|Cr-(Mn)-Ni Austenitics - Applications|
U-202 is a lower cost alternative to the Cr-Ni austenitic stainless steels, such as U-304. About half the nickel of U-304 is replaced with alloy additions of manganese and nitrogen. Copper is added to compensate for the increased work hardening rate, caused by the nitrogen additions. U-202 thus has very similar mechanical properties to U-304 and has excellent ductility and formability. It also has excellent low temperature properties and responds similarly to U-304DQ in drawing operations. U-202 has good welding characteristics. Post weld annealing is not normally required. The pitting resistance of U-202 is not as good as the U-304 types and is similar to U-430.
The low carbon grades (U-304L-ASTM, U-304L-ASME and U- 304LDDQ) do not require post weld annealing and find extensive use in heavy gauge components where freedom from carbide precipitation is often required.
The high carbon grades (U-304, U-304H, U-304DQ and U-304DDQ) have good welding characteristics. There is, however, a risk of carbide precipitation and sensitisation with a possible concomitant loss in intergranular corrosion resistance in corrosive environments in thicker gauges. Post weld annealing is not normally required to restore the excellent performance of these grades in a wide range of corrosive conditions. U-304H, with a higher carbon content, is normally specified where good mechanical properties at elevated temperatures are required. These grades also have a larger grain sizes (ASTM 7 or coarser) to improve the high temperature creep resistance. The drawing quality (U-304DQ) and deep drawing quality (U- 304DDQ and U-304LDDQ) grades have increased levels of nickel to improve their formability and are thus used in forming operations where the depth of the formed part exceeds the diameter.
U-304LS has a high sulphur level, although still within the U-304L specification. This is to improve the machinability, weldability and forgeability. U-304LS should not be confused with the free-machining grades that have much higher levels of sulphur.
U-304LN has a higher nitrogen content to improve the strength and is also particularly suitable for cryogenic applications.
U-321 is a titanium stabilised version of U-304 and is used in applications exposed to the temperature range 450°C to 850°C where there is a high risk of sensitisation. U-321 has higher elevated temperature properties than U-304. Although having good resistance to oxidation and sensitisation, CS321 is unsuitable for use in highly oxidising environments due to possible ‘knifeline’ attack. Typical applications for CS321 would include furnace parts, after burners, expansion bellows, compensators, catalytic converters, etc.