welding vs fastening

welding and fastening both are nouns.

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

As nouns, fastening is a hypernym of welding; that is, fastening is a word with a broader meaning than welding:

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

Other hypernyms of welding include attachment.

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