hardness vs severeness

hardness and severeness both are nouns.

Word	Noun	Adjective	Verb	Adverb
hardness	Yes	No	No	No
severeness	Yes	No	No	No

As nouns, hardness and severeness are synonyms defined as:

Other synonyms of hardness include harshness, inclemency, rigor, rigorousness, rigour, rigourousness, severity, stiffness.

hardness (noun)	severeness (noun)
a quality of water that contains dissolved mineral salts that prevent soap from lathering	extreme plainness
the property of being rigid and resistant to pressure; not easily scratched; measured on Mohs scale	excessive sternness
the quality of being difficult to do	something hard to endure
devoid of passion or feeling; hardheartedness	used of the degree of something undesirable e.g. pain or weather
excessive sternness