Shielded Metal Arc Welding Basics
Fundamentals of this long-used and versatile process are explained In shielded metal arc welding (SMAW), an arc between a covered electrode and a weld pool is used to accomplish a weld. As the welder steadily feeds the covered electrode into the weld pool, the decomposition of the covering evolves into gases that shield the pool. The process is used without application of pressure and with filler metal from the covered electrode. The sound weld metal deposited by the process is used for both joining and for applying a functional surface to metal products. Because of the many possible variations in the composition of the electrode covering and the large selection of core wire chemistry, the process can produce an extensive range of weld metal deposits with desirable mechanical and physical properties, while providing for a smooth arc, uniform metal transfer characteristics, and ease of operation. It is one of the oldest and simplest welding processes and continues to be widely used. The simplicity of the process extends to the number and nature of the circuit components required, which are as follows: 1. A power source of adequate current rating and duty cycle 2. A SMAW electrode compatible with the output of the power source 3. A suitably sized welding cable 4. An electrode holder 5. A workpiece lead. Process Fundamentals The distinctive feature of SMAW is the physical presence of the covering/ coating that surrounds the core wire of the consumable electrode. The covered rod is called an electrode because it functions as the terminal from which the electric flow changes from the conducting solid to the conducting plasma of the welding arc. For any given application, the electrode must meet the following three criteria: 1. It must shield the arc and the weld metal. 2. It must add metal to the weld. 3. It must sustain a welding arc. The constituents of the covering accomplish these functions. When sufficiently heated, the covering’s ingredients do the following: 1. Decompose into gases and displace the air at the weld site, thus providing a shield for the arc and the weld metal; 2. Ionize to support the arc plasma; 3. Flux the molten metal and, on cooling, form a protective slag cover on the weld bead. The covering may also contain metal powders that enhance the metal contribution of the electrode to the weld pool. Principles of Operation The SMAW process uses an electric circuit that supports a welding arc to convert electric line power or fuel into heat. The heat from the welding arc is intense and extremely concentrated. It immediately melts a portion of the workpiece and the end of the electrode. The welder maintains the arc length by holding a consistent space between the electrode and the weld pool that forms on the workpiece. When the arc is removed, the liquid fuses and the melt solidifies into continuous metal.Elements of a typical welding circuit for shielded metal arc welding.