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The entire family of stainless steels may be divided into the following groups:
1. MARTENSITIC: Straight chromium, hardenable by heat treatment. Representative types are 410. 420, 431, and 440C.
2. FERRITIC: Straight chromium, non-hardenable by heat treatment. Representative types are 405 and 430.
3. AUSTENITIC: Chromium-Nickel, non-hardenable by heat treatment. Representative types are 303, 304, and 316.
4. PRECIPITATION HARDENING
A)The martensitic chromium types are hardenable by heat treatment and are produced in bars and wire only, with the exception of Type 410. They are hardened by heating above the critical or transformation temperature and then rapidly cooled in oil or air. By proper selection of grades and analyses a wide range of ''as quenched- hardness is available, depending primarily on carbon and chromium content. These grades attain maximum corrosion resistance in the hardened condition. They may be annealed for best cold working and machining characteristics. All martensitic types are . consistently magnetic.
B) The ferritic chromium types are non-hardenable by heat treatment. Also, they cannot be hardened to any appreciable extent by quenching from high temperature. They are generally used in the annealed or soft condition. Called the 'straight' chromium grades, they contain no nickel, but are quite adequate for many applications.
C) The austenitic chromium-nickel grades have the highest corrosion resistance of all stainless steels, as well as very fine mechanical properties. They cannot be hardened by heat treatment, but great tensile strength and hardness may be developed by cold working. While these alloys are normally non-magnetic, they become slightly magnetic as they are cold worked. They are widely used, with or without additional alloying elements, in sheet strip, and plate form, as well as in bar and wire products. In the annealed state, the austenitic types have a tensile strength of 75,000 to 100,000 psi, considerably higher than mild steel. Grades with a high chromium-to-nickel ratio, such as Type 301, can be cold work hardened to extremely high tensile strengths and still maintain good ductility.
D) The precipitation hardening groups contain steels that may be either martensitic or austenitic as used but they
develop strength during heat treatment by utilizing precipitation hardening reactions rather than phase transformations such as is employed in heat treating steel. None of these types have an AISI Type number at present because each is a proprietary, patented by Armco Steel Corporation.