Materials
Steel Crystal Structures: Austenite, Ferrite, Martensite
Steel's hardness comes from trapping its crystal structure mid-transition. Pure iron is BCC (body-centered cubic) = ferrite, soft. Heated above 912°C it becomes FCC (face-centered cubic) = austenite, can dissolve carbon. Quench rapidly and the carbon-trapped lattice snaps into martensite — the hard, brittle phase that makes steel a tool material. Tempering balances hardness vs toughness.
Case Hardening and Cementation Steel
Case hardening: bury iron in charcoal at 950°C, carbon diffuses into surface at ~0.1-0.2 mm/hour creating a hard steel skin over a soft iron core. Extended days-long cementation produces 'blistered steel' with excess surface carbon. Pattern-welding cemented steel with fresh iron evens out carbon AND creates the visible Damascus pattern.
Salt Bath Nitriding
Molten KNO3/KCl bath at 650°C introduces nitrogen atoms into steel surface, forming a hard nitrided layer. Works on stainless steel where carbon hardening doesn't. No quench needed — the metal-nitrogen structure is hard as-formed. Industrial cyanide versions phased out since the early 2000s for safety; KNO3-based home-accessible version works.
Plasma Nitriding
Plasma nitriding uses a vacuum chamber with N₂+H₂ gas at 50-600 Pa and several hundred volts DC — the plasma ionises nitrogen, which accelerates into the workpiece surface and forms a nitrided layer. Industrial replacement for toxic cyanide-salt nitriding. Key failure mode in home attempts: too-deep vacuum (plasma fills entire chamber instead of hugging the workpiece).