2015_Cha02_Manual Electrode Welding_L

Manual Metal Arc welding is the first arc welding method developed.Its simple setup and high flexibility makes it an ideal welding method in handicraft and building site applications.As the name indicates it is carried about manually and is hard to mechanise or automise,The general setup is simple:An electric arc burns between a melting stick electrode and the workpiece, that is also locally melted. The energy for the Arc is supplied by a power source, (DC or AC, constant current characteristic) connected to the workpiece and the electrode by cables. The connection between electrode and cable is done by an insulated electrode holder, that also serves as a handpiece for the welder.The welder is dressed in uninflammable protection clothing (heat and radiation), wears gloves and has a welding mask, that protects his eyes and face from the radiation of the arc. Further Equipment include a slag hammer and a steel brush for slag removal.The electrode itself consists of a massive core stick an a coating, that forms a liquid slag during the process.The electrode coating stabilises the electric arc, protects the liquid metal from atmosphere, influences the metal transfer and the properties of the resulting beat.The coating consists of different minerals, which serve special propeties in the coating and in different combinations define the welding properties as well as the mechanical properties of the resulting beat.The different compositions of electrodes lead different welding and metallurgical propoerties of the stick electrode as indicated in the table. Furthermore it is possible to vary the composition between the types for special purposes. An example is a rutile type electrode with added cellulose, which will result in an electrode with medium sized droplets and an reduced amount of slag.The table shows further properties of electrode types. The mix-types try to combine the advantages of the standard types without including the disadvantages.The information about the properties allows the selection of electrodes for a certain application:The rutile type is for universal application with mild and building steels, providing good arc striking and restriking properties as well as good arc stability. The fine to medium droplet transfer in combination with lots of liquid slag, that forms the beat leads to good weld appearances, which in turn helps with slag removal. Because of the high amount of slag, they are not suited for vertical down weldingThe basic type produces good toughness at (when properly dried) low hydrogen levels, resulting in very low cold cracking sensitivity. It is weldable in all positions but requires an DC power source. The basic type should be taken into account when high strength steels are to be welded.The C Type produces almost no slag, making the electrode highly suitable for vertical down welding. Protection against atmosphere is provided the CO2 generated from the burning cellulose. The C-type produces a lot of smoke and fumes so that it only can be used outdoors. Its classic application is the welding of pipelines.The pure acid type is not used today. Nevertheless the very slag detachabilty leads to the use in mixed type electrodes mainly for narrow gap welding .Striking the arc on an electrode is required every time, when welding has been interrupted.While the initial strike is assisted by a bit of coating with high carbon content on the tip, this is not the case in an restrike. Here a certain amount of conductivity is necessary in order to preheat the slag for emission of arc stabilising substances. This is only possible with high rutile containing slags.Restriking of basic electrodes therefore should not be attempted, the left over electrodes should be disposed. Because of the bad restriking behaviour they are furthermore not suited for tack welding. R-Type electrodes are to used for this even on high strength steels.Stick electrodes for mild and low alloyed steels are standardised in ISO 2560.The indentification code contains the applied standard as well as coded vaules for yield strength, thoughness, alloying concept, coating, current type and recovery rate, welding position and hydrogen content (all in deposited pure weld metal).The key number for yield strength codes the minimum yield strength (1/10th when measured in (N/mm2), tensile strength, as well as a minimum elongation.The code for impact energy codes the lowest temperature (multiply code by -10) where a minimum impact energy of 47 J is reached in Charpy-V notch test.Even numbers are used for stick electrodes, that must be welded with DC-power sources. Odd numbers for elctrodes that can also be welded on AC-power sources.The higher the number, the higher the deposition effieciency of high deposition elektrodes.5 codes an electrode for pipe weldingOther than in ISO 2560 –A, where a minimum Charpy-V Toughness of 49 J is demanded, ISO 2560 –B only requires 27 J at the specified temperature to meet the demands of the standard.Most of the other codes differ too.Furthermore only the minimum yield strength is standardised, there are no specified values for elongation and tensile strength.The welding current for a particular stick electrode depends on the diameter of the core wire.Small core diameters and long electrodes will have a larger internal resistance, which in turn will lead to strong heating of the electrode during welding, which in turn will soften the core stick and damage the coating. For this reason, the length is restricted to certain standard lengths.For stick electrode welding power sources with constant currentcharacteristic are employed. This guaranties constant weld pentration and a constant melting behaviour of the electrode.Changes in arc length, that will result in changes of voltage, can be compensated on sight by experienced welders.Power sources for MMAW often are simple transformers for AC welding. Adding a rectifier makes them suitable for DC welding as well.Inverters produce DC Current as well, but are generally lighter and small as transformer type power sources and due to their function principle easy to adapt to digitised controllers.The arc welding converter is a DC-generator driven by an electric AC motor. Due to the mechanical inertia of the system, these converters produce stable arcs. The electric motor may be replaced with an diesel engine and is sometimes even combined with an air compresssor and a electric generator. Such machines are typical for pipeline applications, where electric energy is not avialable.The deposition rate of stick electrode reaches from approximately ½ kg/h of deposited weld material for 2mm standard electrodes up to approximately 6 kg/h for high performance electrodes, which may contain up to 120% of the cores mass in form of iron powder in the coating.These values are true for duty cycles of 100% , representing 100% of welding time.In practice, a 100% duty cycle is never reached, as the welder has to exchange electrodes, remove slag, grind the end craters and so on. Realistic duty cycles for MMAW are about 30%, which reduces the effective deposition rate for MMAW to a 1/3 of the values above.MMAW welding may be adapted to a wide range of materials and plate thicknesses simply by selecting a suitable electrode with a suitable diameter and the adjustment of the welding current.Minimum plate thickness is about 2 mm, very thick plates can be welded in multilayer technique.The arc blow effect is a problem not only happening with MMAW but with all electric arc welding processes. The concentration off the electromagnetic field lines when the flow of current is not straigth delectes the Arc, which has its own magnetic field. This can make the control of the arc difficult (specially in mechanised processes without visual control) and lead to lack of Fusion and root defects.The slide shows possible scenarios for magnetic blow effect, either originating from electromagnetic or ferromagnetic effects.Possible countermeasures can be the tilting of the electrode and/or the even distribution of current over the part by several sturdy tack welds.Adding ferrous masses to the workpiece may compensate other blow effects as well as the relocation of the mass contact may help to rediribute the current flow on the workpiece.The most effective method, but not usable for all electrode types is the use of AC-power sources.Hydrogen in the deposited weld material may lead to cold cracking in high strength steels and should be avoided. One source of hydrogen can be moisture in the coating of stick electrodes.Basic coated electrodes tend to pick up from the air quite fast (even in standard packaging and under normal room conditions.They have to be dried by baking them in special ovens and be held warm until use in heated storage boxes.Stick electrodes sealed in cans in small amounts may be an alternative. When using these, the whole contents of a can must be used up rapidly in order to prevent pickup of moisture from air. Electrodes that were not used within the time specified by the producer (normally 1-2 hours) must be disposed or rebaked.。