UNS, XM, AISI, AWS, ASTM, AMS… What do they mean and why are there multiple names for the same materials? There’s often confusion about the naming system and frequent usage of abbreviations in the alloy industry. However, the alloy industry has been getting better after a disorganized start, but the remnants of defunct systems still remain in the alloy industry. In the current state of the industry, the Unified Numbering System (UNS) is the recognized standard for American producers as well as most producers internationally.
UNS (Unified Numbering System)
- Today’s current numbering system
XM (here is a list of the XM designations noted by Allegheny Technologies Incorporated)
- Temporary naming system
AISI (American Iron and Steel Institute)
- Early naming system and still used today
AWS (American Welding Society)
- Early naming system and still used today
ASTM (American Society for Testing and Materials)
- Still used today but mostly for application specifications rather than material identification
AMS (Aerospace Material Specification)
- Today’s aerospace standards, also utilizes specific UNS identities per spec.
How did we end up with so many naming systems and why do we use the UNS system? According to The History of Stainless Steel by Harold Cobb (2010), who was also a member of the board that created the Unified Numbering System while working with the American Society for Testing and Materials (ASTM), the need for a naming system started to become apparent in the 1930s as the American Iron and Steel Institute (AISI) and the Alloy Casting Institute (ACI) had both started to tackle this issue of naming systems but producers retained their trade names. Initially in 1931, Clayton Plummer undertook the preparation of a classification system but that was superseded by the three-digit AISI numbering system. Plummer’s system grouped the alloys by chromium ranges.
In the 1950’s, the U.S. Department of Defense (DoD) began to produce the first catalog of all metals and alloys with the help of the Society of Automotive Engineers (SAE) and the American Society for Testing and Materials (ASTM). These standards were incorporated into the Federal QQ Standards and Military MIL standards. “It was found that the same alloys were sometimes listed with different designations and slightly different composition by the various standards organizations” (Cobb H., 2010, pg. 235).
Early in the 20th century, naming nickel alloys was a specific issue as there had never been a numbering system for alloys with a nickel base composition. Initially, nickel alloys were designated by ASTM such as “Nickel-Chromium-Molybdenum-Iron-Cobalt Alloy.” Remnants of this system are still used today in general nickel specifications by the SAE organization’s Aerospace Material Specification (AMS) that calls out variations in nickel alloys by the base elements such as in AMS 2261: TOLERANCES Nickel and Nickel Base Alloys Bars that designates a multitude of base alloy types like nickel, nickel-chromium, and nickel-copper.
As well, the American Welding Society (AWS) still has remnants of this naming system with their designations such as alloy 718, UNS N07718, weld wire often designated as AWS A5.14 ERNiFeCr-2 that uses the base chemistry in their naming mechanism to respectively note the main elements nickel, iron, and chromium. Note that most welding materials have different UNS designations from the original alloy as many compositions are adjusted slightly for better welding properties.
Early in the 1960’s, the AISI announced they were discontinuing assigning numbers to steels. This led ASTM to meet the need for proprietary numbers for steels with their makeshift system of XM numbers. That established the initial XM-1 through XM-24, here is a list of the XM designations as noted by Allegheny Technologies Incorporated. While the XM naming is still used in some quote requests today, the system was temporarily needed to fill a naming system designation and has been replaced by the current standard UNS system.
Another group within ASTM attempted to mimic the AISI numbers with a 600 series of numbers, which would never be officially recognized by ASTM. These changes led to significant confusion in the metals industry. By 1967, ASTM and SAE began to explore possibilities for creating a new numbering system, and in 1969, the U.S. Army issued a contract to ASTM and SAE to create a “Feasibility Study of a Unified Numbering System for Metals and Alloys.”
In 1972, ASTM and SAE established an Advisory Board that would decide the official name of the new system to be the Unified Numbering System (UNS) that we use today. The UNS system has led to many tradenames such as nickel 201’s UNS of N02201 and alloy 718’s UNS of N07718. Another instance of UNS derived trade names is alloy N60 that is occasionally called alloy 218 due to its UNS of S21800. Today, common trade names for nickel alloys sometimes revolve around their UNS number but it is not a congruent effort.
If you’re interested in learning more about trade names and naming systems, then check out our website’s alloys to see some of the names used to identify materials. To see an example of various naming mechanisms, please check out our website for UNS S21800, common trade name N60 and trademarked Nitronic® 60. Remnants of the ASTM chemistry composition can still be seen in the description of nitrogen & manganese strengthened stainless steel. The weld wire version of N60 is called N60W and has the UNS S21880 and AWS A5.9 Class ER218. Overall, XM is a defunct naming system yet it is still used today as it has been in past decades. The current system to ensure the material you’re looking for is the Unified Numbering System.
Nitronic® is a registered trademark of Cleveland-Cliffs Steel Corp.
References
Cobb, H. (2010, June). The history of Stainless Steel. Google Books. https://books.google.com/books?id=E30rCBeM8nkC&pg=PA236&lpg=PA236&dq=naming%2Bsystems%2Bfor%2Balloys%2Borigin%2Bof%2B%22xm%22&source=bl&ots=DYSmtA3ilh&sig=ACfU3U218n_GWZDzYCAH3C5gFj8CoIN2EA&hl=en&sa=X&ved=2ahUKEwio07L385aCAxUplIkEHY-PAE84HhDoAXoECAIQAw#v=onepage&q=naming%20systems%20for%20alloys%20origin%20of%20%22xm%22&f=false
