Welding Fundamentals and Processes (Part A)
Process families you must be able to compare
SMAW (stick): covered electrode, slag shielding, wide alloy range, low deposition rate, tolerant of wind. FCAW: continuous tubular wire with a flux core; self-shielded (no external gas) versus gas-shielded (usually CO2). GMAW: solid wire plus external shielding gas, with transfer modes - short-circuit (low current, thin material), globular, spray (argon-rich mixtures, high current, flat and horizontal), and pulsed spray. SAW: arc buried under granular flux, highest deposition, mechanized, flat position. GTAW: non-consumable tungsten electrode, inert gas, precise, used for thin material and root passes.
Electrode classification
The digits are where candidates lose points. In E7018: E = electrode, 70 = 70,000 psi minimum tensile strength, 1 = usable in all positions, 8 = low-hydrogen coating with iron powder, usable on AC or DCEP. The third digit is position and the fourth is coating/current type; neither describes strength. ER70S-6: ER = electrode or rod for GMAW/GTAW, 70 = tensile, S = solid, 6 = chemistry (more deoxidizers than S-2).
Heat input and metallurgy
Heat input drives cooling rate, and cooling rate drives toughness and cracking risk:
HI (J/in) = (volts × amps × 60) ÷ travel speed (in/min)
Worked example: 25 V, 200 A, 8 in/min gives (25 × 200 × 60) ÷ 8 = 300,000 J/in, or 300 kJ/in - a hot, slow pass. Doubling travel speed halves heat input.
Preheat and interpass limits are essential variables. Preheat slows cooling on thick or high-carbon-equivalent steel; excessive interpass temperature can degrade toughness. Carbon equivalent (CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15) flags steel that needs preheat before an arc is struck.