P-20 is a prehardened steel that comes in a Rc of 30-36. Easily machined and polished, it is used for mold base plates, large cores and cavities, and manifold blocks. This steel is not recommended if you’re looking for abrasion and/or chemical resistance.
H-13 steel can be hardened after machining to a 46-50Rc. It is used for medium cores and cavities, ejector lifts, and pins. It is excellent for abrasive wear from glass filled materials and it polishes and machines very well.
S-7 and D-2 are very hard steel with a 54-58Rc. They offer excellent abrasion resistance, and, due to their hardness, some corrosion resistance. Recommended DSM materials include all glass fiber reinforced material. Higher hardness also means they’re more brittle. They can be used for core, cavities, runner and gate inserts, and stripper rings.
420-SS is a stainless steel that can be hardened to 46-50Rc. It can be used for mold base plates, large cores, and cavities. In the hardened condition you will get good corrosion and abrasion resistance from glass filled materials. Machining and polishing are very favorable with this steel.
CSM 21-SS is a stainless steel furnished in the prehard condition around 32Rc and can be heat treated to a 44Rc. CSM 21, which can be used for mold base plates, main core, and cavity blocks or inserts, is easily machined and polished.
