butt welding vs welding

butt welding and welding both are nouns.

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

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

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

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