Hangbo Alloy Group
Shanghai · Est. 2012
A practical NACE MR0175 / ISO 15156-compliant material selection guide for H₂S, CO₂ and chloride service — comparing Incoloy 925, Incoloy 945X, Custom Age 625 Plus, Inconel 718, Inconel 625 and Monel K-500 across strength, sour-gas envelope, corrosion and temperature.
Downhole and wellhead hardware in sour oil and gas fields lives in one of the harshest material environments on earth: hydrogen sulfide (H₂S), carbon dioxide (CO₂), chlorides, free sulfur, high pressure and high temperature (HPHT). The dominant failure mode is environmental cracking — sulfide stress cracking (SSC), stress corrosion cracking (SCC) and hydrogen-induced cracking — which can rupture a low-alloy steel component in hours. Specifying a NACE MR0175 / ISO 15156-qualified nickel alloy is the standard defense.
This guide helps completion, materials and procurement engineers choose the right nickel-base corrosion-resistant alloy (CRA) for sour-gas and downhole service. As a direct nickel alloy manufacturer and supplier, Hangbo Alloy Group produces and stocks Incoloy 925, 945X, Custom Age 625 Plus, Inconel 718/625 and Monel K-500 in bar, forging, tube, plate and wire forms, with NACE MR0175 documentation and third-party inspection.
Hydrogen sulfide generates atomic hydrogen at the metal surface. In susceptible steels this hydrogen drives SSC and blistering. Two metallurgical factors control resistance:
Rule of thumb: Once H₂S partial pressure exceeds roughly 3–5 psi (and chlorides are high), low-alloy and martensitic stainless steels fall out of qualification. That is the point where Incoloy 925 / 945X and Inconel 625/725 become the default CRAs.
The table below shows nominal chemistry of the principal NACE-qualified grades used downhole and at the wellhead.
| Grade | UNS | Ni % | Cr % | Fe % | Mo % | Cu % | Hardening |
|---|---|---|---|---|---|---|---|
| Incoloy 925 | N09925 | 42–46 | 19.5–23.5 | Bal | 2.5–3.5 | 1.5–3.0 | Ti, Al (γ′) |
| Incoloy 945X | N09945 | 45–55 | 19.5–23 | Bal | 3.0–4.0 | 1.5–3.0 | Nb, Ti, Al |
| Custom Age 625 Plus | N07716 | 59–63 | 19–22 | Bal | 7.0–9.5 | — | Nb, Ti, Al |
| Inconel 718 | N07718 | 50–55 | 17–21 | Bal | 2.8–3.3 | — | Nb, Ti (γ″/γ′) |
| Inconel 625 | N06625 | ≥58 | 20–23 | ≤5 | 8–10 | — | Nb (solid-sol.) |
| Monel K-500 | N05500 | 63–70 | — | ≤2 | — | 27–33 | Al, Ti |
The copper in Incoloy 925/945X and Monel K-500 improves general corrosion in reducing media; molybdenum in 625, 718 and Custom Age 625 Plus drives pitting/crevice resistance (PREN up to ~49–76).
Values are for the age-hardened (precipitation-strengthened) condition unless noted. Yield strength drives the NACE qualification level.
| Grade | Density (g/cm³) | Yield (min) | Tensile (min) | HRC (NACE) | Max Service Temp |
|---|---|---|---|---|---|
| Incoloy 925 | 8.08 | 758 MPa (110 ksi) | 965 MPa (140 ksi) | ≤35–40 | ~450°C |
| Incoloy 945X | ~8.1 | 966 MPa (140 ksi) | ~1100 MPa | ≤40 | ~450°C |
| Custom Age 625 Plus | ~8.4 | 827 MPa (120 ksi) | ~1035 MPa | ≤40 | ~650°C |
| Inconel 718 | 8.19 | 1035 MPa (150 ksi) | 1275 MPa | ≤40 | ~650°C |
| Inconel 625 (ann.) | 8.44 | 414 MPa (60 ksi) | 830 MPa | ≤35 | ~650°C |
| Monel K-500 | 8.44 | 620 MPa (90 ksi) | 895 MPa | ≤35 | ~315°C (sour) |
Incoloy 945X and Inconel 718 offer the highest strength; Inconel 625 trades strength for an unrestricted H₂S envelope and excellent pitting resistance.
