Welding of Carbon Steels
There are three broad categories of carbon steels:
(i) low carbon steels (carbon up to 0.3%),
(ii) medium carbon steels (carbon up to 0.5%) and
(iii) high carbon steels (carbon up to about 1.5%).
Welding of low carbon steels
These are easily weldable by any common method of welding, e.g. flux shielded metal-arc welding, submerged arc welding, TIG, MIG, CO, welding, plasma arc welding, thermit welding, resistance welding processes, oxy-acetylene welding, brazing and soldering.
Flux shielded manual metal-arc welding is the most common method of welding mild steels. Mild steel and low hydrogen electrodes are used. Oxy-acetylene welding is frequently used. No flux is needed. A neutral flame is employed and filler rod is of mild steel. No pre-heating or post-heating is needed.
Welding of medium carbon steels
Welding of medium carbon steels poses problems as the weld joint tends to harden when mastensite is formed in the heat-affected zone (HAZ) due to rapid cooling. Pre-heating at 150 to 260°C is recommended to eliminate and reduce the hard and brittle areas.
Post-heating is also done between 595 and 675°C. Medium carbon steels can be welded by flux shielded metal-arc welding, resistance welding, submerged arc welding, thermit welding and oxy-acetylene welding. With arc welding, low hydrogen electrodes should be used. In case of oxy-acetylene welding, an excess of acetylene should be used in the flame. No flux is used. High tensile steel rods are recommended.
Welding of high carbon steels
Flux shielded metal-arc welding, resistance welding, thermit welding and oxy-acetylene welding are often used for welding high carbon steels. In case of oxy-acetylene welding, a carburizing flame is used. Filter metal is of high carbon steel.
When flux shielded metal-arc welding is done, pre-heating up to 205°C before welding is essential and post-heating after welding is required between 700 and 790°C. Electrodes used may be mild steel electrodes, austenitic electrodes or low hydrogen electrodes. Flash and upset butt welding and spot welding are also used for welding high carbon steels.
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