Why does “Inconel 625” also go by “N06625”? And does it matter? If you’ve ever stared at a purchase order and wondered whether you’re buying the right material, this guide is for you.
Introduction: The Problem with Trade Names
Every year, procurement errors caused by material designation confusion cost the industry millions. A purchase order for “Inconel” might return three different alloys depending on which supplier reads it. This is exactly the problem the Unified Numbering System (UNS) was designed to solve.
Developed jointly by ASTM and SAE in the 1970s, UNS provides a universal shorthand that engineers and buyers across the world can use without ambiguity. If you’re specifying, purchasing, or working with nickel alloys, understanding UNS numbers is not optional — it’s essential.
This guide explains how UNS works specifically for nickel alloys, how to read the codes, how they relate to ASTM/ASME specifications, and how to avoid the most common mistakes.
What Is the UNS Numbering System?
The Unified Numbering System for Metals and Alloys is an alphanumeric identification system widely accepted globally, particularly in North America. Every commercially significant metal and alloy is assigned a unique UNS designation that:
- Replaces the dozens of competing national numbering systems that existed in the 1960s
- Cross-references existing trade names, brand names, and historical designations
- Provides a common language for international procurement and specification
The system was formally established in 1974 after years of coordination between industry groups. By 1975, the first UNS handbook was published, covering over 1,000 materials. Today it encompasses virtually every commercially traded metal and alloy worldwide.
How the Format Works
Every UNS designation follows this structure:
[PREFIX] + [5-DIGIT NUMBER] = UNS designation
↓ ↓ ↓
Letter Serial number Final code
indicating assigned in e.g., N06625
metal family order of N10276
discoveryFor nickel alloys, the prefix is always “N”.
| UNS Series Prefix | Metal Family |
|---|---|
| N | Nickel and nickel alloys |
| S | Stainless steels and heat-resisting steels |
| A | Aluminum and aluminum alloys |
| C | Copper and copper alloys |
| F | Cast irons |
| G | Carbon and alloy steels |
This means if you see a designation starting with “N”, you immediately know it refers to a nickel-base material — regardless of what trade name it carries.
Reading Nickel Alloy UNS Numbers: The Key Series
Not all “N” numbers are created equal. The first digits often carry historical meaning — many nickel alloy UNS numbers derive from legacy numbering systems. Here’s how to read the most important series:
Core Nickel Alloy UNS Series
| UNS Range | Typical Alloy Type | Example |
|---|---|---|
| N02200–N02299 | Commercially pure nickel | N02200 (Nickel 200) |
| N04000–N04999 | Copper-nickel alloys (Monel family) | N04400 (Monel 400) |
| N06000–N06999 | Nickel-chromium and nickel-molybdenum alloys | N06625 (Inconel 625) |
| N07000–N07999 | Precipitation-hardening nickel alloys | N07718 (Inconel 718) |
| N08000–N08999 | Iron-nickel-chromium alloys (Incoloy family) | N08825 (Incoloy 825) |
| N10000–N10999 | Nickel-molybdenum alloys (Hastelloy B family) | N10001 (Hastelloy B) |
| N11000–N11999 | Nickel-chromium-molybdenum alloys (Hastelloy C family) | N10276 (Hastelloy C-276) |
Key insight: Knowing which series a UNS number falls into tells you the alloy’s general family and properties — before you even look up the full datasheet.
Decoding the Number: Why N06625 Is Inconel 625
The five-digit numbers aren’t random — they preserve relationships with older designation systems. For example:
- N06625: The digits “066” derive from the original Inconel 600-series numbering, and “.625” directly references the nominal composition percentage of the key element (niobium/tantalum). This pattern exists across the Inconel family.
- N10276: The “276” is the standard identifier for Hastelloy C-276 across all global standards.
- N07718: The “718” is the defining identifier for Inconel 718, one of the most widely used nickel alloys in aerospace and oil & gas.
This consistency means that once you learn a few key numbers, you can often guess what a material is from its UNS alone.