Under NACE MR0175 / ISO 15156-3, each alloy is qualified to a defined environmental envelope (partial pressure of H₂S, temperature, chloride). Representative qualification levels:
| Service Condition | Recommended First Choice | Upgrade When… |
|---|---|---|
| High H₂S, high load (hangers, tubing) | Incoloy 925 | Need >110 ksi → 945X or 725 |
| Very high strength sour equipment | Incoloy 945X (Level VII) | Already at top of Fe-Ni-Cr range |
| Corrosion-dominated, moderate load | Inconel 625 (annealed) | Need strength → Custom Age 625 Plus |
| Large/complex shape, no warm work | Custom Age 625 Plus | — |
| Low H₂S, very high strength | Inconel 718 | H₂S >15 psi → step down |
| Fasteners, pump shafts, marine+sour | Monel K-500 | High H₂S + sulfur → 925/945X |
| High H₂S, high strength, max corrosion | Inconel 725 | Premium cost tier |
Hangbo Alloy Group manufactures and supplies NACE-oriented nickel alloys with mill test certificates and third-party inspection (SGS, TUV, NORSOK M-650 where required):
| Standard | Scope |
|---|---|
| NACE MR0175 / ISO 15156-3 | Sour (H₂S) service material qualification |
| API 6A | Wellhead and Christmas tree equipment |
| API 17D | Subsea wellhead and tree equipment |
| ASTM B805 / B983 | Incoloy 925 / 945X bar, rod, tube |
| ASTM B637 | Inconel 718 bar, forgings, rings |
| ASTM B443 / B446 | Inconel 625 plate & bar |
| ASTM B865 | Monel K-500 rod, bar, forgings |
| NORSOK MDS / M-650 | Qualification & manufacturing of CRAs |
| NACE TM0177 / TM0316 | SSC / SCC test methods |
Need Incoloy 925, 945X, Custom Age 625 Plus, Inconel 718/625 or Monel K-500 for a sour-gas or downhole project? Our metallurgists confirm the grade against your H₂S, CO₂, chloride, temperature and hardness envelope and prepare NACE MR0175 documentation. As a direct manufacturer and supplier, we offer competitive price quotes, full certification, and worldwide delivery.
Email: [email protected] | Phone / WhatsApp: +86-136-1165-6360
Common NACE-qualified nickel-base CRAs for sour (H2S) service include Incoloy 925 (Level VI, up to 35–40 HRC), Incoloy 945X (Level VII and VI-450°F), Custom Age 625 Plus (N07716), Inconel 625 (annealed, unrestricted H2S but moderate strength), Inconel 725, and Inconel 718 (restricted to about 15 psi H2S partial pressure). Monel K-500 is accepted with caution for many sour programs. Each grade is qualified only within its hardness and environmental envelope in Annex B of the standard.
Inconel 718 is an excellent high-strength alloy but under NACE MR0175/ISO 15156 it is restricted to roughly 15 psi H2S partial pressure, so it is not suitable for most high-H2S deep sour-gas completions. For H2S above about 3–5 psi with high chloride, operators typically step down to Incoloy 925 or 945X, or to Inconel 625/725, which carry unrestricted or much higher H2S envelopes.
NACE MR0175 and ISO 15156 are the same harmonized standard published jointly as ANSI/NACE MR0175 / ISO 15156. North America often cites 'NACE MR0175' while Europe, the Middle East and Asia cite 'ISO 15156-3'. A single document with dual logos covers material limits for H2S-containing oil and gas production environments.
Incoloy 925 (UNS N09925) delivers a minimum aged yield strength of ~758 MPa (110 ksi) and is qualified to NACE Level VI. Incoloy 945X (UNS N09945) contains higher hardening additions (more Nb and Ti) and reaches ~966 MPa (140 ksi) minimum yield, qualified to Level VII and VI-450°F. 945X is chosen when the strength budget demands more than 925 can provide; 925 is the cost-effective high-volume workhorse for HPHT sour completions.
