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