treatise vs piece of writing

treatise and piece of writing both are nouns.

Word	Noun	Adjective	Verb	Adverb
treatise	Yes	No	No	No
piece of writing	Yes	No	No	No

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

treatise: a formal exposition
piece of writing: the work of a writer; anything expressed in letters of the alphabet (especially when considered from the point of view of style and effect)

Other hypernyms of treatise include writing, written material.

treatise (noun)	piece of writing (noun)
a formal exposition	the work of a writer; anything expressed in letters of the alphabet (especially when considered from the point of view of style and effect)