The Most Important Nickel Alloy UNS Numbers: Complete Reference Table
This table covers the nickel alloys most commonly encountered in industrial applications:
| UNS Number | Common Trade Name(s) | Ni% (approx.) | Key Features | Primary Applications |
|---|---|---|---|---|
| N02200 | Nickel 200 | 99.0% | Pure nickel, excellent corrosion resistance | Chemical processing, food handling |
| N02201 | Nickel 201 | 99.0% | Low-carbon version of N02200 | Applications requiring welding |
| N04400 | Monel 400 | 63% | High strength, excellent seawater resistance | Marine, oil & gas, chemical processing |
| N05500 | Monel K-500 | 63% | Precipitation-hardened Monel, higher strength | Pumps, valves, marine hardware |
| N06600 | Inconel 600 | 76% | Good high-temperature strength, oxidation resistance | Heat exchangers, furnace components |
| N06625 | Inconel 625 | 58% | High Mo + Nb, excellent for fabrications | Aerospace, chemical processing, marine |
| N07718 | Inconel 718 | 52.5% | Precipitation-hardened, age-hardenable | Gas turbines, oil & gas, aerospace |
| N07750 | Inconel X-750 | 70% | High-temperature precipitation-hardened | Gas turbines, rocket engines |
| N08800 | Incoloy 800 | 32% | Fe-Ni-Cr, good creep resistance | Heat exchangers, thermal processing |
| N08825 | Incoloy 825 | 42% | High Cr + Mo + Cu, excellent H₂S resistance | Sour gas, chemical processing, pollution control |
| N09925 | Incoloy 925 | 44% | Lower-cost alternative to 718, precipitation-hardened | Oil & gas completions |
| N10001 | Hastelloy B | 62% | Excellent HCl acid resistance | Chemical processing |
| N10276 | Hastelloy C-276 | 57% | Most versatile C-family alloy, low C prevents sensitization | Chemical, pharmaceutical, flue-gas desulfurization |
| N06022 | Hastelloy C-22 | 56% | Better Cr than C-276, superior oxidizing environments | Chemical, pharmaceutical, marine |
UNS vs ASTM vs ASME: What’s the Difference?
A common source of confusion: UNS is not a specification — it’s a designation system. Think of it this way:
| System | What It Does | Who Manages It | Example |
|---|---|---|---|
| UNS | Identifies which material you’re talking about | ASTM International / SAE International | N06625 |
| ASTM | Defines the minimum requirements the material must meet | ASTM Committee B02 | ASTM B444 |
| ASME | Defines how the material is used in pressure vessel codes | ASME BPVC | ASME SB-444 |
In practice: A purchase order should always specify both:
- The UNS number — to identify the exact alloy chemistry
- The ASTM/ASME specification — to define the form (plate, pipe, bar), condition (solution-annealed, aged), and test requirements
Example:
"Inconel 625, UNS N06625, ASTM B444 Grade 2, solution-annealed, 6" NPS seamless pipe"
This specification tells the supplier exactly what chemistry (UNS), what form and condition (ASTM), and prevents ambiguity.
Always Request a Material Test Certificate (MTC)
Any reputable supplier should provide a Material Test Certificate (MTC) — also called a Mill Test Report — that confirms:
- Exact chemical composition (matching the UNS requirement)
- Mechanical properties (yield, tensile, elongation)
- Heat number for full traceability
- Results of any required corrosion or hardness tests
If an MTC shows a material with UNS N06625 but the chromium content is 18% instead of 20–23%, that’s a red flag — you may not have received what you ordered.
The Five Most Common UNS-Related Mistakes
Mistake 1: Assuming Trade Names Are Universal
“Inconel” is a registered trademark of Special Metals Corporation. “Hastelloy” is a registered trademark of Haynes International. These names do not describe a single specific alloy — they describe product families with multiple compositions.
| Trade Name | Covers These UNS Numbers |
|---|---|
| Inconel 600 | N06600 |
| Inconel 625 | N06625 |
| Inconel 718 | N07718 |
| Hastelloy C-276 | N10276 |
| Hastelloy C-22 | N06022 |
| Incoloy 825 | N08825 |
Always specify the UNS number alongside any trade name. A supplier receiving an order for “Inconel” might interpret it as any of a dozen alloys.
Mistake 2: Confusing UNS with Mechanical Property Grades
ASTM specifications often have multiple “grades” or “types” — these refer to manufacturing or condition variations, not different alloys. For example:
- ASTM B444 Grade 1 = Inconel 625 (N06625), normalized grain size
- ASTM B444 Grade 2 = Inconel 625 (N06625), fine grain size (better for forming)
Both are UNS N06625. Neither is “better” than the other — they serve different fabrication needs. Confusing grades with separate alloys leads to over-specification and unnecessary cost.
