Welding Fundamentals
Current, voltage and the arc
The power source sets current; your arc length sets voltage. SMAW arc voltage typically runs 20-30 V and rises as the arc lengthens. For a fixed current, a longer arc raises voltage and heat input but reduces penetration, widens the bead and increases spatter.
Polarity and penetration
DCEN (electrode negative) puts roughly two-thirds of the arc heat into the workpiece: deep penetration, narrow bead, which is why it suits heavy root passes. DCEP puts more heat into the electrode: shallower penetration, wider bead, better oxide cleaning. AC splits the difference and helps against arc blow. The electrode classification overrides preference - E6010 is DCEP-only, E6011 runs AC or DC.
Heat input and bead geometry
Heat input (kJ/mm) = (volts x amps x 60) / (1000 x travel speed in mm/min). Slow travel raises heat input, widens the bead and increases dilution; fast travel narrows the bead and risks lack of fusion.
Arc blow
Magnetic deflection appears with DC, poor work-lead placement, or welding at the end of a joint. Counter it by shortening the arc, moving the work lead, splitting the ground, backstepping, or switching to AC.
Worked example
200 A, 24 V, 150 mm/min: (24 x 200 x 60) / (1000 x 150) = 1.92 kJ/mm. If the WPS caps heat input at 1.5 kJ/mm, hold current and voltage and raise travel speed to (24 x 200 x 60) / (1000 x 1.5) = 192 mm/min. Slowing down instead would break the procedure.
Traps that cost marks
- Believing current alone controls penetration; travel speed and polarity matter equally.
- Confusing arc voltage (20-30 V under the arc) with open-circuit voltage (50-80 V).
- Lengthening the arc to see the puddle - this raises voltage, spatters, and undercuts.