Pb vs hard lead

Pb and hard lead both are nouns.

Word	Noun	Adjective	Verb	Adverb
Pb	Yes	No	No	No
hard lead	Yes	No	No	No

As nouns, hard lead is a hyponym of Pb; that is, hard lead is a word with a more specific, narrower meaning than Pb:

Pb: a soft heavy toxic malleable metallic element; bluish white when freshly cut but tarnishes readily to dull grey
hard lead: unrefined lead that is hard because of the impurities it contains

Other hyponyms of Pb include antimonial lead, pig lead.

Pb (noun)	hard lead (noun)
a unit of information equal to 1000 terabits or 10^15 bits	a lead alloy that contains about 5% antimony
a soft heavy toxic malleable metallic element; bluish white when freshly cut but tarnishes readily to dull grey	unrefined lead that is hard because of the impurities it contains