hardening vs natural action

hardening and natural action both are nouns.

Word	Noun	Adjective	Verb	Adverb
hardening	Yes	No	No	No
natural action	Yes	No	No	No

As nouns, natural action is a hypernym of hardening; that is, natural action is a word with a broader meaning than hardening:

hardening: the process of becoming hard or solid by cooling or drying or crystallization
natural action: a process existing in or produced by nature (rather than by the intent of human beings)

Other hypernyms of hardening include action, activity, natural process.

hardening (noun)	natural action (noun)
the act of making something harder (firmer or tighter or more compact)	a process existing in or produced by nature (rather than by the intent of human beings)
the process of becoming hard or solid by cooling or drying or crystallization
abnormal hardening or thickening of tissue