The high temperature strength of materials is generally expressed in terms of their “creep strength” – the ability of the material to resist distortion over long term exposure to a high temperature. In this regard the austenitic stainless steels are particularly good. Design codes such as Australian Standard AS1210 “Pressure Vessels” and AS4041 “Pressure Piping” (and corresponding codes from ASME and other bodies) also stipulate allowable working stresses of each grade at a range of temperatures. The low carbon versions of the standard austenitic grades (Grades 304L and 316L) have reduced strength at high temperature so are not generally used for structural applications at elevated temperatures. “H” versions of each grade (eg 304H) have higher carbon contents for these applications, which results in significantly higher creep strengths. “H” grades are specified for some elevated temperature applications.
Although the duplex stainless steels have good oxidation resistance due to their high chromium contents, they suffer from embrittlement if exposed to temperatures above about 350°C, so they are restricted to applications below this.
Both martensitic and precipitation hardening families of stainless steels have high strengths achieved by thermal treatments; exposure of these grades at temperatures exceeding their heat treatment temperatures will result in permanent softening, so again these grades are seldom used at elevated temperatures.
Although this material requires higher initial forces than 304 stainless, it is quite tough and ductile and can be readily stamped, blanked, spun and drawn.
Annealing 1800-2000 F (928-1093 C), air cool. This procedure will result in maximum ductility. For maximum corrosion resistance, see the note on stabilizing anneal under corrosion.
Hardening This alloy does not harden by heat treating. Elevated properties may only be obtained through cold reduction.
321 is a titanium stabilised chromium-nickel austenitic stainless steel with good strength and excellent corrosion resistance, as supplied in the annealed condition with a typical brinell hardness of 175. Characterised by high corrosion resistance in general atmospheric corrosive environments it exhibits excellent resistance to most oxidizing agents, general foodstuffs, sterilizing solutions, dyestuffs, most organic chemicals plus a wide variety of inorganic chemicals, also hot petroleum gases, steam combustion gases, nitric acid, and to a lesser extent sulphuric acid. It displays good oxidation resistance at elevated temperatures has excellent resistance to intergranular corrosion and has excellent weldability. 321 cannot be hardened by thermal treatment, but strength and hardness can be increased substantially by cold working, with subsequent reduction in ductility.
