The Unified Numbering System

The Unified Numbering System (UNS) is an alloy identification system created in the early 1970’s and formalized with the issuance of joint standard ASTM E527 and SAE J1086 in August of 1974. The system assigns numbers to alloys on the basis of composition only. There is no definition of mechanical or other properties. With the introduction of many new alloys in the 60’s and 70’s, the original identification systems used for steel, aluminum and copper alloys were running out of options with only three or four digit identifiers.

In the steel industry, the designations for steels and stainless steels had been originally assigned by the American Iron and Steel Institute (AISI). This group was responsible for the formulation of the 200, 300 and 400 series designations of stainless alloys that were first assigned as early as the 1930’s. As this organization transformed from a technical society to a lobby group, the history of these number assignments appears to have been lost. Nickel alloy designations were essentially trade names from either Inco (i.e. 400, 600, 625, 718) or Haynes (i.e. B, C, D, X). The adoption of the UNS transferred the naming authority for these alloys to the independent organizations, ASTM and SAE. A more complete list of alloys and naming authorities includes:

Axxxxx Aluminum Alloys

Aluminum Association

Cxxxxx Copper and Copper Alloys Copper Development Association
Exxxxx Rare Earths ASTM
Fxxxxx Cast Irons ASTM
Gxxxxx AISI and SAE steels SAE
Jxxxxx Cast Steels ASTM
Nxxxxx Nickel Alloys SAE
Rxxxxx Reactive Metals SAE
Sxxxxx Stainless Steels SAE
Txxxxx Tool Steels SAE
Wxxxxx Weld Fillers American Welding Society
Zxxxxx Zinc Alloys Zinc Institute

In Europe, steel alloy designations are more commonly recognized by the names assigned by the German standards organization, DIN. The original AISI designation of 304 stainless became S30400 under the UNS, but in Europe the 304 counterpart is identified as X5CrNi18-10 or perhaps as 1.4301, a carryover from another numbering system used by DIN. This number is often referenced as a Werkstoffe or Wk. number. Note that while 1.4301 and S30400 are similar, there are differences in chemistry. The most notable difference is that S30400 requires a minimum of 18% chromium, while 1.4301 requires only 17% Cr. There have been serious attempts to unify or “harmonize” European and North American standards with no real success. Some of these differences are philosophical and some are political. Very few of these differences can be justified on technical reasons.

The UNS Book, a listing of all assigned numbers and alloy compositions, is now in its twelfth edition and is available from the ASTM Bookstore for $205 to non-members. The UNS book does list North American standards in which the alloy appears. It does not cross reference alloys with other identification systems. There are several resources that do provide cross references. One of the most complete and reliable is the ASTM Passport to Steel, which is available on-line for an annual fee of $995.