There are two systems for identifying different metals. One is the combined standards of American Iron & Steel Institute (AISI) and Society of Automotive Engineers (SAE). The second system is the newer United Numbering System.

AISI/SAE Metal Designation System

With one exception the first digit signifies the major alloy used in the steel. Carbon steels are not alloyed.

Key to the AISI/SAE steel numbering system

The first two numbers in this designation signify the primary alloy(s) of the steel. The next two numbers in the designation signify the amount of carbon present in the steel.

As shown, the AISI / SAE steel designation system gives information about the chemical composition of the steel (alloy type and carbon content). However, in many cases, this is not enough information for the purchasing company to procure the steel. The ASTM specification of fabrication methodology will often be added to the material specifications demonstrated but fabrication methods will not be discussed here. This is one reason why the Unified Numbering System (UNS), which provides more specific information of a metal’s content, emerged for more precise manufacturing requirements.

Following are two examples of UNS designators and how they relate to AISE/SAE designations:

1. A resulfurized carbon steel containing 0.21% carbon would be UNS G11210 or AISI/ASE 1121
2. A steel alloyed with 20% chromium and vanadium and containing 0.75% carbon would be UNS G62750 or AISI/SAE 6275

Unified Numbering System (UNS)

The Unified Numbering System (UNS) Standard is a relatively new standard. It is gaining in acceptance and will soon be the single metals Standard in the USA. But some history is in order before commencing:

The American Iron and Steel Institute (AISI) has a standard that is widely accepted in the United States and other countries. However, it does not cover all metals and in many instances, it does not convey enough information to speak to all of the attributes of some metals. Therefore, two United States standards organizations, ASTM (American Society for Testing and Metals) and SAE (Society of Automotive Engineers), have developed a standard for classification of metals that is independent of AISI standards, the UNS. It is composed of seventeen letters that represent seventeen metal types. Each letter is followed by five digits that signify the specific makeup of each of these metal types. The first three or four digits are derived from different standards.

Above is the complete list of UNS metals designations.

Below are the basic metals important to machinists and welders.

Following are two examples of UNS designators and how they relate to AISE/SAE designations (notice how the older standard numbers have been incorporated into the UNS)

1. UNS designation “G10300” represents an AISI/SAE 1030 carbon steel
2. UNS designation “S31600” represents an AISI/SAE 316 stainless steel

Since the AISE/SAE standards have only three or four digits the last unused digit(s) in the UNS system can be used for added information such as tempering or manufacturing methods. At this writing all of these digits are not fully utilized but will surely be used in the near future.

Note that the UNS standard is not yet widely used in the machine shop. However, since it is gaining acceptance with engineers and purchasing agents it will soon become the standard on the shop floor. In the interim UNS can be easily translated from AISI and SAE designators and vice versa.

Derived from Unified numbering system – Wikipedia. Available and accessed online 30 January 2024, and The Virtual Machine Shop (http://www.jjjtrain.com/vms/eng_metal_stds/eng_metal_stds_02.html and http://www.jjjtrain.com/vms/eng_metal_stds/eng_metal_stds_03.html) accessed and available via the WayBack Machine 15 January 2024.