Unified Numbering System (UNS) for Metals and Alloys

Topics Covered

Introduction

The traditional designation systems for metals and alloys in the United States have been developed by a large group comprising trade associations (Aluminum Association (AA), the American Iron and Steel Institute (AISI), and the Copper Development Association (CDA)), metal producers, professional societies (ASTM International, the American Welding Society (AWS), and the Society of Automotive Engineers (SAE)), and the U.S. government.

In the 1960’s the traditional system was found to have some problems for which solutions could not be found. The problems included use of same number for different alloys, use of different numbers for the same alloy and use of trade names for many alloys. This gave rise to the need for a new metals designation system.

Development of the Unified Numbering System (UNS)

An 18-month feasibility study into developing a new system consisted of discussions with major trade associations (Aluminum Association, the American Iron and Steel Institute, the Copper Development Association, and the Steel Founders’ Society of America (SFSA)).

After general agreement from all the groups involved, the study was completed in January 1971. The resultant report, which stated that a unified numbering system for metals and alloys was definitely possible and beneficial, was submitted to the U.S. Army. An advisory board was set up by ASTM and SAE in April 1972 to further work on the numbering system. The term Unified Numbering System (UNS) was coined by the board.

Guidelines for System Formation

The guidelines for forming the system are as follows:

• The designation for a metal or alloy must refer to a specific metal or alloy as determined by its special chemical composition, or to its mechanical properties or physical features when these are the primary defining criteria and the chemical composition is secondary or not critical.
• The numbers assigned must integrate numbers from existing numbering systems whenever possible so as to enable easy recognition.
• The numbering system must be designed to adapt current metals and alloys, and to anticipate the need to furnish numbers for new alloys for the future.
• The system must be suited for computer use and for general indexing of metals and alloys.

Summary of the System

The UNS Advisory Board had completed “SAE/ASTM Recommended Practice for Numbering Metals and Alloys” in March 1974. Though the Advisory Board worked on assigning numbers for the 18 series of numbers they developed, the specific details for each series was formulated by field experts. By the end of 1974, specific UNS designations for more than 1000 metals and alloys were formulated. The designations were listed in a UNS Handbook published in 1975. A sample entry is shown in the table below

The Unified Numbering System (UNS) is a composition-based system of commercial materials. Each metal is indicated by a letter that is followed by five numbers. Precise composition with impurity limits and performance specifications are not guaranteed by the UNS system. The UNS has included other nomenclature systems in order to avoid confusion, for example the conversion of aluminum 6061 (AA6061) into UNS A96061.

The UNS numbering categories are as follows:

Importance and Use of the UNS

The UNS allows correlation of many nationally recognized numbering systems currently used by trade associations, societies and individual users and producers of metals and alloys, thereby avoiding confusion. It provides the required uniformity for proficient indexing, record keeping, data storage and retrieval, and cross referencing. A UNS number cannot be considered as a specification by itself as it does not provide the requirements for form, condition, and quality.

The UNS numbers are to be used only to identify metals and alloys that are regularly produced and used. It will not be issued for a material that has just been conceptualized or that is still in experimental stage.