To harden the outer surface of steel or metal through heat treatment while keeping the core soft and flexible.
Compound of 'face' (meaning outer surface) and 'harden' (from Old English 'heard', meaning hard). This metallurgical term developed as industrial metalworking became more sophisticated in the 19th century.
Machinists discovered that if you harden just the outer 'face' of steel while leaving the inside soft, you get the perfect armor—tough on the outside to resist scratches but flexible inside to prevent shattering, like making the ultimate protective gear!
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