Welding Process and Essential Variables
Know your essential variables
Every process is qualified through a welding procedure specification (WPS) built on essential variables. Change one beyond the code's allowance and the qualification no longer covers the work. For SMAW that means electrode classification (E6010, E7018, E7018-1), current type and polarity, and in many codes the vertical-up versus vertical-down progression. For GMAW you must track transfer mode (short-circuit, globular, spray, pulsed), shielding gas composition and flow rate, and filler classification. For FCAW, gas-shielded versus self-shielded is the fundamental split. For GTAW, current type (AC for aluminum, DCEN for steel), filler and torch gas matter.
Settings and their consequences
Candidates should relate settings to results: too little current gives lack of fusion and a crowned, cold bead; too much gives undercut, spatter and burn-through. Low-hydrogen electrodes such as E7018 must be drawn from a holding oven and carried in a heated quiver; a damp electrode produces porosity and hydrogen cracking. Electrode extension, travel angle and arc length all shift penetration.
Rule of thumb: heat input
Heat input (kJ/in) = (volts x amps x 60) / (travel speed in in/min x 1000). At 25 V, 200 A and 6 in/min: (25 x 200 x 60) / 6000 = 50 kJ/in. Excess heat input can cost toughness; too little invites lack of fusion.
Traps
Questions swap polarity meanings, or present a change of shielding gas or filler classification as if it were minor. Treat gas, polarity and filler group as essential until the code proves otherwise.