17-4 is a martensitic precipitation hardened stainless steel. The name comes from the chemical makeup which is approximately 17% chromium and 4% nickel. SAE Type 630 or Alloy 630 stainless steel is the same as 17-4PH, and they both refer to the same grade. 17-4 can be heat treated to high levels of strength and hardness, and features corrosion resistance and machinability comparable to austenitic 304 stainless. Being martensitic, 17-4 is magnetic. 17-4 is capable of being hardened up to approximately 44 Rc when heat treated to condition H900. (UNS S17400)