To harden the outer surface of steel while keeping the interior softer, done through heating with carbon-rich materials.
Compound word: 'case' (outer surface) plus 'harden' (to make hard). The metallurgical technique dates to medieval craftspeople.
Medieval swordsmiths casehardened blades so carefully that the difference between life and death in battle often came down to a millimeter of hardened surface!
Complete word intelligence in one call. Free tier — 50 lookups/day.