Grain size is usually in reference to the prior austenitic grain size. Just to be clear it does not normally refer to the martensite needles themselves. What is the grade of steel that you are attempting to measure the grain size in? The grade of steel may influence the type of grain size measurement that is performed. And there are many different types of grain size tests. These include the Shepherd fracture grain size test, the Mcquaid-Ehn test, Snyder-Graff grain size test, etc.
And yes, it may be possible to determine the grain size of a heat treated and tempered part depending upon what it is and how it was heat treated and prepared.
Yes, it is possible to reveal the prior austenite GS in Q&T 4150. Generally, an aqueous picral, perhaps with some HCl and a surfactant (such as dishwashing liquid) is used with various levels of success. In general, the lower the tempering temperature the better the results. Often, multiple attempts of polishing, etching, polishing, etc..., is needed. Sometimes light polishing after deep etching works.
Revealing the prior austenite GS with a picral really is an art. I have tried many different concoctions with varying levels of success over the decades. One thing I have learned is that old, nearly black etchant often works better than a fresh solution.
