The Manual Metal Arc process
Manual metal arc welding was first invented in Russia in 1888. It
involved a bare metal rod with no flux coating to give a protective gas
shield. The development of coated electrodes did not occur until the
early 1900s when the Kjellberg process was invented in Sweden and the
Quasi-arc method was introduced in the UK. It is worth noting that
coated electrodes were slow to be adopted because of their high cost.
However, it was inevitable that as the demand for sound welds grew,
manual metal arc became synonymous with coated electrodes. When an arc
is struck between the metal rod (electrode) and the workpiece, both the
rod and workpiece surface melt to form a weld pool. Simultaneous melting
of the flux coating on the rod will form gas and slag which protects the
weld pool from the surrounding atmosphere. The slag will solidify and
cool and must be chipped off the weld bead once the weld run is complete
(or before the next weld pass is deposited).
The process allows only short lengths of weld to be produced before a
new electrode needs to be inserted in the holder. Weld penetration is
low and the quality of the weld deposit is highly dependent on the skill
of the welder.
Types of flux/electrodes
Arc stability, depth of penetration, metal deposition rate and
positional capability are greatly influenced by the chemical composition
of the flux coating on the electrode. Electrodes can be divided into
three main groups:
Cellulosic electrodes contain a high proportion of
cellulose in the coating and are characterised by a deeply penetrating
arc and a rapid burn-off rate giving high welding speeds. Weld deposit
can be coarse and with fluid slag, deslagging can be difficult. These
electrodes are easy to use in any position and are noted for their use
in the 'stovepipe' welding technique.
Features:
- deep penetration in all positions
- suitability for vertical down welding
- reasonably good mechanical properties
- high level of hydrogen generated - risk of cracking in the heat
affected zone (HAZ)
Rutile electrodes contain a high proportion of
titanium oxide (rutile) in the coating. Titanium oxide promotes easy arc
ignition, smooth arc operation and low spatter. These electrodes are
general purpose electrodes with good welding properties. They can be
used with AC and DC power sources and in all positions. The electrodes
are especially suitable for welding fillet joints in the horizontal/vertical
(H/V) position.
Features:
- moderate weld metal mechanical properties
- good bead profile produced through the viscous slag
- positional welding possible with a fluid slag (containing fluoride)
- easily removable slag
Basic electrodes contain a high proportion of
calcium carbonate (limestone) and calcium fluoride (fluorspar) in the
coating. This makes their slag coating more fluid than rutile coatings -
this is also fast-freezing which assists welding in the vertical and
overhead position. These electrodes are used for welding medium and
heavy section fabrications where higher weld quality, good mechanical
properties and resistance to cracking (due to high restraint) are
required.
Features:
- low weld metal produces hydrogen
- requires high welding currents/speeds
- poor bead profile (convex and coarse surface profile)
- slag removal difficult
Metal powder electrodes contain an addition of metal
powder to the flux coating to increase the maximum permissible welding
current level. Thus, for a given electrode size, the metal deposition
rate and efficiency (percentage of the metal deposited) are increased
compared with an electrode containing no iron powder in the coating. The
slag is normally easily removed. Iron powder electrodes are mainly used
in the flat and H/V positions to take advantage of the higher deposition
rates. Efficiencies as high as 130 to 140% can be achieved for rutile
and basic electrodes without marked deterioration of the arcing
characteristics but the arc tends to be less forceful which reduces bead
penetration.
Power source
Electrodes can be operated with AC and DC power supplies. Not all DC
electrodes can be operated on AC power sources, however AC electrodes
are normally used on DC.
Welding current
Welding current level is determined by the size of electrode - the
normal operating range and current are recommended by manufacturers.
Typical operating ranges for a selection of electrode sizes are
illustrated in the table. As a rule of thumb when selecting a suitable
current level, an electrode will require about 40A per millimeter
(diameter). Therefore, the preferred current level for a 4mm diameter
electrode would be 160A, but the acceptable operating range is 140 to
180A.
What's new
Transistor (inverter) technology is now enabling very small and
comparatively low weight power sources to be produced. These power
sources are finding increasing use for site welding where they can be
readily transported from job to job. As they are electronically
controlled, add-on units are available for TIG and MIG welding which
increase the flexibility. Electrodes are now available in hermetically
sealed containers. These vacuum packs obviate the need for baking the
electrodes immediately prior to use. However, if a container has been
opened or damaged, it is essential that the electrodes are redried
according to the manufacturer's instructions.
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