Q. 1 What is the Electric Arc? How does it support Welding? [back to top]

The arc is a means of transforming electrical energy into heat. The arc is an electrical discharge sustained through a gap in the electric circuit. The resistance offered by the air or gas in this gap to the passage of current creates high temperature which is concentrated at and between the terminals between which the gap is made, namely the work and the electrode tip. In an arc, the temperature attained can be between 3300 - 5500°C

The heat generated in the welding arc is utilized to melt the workpieces and electrode and form the welded joint.

In the welding arc, the amount of energy present is the product of the voltage across the arc and the amperes flowing in the current. This is measured in watts or kilowatts. Therefore, a 200 amp 30 volt arc will consume 6000 watts. For all practical purposes it can be considered that the amount of heat generated in the arc is proportional to the amperes. The arc is sustained across the gap or arc length by the voltage. In actual welding, however, the arc voltage will be influenced by other factors such as the electrode coating constituents, current, polarity of the terminals etc.

The difficulty encountered when trying to weld with bare wires is that the melted metal in the arc and work piece molten crater is exposed to the atmosphere causing the metal to oxidise leading to loss of ductility. In addition nitrogen pick-up causes embrittlement.

The arc also tends to be unstable, burn at a low rate, is less concentrated and therefore has less penetrating power.

The above facts, chemical and electrical are the reasons for the coating of the electrodes. It is essential that the molten metal is protected from the atmosphere. The arc has to be stabilized and concentrated to make it more effective.

In the early days of World War I, arc welding was used in England. The electrodes were made by tightly wrapping a high grade asbestos cloth around a lime washed wire. America supplied electrodes by replacing the asbestos with cotton cloth dipped in water-glass (sodium silicate); a standard fireproofing substance, to minimize flaming of the cotton cloth in the arc. These electrodes worked well and in fact, are essentially the same in chemical composition as some modern electrodes. Cotton is almost pure cellulose. Cellulose quickly decomposes in the arc temperature to carbon monoxide and hydrogen. These gases, surrounding the arc stream and molten pool, provides an excellent protective atmosphere, shielding the molten metal from oxygen and nitrogen in the surrounding air. Water glass is an aqueous solution of sodium silicate. This readily ionizes, which means that it assists the flow of electricity across the arc gap. Furthermore, sodium silicate acts as a flux, spreading over the face of the molten pool, protecting it from atmosphere as it solidifies.

These are fundamentally the same basic constituents of the modern day E-6010 type electrode, which is widely used in construction and pipeline welding because of its digging, deep penetrating arc. It makes very little slag making it ideal for out-of-position welding.

From this first type of coated electrodes, others have been developed by changing the formulation of the coating to give other desired operating characteristics and weld properties. By adding oxides of certain refractory metals, such as titanium, the harsh digging action of the arc is modified to give an arc that is softer and less penetrative. This is needed for welding sheet metal or bridging gap where metal fit-up is poor. Other elements, such as ferromanganese are added to control the chemical reactions that take place in the arc, just as certain elements are added to the open hearth furnace and Bessesmer converter in steel making.

Also, just as in steel making, slag is used to remove impurities from the molten metal, some electrodes are made to produce heavy slag around the arc and crater for this same purpose. These electrodes do not use volatile matter in the coating, but rely on the slag to control the chemistry of the steel making that takes place in the arc. The heavy slag has several advantageous features. It slows the cooling rate which allows gas to escape and the slag particles to rise to the surface; it reduces cooling stresses and allows more time for all the necessary chemical reactions to take place in the weld metal. The burn-off rate is increased due to the coating, resulting in a large molten pool and hence faster deposition rate. The heavy slag also ensures that the slag extends beyond the wire end, creating a crucible effect. This tends to concentrate the arc energy, making more efficient use of it.

The coating has also been utilised in making possible the use of electrodes with alternating current. The AC arc changes polarity 100 times per second, which makes arc stability a problem. It becomes necessary to have in the arc stream a gas which remains ionized during the reversal of current. This will permit the reignition of the arc. Potassium compounds are therefore incorporated in the flux coating for satisfactory operation with AC.

Iron Powders are used in electrode coatings in relatively large amounts, to increase the speed of welding and for better bead appearance. The iron powder increases the deposition efficiency of the electrode. The thick coating increases the crucible effect, it also allows the electrodes to be dragged upon the work piece without having to hold the electrode at a certain height above the work piece.

