To harden the outer surface of steel or iron while keeping the inside softer and more flexible.
From 'case' (outer layer) + 'harden' (make hard). Developed as a metallurgical term in the 17th century to describe the process of carburizing steel for increased durability.
Medieval sword-makers discovered that heating steel in a carbon-rich environment created an incredibly hard edge that wouldn't shatter—this case-hardening technique gave knights a competitive advantage in battle and is still used in industrial manufacturing today.
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