The heat in shielded metal arc welding (SMAW) comes from an electric arc formed between the coated electrode (the “stick”) and the workpiece. This arc converts electrical energy into intense thermal heat, reaching temperatures above 6,000°F, which is hot enough to melt both the electrode and the base metal simultaneously. The process is straightforward in principle: electricity jumps across a small gap, and the resistance of that gap generates enormous heat.
How the Electric Arc Generates Heat
When you strike the electrode against the workpiece and pull it slightly away, current flows across the air gap and creates a sustained arc. This arc is essentially a column of superheated, electrically conductive gas called plasma. The total heat produced is governed by the power of the arc, which is the product of welding current (amps) and arc voltage (volts).
The arc isn’t a single uniform heat source. It has three distinct zones, each contributing to the overall thermal energy. At the cathode (negative terminal), electrons are emitted and generate heat as they escape the surface. In the plasma column between the electrode and workpiece, the flowing current heats the ionized gas. At the anode (positive terminal), incoming electrons slam into the surface and release their kinetic energy as heat. The heat generated in each zone equals the current multiplied by the voltage drop in that specific region.
Where the Heat Goes: Polarity Matters
Not all of that thermal energy is distributed evenly. The polarity you select, meaning which terminal the electrode is connected to, determines where most of the heat concentrates.
With direct current electrode negative (DCEN), roughly 70% of the heat develops at the workpiece and 30% at the electrode. This makes sense physically: the electrons flow from the electrode to the plate, and the larger positive surface of the workpiece absorbs most of their energy. The result is deeper penetration into the base metal, while the electrode melts more slowly and maintains its conical shape.
Flip the polarity to direct current electrode positive (DCEP), and the heat balance shifts. The electrode tip now receives a greater share of energy, causing it to melt faster and produce wetter, more fluid droplets. Penetration into the base metal is shallower, but the arc has a cleaning action that’s useful for certain applications. Many common SMAW electrodes are designed to run on DCEP or AC, and the electrode manufacturer specifies which polarity works best for each rod type.
How Much Heat Actually Reaches the Weld
A significant portion of the arc’s energy never makes it into the weld joint. Heat escapes through radiation (you can feel this as the intense warmth on exposed skin near the arc), convection into the surrounding air, conduction into the base metal away from the joint, and spatter that carries molten droplets away from the weld pool.
For SMAW specifically, arc efficiency depends heavily on arc length. A short, tight arc with lower voltage achieves 78% to 93% efficiency, meaning that percentage of the generated heat actually goes into melting and fusing the metal. A longer, higher-voltage arc spreads the plasma column wider, losing more energy to radiation, and drops efficiency to around 62% to 72%. This is one reason welding instructors emphasize keeping a short arc length: you’re not just improving weld quality, you’re putting more of your heat where it needs to go.
For comparison, submerged arc welding (where the arc is buried under granular flux) consistently achieves the highest efficiency among common arc processes, up to about 85%, because the flux blanket traps heat that would otherwise radiate away. Gas tungsten arc welding (TIG) sits at the low end because it operates at lower power with more arc spread.
Calculating Heat Input
The standard formula for heat input in arc welding is amps multiplied by volts, multiplied by 60, then divided by travel speed in inches per minute. The result is expressed in joules per inch. In practice, this calculation is difficult to apply precisely to SMAW because travel speed varies constantly as the welder moves the electrode by hand and the electrode gets shorter. Still, the formula illustrates the key variables you can control: higher amperage or voltage increases heat input, and moving faster reduces it.
This matters because heat input directly affects the size of the weld pool, the depth of penetration, and how much the surrounding metal is thermally altered.
What the Heat Does to Surrounding Metal
The intense, localized heat of the arc doesn’t just melt the joint area. It also heats the base metal on either side of the weld to temperatures high enough to change its internal grain structure, even though this metal never actually melts. This thermally altered region is called the heat-affected zone, or HAZ.
The HAZ forms in layers based on how hot each area got. Closest to the molten weld pool, where temperatures were highest, the metal’s grain structure coarsens, producing larger crystals that can be more brittle. Slightly farther out, the grains are refined into a finer, often tougher structure. Beyond that is a partially transformed zone where only some of the grain structure changed, and finally a tempered zone where the heat was just enough to slightly soften the metal. In materials that don’t undergo the same type of solid-state transformation during cooling, you’ll typically see a grain growth zone and a recrystallized zone instead.
The width and severity of these zones depend directly on how much heat you put into the weld. Higher heat input means a wider HAZ with more dramatic property changes. This is why controlling amperage, voltage, and travel speed isn’t just about getting good fusion. It’s about managing what happens to the metal you’re not melting.