Q. 3 Is there a method of calculating the Arc Voltage? [back to top]

The welding current determines the Arc Voltage and it can be calculated using the following formula.

Both AC and DC are used. DC is suitable for all types of electrodes and for welding all ferrous and non-ferrous metals. AC is not suitable for certain type of electrodes and for certain types of metals.

To make a choice between AC and DC, the following points must be considered:

1. All type of electrodes work on DC while certain non-ferrous type and basic low-hydrogen ferritic type electrodes may not give a stable arc with AC. In AC welding, because of the reversing nature of the current, flux coatings must contain enough arc stabilizers to re-ignite the arc immediately after the current comes to zero during each cycle.
2. When using low diameter electrodes at low amperages, DC has a definite advantage over AC, both for starting and maintaining the arc.
3. For all electrodes maintaining a short arc is easer with DC than with AC, except when iron-powder electrodes are used.
4. Vertical and overhead welding on thick sections is easier with DC than AC, because a stable arc can be maintained at low currents.
5. Effect of polarity: DC has the advantage of two polarities which means that the electrode can be made negative or positive. Straight polarity (electrode negative) can be used for MMAW for all steels (except when low-hydrogen electrodes are used), but not for most non-ferrous metals. Using straight polarity ensures that more of the heat is concentrated on the electrode and hence the melting and deposition rates are higher, welding is faster and the work pieces less susceptible to distortion. However penetration is shallower and narrower.

   Reverse polarity (electrode positive) is always used for basic low-hydrogen electrodes and for most non-ferrous metals. This produces maximum penetration for a given set of parameters. This characteristics also makes it a better choice for root passes in groove welds made with backing bars and also for vertical and overhead welding.

   In AC welding the choice is non existent because AC has both straight and reverse polarity in regular cycles. Therefore AC gives penetration and electrode melting rates intermediate between those given by DC straight and reverse polarity.

6. DC can cause problems of "arc blow", specially when welding near or at corners, near the end of joints or on structures involving several pieces coming together. Welding with high currents on heavy structures may also give rise to arc blow. AC does not have this problem.
7. DC is preferred to AC for sheet metal welding. DC straight polarity minimizes burn-through, because of shallow penetration.

This happens due to the voltage drop due to the long cables. This can happen both in case of AC and DC welding. Such voltage drop added to that of the arc may overload the welding power source and weaken the arc. AC is preferred where the distance between the welding area and power sources are considerable as in shipyards. Long cables should not be coiled excessively, because induction set up by the coils will reduce the output of the power source and may overload the transformer. It may be good sense to keep the cables as short as required for the job.

Q. 6  How do we choose how much current we should use during MMAW welding? [back to top]

Every electrode has a certain current range within which it operates satisfactorily. On a light job where overheating must be avoided, current on the lower side should be used. For heavy work the maximum current within the range is to be used. Current in the mid range is to be used for normal work. Welding current increases with electrode size. For a given electrode size, the current increases with coating thickness, because as the coating thickness increases, a higher current is required to melt it.

Too low a current gives an unstable arc and results in lack of fusion in the welded joint. Too high a current overheats the electrode, increases spatter and results in weld metal porosity. Currents recommended by the electrode manufacturer must be used.

The 'red hotness' is a sure sign of using too high a current and most probably much above the recommended current range for that particular electrode. Many fabricators in a bid to increase production use higher than the recommended current. This does not in any way increase the production rate and in fact hampers smooth production.

Use of higher currents cause the following problems:

1. will cause burn throughs in thin sheets.
2. will cause large craters and over dilution with the base metal. This is not desirable as higher carbon or alloy pickup from the base metal may cause weld cracks.
3. the 'red hotness' may lead to loss of valuable alloying ingredients from the weld metal due to vapourisation. This will in all probability reduce the strength properties of the weld metal.
4. Will cause the slag to be ingrained and stick to the weld metal and prevent easy removal causing loss of production.
5. Loss of weld metal due to large stub ends loss.

