Most ASTM and ASME standards list the steel grades by their UNS (Unified Numbering System) numbers but also make reference, where appropriate to the more general AISI (American Iron And Steel Institute) grade designations.These grade numbering systems are widely used in the USA,where they originated and are recognized by most stainless steel specifiers and users.
The American Iron And Steel Institute (AISI) developed designations such as 304, 430 etc and published compositions for these in their "Steel Products Manual" (1974)
These are NOT specifications,but steel grade composition ranges only.
These grades were used by the American Society Testing And Materials (ASTM) to identify grades in a wide range of standards they published for stainless products, such as sheet and plates (ASTM A240), bars (ASTM A276) and tubes (ASTM A269).The compositions of the AISI grades were made more specific with the introduction of the "Unified Numbering System",jointly established by ASTM and SAE (Society Of Automotive Engineers).
This five digit number, preceded by the letter 'S' for most stainless steels,identified the specific variant of the grade.
e.g. 420 being UNS S42000, 630 or 17/4PH being UNS S17400
e.g. 304 being UNS S30400, 304L being UNS S30403
American Society Of Mechanical Engineers (ASME) publish design codes and standards for pressure applications.ASME standards are generally based on the appropriate ASTM standards,but the standard numbers are preceded with the letter 'SA'; rather than just the 'A' of the ASTM standards.
Where UNS grade numbers are used, these are universal, regardless of the product type or the standard in which they appear.
The AISI designations have been adopted in Japan.The aisi number is preceded with the letters SUS e.g. SUS 304.The Japanese have developed variants on some grades for certain products forms e.g. SUS 304J1 for sheet productss.
The composition ranges should always be checked in the appropriate standard.
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