workpiece vs piece of work

workpiece and piece of work both are nouns.

Word	Noun	Adjective	Verb	Adverb
workpiece	Yes	No	No	No
piece of work	Yes	No	No	No

As nouns, piece of work is a hypernym of workpiece; that is, piece of work is a word with a broader meaning than workpiece:

workpiece: work consisting of a piece of metal being machined
piece of work: a product produced or accomplished through the effort or activity or agency of a person or thing

Other hypernyms of workpiece include work.

workpiece (noun)	piece of work (noun)
work consisting of a piece of metal being machined	a product produced or accomplished through the effort or activity or agency of a person or thing