AISI vs UNS Stainless Steel Designation Systems: Comprehensive Comparison with Tables
The designation of stainless steel grades relies on two major systems: the traditional AISI (American Iron and Steel Institute) numbering system and the modern UNS (Unified Numbering System). While AISI designations remain widely used in commercial communication, UNS numbers dominate formal standards, material certificates, and engineering specifications. The following detailed comparison presents all key relationships in structured tables for clarity and ease of reference.
1. System Overview Comparison
| Aspect | AISI System | UNS System |
|---|
| Full Name | American Iron and Steel Institute | Unified Numbering System |
| Developed By | American Iron and Steel Institute | ASTM International + SAE International |
| Introduction Period | Mid-20th century | 1970s onward |
| Primary Purpose | Practical commercial identification | Precise, standardized identification of all alloys |
| Current Status | Still used in daily communication | Preferred in modern standards (ASTM, ASME, etc.) |
| Number Format | 3-digit number + optional suffix | Letter + 5 digits (Sxxxxx for stainless) |
| Expandability | Limited (no longer actively updated) | Highly expandable |
| Ambiguity Level | Higher (same number can cover slight variants) | Very low (unique designation for each grade) |
| Typical Example | 304, 316L, 410 | S30400, S31603, S41000 |
2. Numbering Structure Comparison
| Feature | AISI | UNS |
|---|
| Series Indication | First digit (2xx, 3xx, 4xx) | Letter prefix (S = stainless) |
| Carbon Level Indication | Suffix letters (L = low, H = high) | Last digits of the number |
| Special Additions | Suffix (Ti, N, Se, Cu, etc.) | Incorporated into the unique 5-digit number |
| Coverage of New Alloys | Poor | Excellent |
| Formal Standards Adoption | Secondary | Primary |
3. Austenitic Stainless Steels – Detailed Correspondence Table
200 Series (Chromium-Nickel-Manganese)
| AISI Designation | UNS Number | Common Notes |
|---|
| 201 | S20100 | Standard 201 |
| 201L | S20103 | Low carbon |
| 201LN | S20153 | Low carbon + nitrogen |
| 202 | S20200 | Standard 202 |
| 205 | S20500 | Higher manganese |
300 Series (Chromium-Nickel) – Most Common Grades
4. Ferritic Stainless Steels Correspondence Table
5. Martensitic Stainless Steels Correspondence Table
6. Duplex Stainless Steels Correspondence Table
7. Practical Usage Comparison
| Application Scenario | Preferred System | Reason |
|---|
| Daily commercial discussion | AISI | Short and widely recognized |
| Mill Test Certificates (MTC) | Both | Usually listed together |
| ASTM / ASME Standards | UNS (primary) | Higher precision |
| Purchase Orders & Contracts | Both | AISI for familiarity + UNS for accuracy |
| New or specialty alloys | UNS only | Many modern grades never received AISI numbers |
8. Advantages and Disadvantages Summary
| Criterion | AISI Advantages | AISI Disadvantages | UNS Advantages | UNS Disadvantages |
|---|
| Ease of use | Very high | — | Moderate | Longer numbers |
| Precision | Moderate | Possible ambiguity | Very high | — |
| Coverage of new grades | Poor | Limited expansion | Excellent | — |
| Industry familiarity | Extremely high | — | High among technical users | Lower among non-technical staff |
| Formal standards adoption | Secondary | Declining | Primary | — |
9. Summary Recommendations
| Situation | Recommended Practice |
|---|
| Verbal or informal communication | Use AISI designation (e.g., “304L”) |
| Formal specifications and drawings | List both (e.g., Type 304L / UNS S30403) |
| Material certificates and test reports | Always include UNS number |
| Global supply chain documentation | Prefer UNS with AISI reference in parentheses |
The tables above provide a complete and structured view of the relationship between the AISI and UNS systems across all major stainless steel families. For detailed chemical composition, mechanical properties, and available product forms of specific grades, please refer to the individual product pages linked throughout this article on SHH STEEL AISI AND UNS STAINLESS SUPPLIER.
This dual-system approach ensures both commercial convenience and technical precision in today’s global stainless steel market.
