Characteristics: Alloy steels contain 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. Includes alloys 303, 304/304L, 316/316L, 321, and 347. (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 are ferromagnetic.