Shielded metal arc welding (SMAW) is a
process that melts and joins metals by heating them with an arc established
between a sticklike covered electrode and the metals. It is often called stick
welding. The electrode holder is connected through a welding cable to one
terminal of the power source and the workpiece is connected through a second
cable to the other terminal of the power source.
The core of the covered electrode, the
core wire, conducts the electric current to the arc and provides filler metal
for the joint. For electrical contact, the top 1.5 cm of the core wire is bare
and held by the electrode holder. The
electrode holder is essentially a metal clamp with an electrically insulated
outside shell for the welder to hold safely. The heat of the arc causes both
the core wire and the flux covering at the electrode tip to melt off as
droplets. The molten metal collects in the weld pool and solidifies into the
weld metal.The lighter molten flux, on the other hand, floats on the pool
surface and solidifies into a slag layer at the top of the weld metal.
Functions of
the electrode Covering :
The covering of the electrode contains various
chemicals and even metal powder in order to perform one or more of the
functions.
Protection :
•
It provides a gaseous shield to protect the molten metal from air. For
a cellulose-type electrode, the covering contains cellulose, (C6H10O5)x. A
large volume of gas mixture of H2, CO, H2O, and CO2 is produced when cellulose
in the electrode covering is heated and decomposes.
•
For a limestone- (CaCO3) type electrode, on the other hand,CO2 gas and
CaO slag form when the limestone decomposes. The limestone-type electrode is a
low-hydrogentype electrode because it produces a gaseous shield low in
hydrogen. It is often used for welding metals that are susceptible to hydrogen
cracking, such as high-strength steels.
Deoxidation
:
It
provides deoxidizers and fluxing agents to deoxidize and cleanse the weld
metal. The solid slag formed also protects the already solidified but still hot
weld metal from oxidation.
Arc
Stabilization
:
It provides arc stabilizers to help maintain a
stable arc. The arc is an ionic gas (a plasma) that conducts the electric
current. Arc stabilizers are compounds that decompose readily into ions in the
arc, such as potassium oxalate and lithium carbonate. They increase the
electrical conductivity of the arc and help the arc conduct the electric
current more smoothly.
Metal
Addition :
It provides alloying elements and/or metal powder
to the weld pool. The former helps control the composition of the weld metal
while the latter helps increase the deposition rate.
Advantages/Disadvantages
:
The welding equipment is relatively simple,
portable, and inexpensive as compared to other arc welding processes. For this
reason, SMAW is often used for maintenance, repair, and field construction.
However, the gas shield in SMAW is not clean enough for reactive metals such as
aluminum and titanium. The deposition rate is limited by the fact that the
electrode covering tends to overheat and fall off when excessively high welding
currents are used.The limited length of the electrode (about 35 cm) requires
electrode changing, and this further reduces the overall production rate.
Current
Supply for SMAW :
References :
Haynes
welding Manual 1994
Welding Metallurgy Sindo Kou
