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How do stainless steel grade numbers work?
Short answer
Stainless grades carry several names for the same steel. The AISI number (SS 316L) is the common name, the UNS number (S31603) is the unique code used in ASTM specifications, and the EN number (1.4404) is the European designation. Suffixes like L, H and Ti show low carbon, high carbon or titanium stabilisation.
AISI numbers and suffixes
The three-digit AISI numbers group grades by family: 200 series (Cr-Mn-Ni), 300 series (Cr-Ni austenitic), 400 series (ferritic and martensitic). Suffixes modify the base grade: L = low carbon (≤ 0.030%, e.g. SS 316L), H = controlled high carbon for creep strength (e.g. SS 304H), Ti = titanium-stabilised (SS 316Ti), N = nitrogen-strengthened.
UNS numbers
The Unified Numbering System gives each alloy a letter plus five digits: S for stainless (S30400, S31603), N for nickel alloys (N08904 for SS 904L, N06625 for Inconel 625), K for many alloy steels, and C for copper alloys. The last digits often encode the variant: S31600 → S31603 (L) → S31609 (H).
EN (Werkstoff) numbers
European steel numbers such as 1.4404 (SS 316L) or 1.4571 (SS 316Ti) come with a descriptive name like X2CrNiMo17-12-2 (≈0.02% C, 17% Cr, 12% Ni, 2% Mo). EN and ASTM limits overlap but aren't identical, so treat cross-references as approximate.
Specification prefixes
Product specifications add their own prefixes: TP (tubular product, ASTM A312/A213), F (forging, A182), WP (wrought fitting, A403), B8/B8M (bolting, A193). So SS 316L pipe is TP316L, SS 316L flanges are F316L and SS 316L butt-weld fittings are WP316L.
One grade, many designations: SS 316L
Decoding a UNS number
A UNS number has a letter for the metal family and five digits. S covers stainless and heat-resisting steels, N nickel alloys, K most low-alloy steels and R reactive metals such as titanium. For stainless, the first three digits usually echo the AISI number and the last two mark variants: S31600 is SS 316, S31603 is SS 316L, S31609 is SS 316H and S31635 is SS 316Ti.
When names do not line up
Cross-references are approximate. EN 1.4404 and ASTM 316L overlap but have slightly different limits, and 1.4571 has no exact ASTM twin beyond SS 316Ti. Check the equivalent grades chart and order by the specification your design uses.
Materials in this guide
Related guides, comparisons and articles
Frequently asked questions
What does the L in SS 316L mean?
Low carbon, 0.030% maximum, which prevents sensitisation during welding.
Is AISI still a standard?
AISI numbers are no longer maintained as a standard but remain the common trade names; ASTM and UNS designations govern orders.
What does the L mean in SS 316L?
Low carbon: 0.030% maximum, which reduces sensitisation during welding.
Is 1.4404 exactly the same as SS 316L?
It's the closest EN equivalent, but the limits aren't identical. Confirm against the governing specification.
How do I choose between grades?
Start from the service environment (chlorides, acids, temperature, pressure), then check the governing code and cost. Our comparisons and grade pages list typical choices, and our team can help shortlist grades for your conditions.
Does Bright Tubes supply the materials in this guide?
Yes. SS 316L Stainless Steel, SS 304H Stainless Steel, SS 316Ti Stainless Steel, SS 904L Stainless Steel and related products are supplied from stock or mill-sourced, with EN 10204 certificates.