Metals and Alloys
Phase diagrams and the Fe–C system
Be able to read binary phase diagrams, apply the lever rule, and distinguish eutectic from eutectoid reactions. In the Fe–Fe3C diagram, know the eutectoid at 0.77 wt% C and 727 C, proeutectoid ferrite or cementite, pearlite as a lamellar two-phase constituent, and the difference between equilibrium (slow-cool) and non-equilibrium (martensite) products. Martensite is a diffusionless, carbon-supersaturated, body-centered tetragonal phase; its hardness depends mainly on carbon content, not on alloy content.
Strengthening and heat treatment
Name the four strengthening mechanisms: solid-solution, grain-boundary (Hall–Petch), precipitation (GP zones to coherent precipitates, as in 2024/7075 or 17-4PH), and strain hardening. Distinguish hardenability (Jominy end-quench, the ability to form martensite at depth) from hardness. TTT and CCT diagrams explain critical cooling rate; tempering trades strength for toughness.
Alloy families
Austenitic 304 versus 316 (Mo for chloride pitting), sensitization and weld decay in unstabilized austenitics, 2024 versus 7075 versus 6061 and temper designations (T6 = solution treat and artificially age), dezincification of brass, alpha-beta titanium, and nickel superalloys for creep.
Traps
Inverting the lever-rule arms; treating martensite as an equilibrium phase; assuming a higher-alloy steel is always harder after quenching; forgetting that 2024 is generally not fusion-welded; and mixing up annealing (soften), normalizing, and solution treating.
Worked example
At 730 C a 0.40 wt% C steel sits in the austenite + ferrite field with tie-line ends at 0.022 and 0.77 wt% C. Ferrite fraction = (0.77 - 0.40)/(0.77 - 0.022) = 0.49, so about 51% transforms to pearlite on slow cooling.