What's The Difference Between ASTM And AISI Steel Standards?
If you work in procurement for large structural construction projects, you're probably familiar with A36 steel. This designation refers to the A36 standard put forth by the American Society for Testing and Materials (ASTM). However, raw material standards are far from straightforward, and you're equally likely to come across steel that uses designations from the American Iron and Steel Institute (AISI).
While regulations and engineering requirements will dictate the appropriate material for any project, it can still pay to understand how these two standards differ and why you may use one or the other.
Understanding 4-Digit AISI Steel Designations
AISI steel classifications follow the four-digit unified numbering system (UNS), which can tell you a lot about the material if you're familiar with the standard. The first two digits of each code tell you the alloying material, while the last two digits tell you the percentage of carbon. For example, 4340 steel is a medium-carbon steel alloy containing nickel, chromium, and molybdenum.
Note that these standards focus entirely on the chemical composition of the steel and not its material or engineering properties. While the elements present in steel will substantially impact its suitability for various projects, the final product will depend on additional factors. For example, heat treatment can significantly alter the hardness of specific steel categories.
Understanding ASTM Steel Designations and A36 Steel
ASTM provides its own set of standards, many of which are interchangeable with AISI classifications since both follow UNS conventions. However, there may be some differences, so it's critical to always consult with a supplier that has a deep understanding of steel grading. An experienced supplier can understand your needs based on the specific grade you're requesting.
However, A36 is a unique ASTM standard that does not follow UNS conventions. Instead, this standard covers the mechanical properties of a particular steel alloy instead of its chemical composition. While A36 steel is low in carbon, it must also meet various other stringent requirements, including yield strength, tensile strength, and electrical resistance.
The milling process used to fabricate A36 steel results in a rougher surface, making it more suitable for structural elements than finished products that require a smooth surface. Since the A36 standard specifies various minimum mechanical properties, it's highly suitable for engineering projects that require known material specifications.
Ultimately, it's critical to understand both the type of steel you require and the how standards organizations create and use steel designations. By understanding the similarities and differences between these classifications, you'll be able to choose a steel alloy with the appropriate mechanical characteristics for your project.
