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MMA - Arc welding with coated electrodes
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1. Description of the process
Arc welding with a coated electrode is a process in which the fusion of metal is produced thanks to the heat generated by an electric arc established between the end of a coated electrode and the metal base of the joint to be welded.
The filler material is obtained through electrode fusion in the shape of small drops. The protection is obtained through the decomposition of the coating in the form of gases and in the form of liquid slag that floats over the molten pool, afterwards becoming solid.
2. Applications
Electric arc welding with coated electrodes is one of the most used processes due to its great versatility and possibilities of use.
The process is applicable to carbon steel, alloy steel, stainless steel, molten and non ferrous metals, such as aluminium, copper, nickel and its alloys.
The sectors of major application are construction, installations, industrial maintenance, field work and private uses.
3. Selection of the type of power supply (AC / DC)
Arc welding with coated electrodes can be carried out in alternating current (CA), as well as in direct current (CC). The election of the type of power will depend on the type of equipment available, the type of electrode to be used and the material to be welded.
The traditional welding equipment of SOLTER proportions alternating current (AC).The SOLTER equipment with INVERTER technology proportions direct current (DC).
4. Polarity selection (+ / -)
If we weld using SOLTER equipment that proportions direct current (DC), as for example any equipment of an INVERTER type, we will be able to verify that there are two connection points (+ o -) available.
- Inverse polarity: Electrode holding clips connected to the positive pole (+) and the ground clip to the negative one (-).
- Direct polarity: Electrode holding clip to the negative pole (-) and ground clip to the positive one (+).
The use of one or the other option depends on the technical specification of each type of electrode that we use.
“Rutile” type electrodes: These are the most common ones and the principally most used ones due to their easy welding. They can be used by equipment that proportions alternating current (AC), as well as by equipment that proportions direct current (DC). By welding with direct current, normally the polarity is indifferent.
“Basic” electrodes and special ones (stainless steel, molten, aluminium...): Normally they need direct current (DC), since this makes welding easier. The polarity will depend on the characteristics of each electrode.
5. Welding equipment
The equipment that is used in the welding process is a source of energy that modifies the parameters we have in the network (Ex. 230V 50hz) to the necessary ones so that an electrode melts correctly and homogeneously.
- Transformers: Transformers: They modify the voltage (V) and the intensity (A) parameters, but continue to proportion alternating current (AC)
- Rectifiers – INVERTER: They consume alternating current and at the same time they modify the voltage (V) and intensity (A) parameters, they also rectify the current that passes from alternating current (AC) to direct current (DC).
5.1. Electrode holding clip
It has the mission of conducting the electricity to the electrode and keep it in place. In order to avoid overheating in the clamp, these should be kept in perfect condition; overheating would mean a reduction of quality and would make welding execution very difficult. An electrode holder should always be selected in function of the electrode to be used and the welding intensity.
5.2. Ground clip
The correct connection of the ground clip is of extreme importance. The location of the corresponding cable is especially relevant in the welding process. A poorly fastened cable cannot provide a constant good electrical connection and will heat up, leading to a break in the circuit and consequent loss of arc.
The ground clip contact area must be thoroughly clean and free from any substances that could prevent good contact, such as paint, varnish, oils....
6. Welding parameters
6.1. Electrode
The fundamental element of this process is the electrode, which establishes the arc, protects the molten pool and which, upon being consumed, produces the contribution of material that, joined do the founded material of the base metal, will constitute the welding.
The electrodes have normalised diameters, whereby the most common ones are 1.6mm; 2.0mm; 2.5mm; 3.25mm; 4.0mm; 5mm;....
On the other hand, the most common longitudes are 150, 200, 250, 300, 350 and 450mm.
Generally, the major diameter possible should be selected that assures the requisite of thermal contribution and allows easy use, in function of the position, the thickness of the material and the type of joining, which are the parameters on which the electrode diameter selection depends.
Normally, the welding intensities would be:
ELECTRODE DIAMETER
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WELDING CURRENT
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1,6 mm.
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40-60 Amp.
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2,0 mm.
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60-80 Amp.
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2,5 mm.
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70-90 Amp.
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3,25 mm.
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90-130 Amp.
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4,0 mm.
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130-160 Amp.
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The major diameter electrodes are selected for the welding of materials of great thicknesses and for welding in a flat position.
When welding in a cove, vertical and indoor position, the molten pool tends to fall due to the effect of gravity. This effect is ever so more accused, and it is ever so more difficult to maintain the molten pool in its place, the bigger its volume is. In other words, the bigger the diameter of the electrode, due to which I these positions it would be convenient to have electrodes of a lesser diameter and, therefore, lesser welding current.
6.1.1. Types of electrodes
Electrodes
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Characteristics
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Rutile
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The slag, composed of titanium oxide, is very dense and viscous.
These electrodes are of easy arc ignition and handling. Smooth melting. A very regular, good looking welding cord. Especially indicated for difficult positions. It is a matter of the most commonly used electrode. It is used above all for welding steel.
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Basic
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Dense slag of a brilliant aspect and not very abundant.
The welding metal is very resistant to fissuring. Very much used in the case of responsible welding, great thicknesses, rigid structures.
Its handling is somewhat difficult, due to which it is recommended to use direct current (DC) equipment.
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Cellulose
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The slag is formed by organic substances that generate a great quantity of gases. These gases proportion a great coating to the molten pool, giving the process a great penetration.
Its application is especially based on the welding of tubing in a vertical descending position.
Normally they need direct current and direct polarity.
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Acids
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They are some elevated fusion velocity electrodes and of great penetration.
They are used with base metals with good welding ability, contents very low in sulphur, phosphorous and carbon.
Especially indicated for flat positions.
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7. Devices
During the coated electrode welding, there is a series of processes that make the application quite easy and at Solter we incorporate Inverter in our equipment:
Anti-Stick:
Avoids the electrode sticking to the piece whilst welding is carried out.
Hot-Start:
Makes the beginning of each welding easy by incrementing the intensity at the beginning of each electrode. Very valid, above all when difficult electrodes are used.
Arc-Force:
This device makes the special electrode welding easy, since it maintains the electrode filler material to the molten pool in a constant way, thus avoiding that the arc is cut off.
Arc-Sense:
An extremely stable welding arc, above all when working at low intensities in critical welding processes.
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