Job Knowledge 82
The next series of articles will cover welding electrodes and filler metals, beginning with a brief look at the requirements for a flux. Whether a flux is in an electrode coating or is in granular form, as in a submerged arc flux, the requirements are the same.
- The flux must be capable of providing a protective shield to prevent atmospheric contamination of the electrode tip, the filler metal as it is transferred across the arc and the molten weld pool. Generally, it does this by decomposing in the heat of the arc to form a protective gaseous shield.
- It must be capable of removing any oxide film (failure to do so will result in lack of fusion defects and oxide entrapment). It does this by reacting chemically with the oxide.
- It should improve mechanical properties by providing clean, high quality weld metal and perhaps by transferring alloying elements across the arc.
- It must be capable of providing the desired weld metal composition, again by transferring alloying elements across the arc.
- It should aid arc striking and arc stability.
- It should produce a slag that will shape the molten pool and hold the pool in place during positional welding if required.
- Any slag should be readily removable and preferably self-detaching.
- It should not produce large amounts of fume and any that it does should not be harmful to the welder.
These requirements have resulted in a multitude of different consumables, many being apparently identical, and this can make filler metal selection a difficult and confusing task. This article attempts to give some insight into the various types of flux coated manual metal arc (MMA) electrodes before moving on in later articles to look at other types of welding fluxes.
Most MMA electrodes can be conveniently divided into three groups by their coating composition. These are cellulosic, rutile and basic coatings, each of which gives the electrode a distinctive set of characteristics.
Cellulosic electrodes contain a large proportion of cellulose, over 30% and generally in the form of wood flour. This is mixed with rutile (titanium dioxide, TiO2 ), manganese oxide and ferro-manganese and is bonded onto the core wire with sodium or potassium silicate. Moisture content of these electrodes is quite high, typically 4 to 5%. The cellulose burns in the arc to form a gas shield of carbon monoxide, carbon dioxide and, in conjunction with the moisture in the coating, produces a large amount of hydrogen, typically 30 to 45ml hydrogen/100gm weld metal.
The hydrogen raises arc voltage and gives the electrodes their characteristics of deep penetration and high deposition rates. The high voltage requires a high open circuit voltage of around 70 volts to allow easy arc striking and to maintain arc stability. The forceful arc also results in appreciable amounts of weld spatter and this limits the maximum current that can be used on the larger diameter electrodes. A thin, friable and easily removed slag is produced, giving a rather coarsely rippled weld profile. The slag is also fast freezing so that, unlike most other electrodes, they can be used in the vertical down position - 'stove piping'.
Electrodes with a sodium silicate binder can be used only on DC electrode positive (reverse polarity). Those with a potassium silicate binder can be used either DC electrode positive or on AC. The electrodes require some moisture in the coating to aid the running characteristics and they must never be baked, as may be done on basic coated electrodes. This has the advantage that the electrodes are tolerant to site conditions. If they become damp, drying at a temperature of around 120°C will be sufficient.
Electrode compositions are only available for welding low carbon non-alloyed steels although nickel additions may be made to improve notch toughness. Charpy-V values of around 27J at -20°C are possible in the unalloyed electrodes. The high hydrogen level means that any steel welded with these electrodes should be selected to have a very high resistance to hydrogen induced, cold cracking (see Connect articles numbers 45 and 46). They should not be used without giving due consideration to the steel composition, restraint and the need for preheat. The characteristics of deep penetration, high deposition rates and the ability to be used vertically down means that the main use for these electrodes is for cross country pipelining although they are used to a more limited extent for welding storage tanks.
Rutile coatings, as the name suggests, contain a large amount of rutile, titanium dioxide, typically around 50%, in addition to cellulose, limestone (calcium carbonate), silica (SiO2) mica (potassium aluminium silicate), ferro-manganese and some moisture, around 1 to 2%. Binders are either sodium or potassium silicate. The cellulose and the limestone decompose in the arc to form a gas shield containing hydrogen (around 20ml/100gm weld metal) carbon monoxide and carbon dioxide. The electrodes have medium penetration characteristics, a soft, quiet but stable arc and very little spatter, making them a 'welder friendly' electrode. Striking and re-striking is easy and the electrodes will run on very low open circuit voltages. The electrodes produce a dense covering of slag that is easily removed and gives a smooth evenly rippled weld profile.
The presence of cellulose and moisture means that the electrodes produce relatively high levels of hydrogen, perhaps 20 to 25ml/100gm weld metal. This restricts their use to mild steels less than 25mm thickness and thin section low alloy steels of the C/Mo and 1Cr1/2Mo type. Mechanical properties are good and Charpy-V notch toughnesses of 40J at -20°C are possible. They are probably the most widely used general purpose electrode. Rutile coated austenitic stainless steel electrodes can be obtained and can be used in all thicknesses as cold cracking is not a problem with these alloys.
Rutile electrodes, like cellulosic electrodes, require some moisture in the coating and they should not be baked. If they become damp, re-drying at around 120°C should be sufficient. Those electrodes with a sodium silicate binder can be used on DC electrode negative or AC. Electrodes with the potassium silicate binder can be used on both polarities and on AC. The potassium silicate binder electrodes generally have better arc striking and stability characteristics than the sodium silicate binder types and a more readily detachable slag.
The next article will look at the basic, low hydrogen electrodes and some of the other less common types of coatings.
This article was written by Gene Mathers.