WordNet records no synonym and no antonym for case-hardened steel. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
steel with a surface that is much harder than the interior metal
Synonyms, antonyms and the broader and narrower relations for case-hardened steel come from Princeton University’s WordNet. Full sources and licences.