Aircraft Materials - Ferrous
Core concepts
Ferrous airframe materials fall into three families: low-alloy steels such as 4130 chromium-molybdenum; corrosion-resistant steels, including the austenitic 300 series (302, 304, 321, 347), the martensitic 400 series (410, 440C) and precipitation-hardening grades such as 17-4PH; and maraging steels for highly stressed landing-gear parts.
Learn the heat-treatment vocabulary precisely. Annealing softens and relieves internal stress; normalising refines grain structure; hardening is austenitise-then-quench; tempering follows hardening to trade hardness for toughness. Case hardening gives a hard wear surface over a tough core: carburising diffuses carbon into low-carbon steel and requires a quench, while nitriding diffuses nitrogen at a lower temperature and needs no quench.
Traps in questions
Austenitic stainless steels cannot be hardened by heat treatment at all - only by cold working - and in the annealed condition they are essentially non-magnetic. Martensitic 400-series steels are magnetic and hardenable. Nitrided surfaces must not be ground or machined afterwards, because the thin case is the entire benefit. Quenching a high-strength forging to "fix" a defect destroys its temper. Never confuse hardening temperature with tempering temperature; a part overheated during tempering has lost strength permanently.
Worked example
A fastener is strongly attracted to a magnet, shows light surface rust after a damp winter and is specified as heat-treatable. Rule out 304 stainless immediately: it would be non-magnetic and non-hardenable. The material is a 400-series stainless or a low-alloy steel. Confirm from approved data - never by the magnet alone.