Shielded Metal Arc Welding: (SMAW)
It is also known as Manual metal arc welding; stick welding or electric arc welding. It is a fusion welding process. Welding is performed with the heat of an electric arc that is maintained between the ends of a coated metal electrode and the work piece.
The electric energy from the arc thus formed produces temperatures of 10000 degree Fahrenheit or higher. A pool of molten metal consisting of base metal and filler metal is formed near the tip of the electrode.
Both ferrous and non-ferrous metals and alloys can be welded by arc welding processes with proper electrodes. The arc and the weld metal pool are protected by the arc flame and slag produced and controlled by the electrode coating. The arc and the weld metal pool are protected by the arc flame and slag produced and controlled by the electrode coating.
Arc time = (time arc is on)/ (Hours Worked)
Arc welding equipment:
1. Arc welding equipment (AC or DC)
2. Welding cables
3. Electrode holder
4. Ground clamp
5. Welding electrodes
6. Welding helmet and hand shield
7. Protective clothing including hand gloves
8. Chipping hammer, wire brush etc
Electrode coatings are generally electrical insulators and hence permit welding in narrow grooves and limited spaces without short circuiting. Electrode coatings provide an excellent means of introducing alloying elements in the weld metal.
Functions of the electrode coating:
Depending upon its composition the coating performs some or all of the following functions:
1. The coating is consumed at a slower rate than the electrode core wire forming a sheath of the un-burnt coating material over the arc. The length of this sheath depends upon the refractoriness of the coating material and its thickness. The existence of this sheath over the arc helps in directing the arc over the weld area making it more stable, reducing thermal losses, raising the temperature at the tip of the electrode and increasing the metal deposition rate.
2. The gas producing materials in the coating give out lot of gases due to dissociation and combustion. The gases so produced form a protective gaseous shield around the arc. This helps is shielding the arc and the welded metal from atmospheric oxygen, hydrogen and nitrogen. In addition to the shielding provided by the coating gases, shielding is also provided by the vapours of metallic oxides and silicates formed by the heat of the arc. The less volatile mineral ingredients in the coating help in the removal of oxides by forming a slag. The slag floats over the molten weld metal protecting it from atmospheric gases during welding and cooling. The removal of oxides by the slag improves the fluidity of the weld metal which in turn results in a sound, inclusion free joint.
3. The coating helps improve arc stability and makes possible a weld free from overlap, spattering and gas bubbles. Arc stability results from the sheath formation mentioned above and the addition of coating constituents like titanium dioxide, calcium carbonate and potassium silicates.
Advantages of Carbon electrodes:
1. Less expensive
2. Carry less current
3. Short life
4. Simple arc control
5. Soft material
6. Higher electrical resistance
Advantages of Graphite electrodes:
1. Comparatively costlier
2. Carry larger current
3. Long life
4. Arc control is comparatively difficult
5. Material is hard and brittle
6. Lesser electrical resistance
· The process is mostly manual and makes use of either a DC or an AC source.
· Current usually ranges from 50A to 300A
· Power requirements less than 10 KW
· Work piece thickness 3 to 19mm
Advantages of the SMAW process:
· The shielded metal arc welding process is simple, versatile and requires a smaller variety of electrodes.
. The equipment cost is low and it can be easily maintained. Welds by this process can be made in any position.
Limitation of the SMAW process:
· The process is slow
· Lot of electrode material is wasted in the form of unused ends, slag, arc gases
· If the slag is not removed properly, it remains in the bead leading to poor quality-welds.