Mistake 3: Ignoring the Carbon Suffix
Some UNS numbers have variants indicating carbon content. The most critical example in stainless steel is the “L” grades (low carbon):
| UNS | Alloy | Carbon Level | Key Difference |
|---|---|---|---|
| S30400 | 304 | ≤ 0.08% | Standard grade |
| S30403 | 304L | ≤ 0.03% | Low carbon, better for welding |
| S31600 | 316 | ≤ 0.08% | Standard grade |
| S31603 | 316L | ≤ 0.03% | Low carbon, better for welding |
For nickel alloys, the C-276 family shows this distinction clearly:
| UNS | Carbon Level | Sensitization Risk |
|---|---|---|
| N10276 | ≤ 0.01% | Low (stabilized) |
| N06455 | Same family | Alternative low-C version |
Using the wrong carbon grade in a welded fabrication can result in intergranular corrosion — a failure mode that occurs months or years after installation.
Mistake 4: Using UNS Alone Without an ASTM/ASME Spec
UNS tells you what the material is chemically. ASTM/ASME tells you what form it’s in, what condition it’s in, and what tests it passed. Without the specification:
- A bar might arrive rough-turned instead of ground
- Pipe might arrive in the wrong wall thickness tolerance
- A plate might not have passed the required charpy impact test for low-temperature service
Mistake 5: Assuming International Equivalence Without Verification
Many engineers assume that UNS N06625 and its European equivalent are chemically identical — but they’re not always. Key differences in specified ranges can affect final performance.
| Standard System | Designation for Inconel 625 |
|---|---|
| UNS (US) | N06625 |
| EN (Europe) | 2.4856 (NiCr22Mo9Nb) |
| Werkstoff (Germany) | 2.4856 |
| GB (China) | NS336 |
| JIS (Japan) | NCF 625 |
When sourcing internationally, always verify that the specified ranges in each standard are compatible with your application — not just that the names are equivalent.
UNS Numbers and International Sourcing: The Practical Checklist
If you’re procuring nickel alloys internationally, use this checklist:
- Specify the UNS number as the primary identifier in all RFQs and purchase orders
- Add the ASTM or ASME specification for form, condition, and test requirements
- Include a required MTC with heat number, chemistry, and mechanical properties
- For European suppliers, cross-reference the UNS to EN or Werkstoff number (and verify the ranges are compatible)
- For Chinese suppliers, clarify whether they reference GB standards, ASTM equivalents, or their own mill specifications
- For critical applications, consider third-party material verification (PMI — Positive Material Identification) before installation
PMI tip: Handheld XRF (X-ray fluorescence) analyzers can verify the chemical composition of a nickel alloy in minutes — catching UNS mismatches before the material enters fabrication.
Quick Reference: Your At-a-Glance UNS Cheat Sheet
| Need This | Start With This UNS |
|---|---|
| Seawater / marine environment | N04400 (Monel 400) |
| Sour gas / H₂S service | N08825 (Incoloy 825) or N10276 (C-276) |
| Strong oxidizing acids (nitric, acetic) | N06625 (Inconel 625) |
| Strong reducing acids (HCl, sulfuric) | N10001 (Hastelloy B) |
| Mixed oxidizing/reducing environments | N10276 (Hastelloy C-276) |
| Best overall corrosion resistance | N06022 (Hastelloy C-22) |
| High-temperature strength (>600°C) | N07718 (Inconel 718) |
| Best value for general purpose | N06600 (Inconel 600) |
| Fabricated components (pipe, vessel) | N06625 (Inconel 625) |
Conclusion
UNS numbers are the backbone of clear nickel alloy specification. They eliminate ambiguity between trade names, connect international standards, and protect both buyers and suppliers from costly errors.
The three rules to live by:
- Always pair a UNS number with an ASTM/ASME specification
- Always require a Material Test Certificate
- Always verify internationally sourced materials against your exact requirements — equivalence is not identity
Master these three rules and you’ll eliminate the most common and expensive material mistakes in your projects.
Related Articles
- Inconel 625 vs 718 — Which to Choose for Your Application? — Deep-dive into the two most widely specified precipitation-hardened nickel alloys
- The Ultimate Nickel Alloy Selection Guide — The comprehensive decision framework for choosing the right nickel alloy for any environment
