openwork vs piece of work

openwork and piece of work both are nouns.

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

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

openwork: ornamental work (such as embroidery or latticework) having a pattern of openings
piece of work: a product produced or accomplished through the effort or activity or agency of a person or thing

Other hypernyms of openwork include work.

openwork (noun)	piece of work (noun)
ornamental work (such as embroidery or latticework) having a pattern of openings	a product produced or accomplished through the effort or activity or agency of a person or thing