API TR 938-C: Second Edition
DOD Adopted ANSI Approved Approved
This report covers many of the "lean," "standard," and "super" grades of duplex stainless steels (DSS) most commonly used within refineries. The definitions of these terms have not been firmly established by the industry, and vary between literature references and materials suppliers. Table 1 shows how the various grades are being classified into "families" for the purposes of this report. The UNS numbers of the standard grades being used for corrosive refining services include S31803 and S32205, while the super grades include S31260, S32520, S32550, S32750, S32760, S39274, and S39277. The grades which are labeled as "semi-lean" include S32304 and UNS S32003, have either lower Cr or Mo than the standard grades, and are used in some process services that are less aggressive. These alloys and lean duplexes, such as S32101, have also been used for storage tanks and structural applications primarily for their higher strength compared to carbon steel (CS). It is observed that new DSS alloys are being introduced and are likely to continue to be introduced. These new grades can be reasonably placed in the context of this discussion based on their composition.
The product forms within the scope are tubing, plate, sheet, forgings, pipe, and fittings for piping, vessel, exchanger, and tank applications. The use of DSS for tanks is addressed by API 650, Appendix X. Later revisions of this report may consider expanding the scope to include castings and other product forms for pumps, valves, and other applications. Use of DSS as a cladding is also not included within the scope of this document.
The majority of refinery services where DSS are currently being used or being considered in the refining industry contain:
a) a wet, sour (H2S) environment which may also contain hydrogen, ammonia, carbon dioxide, chlorides, and/or hydrocarbons;
b) water, containing chlorides, with or without hydrocarbons; or
c) hydrocarbons with naphthenic acids at greater than 200 °C (400 °F) but below the maximum allowable temperatures in the ASME Code for DSS (260 °C to 343 °C [500 °F to 650 °F] depending on the grade).
The specific plant locations containing these services are described in a later section and the report scope will be limited to these environments.
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