by Harold M. Cobb

Designation systems for metals and alloys in the United States have traditionally been those established by various groups, including metal producers; trade associations such as the Aluminum Association (AA), the American Iron and Steel Institute (AISI), and the Copper Development Association (CDA); professional societies such as ASTM International, the American Welding Society (AWS), and the Society of Automotive Engineers (SAE); and the U.S. government.

By the 1960s, it was becoming increasingly apparent that the traditional approach to the designation of metals left some problem areas that could not be satisfactorily solved. The following list of some of these problems should make it clear that a case could be made for developing an entirely new metals designation system.

Trade names—Many alloys were known only by trade names in cases where no central organization assigned numbers. However, the technical societies had policies prohibiting the use of trade names in their specifications. Therefore, it became the practice, for nickel alloys, to list the principal alloying elements as the designation. For example, a well-known nickel alloy was given the designation “Nickel-chromium-cobalt-molybdenum alloy,” a designation which was not particularly meaningful to most readers.

Same number for different alloys—It was not unusual for the various trade associations to assign the same number to different alloys. For example, Alloy 205 could be either a copper alloy, a nickel alloy, or a type of stainless steel.

Different numbers for the same alloy—The AISI and SAE designated the same stainless steels by different numbers, three digits for AISI and five digits for SAE.

Discontinued numbers—The AISI decided to discontinue the practice of designating numbers for steels, and this was a particular problem with regard to the assignment of designations to new stainless steels. ASTM was attempting to fill the gap by issuing a new series of numbers for proprietary stainless steels (e.g., XM-1, XM-2, XM-3, etc.)

Outmoded systems—CDA’s three-digit system for the numbering of copper alloys was outmoded, and a new system was being considered.

Variety of designations—The U.S. government often had metal designations that differed from those of other specification-writing bodies.

A New Numbering System

Because of these problems, in 1967 ASTM and SAE began to explore the possibility of developing a new numbering system for metals that would address the difficulties described above and provide numbers for all alloys.

The U.S. Army was especially interested in this subject, and in May 1969 the Army Materials and Mechanics Research Center (AMMRC) issued a contract to SAE to conduct a “Feasibility Study of a Unified Numbering System for Metals and Alloys.” This project was jointly sponsored by ASTM and SAE, and a committee was appointed to conduct the study. The committee was chaired by Norman L. Mochel, a past president of ASTM, and consisted of the following members:

• Herbert F. Campbell, Army Materials and Mechanics Research Center;
• Harold M. Cobb, ASTM;
• Alvin G. Cook, Allegheny Ludlum Steel Corp.;
• Henry B. Fernald, Technical Consultant;
• Muir L. Frey, Engineering Consultant;
• S. T. Main, Grumman Aircraft Corp.;
• Norman L. Mochel, Engineering Consultant;
• R. Thomas Northrup, Society of Automotive Engineers;
• Bruce A. Smith, General Motors Engineering Staff; and
• Harry H. Stout, Phelps Dodge Copper Products Corp.

Development of the UNS

Some individuals consulted during the course of the 18-month feasibility study expressed grave doubts about the possibility of establishing an overall numbering system, but others thought it was worth exploring. The major trade associations concerned with metals numbering systems were also consulted, including the Aluminum Association, the American Iron and Steel Institute, the Copper Development Association, and the Steel Founders’ Society of America (SFSA). It was recognized at the outset that any new system could be successful only if these organizations were in general agreement with the concept. In January 1971, the study was completed and a report was submitted to the U.S. Army stating that it had been determined that a unified numbering systems for metals was feasible and desirable. The report included a general proposal of how such a system could be established to provide a coherent designation system for all current and future metals and alloys.

In April 1972, ASTM and SAE established an Advisory Board to further develop and refine the proposed numbering system, The Advisory Board consisted of the following members:

• Chairman: Bruce A. Smith, General Motors Engineering Staff;
• Secretaries: Harold M. Cobb, ASTM Staff;
• R. Thomas Northrup, SAE Staff;
• John Artman, Defense Industrial Supply Center;
• Lawrence H. Bennett, National Bureau of Standards;
• Alvin G. Cook, Allegheny Ludlum Steel Corp.;
• Henry B. Fernald, Jr., Technical consultant;
• John Gadbut, International Nickel Co.;
• Joseph M. Engel, Republic Steel Corp. (representing AISI);
• W. Stuart Lyman, Copper Development Association;
• Robert E. Lyons, Federal Supply Service;
• Norman L. Mochel, Metallurgical Consultant;
• Edward F. Parker, General Electric Co.;
• Richard R. Senz, The Aluminum Co. of America (for the Aluminum Assoc.);
• Whitney Snyder, American Motors Corp.;
• Rudolph Zillman, Steel Founders’ Society of America;

This board decided that the official name of the new all-encompassing system would be the “Unified Numbering System for Metals (UNS).” The major guiding principles of the system would be as follows:

• Each designation for a metal or alloy should pertain to a specific metal or alloy as determined by its unique chemical composition, or to its mechanical properties or physical characteristics when these are the primary defining criteria and the chemical composition is secondary or not significant.
• For ease of recognition, the numbers assigned should incorporate numbers from existing numbering systems whenever possible.
• The numbering system should be designed to accommodate current metals and alloys, and to anticipate the need to provide numbers for new alloys for the foreseeable future.
• The system should be ideally-suited for computer use and for the general indexing of metals and alloys.

Description of the System

In March 1974, the UNS Advisory Board completed the “SAE/ASTM Recommended Practice for Numbering Metals and Alloys.” This established 18 series of designations, which consisted of a prefix letter and five digits, as shown in Table 1. Note that in most cases the letter is suggestive of the family of metals identified.

The procedures for assigning numbers to each of the 18 series of numbers was to be coordinated by the Advisory Board, but the specific details for each series would be developed by experts in each of those fields. Examples of some of these designations are shown in Table 2.

By 1974, the Advisory Board had coordinated the establishment of specific UNS designations for over 1000 metals and alloys, including steel, stainless steel, tool steel, superalloys, aluminum, copper, cobalt, magnesium, and nickel. These designations were listed in the first edition of a UNS Handbook, which was published in 1975. Each entry in the book included the UNS designation, a brief description of the alloy, the chemical composition of the alloy, and a list of the national specifications in which the alloy appeared. A typical entry from the book is shown in Table 3.

UNS Achievements Over 25 Years

The UNS designation system has been a highly successful venture from many standpoints. It has provided satisfactory solutions to the six problem areas discussed earlier in this article. Metals and Alloys in the Unified Numbering System, published jointly by ASTM and SAE, lists over 5000 UNS designations in the recently-published ninth edition. It is also significant to note that this publication has been adopted by the Department of Defense (DoD) as a replacement for MIL-HDBK-H1, Cross Index of Chemically Equivalent Specifications and Identification Code (Ferrous and Nonferrous Alloys).

The Copper Development Association, in a sweeping change, replaced all of the traditional three-digit CDA numbers with UNS designations, and the American Welding Society now references UNS designations in all AWS specifications for welding filler metal and electrodes. A large percentage of ASTM specifications include UNS references and, for new stainless steel alloys and most nickel alloys, UNS is the only designation listed.

Virtually all of the metals reference books published in the last 20 years have adopted UNS as their principal indexing systems and many metal producers now reference the UNS designations in their literature. //

For More Information

James D. Redmond, TMR Stainless, Pittsburgh, Pa. 15327-6423 (phone: 412/369-0377).

Copyright 2002, ASTM