Metallurgy and Materials Science
Core Concepts
Understand the iron-carbon diagram, the effect of carbon content on hardenability and weldability, and the phases ferrite, austenite, cementite, martensite, and pearlite. Know heat-treatment basics - annealing, normalizing, quenching, tempering, stress relief - and how cooling rate, preheat, and interpass temperature control microstructure. Study alloy families (carbon steel, low-alloy, stainless 300/400 series, aluminum, nickel, copper), the significance of carbon equivalent, hydrogen embrittlement, and sensitization of stainless steel. Recognize common defects rooted in metallurgy: cold cracking, hot cracking, lamellar tearing, and porosity from surface contamination.
Traps
Watch for questions that confuse hardenability with hardness, or that imply faster cooling always improves toughness. Carbon equivalent formulas and the 0.4% threshold for preheat appear often. Remember stainless steel 304 is austenitic and non-magnetic; 410 is martensitic and hardenable.
Worked Example / Rule of Thumb
Carbon equivalent (CE) rule: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. A CE above about 0.40% signals preheat is needed to avoid cold cracking. Rule of thumb: higher carbon and faster cooling mean harder, more crack-prone welds.