Alloy steels containing high percentages of chromium, from less than 10% to more than 25%. There are three groups: (1) Austenitic, which contain both chromium (16% min) and nickel (7% min); a stress-corrosion resistant type contains 2% silicon. (2) Ferritic, which contain chromium only and cannot be hardened by heat treatment. (3) Martensitic, which contain chromium and can be hardened by heat treatment; these last are ferromagnetic. A subgroup of the martensitic steels comprises the precipitation-hardening types.See SAE Steel.