Nickel-base alloys resist chloride stress corrosion cracking (Cl-SCC) far better than austenitic stainless steels because of their high nickel content (typically 42%+). Incoloy 925, 945X, Custom Age 625 Plus, Inconel 625 and Inconel 718 all resist Cl-SCC in neutral brines up to their yield strength at temperatures to about 200°C, and most also carry H2S qualification under NACE MR0175.
In aged condition, Incoloy 925 and 945X are typically limited to about 450°C (840°F) before gamma-prime over-aging reduces strength; above this, Inconel 718 or 725 should be evaluated. Inconel 625 remains useful to roughly 650°C in oxidizing service. Monel K-500 is generally limited to below about 315°C in sour service because of sulfur-compound sensitivity.
Yes, but procedure matters. Incoloy 925 is welded with matching ERNiCrMo-16 filler and requires a post-weld solution anneal plus re-age to restore strength (as-welded strength is only about 60–70% of aged). Inconel 718 is one of the most weldable age-hardening nickel alloys (ERNiFeCr-2 filler) because its gamma-double-prime precipitates slowly, resisting strain-age cracking. All welds must stay within the NACE hardness envelope.
Sour-service nickel alloys are supplied as bar and forged stock for valves, hangers and tree components; mechanical tubing and tubulars for completions; plates for pressure-containing parts; and round bars for fasteners, pump shafts and mandrels. Bars, forgings, seamless tubes, plates and welding wire are all available from Hangbo Alloy Group.
Key standards are NACE MR0175 / ISO 15156-3 (sour service qualification), API 6A (wellhead and tree equipment), API 17D (subsea), ASTM B805 / B983 (Incoloy 925/945X bar and tube), ASTM B637 (Inconel 718), ASTM B443/B446 (Inconel 625), ASTM B865 (Monel K-500), and NORSOK MDS D-series / M-650 for qualification.
Choose Inconel 625 when corrosion (especially pitting/crevice in chlorides) dominates and moderate strength (~60 ksi yield) is acceptable — it has an unrestricted H2S envelope and high PREN (~49). Choose Incoloy 925 when the component must carry high structural load (110 ksi yield) such as hangers, tubing and fasteners, while still meeting NACE Level VI.
Monel K-500 (UNS N05500) is widely used for sour-service fasteners, pump shafts and mandrels and resists chloride SCC, but it is sensitive to elemental sulfur and very low-pH high-H2S conditions. It is generally accepted for NACE sour programs with hardness kept at or below 35 HRC and with project-specific qualification for sulfur-bearing or high-H2S wells.
Hardness limits depend on the alloy and its qualification level. Typical maxima are 35 HRC for Inconel 625 (annealed) and Monel K-500, around 35–40 HRC for Incoloy 925, up to about 40 HRC for some Incoloy 945X and Inconel 718 conditions, and up to 43–45 HRC for Inconel 725. Hardness is controlled by heat treatment and verified by batch testing with NACE TM0177 methods.
The main families are age-hardenable iron-nickel-chromium alloys (Incoloy 925, 945X, 945), age-hardenable nickel-chromium superalloys (Inconel 718, 725, Custom Age 625 Plus), solid-solution Ni-Cr-Mo alloys (Inconel 625), and age-hardenable nickel-copper alloys (Monel K-500). Each trades strength, sour-gas envelope and cost differently.
Contact Hangbo Alloy Group at [email protected] or call/WhatsApp +86-136-1165-6360 with your grade, UNS, specification (ASTM/API/NACE), product form, size and quantity, plus service H2S partial pressure, temperature and chloride level. As a direct manufacturer and supplier we provide competitive price quotes, NACE MR0175 documentation and worldwide delivery.
For help selecting between Incoloy 925, 945X, Custom Age 625 Plus, Inconel 718/625, or Monel K-500 — or for a quotation on bars, forgings, tubes, plates or welding consumables — contact Hangbo Alloy Group. We confirm the grade against your NACE MR0175 envelope and typically respond within 10 minutes.
Email: [email protected]
Phone: +86-136-1165-6360
WhatsApp: +86 13611656360
Send us your well conditions (H₂S, CO₂, chloride, temperature) and we will confirm the NACE MR0175 grade and deliver a competitive quote with full certification.