Q. 8 What is the function of the flux coating in MMAW electrodes? [back to top]

1. It helps to strike and maintain the arc.
2. It generates gases which displace the oxygen and nitrogen of the atmosphere and provide a gaseous shield around the arc to protect the molten droplets during their passage across the arc as well as the molten weld puddle. It produces slag which further protects the molten droplets in the arc and the molten weld puddle from atmospheric attack and also provides an insulating blanket over the weld bead.
3. It helps to deoxidize and refine the weld metal.
4. It helps to modify the chemistry by providing alloying elements for the weld metal, either by reducing metallic oxides or by means of powdered ferro-alloys and metals. Thereby the mechanical properties of the weld metal are also suitably modified.
5. It helps to control the weld bead profile and to obtain smooth weld surface with even ripples.
6. It helps us to reduce weld spatter.
7. It insulates the electrode so that it can operate at sufficiently high current without getting overheated.
8. It makes vertical and overhead welding possible by controlling the viscosity of the slag.
9. It influences the operating characteristics of the electrode and to some extent, its burn-off rate and the depth of penetration of the weld bead into the base metal.
10. The coating having a low thermal conductivity, melts slower than the core wire. Thus, in the case of heavy-coated electrodes, the coating forms a cup around the upper portion of the arc and prevents air being drawn into the arc stream. The cup formation also makes touch or contact welding possible.
11. The coating increases deposition efficiency and weld metal deposition rate through iron powder and ferro-alloy additions.

Q. 9 What are Cellulosic type of MMAW electrodes ? [back to top]

Q. 10 What are Rutile type of MMAW electrodes? [back to top]

The difficulty encountered when trying to weld with bare wires is that the melted metal in the arc and work piece molten crater is exposed to the atmosphere causing the metal to oxidise leading to loss of ductility. In addition nitrogen pick-up causes embrittlement.

The slag formed during welding is viscous and quick freezing and this renders the electrode particularly suitable for welding in the vertical and overhead positions. This is the most popular type in most countries and is described as the general purpose mild steel electrode.

Q. 11 What are Basic Low-Hydrogen type of MMAW electrodes? [back to top]

This covering has high content of Calcium Carbonate (in the form of limestone, calcite or marble) and Calcium Fluoride (in the form of Flourspar). This results in a chemically basic slag which is fairly fluid. Use of organic materials such as cellulose and minerals with chemically combined water (such as china clay, talcum and mica) are avoided in order to keep the arc hydrogen free. This type of coating is called the low-hydrogen type.

The gaseous atmosphere formed around the arc consists of CO, CO2 and F compounds. The basic characteristics of the slag results in good transfer efficiency of the alloying elements from the coating to the weld deposit. The so-called hydrogen controlled mild steel electrodes are generally made with this type of coating, and this coating is also preferred in low-alloy high tensile electrodes, in order to minimise the risk of hydrogen-induced cracking or cold cracking in the base metal. The popular Basic coated electrodes use potassium as binders for use in both DC and AC.

1. Low Carbon mild steels : Carbon 0.10 to 0.25%. These are construction steels - most widely used.

   Sections less than 1" thick - easily welded using E6013 range of electrodes. No special pre-heat or post-heat or special welding techniques required.

   Sections above 1" - Easily welded using E7016/E7018 electrodes. Pre heat/post heat may be required depending on size, sections, joint restrictions etc

2. Medium Carbon steels : Carbon 0.25% to 0.5%, manganese 0.6% to 1.65

   These are hardenable steels, i.e. martensite may form in the weld zone. Therefore pre-heat or post-heating or both may be necessary.

   Selection of electrode for welding becomes more critical as the carbon content of steel increases. Low hydrogen electrodes must be used.

3. High carbon steels - Carbon 0.5 to 1.3% manganese 0.3 to 1.0%

   These are very difficult to weld because they are susceptible to weld cracking. Excessive hardness and brittleness are probable. Use of low hydrogen electrodes is mandatory. Both pre-heating and post weld stress relieving or tempering are necessary, specially for heavier sections.

4. Low alloy high strength steels: These include low-Manganese steels, low-to-medium Nickel Steels, low Nickel-Chromium steels, Molybdenum, Chromium-Molybdenum steels etc.

   These are welded with E80XX, E90XX, E-100XX class of covered electrodes. In all cases low hydrogen coatings should be used. The strength levels and compositions should be slightly overmatched with the base metal being welded. Pre-heating and Post weld heat treatment will be necessary in most cases.

