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Manual Metal Arc Welding


MMA Manual Metal Arc UK
STICK Manual Metal Arc UK
ELECTRIC ARC Manual Metal Arc UK
SMAW Shielded Metal Arc Welding US

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.

Application

Click to see complete photograph Widely used in the fabricating industries for the construction and repair of plain carbon and low alloy steels. Stainless steel, inconel, nickel and cast iron electrodes are also commonly used, as are surfacing type electrodes for the building up of wear resistant surfaces. The equipment is of initial low cost, and the process is flexible and safe to use. Good ventilation and protective clothing is essential.

Equipment

The basic equipment consists of:-
Power source Requires a controllable supply of AC/DC current at between 50 and 500 amps, and an arc voltage of 20 to 50 volts. Open circuit voltage must be high enough to initiate the arc, (60-100 volts), therefore a drooping or constant current power source would be appropriate.
Electrode holder Designed to insulate the operator from the currents and voltages used during welding and to enable the electrode to be firmly clamped in position, to facilitate access to joints. The electrode holder is connected to the power source.
Cables (Welding lead return) Designed with heavy insulation to carry current and also to protect from electric shock, fixed with a secure metal clamp to connect to the workpiece from the power source.

Control of the process

The control of the process is dependent upon :-

  • Operator
  • Current

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 :-

  • Arc characteristics
  • Shielding
  • Weld pool control
  • Alloying elements

Arc Characteristics

To improve metal transfer and reduce spatter, certain chemicals are added to the coating. These chemicals also reduce operating voltage for the electrode.

Shielding

This is provided by gases produced by the flux and slag covering during welding.

Weld Pool Control

Slag fluidity determines the ease of positions welding. Fast freezing slags are more suitable for welding in the vertical and overhead positions.

Alloying Elements

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.

Electrodes

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 :-

  • Rutile (titanium dioxide)
  • Basic (calcium carbonate and fluoride)
  • Cellulosic (cellulose)
  • Iron powder
  • Surfacing and non-ferrous alloy types

Rutile (titanium dioxide) Gives arc stability, low spatter. Most common general purpose electrode.
Basic (calcium carbonate and fluoride) Moisture resistant flux coating, low spatter, good arc striking. Used on carbon steels giving excellent mechanical properties and low crack risk.
Cellulosic (cellulose) Deeply penetrating all positions electrodes. Light slag covering, used mainly for high speed welding of pipe.
Iron powder Added to coating/flux to produce deep penetrating welds. 110-170% efficient. Used mainly on structural steels in the flat or horizontal-vertical position. Good arc striking characteristics.
Surfacing and non-ferrous alloy types Special applications such as building up of worn surfaces, providing a wear resistant finish. Nickel type electrodes for welding of cast iron, giving a imaginably deposit.

Care and storage of electrodes

The 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 Electrodes

Low 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 Considerations

To ensure that the correct electrodes are used and that these are in the required condition, the following practical steps should be taken:-

  • Ensure that the electrodes are stored in clearly marked groups
  • Open new packets of electrodes only when they are required for use
  • Keep electrode types separate when in storage or in drying areas
  • Avoid overloading drying rooms
  • Issue a limited quantity of electrodes particularly low hydrogen types
  • Rotate stock to ensure that electrodes are not stored for long periods
  • Discard electrodes with any visible damage to electrode coating

Quality Assurance

Application 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:-

  • A purchasing specification which defines the required electrode
  • Procedures for checking goods received against specification
  • Written storage procedures covering the requirement listed previously
  • Written drying procedures and calibration systems for drying ovens

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