Reading Alloy Designations: AISI, AA, UNS, and More

## Why Designation Systems Matter An alloy name like "stainless steel" or "6061 aluminum" tells you almost nothing about whether a specific piece of metal meets the properties your application requires. Designation systems provide a standardized shorthand that links a material to its composition range, processing history, and measured properties—all of which are defined in published standards that suppliers must certify. When a drawing specifies ASTM A276, AISI 316, Condition A, the engineer and the supplier both know exactly what composition, finish, mechanical property minimums, and test documentation are required. That specificity is the entire point of these systems. ## AISI/SAE Steel Designations The American Iron and Steel Institute (AISI) and Society of Automotive Engineers (SAE) jointly developed the most widely used steel designation system for wrought steels. The basic structure is a four-digit number: - **First two digits**: alloy group (10xx = plain carbon, 13xx = manganese steel, 41xx = Cr-Mo, 43xx = Ni-Cr-Mo, 31xx = Ni-Cr, 51xx = chromium, 86xx = Ni-Cr-Mo) - **Last two digits**: nominal carbon content in hundredths of a percent **AISI 1045** = plain carbon steel (10), 0.45% carbon (45). Simple and unambiguous. **AISI 4140** = chromium-molybdenum steel (41), 0.40% carbon (40). **AISI 52100** = chromium steel (52), 1.00% carbon—a bearing steel used for ball and roller bearings due to its extreme hardness and wear resistance. Stainless steels add a three-digit prefix: 304, 316, 410, 440C. These do not follow the same hundredths-of-carbon convention. ## Aluminum Association (AA) Designations The Aluminum Association uses a four-digit system for wrought alloys: - **First digit**: alloy group by major alloying element (1xxx through 8xxx) - **Second digit**: modification of original alloy or impurity limit (0 = original) - **Last two digits**: specific alloy identification within the group (for 1xxx, the last two digits indicate purity level) For cast aluminum alloys, a three-digit plus decimal format is used: A356.0 (foundry alloy), where the letter prefix indicates a modification and the digit after the decimal indicates the product form (0 = casting, 1 = ingot). ### Temper Designations The temper code appended with a hyphen is often more important than the alloy number itself: | Temper | Meaning | |--------|---------| | -O | Annealed, lowest strength | | -H12 | Strain hardened 1/4 hard | | -H32 | Strain hardened 1/4 hard + stabilized | | -T4 | Solution heat treated + naturally aged | | -T6 | Solution heat treated + artificially aged | | -T73 | Overaged for maximum corrosion resistance | 6061-T6 and 6061-O have the same composition but tensile strengths of 310 MPa and 125 MPa respectively. Specifying just "6061" without a temper is incomplete. ## UNS: The Universal Numbering System The Unified Numbering System (UNS), administered jointly by SAE and ASTM, provides a single alphanumeric code that maps across multiple existing designation systems and covers all commercial alloy families: | Prefix | Alloy Group | |--------|-------------| | A | Aluminum | | C | Copper | | D | Steels with specific mechanical properties | | E | Rare earth metals | | F | Cast irons | | G | AISI/SAE carbon and alloy steels | | H | AISI/SAE H-steels (hardenability guaranteed) | | J | Cast steels | | K | Miscellaneous steels | | N | Nickel alloys | | P | Precious metals | | R | Reactive and refractory metals | | S | Heat and corrosion resistant steels (stainless) | | T | Tool steels | | Z | Zinc alloys | **Examples**: G10450 = AISI 1045 carbon steel. S30400 = AISI 304 stainless. A96061 = AA 6061 aluminum. C36000 = free-cutting brass. R56400 = Ti-6Al-4V. UNS numbers are useful when cross-referencing materials across national standards (ASTM, ISO, DIN, JIS), because a UNS number is a stable identifier that can be looked up in any database. ## ASTM Standards While AISI/AA/UNS define composition, ASTM standards define the whole product: composition, mechanical properties, dimensions, test methods, and documentation. When specifying material for purchase, always reference the ASTM standard, not just the alloy designation: - **ASTM A36**: Structural steel plate and shapes - **ASTM A276**: Stainless steel bars and shapes - **ASTM B209**: Aluminum sheet and plate - **ASTM B16**: Brass rod, bar, and shapes - **ASTM B265**: Titanium strip, sheet, and plate ## EN and ISO Systems European standards use the EN 10027 system for steels, which includes both a numerical designation and a symbolic designation. The symbolic system encodes properties directly into the name: **S355J2** means structural steel (S), 355 MPa minimum yield strength, J2 impact toughness grade at -20°C. For stainless, **1.4301** is the numerical designation for what AISI calls 304. ISO/EN designations are frequently encountered on materials sourced from European suppliers and are increasingly specified in global supply chains alongside or in place of AISI/SAE designations.