welding vs butt-welding

welding and butt-welding both are nouns.

Word	Noun	Adjective	Verb	Adverb
welding	Yes	No	No	No
butt-welding	Yes	No	No	No

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

welding: fastening two pieces of metal together by softening with heat and applying pressure
butt-welding: creating a butt joint by welding

Other hyponyms of welding include butt welding, spot-welding, spot welding.

welding (noun)	butt-welding (noun)
fastening two pieces of metal together by softening with heat and applying pressure	creating a butt joint by welding