25
The Manual Metal Arc Process
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
• Rutile
• Basic
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 (vertical down position) 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
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 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 hydrogen weld metal
•
Requires high welding currents/speeds
•
Poor bead profile (convex and coarse surface
profile)
•
Slag removal difficult
Iron 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 horizontal/vertical positions to take
advantage of the higher deposition rates.
MMA OPERATION