizontal positions only.
Fill-freeze electrodes have the characteristics of both fast-fill and fast-freeze electrodes. They solidify faster than fast-fill class and they have higher deposition rates than fast- freeze class.