Welding process
Shielded metal arc welding (SMAW)
Manual stick welding, where a consumable covered electrode carries the current, melts to supply filler metal, and burns its own covering to shield and refine the weld pool.
How it works
An arc is struck between the tip of a flux-covered electrode and the workpiece. The arc column reaches temperatures far above the melting point of steel, melting both the base metal and the core wire. Droplets of molten core wire transfer across the arc into the pool, so the electrode supplies filler metal and carries welding current at the same time.
The covering does most of the metallurgical work. As it burns it decomposes into a gas shield that pushes air away from the arc, releases arc stabilisers that keep the column steady, and forms a molten slag that floats on the pool. Ingredients in the covering also deoxidise the metal and can add alloying elements to the deposit.
The slag freezes over the bead and slows the cooling rate while protecting the hot metal from the atmosphere. It must be chipped and brushed off before the next pass, because slag left in place becomes a trapped inclusion. Each electrode burns down to a stub, so the welder stops, reloads, and restarts repeatedly along the joint.
Strengths
- Equipment is simple, portable, and inexpensive, needing only a power source, leads, and electrodes
- Works outdoors and in wind because the shielding is generated at the arc rather than blown from a nozzle
- Handles rusty, painted, or poorly prepared material better than most gas-shielded processes
- Covers a huge range of base metals and positions by simply changing electrode type
Limitations
- Low deposition rate and low operating factor, since the welder stops at every stub change
- Every pass produces slag that must be removed, adding labour and creating inclusion risk
- Highly dependent on welder skill for arc length, angle, and travel speed
- Low-hydrogen electrodes need controlled storage and limited exposure time to stay low-hydrogen
Key variables
- Amperage
- Set by electrode diameter and position. Too high causes undercut and burn-through; too low gives incomplete fusion and slag entrapment.
- Arc length
- Controlled by the welder's hand rather than the machine. A long arc loses shielding, widens the bead, and lets in nitrogen and hydrogen.
- Polarity
- The electrode covering dictates whether the rod runs on direct current electrode positive, electrode negative, or alternating current.
- Travel speed
- Governs bead size and heat input. Too slow lets slag run ahead of the pool; too fast leaves a narrow, poorly fused bead.
- Electrode angle
- Work and travel angles determine where arc force lands, controlling penetration and whether slag trails behind the pool.
What an inspector watches for
- Check electrode identification, condition, and oven or quiver control, since damp low-hydrogen rods are a leading cause of cracking
- Watch interpass cleaning closely, because slag inclusions are the characteristic defect of this process
- Examine starts and stops for crater cracks and restart porosity, which cluster at every stub change
- Verify preheat and interpass temperature on hardenable steel, since heat input is entirely in the welder's hands