Job Knowledge 83
The previous article, Part 1, dealt with the cellulosic and rutile electrodes. This article will cover the basic, iron powder and acid electrodes.
The description 'basic' originates from the chemical composition of the flux coating which contains up to perhaps 50% of limestone, calcium carbonate (CaCO3). This decomposes in the arc to form a gas shield of carbon monoxide/dioxide.
In addition to the limestone there may be up to 30% of calcium fluoride (CaF2) added to lower the melting point of the limestone and to reduce its oxidising effect. Also deoxidants such as ferro-manganese, ferro-silicon and ferro-titanium are added to provide de-oxidation of the weld pool.
Other alloying elements such as ferro-chromium, ferro-molybdenum or ferro-nickel may be added to provide an alloy steel deposit. Binders may be sodium silicate, only for use on DC+ve current, or potassium silicate which enables the electrodes to operate on both direct and alternating current.
The gas shield from basic electrodes is not as efficient as that from the rutile or cellulosic types and it is necessary to maintain a constant short arc if porosity from atmospheric contamination is not to be a problem. The electrodes are particularly sensitive to start porosity because of the length of time taken to establish an efficient protective shield. An essential part of welder training is familiarisation with the technique of starting the weld ahead of the required start position and moving back before proceeding in the direction of welding.
The penetration characteristics of basic electrodes are similar to those of rutile electrodes although the surface finish is not as good. The slag cover is heavier than rutile electrodes but is easily controlled, enabling the electrodes to be used in all positions. High limestone coatings have been developed that enable a limited range of electrodes to be used in the vertical-down (PG) position. The weld pool blends smoothly into the parent metal and undercutting should not occur.
The slag is not as easily removed as with rutile or cellulosic electrodes but the low melting point means that slag entrapment is less likely. The chemical action of the basic slag also provides very clean, high quality weld metal with mechanical properties, particularly notch toughness, better than that provided by the other electrode types. A further feature of these electrodes is that the welds are more resistant to solidification cracking, tolerating higher levels of sulphur than a rutile or cellulosic electrode. This makes them valuable if it becomes necessary to weld free cutting steels.
The basic electrode is also known as a low hydrogen rod ('lo-hi'). The coating contains no cellulose and little or no moisture provided the electrodes are correctly handled. When exposed to the atmosphere, moisture pick-up can berapid. However, baking the electrodes at the manufacturers' recommended baking temperature, generally around 400°C, will drive off any moisture and should provide hydrogen levels of less than 5ml/100g weld metal. After baking the electrodes need to be carefully stored in a holding oven at a temperature of some 120°C to prevent moisture pick-up.
Many manufacturers now provide electrodes in hermetically sealed vacuum packs with hydrogen levels guaranteed to be less than 5ml/100g weld metal. These are particularly useful in site applications where there is a need to maintain very low hydrogen levels and baking and storage facilities are not available. The electrodes are taken directly from the pack and can be used for up to 12 hours from opening before sufficient moisture has been absorbed to require baking.
Basic, low hydrogen electrodes are therefore widely used in a variety of applications where clean weld metal and good mechanical properties are required. They can be obtained with alloyed core wires and/or ferro-alloy additions to the coating to give very wide selection of weld metal compositions, ranging from conventional carbon steels, creep resistant and cryogenic steels and duplex and stainless steels. Where high quality, radiographically or ultrasonically clean weld metal is a requirement, such as on offshore structures and pressure vessels, basic electrodes will be used.
Developments over the last 20 or so years have enabled carbon-manganese steel consumables to give good Charpy-V and CTOD values at temperatures down to -50oC. The low hydrogen capabilities also mean that basic electrodes would be used for the welding of thick section carbon steels and high strength, high carbon and low alloy steels where cold cracking is a risk (see Job knowledge articles Nos. 45 and 46).
In addition to the 'standard' cellulosic, rutile and basic electrodes discussed above, electrodes may be classified as 'high recovery'.
By adding substantial amounts of iron powder, up to 50% of the weight of the flux coating, to either basic and rutile electrode coatings it is possible to deposit a greater weight of weld metal than is contained in the core wire. These electrodes are described as having an efficiency above 100% eg 120%, 140% etc and this 3 digit figure is often included in the electrode classification.
The electrodes have thicker coatings than the 'standard' electrodes which can make them difficult to use in restricted access conditions. They are, however, welder friendly with good running characteristics and a smooth stable arc. The iron powder not only melts in the heat of the arc to increase deposition rate but also enables the electrode to carry a higher welding current than a 'standard' electrode.
The iron powder is electrically conducting, so allowing some of the welding current to pass through the coating. High welding currents can therefore be used without the risk of the core wire overheating, thus increasing both the burn-off and the deposition rates. The high recovery electrodes are ideally suited for fillet welding, giving a smooth, finely rippled surface with a smooth blend at the weld toes. They are generally more tolerant to variations in fit-up and their stability on low open circuit voltages means that they are very good at bridging wide gaps. However, the large weld pool means that they are not suited to positional welding and are generally confined to welding in the flat (PA) and horizontal-vertical (PC) positions.
The last type of electrode covering is described as 'acid'. These electrodes have large amounts of iron oxides in the flux coating which would result in a high oxygen content in the weld metal and poor mechanical properties. It is therefore necessary to incorporate large amounts of de-oxidants such as ferro-manganese and ferro-silicon in the flux. Although they produce smooth flat weld beads of good appearance and can be used on rusty and scaled steel items the mechanical properties tend to be inferior to the rutile and basic coated electrodes. They are also more sensitive to solidification cracking and are therefore little used.
The next articles will cover specification and classification of MMA (SMAW) electrodes.
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