1. Undercutting - this is caused due to the use of excessive high current or incorrect electrode movement. If the welder in not successful in avoiding this during welding, he should fill in the undercut with the weld deposit. The method of rapid weaving also causes undercuts but if a small pause is made on both sides of the bead, the electrode will deposit material and the undercut will be avoided.
2. Slag Inclusion - In multiplayer welds, care should be taken to see that there are no acute angles between the beads, otherwise it will be impossible to remove the slag completely and to achieve sufficient fusion at the bottom of the groove when the next bead is deposited. This problem occurs when the welder has been careless in cleaning out the slag in narrow grooves between the weld bead and the weld plate. This risk is greater when the bead is convex due to the use of lower current.
3. Root Fault - these are nothing but welding with faulty fusion or slag in the root. When this occurs it is generally the welder's fault. Unsuitable type or size of electrode and insufficient current use are factors which may cause root faults.
4. Porosity - One of the main reasons for porosity is the use of damp electrodes and a long welding arc. Low hydrogen electrodes are most susceptible to dampness.
   The use of wrong polarity during welding with low hydrogen electrode may give rise to porosity. Low hydrogen (Basicote) electrode should be welded with Reverse Polarity(DCRP).
   The workpiece may be of such a composition that the weld metal becomes porous inspite of good electrodes being used. The pores are mainly a result of the material's surface content.
5. Cracks - these can occur due to various reasons:
   
   • When a welder stops welding midway, cracks can occur due the development of high stresses in a small cross section. Therefore, when good welds are required, tack welds should be removed as the main welding proceeds. This is done to avoid cracks occurring in tack welds due to shrinkage stresses. When welding is done over a tack, there is a chance that it may not fuse and cracks may occur in the weld metal.
   • In some cases it has been seen that cracks occur in the weld or parent metal adjacent to the welds. The most likely cause of this type of crack lies with the parent metal which may not be easily weldable due the high content of carbon, sulphur or phosphorus. These elements segregate during welding and cause hot cracking. If the parent metal contains more than 0.2% carbon, more than 0.05% sulphur and 0.04% phosphorous, it is safer to use low hydrogen electrodes.
   • Sometimes the Heat Affected Zone (HAZ) cracks due to the fast rate of cooling of the weld metal. This occurs in some compositions of steel in which the HAZ gets hardened due to fast cooling. This can be solved by using a higher diameter electrode which will generate more heat and by ensuring slow cooling. A slow rate of welding can also help here. The pre - heating and post - heating of base metal can also be tried to avoid this problem.

The numbers referred to is a system of classification of MMAW electrodes for Carbon Steel Covered Arc Welding Electrodes under the American Welding Society (AWS 5.1). We will try and give a short explanation as to what the numbers signify. It may be noted that different countries have their own system of electrode designation and the numbers may have different meanings for different specifications. However, by far the most universally accepted Electrode specification is the AWS specification.

The American coding system starts with the prefix E, which designates an electrode. Then comes a two digit number 60 or 70. This number 60 designates tensile strength of at least 60 ksi of the weld deposit. 60ksi means 60,000 psi (lbs/sq in). Similarly, 70 means minimum weld metal tensile strength of 70 ksi or 70,000 psi. the actual stipulated minimum tensile strength values and the associated yield strength values vary according to the type of covering as shown below. The third digit indicates the position in which the electrode can be used satisfactorily. The last two digits taken together indicates current conditions (type of current) and the type of covering.

AWS Classification Tensile Strength min Yield Strength min Elongation min % Impact Energy Charpy-v J/°C Weld-ing Posi- tion Type of coating Type of Current ksi N/mm2 ksi N/mm2 AC DC E 6010 60 414 48 331 22 27/ - 29 1 Cellulosic - + pol E 6013 60 414 48 331 17 Not spec 1 Rutile x + pol E 7014 70 482 58 399 17 Not spec 1 Rutile x +/- pol E 7016 70 482 58 399 22 27/ - 29 1 Rutile x + pol E 7018 70 482 58 399 22 27/ - 29 1 Rutile x + pol E 7018-1 70 482 58 399 22 27/ - 29 1 Rutile x + pol E 7024 70 482 58 399 17 Not spec 2 Rutile x +/- pol

Code Welding Position 1 2 3 All position except vertical-down Flat and H – V fillets All position but in V – down only