In shielded metal-arc welding, intense heat from an electric arc is used to melt and fuse metals to form a weld. It is one of oldest and most widely used welding processes. Although used chiefly for joining iron and mild steels, shielded metal-arc welding is well suited to maintenance tasks because equipment is relatively inexpensive, simple to operate, and can be used for welding many different kinds of metals.
Below, you will find explanations describing shielded metal-arc welding process and how welding machines and accessories are set up and used. You will also find information on selecting an electrode. The personal safety equipment and precautions are also described.
How Process Works
A typical SMAW outfit consists of an electric welding machine, two welding cables, a ground clamp, an electrode holder, and a covered metal electrode. Electric current from welding machine is used to form an electric arc between tip of electrode and work.
Welding is started by touching end of electrode to base metal, then lifting electrode about ľ inch. This forms arc, which produces temperatures up to 5550įC. The intense heat at arc area instantly melts base metal and begins to burn covering off electrode and melt core.
The melted core becomes filler metal for weld and decomposition of flux forms a protective gaseous atmosphere around arc area. The gas forms a shield against contamination from oxygen and nitrogen in surrounding air. Additional shielding is provided by electrode flux, which forms a deposit called slag.
The shielding gas is ionized, and conducts electricity and maintains stability of arc.
Welding Voltage and Current
Either direct current (DC) or alternating current (AC) is used. The arc voltage or working voltage is voltage present in welding circuit while an arc is struck and welding is being done. The arc voltage ranges from 15V to 40V depending on arc length.
The open circuit voltage is voltage generated by welding machine when no welding is being done. Open-circuit voltages are normally set between 50V and 100V, but drops to arc voltage level when an arc is struck and welding begin.
In any electrical circuit, there is a correlation between voltage, current and resistance. The best results are normally obtained with an arc length about diameter of electrode.†
When arc length is increased, less current flow occurs because of increased resistance. The result is a cooler arc and a greater tendency to spatter. There will be less penetration of weld, increased exposure to oxidation and contamination, and an erratic, unstable arc.
When arc length is reduced, less resistance more current flows with less voltage and arc becomes hotter. With thin material, heat can melt a hole in welding, porosity, and undercutting of adjacent base metal.†
For DC machines, this is important. When electrode is negative and work piece is positive, this is called Straight Polarity. The opposite of this is Reverse Polarity.
DCSP or direct current straight polarity is characterized by faster melting of electrode, weld puddle being broad and penetration into base metal is relatively shallow. This is used when fast welding speeds and high deposition rates are required.
DCRP or direct current reversed polarity results in a hotter arc, making deeper, narrower weld puddle. This is used for structural welding, multi pass welds, and applications requiring deep penetrations.
Most electrodes are designed to be used with only one polarity.
Most AC power sources contain a transformer that steps down line voltage to level required for welding (normally less than 100V)