fastening vs welding

fastening and welding both are nouns.

Word	Noun	Adjective	Verb	Adverb
fastening	Yes	No	No	No
welding	Yes	No	No	No

As nouns, welding is a hyponym of fastening; that is, welding is a word with a more specific, narrower meaning than fastening:

fastening: the act of fastening things together
welding: fastening two pieces of metal together by softening with heat and applying pressure

Other hyponyms of fastening include bonding, soldering, doweling, earthing, grounding, linkage, ligature, tying, fixation.

fastening (noun)	welding (noun)
the act of fastening things together	fastening two pieces of metal together by softening with heat and applying pressure
restraint that attaches to something or holds something in place