ASTM B444 — Seamless Inconel 625 (UNS N06625) Pipe and Tube
ASTM B444 (latest edition: B444-22) covers seamless Inconel 625 (Alloy 625, UNS N06625) pipe and tube for general corrosion, cryogenic, and elevated-temperature service. ASME SB-444 is adopted in BPVC Section II Part B. Alloy 625 is arguably the most versatile nickel alloy available — it combines excellent resistance to a wide range of corrosive environments with outstanding strength from cryogenic temperatures (–196 °C) to elevated temperatures (816 °C), making it a single-material solution across multiple demanding industries.
1. Scope
B444 covers seamless pipe and tube in two UNS alloys: N06625 (Inconel 625) and N06219 (Alloy 219, a Cr-Fe-Ni modification). N06625 is overwhelmingly the dominant grade. Tubing is furnished in OD × wall-thickness dimensions; pipe in NPS + schedule per ASME B36.19. Both Grade 1 (annealed) and Grade 2 (higher-strength condition) are defined.
2. Chemical Composition — UNS N06625 (Alloy 625)
| Element |
Composition (wt %) |
Role |
| Ni |
58.0 min (balance) |
Austenitic matrix; corrosion and SCC resistance |
| Cr |
20.0–23.0 |
Passive film; oxidation and pitting resistance |
| Mo |
8.0–10.0 |
Pitting resistance (PREN contribution); solid-solution hardener |
| Nb + Ta |
3.15–4.15 |
Solid-solution hardening; precipitation of γ'' phase (strengthening) |
| Fe |
5.0 max |
Controlled impurity limit |
| C |
0.10 max |
Low carbon prevents sensitisation; 0.10 max in B444 |
| Si |
0.50 max |
Controlled for weldability |
| Mn |
0.50 max |
— |
| Al |
0.40 max |
Deoxidation |
| Ti |
0.40 max |
Minor strengthening; controlled |
3. Grade 1 vs Grade 2 — Mechanical Properties
ASTM B444 defines two property grades based on heat treatment condition:
| Property |
Grade 1 (Annealed) |
Grade 2 (Solution-Annealed / Stress-Relieved) |
| UTS min (MPa) |
690 |
827 |
| 0.2% YS min (MPa) |
275 |
414 |
| Elongation min (%) |
30 |
30 |
| Typical HRC |
≤ 35 |
≤ 35 |
| Anneal temperature |
871–982 °C (stress relief) or 1093–1204 °C (solution) |
1093–1204 °C (full solution anneal) |
| Typical application |
Chemical processing, heat exchangers, general service |
Subsea, oil & gas, aerospace, high-pressure service |
4. Corrosion Resistance
Alloy 625's PREN ≈ 52 (Cr + 3.3×Mo = 20 + 33 ≈ 53) — the highest PREN of any standard nickel alloy pipe specification. This provides exceptional pitting and crevice corrosion resistance in seawater and chloride environments. Key corrosion resistance characteristics:
| Environment |
Performance |
Competing Material |
| Seawater (all temperatures) |
Excellent — no pitting to 135 °C in ASTM G48 testing |
2507 super duplex limited to 316 °C; 625 has no upper limit |
| Sour gas (H₂S + CO₂) |
Excellent — fully compliant NACE MR0175 at all temperatures |
316L not permitted above 60 °C in sour service |
| Cryogenic (LNG, –196 °C) |
Outstanding — FCC structure, no DBT |
Duplex limited to –29 °C minimum |
| Oxidising acids (HNO₃) |
Good (Cr passive film) |
C-22 better for concentrated HNO₃ |
| Reducing acids (HCl) |
Good to moderate |
C-276 or B-2 preferred for concentrated HCl |
| High temp oxidation (to 816 °C) |
Excellent |
310S for temperatures >900 °C |
5. Testing Requirements
| Test |
Requirement |
| Tensile |
One per heat or heat-treatment lot |
| Hardness |
Each pipe/tube; especially for Grade 2 |
| Hydrostatic |
Each piece, or eddy current as alternative |
| Flare / Flattening (tube) |
For small OD tubing per B444 requirements |
| Chemical Analysis |
One ladle per heat; PMI strongly recommended for all 625 products |
6. Related Standards
| Standard |
Form and Scope |
| ASTM B443 / ASME SB-443 |
Alloy 625 plate, sheet, and strip |
| ASTM B446 / ASME SB-446 |
Alloy 625 rod and bar |
| ASTM B564 / ASME SB-564 |
Alloy 625 forgings (flanges, nozzles) |
| ASTM B366 / ASME SB-366 |
Alloy 625 wrought butt-welding fittings |
| ASTM B622 / ASME SB-622 |
Hastelloy C-276/C-22 seamless pipe (B444 complement for C-grades) |
HT PIPE supplies ASTM B444 Alloy 625 seamless pipe and tube in Grade 1 and Grade 2 conditions, from OD 6 mm to NPS 12, with EN 10204 3.1 MTR and NACE MR0175 compliance. Contact us for subsea, LNG, and sour-gas service specifications.
7. Frequently Asked Questions
Q1: What is the PREN of Inconel 625 and how does it compare to super duplex 2507?
PREN for Alloy 625 ≈ %Cr + 3.3×%Mo = 21 + 3.3×9 ≈ 51. For super duplex S32750 (2507): PREN ≈ 25 + 3.3×4 + 16×0.28 ≈ 43. Alloy 625 has a significantly higher PREN, providing greater pitting resistance. However, PREN alone does not tell the whole story: 625 is an austenitic structure with no upper temperature limit for corrosion resistance, while duplex 2507 is limited to 316 °C continuous service. For hot seawater (above ~60 °C), 625 outperforms 2507.
Q2: Can Grade 1 and Grade 2 B444 pipe be welded together?
Yes. Grade 1 and Grade 2 are the same alloy (N06625) with different heat treatment conditions. They can be welded together using matching filler metal (ERNiCrMo-3, which is the 625 composition). After welding, the weld heat-affected zone will typically have properties between Grade 1 and Grade 2. For applications requiring Grade 2 minimum properties throughout, post-weld heat treatment (PWHT) may be required, though this is rare in practice since the heat-affected zone is narrow and the overall structure retains excellent properties.
