I have been working on a little Stirling engine from plans and for one part the drawings call for a 3/16 diam rod of S7 be turned down over a length of 3 inches. I didn't have S7 but did have D2. After several attempts at turning the D2 I am wondering does S7 turns easier than D2? The D2 sort of tore instead of cut and so never managed to get a good cut. Granted, the diameter is small and my follow rest unable to support in more than one place on this small of diameter.
D2 is a tougher material but I always find good chip control. We have always called S7 "Stringer 7", turns better finish than D2 but has horrid chip control.
The characteristics of these materials are for 2 entirely different uses. D2 and S7 are not considered for similar uses at all.