The 400 series of martensitic free machining grade stainless steels can be hardened by heat treatment to achieve elevated strength and hardness. They exhibit better machining characteristics than austenitic grades, however, sacrifices corrosion resistance. UNS S41001, UNS S41600, UNS S41000, UNS S42000, UNS S44004, UNS S4220. Electric Arc Furnace AOD Refined.