My guess is that he is wanting an etchant to reveal the prior austenite grain boundries of some quenched and tempered 5155 and 5160 steels. It can be difficult, but I've seen Nital used, as well as Picral. My preference is Picral, with dishwashing soap added. Even with the best procedure, it can be difficult to get a decent etch. There is definately a knack to it.
rp