hard hat vs safety hat

hard hat and safety hat both are nouns.

Word	Noun	Adjective	Verb	Adverb
hard hat	Yes	No	No	No
safety hat	Yes	No	No	No

As nouns, hard hat and safety hat are synonyms defined as:

hard hat and safety hat: a lightweight protective helmet (plastic or metal) worn by construction workers

Other synonyms of hard hat include tin hat.

hard hat (noun)	safety hat (noun)
a lightweight protective helmet (plastic or metal) worn by construction workers	a lightweight protective helmet (plastic or metal) worn by construction workers
a worker skilled in building offices or dwellings etc.