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Inconel 625 / Alloy 621 Seamless Pipe & Tube | ASME SB-444 UNS N06625
Grade 1 or Grade 2, and why the choice matters
ASME SB-444 covers cold-worked seamless pipe and tube of UNS N06625 (Inconel 625), UNS N06852 (Alloy 621) and UNS N06219. The single most common question we receive on this grade is Grade 1 against Grade 2, and the answer comes down to heat treatment an
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Grade 1 or Grade 2, and why the choice matters
ASME SB-444 covers cold-worked seamless pipe and tube of UNS N06625 (Inconel 625), UNS N06852 (Alloy 621) and UNS N06219. The single most common question we receive on this grade is Grade 1 against Grade 2, and the answer comes down to heat treatment and grain structure rather than chemistry. The chemistry is identical; the difference is whether the material is annealed or solution annealed.
Grade 1 is annealed at 1600 degrees F (871 degrees C) minimum. Grade 2 is solution annealed at 2000 degrees F (1093 degrees C) minimum, with an optional stabilization anneal at 1800 degrees F (982 degrees C) minimum to further improve resistance to sensitization. Grade 2 is the correct default for corrosive service, because it delivers the full chromium-molybdenum-columbium solid solution without the grain-boundary precipitation that annealing leaves behind. Grade 1 is used where the material is subsequently formed or where corrosion is not the governing concern.
Grade comparison
| Property | N06625 Grade 1 | N06625 Grade 2 | N06852 | N06219 |
|---|---|---|---|---|
| Condition | Annealed | Solution annealed | Solution annealed | Solution annealed |
| Tensile, min, ksi (MPa) | 120 (827) | 100 (690) | 85 (586) | 96 (660) |
| Yield (0.2% offset), min, ksi (MPa) | 60 (414) | 40 (276) | 35 (241) | 39 (270) |
| Elongation in 2 in, min, % | 30 | 30 | 30 | 30 |
| Chromium, % | 20.0 – 23.0 | 20.0 – 23.0 | 20.0 – 23.0 | 18.0 – 22.0 |
| Molybdenum, % | 8.0 – 10.0 | 8.0 – 10.0 | 8.0 – 10.0 | 7.0 – 9.0 |
| Columbium + tantalum, % | 3.15 – 4.15 | 3.15 – 4.15 | 0.51 – 1.00 | — |
| Iron, % | 5.0 max | 5.0 max | 15.0 – 20.0 | 2.0 – 4.0 |
| Nickel, % | 58.0 min (bal.) | 58.0 min (bal.) | Bal. | Bal. |
Alloy 621 (N06852) is the low-iron, low-columbium member of the family. With iron at 15 to 20% and columbium at 0.51 to 1.00% it is not a substitute for 625 in most applications, but it is the correct choice in specific sulphuric and hydrochloric acid duties where the higher nickel content of 625 is not required and where 621 has a long service history.
Size range, condition and product forms
We supply SB-444 as cold-worked seamless pipe and cold-worked seamless tube. Pipe is supplied to the dimensional requirements of the general requirements for pipe and tube referenced by SB-444, in nominal sizes from NPS 1/8 through NPS 8 and above on request. Tube is supplied in the finer outside diameter increments used in heat exchanger and instrumentation service, with wall thicknesses down to 0.018 in (0.46 mm). Longer lengths, and tube in coils for specific projects, can be arranged. Minimum strength values apply only to tubing supplied in straight lengths, and material under 1/8 in (3.2 mm) outside diameter and certain wall thicknesses is supplied on a chemical and surface inspection basis without tensile properties, which is normal for this size range.
Certification and inspection
Every order is supplied with an EN 10204 3.1 material test report showing the heat number, the SB-444 Table 2 chemical analysis and the tensile results for the grade and condition supplied. On request we add intergranular corrosion testing to ASTM G28, which is the standard reference test for this alloy family, and we supply NACE MR0175 / ISO 15156 documentation where the service requires sour limits. Hydrostatic testing, eddy current testing and ultrasonic testing can be included and witnessed. PED 2014/68/EU and UKCA certification of conformity are available for European and UK delivery, and third-party inspection by TUV, SGS, BV or Lloyd’s Register is arranged routinely.
Ordering information
Please specify the grade (N06625 Grade 1, N06625 Grade 2, N06852 or N06219), the product form (pipe or tube), the outside diameter or nominal size, the wall thickness, the length, the quantity and the delivery date. Add the destination port and any certification you need on the paperwork. If the material is destined for a specific service, tell us the medium, the temperature and the chlorine or acid content so that our technical team can confirm the grade and condition before you commit. Send us a line list or a cut list and we will quote against it, item by item, with the certificate package stated on each line.
Related specifications and products. The same UNS N06625 material is supplied as butt weld fittings, where the delivery condition decides the grade. See the nickel alloy round pipe and tube range Inconel 625 butt weld fitting supply the Grade 1 against Grade 2 comparison for the dimensional and material basis behind this page.
