What is another word for case-harden?
Pronunciation: [kˈe͡ɪshˈɑːdən] (IPA) 
The term "case-harden" typically refers to the process of hardening the surface layer of a metal object while keeping the inside soft and ductile. Some synonyms for this term include "surface hardening," "carburizing," and "nitriding." Surface hardening involves heating the metal object at high temperatures and then rapidly cooling it to develop a hard outer layer. Carburizing, on the other hand, involves introducing carbon to the surface layer by heating the object in a carbon-rich environment. Nitriding involves introducing nitrogen to the surface layer of the metal object, creating a hard and wear-resistant surface. All of these processes essentially achieve the same result as hardening the surface layer of the metal object while maintaining the internal structure soft and ductile.
