ARC WELDING WITH COATED ELECTRODES
(M.M.A. Manual Metal Arc)
Arc welding with coated electrodes is a manual process where the heat source consists of the electric arc.
When the arc strikes between the coated electrode (by means of an electrode holder clamp) and the piece to be welded (base material), it generates heat which causes rapid melting of both the base material and the electrode (weld material).
- Gaseous protection
- Welding arc
- Melting pool
- Base material
/ THE WELDING CIRCUIT
The welding circuit consists essentially of the following elements:
- POWER SOURCE
- ELECTRODE HOLDER CLAMP
- EARTH CLAMP
- COATED ELECTRODE
1. POWER SOURCE
La polarità della corrente di uscita del generatore identifica altre due categorie di appartenenza:
The purpose of the power source is to feed the electric arc, which is present between the base material and the electrode, through the output of a current sufficient in quantity to keep the arc struck. Electrode welding is based on the constant current principle i.e. the current delivered by the power source should not vary when the operator moves the electrode towards the piece.
The main construction property of the source is therefore to keep the current unchanged in the presence of variations in arc length as the electrode moves closer to or away from the piece: the more constant the current, the more stable results the arc and the simpler the operator's work. Inside the power source, there is usually a welding current adjustment device, of a mechanical (magneticshunt or saturable reactance) or electronic type (SCR systems or inverter systems). This distinction can be used to classify electrode welding machines into three families, depending on their construction technology: electromechanical welding machines, electronic welding machines (SCR), inverter welding machines.
The polarity of the power source output current distinguishes two further categories:
ALTERNATING CURRENT (AC) POWER SOURCE
The power source output current takes the form of a sine wave, which changes its polarity at regular intervals, with a frequency of 50 or 60 cycles per second (Hertz). It is obtained using a transformer, which converts the mains current into a suitable current for welding. This is for electromechanical welding machines.
DIRECT CURRENT (DC) POWER SOURCE
The power source output current has a continuous wave form, which is obtained by means of a device, the rectifier, which is situated at the base of the transformer and can convert from alternating to direct current. This output is typical of electronic SCR and inverter power sources.
If the welding circuit has a direct current (DC) power source, it can be further classified according to the method of connecting the power source poles to the material to be welded:
- Straight polarity connection
Straight polarity connection occurs when the clamp cable (with the electrode holder clamp) is connected to the negative pole (-) of the power source and the earth cable (with the earth clamp) to the positive pole (+) of the power source. The electric arc concentrates the heat produced on the piece and causes its melting. In this way, as the core of the electrode melts, it is deposited and penetrates into the welding joint.
- Reverse polarity connection
Reverse polarity connection occurs when the clamp cable (with the electrode holder clamp) is connected to the positive pole (+) of the power source and the earth cable (with the earth clamp) to the negative pole (-) of the power source. The heat of the electric arc is mostly concentrated at the tip of the electrode.Each type of electrode requires a specific current type (AC or DC) and, in the case of DC current, a specific polarity: the choice of the electrode therefore depends on the type of power source used. Incorrect use will cause arc stability problems and hence also welding quality problems.
2. ELECTRODE HOLDER CLAMP
The primary function of the electrode holder clamp is to support the electrode, guaranteeing a good electrical contact for current passage; it should also guarantee sufficient electrical insulation for the welding operator.
3. EARTH CLAMP
The earth clamp is a tool that, via the earth cable, ensures the electrical circuit to be closed between the welding power source and the piece to be welded. The clamp and earth cables, connected to the electrode holder clamp and earth clamp respectively, permit an electrical connection between the power source and the base material to be welded. The choice of cable section and length should be based on the maximum welding current in amps.
4. COATED ELECTRODE
The coated electrode consists of a core and a coating which have different but complementary functions: the core acts mainly as a conductor for the arc power supply and as supplier of the material that fills the joint; the coating, on the other hand, has the main function to protect the weld pool and stabilise the arc.
/ THE ARC FORCE, HOT START AND ANTI STICK DEVICES
Welding power sources may contain special devices whose functions is to facilitate their use. These devices are described as follows: arc force, hot start and anti-stick devices.
The arc force device eases the transfer of drops of melted material from the electrode to the base material, preventing the arc from extinguishing when the drops cause contact (i.e. a short circuit) between the electrode and the weld pool.
The hot start device facilitates the striking of the electric arc, by supplying an overcurrent every time welding restarts.
The anti-stick device automatically switches off the power source if the electrode sticks to the base material, thus allowing it to be removed manually without ruining the electrode holder clamp.
/ MATERIALS WELDED BY MMA
When the steel composition is easily identifiable, rutile electrodes can be used as they are easier to strike and to weld and give a good-looking seam.
In practice, welding of medium, high carbon steels (>0.25%) can cause the formation of structural defects; application of the electrode procedure is recommended mainly for welding medium to thick joins using basic electrodes: in these cases a high quality weld is obtained with good breakage resistance.
Steel pipe welding is carried out using cellulose electrodes, where high penetration and good electrode workability are required. Bevelling is always recommended, with a bevel angle that is sufficient to allow almost complete electrode insertion into the welding gap.
For special materials such as stainless steel, aluminium and its alloys, cast iron, specific electrodes for the particular material are used.
Stainless steels are welded with direct current (DC) with reverse polarity; special electrodes are used and are differentiated by the metallurgical composition of the material to be welded (presence of chrome (Cr) and of Nickel (Ni) in variable proportions).
Aluminium and light alloys are welded with direct current (DC) with reverse polarity. The machine should be equipped with rather a high strike dynamic to guarantee electrode strike.
Also in this case special electrodes are used and are differentiated by the metallurgical composition of the material to be welded (presence of Magnesium (Mg) and of Silicon (Si) in variable proportions).
Cast iron is welded with direct current (DC) with reverse polarity; the majority of cast iron structures and machine members are obtained by casting, so that welding is used to correct possible casting defects or for repairs. Special electrodes are used and the base material should be heated sufficiently before use.