Technical Information about Stainless Steel - Designation Systems

Stainless steel grades have designations derived from various sources "18/18" or "18/10" for instance, is a non-technical description often seen describing grade 304 or its equivalents.

a. AISI three digit designations such as 304 or 316 are now the most widely used system, being adopted, somethimes with minor variations by Standards bodies in Japan, Britain and Australia. They are also used in ASTM specifications which tend to be used, or at least, understood in almost every country. The system has proven durable, but inadequate to cope with new alloys. Extra letters designate alloy variants, such as 304L, 316LN or 310S.

b. UNS numbers address the inadequacies of the AISI system. One letter ("S" for wrought stainless steels, "J" for cast) plus five digits based on the AISI numbers. Grade 304 is UNS S30400, 304L is S30403 and 304H is S30409. Grade 904L is officially classified as a nickel-based alloy hence designated UNS N08904.

c. European designations originated as German Werkstoff Numbers (eg 1.4301 for Grade 304) or DIN grade designations (eg X5CrNil89 for 304). Similar descriptive deisgnations were used in other European countries. These are now incorporated in "Euronorm" specifications, which replace national specifications throughout the European Union.

d. Proprietary designations have often been given to alloys bytheir original manufacturer. Standard designations are later added when alloys are widely accepted, but the proprietary designations often persist. Examples are "253MA" which is an Avesta Sheffield trademark (UNS equivalent is S30815) and "SAF2304" which is a Sandvik trademark (UNS equivalent is S32304).

e. Cast alloys are often loosley reffered to by equivalent wrought designations, but a cast "304" has different composition and properties.