How To Classify The ASTM Tool Steel? We know ASTM tool steel start from letter, such as H12, A2, D2, O2, S7, L2, F2 and P20 etc, do you know how to classify them? What’s the difference among them?1: Hot work tool steels, Identification H:1): Types H10 to H19 are characterized by a controlled chromium content along with other alloying e