Chemical Composition of AISI and UNS Austenitic Stainless Steel

Most ASTM and ASME standard 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 recognised 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 sheets 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. 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 letters '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 product forms e.g. SUS 304J1 for sheet products.
The composition ranges should always be checked in the appropriate standard.

The tables in this article use the UNS number as their key, but also show the related AISI and SAE numbers. Where appropriate, common names for the steels are also shown.

There are some differences in the composition ranges for some grades between flat and long products. This is due to the harmonisation process which is aimed to bring US and European compositions into line. This process is more advanced for flat products than for long. Consequently, there will be small differences until this process is complete. Where there is a different composition depending on product form, both compositions are shown.

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