What Is UNS Numbering System?

Quick Answer: The Unified Numbering System (UNS) is a standardized identification system for metals and alloys, managed jointly by ASTM and SAE. Each UNS number consists of a single letter prefix followed by five digits (e.g., S31600 for Type 316 stainless steel). The system does not define material specifications; it only identifies the alloy composition.

UNS Number Structure

The letter prefix indicates the alloy family, and the five-digit number typically incorporates existing designation numbers where possible. For example, S30400 maps directly to AISI Type 304.

Prefix	Alloy Family	Examples
A	Aluminum and aluminum alloys	A96061 (6061 Al)
C	Copper and copper alloys	C70600 (90/10 CuNi)
D	Steels with specified mechanical properties	D3753
F	Cast irons	F10004 (gray iron)
G	AISI-SAE carbon and alloy steels	G10180 (1018 steel)
J	Cast steels (except tool steels)	J92900 (CF8M cast)
K	Miscellaneous steels and ferrous alloys	K03504 (ASTM A106 Gr. B)
N	Nickel and nickel alloys	N06625 (Inconel 625)
R	Reactive and refractory metals (Ti, Zr, Ta)	R50400 (Ti Grade 2)
S	Stainless steels	S31600 (316), S31803 (2205 Duplex)
W	Welding filler metals	W07018 (E7018)

Common UNS Numbers in Piping

UNS Number	Common Name	ASTM Specification
K03504	Carbon steel (A106 Gr. B)	ASTM A106
K11562	1-1/4Cr-1/2Mo (A335 P11)	ASTM A335
K21590	2-1/4Cr-1Mo (A335 P22)	ASTM A335
S30400	Type 304 SS	ASTM A312 TP304
S31600	Type 316 SS	ASTM A312 TP316
S31603	Type 316L SS	ASTM A312 TP316L
S31803	Duplex 2205	ASTM A790
S32750	Super Duplex 2507	ASTM A790
N06625	Inconel 625	ASTM B444
N10276	Hastelloy C-276	ASTM B622
N08020	Alloy 20	ASTM B729

UNS vs. ASTM vs. AISI

The UNS is not a specification; it is an identification system. The distinction matters:

• UNS S31600 identifies the chemical composition of Type 316 stainless steel
• ASTM A182 F316 specifies forged fittings and flanges made from UNS S31600, with required mechanical properties, heat treatment, and testing
• AISI 316 is the older trade designation for the same alloy composition

Pro Tip

On mill test certificates (MTRs), the UNS number appears alongside the ASTM specification. Always verify that the UNS number on the MTR matches the material specified in the purchase order; this catches material mix-ups where the ASTM spec is correct but the actual alloy supplied is wrong.

When UNS Numbers Are Required

UNS numbers are explicitly referenced in:

• ASME BPVC Section II (materials)
• NACE MR0175/ISO 15156 (sour service material selection; lists acceptable UNS numbers)
• Client material requisitions and piping specifications
• Positive Material Identification (PMI) programs; PMI verifies that installed material matches the specified UNS composition

The complete UNS system is published in ASTM DS-56, “Metals & Alloys in the Unified Numbering System,” which cross-references over 5,500 alloy designations.

Read the full guide to steel corrosion