Base Material: Identification, Carbon Equivalent, and Weldability
Know the metal before choosing a procedure
Base material questions test whether a candidate can identify a metal and predict how it welds. Carbon steels weld easily; low-carbon grades are most forgiving, while higher-carbon grades need preheat and controlled cooling to avoid cracking. Low-alloy and high-strength steels often require preheat and low-hydrogen electrodes because hydrogen can cause delayed cracking. Stainless steels resist corrosion but distort and can sensitize if overheated; heat input must be limited. Aluminum conducts heat rapidly and forms an oxide layer that must be removed; GTAW with alternating current is common. Cast iron is brittle and prone to cracking; cold welding with short beads and peening is often preferred over full fusion.
Concepts to master
Understand carbon equivalent: as carbon and alloy content rise, weldability falls and preheat becomes necessary. Recognize that low-hydrogen practices (dry electrodes, clean joint faces) matter most on thick, restrained, or high-strength joints. Know that dissimilar metals have different expansion rates and may need a compatible filler.
Traps in questions
A common trap is treating all stainless as identical; austenitic grades behave differently from martensitic grades. Another is assuming aluminum needs preheat like steel; it usually does not. Questions may also imply cast iron can be welded like mild steel, which invites cracking.
Rule of thumb
If the stem names a high-carbon or high-strength steel, expect preheat and low-hydrogen controls. If it names aluminum, expect oxide removal and high heat conductivity. If it names cast iron, expect short beads, low current, and peening.