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Manual Metal Arc Welding
Consists of an AC/DC arc burning between a
consumable electrode and the workpiece to melt the joint area. The arc and
the weld pool are shielded by gases and slags which result from the
decomposition of the electrode coating. The electrode material is
transferred across the arc to fill/fuse the joint and the operator must
feed in the electrode to maintain a constant arc length.
The basic equipment consists of:-
The control of the process is dependent upon :-
These are the main parameters for the MMA process.
Current range is dependent on the size and type of electrode used; and
the thickness of material and welding position. A high level of manual
dexterity is required to co-ordinate the electrode, to match the burn off
rate and to maintain a constant arc length. The quality of the welded
joint depends entirely on the operators skill.
The characteristics of the process are determined by the following
features :-
To improve metal transfer and reduce spatter, certain chemicals are
added to the coating. These chemicals also reduce operating voltage for
the electrode.
This is provided by gases produced by the flux and slag covering during
welding.
Slag fluidity determines the ease of positions welding. Fast freezing
slags are more suitable for welding in the vertical and overhead
positions.
The flux coating can contain elements which can improve mechanical
properties of the joint, without changing the core wire composition. This
is a more economical proposition than changing the core wire.
Fig. 1 MMA Electrode MMA electrodes are available in various diameters and lengths, depending on the position, material thickness and requirements of the welded joint. There are five main groupings of electrodes :-
Care and storage of electrodesThe quality of the welded joint and ease of welding depends entirely on the condition of the electrodes. If the mineral coating is damaged, poor arc stability and inadequate shielding will result. If not properly protected most of the coating materials can absorb water. This could result in the coating deteriorating and subsequent hydrogen pick-up in the weld bead. Special care must be taken with the storage of controlled hydrogen electrodes. The electrodes should be re-dried according to the manufacturers instructions if they absorb damp. Increased spatter, striking difficulties weld bead porosity and harsh arcing characteristics can all be indications of poor electrode condition. The condition of the electrode profoundly influences the ease of welding and weld metal quality. In applications varying from simple fabrications of plain carbon steel to demanding joints in high strength materials, manual metal arc continues to be the process used. Damage to, or deterioration of the electrode coating may be indicated by any one or more of the following: weld metal porosity; arc instability; excessive spatter; poor bead profile; undercut; poor slag release; heat affected zone cracking. Therefore for quality and effectiveness attention to the correct storage and handling of electrodes cannot be over-emphasised. Special Requirements for Hydrogen Controlled ElectrodesLow hydrogen electrodes are used where there may be a risk of cold cracking. It is essential that the moisture level of the coating is kept at a very low level, this may entail special packaging, storage conditions, re-drying and the use of heated quivers. Practical ConsiderationsTo ensure that the correct electrodes are used and that these are in the required condition, the following practical steps should be taken:-
Quality AssuranceApplication of quality systems such as BS5750 usually entails control of the purchase, receipt, storage and issue of MMA electrodes. A quality system will usually involve:-
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