Hangbo Alloy Group
Shanghai · Est. 2012
Inconel 625 and 718 are both used in sour-gas service but on different logics: 625 is the corrosion-first solid-solution alloy for tubing, 718 is the precipitation-hardened high-strength alloy for wellhead parts that must stay within NACE hardness limits. This comparison shows where each belongs.
For sour-gas (H₂S-containing) downhole tubing and wellhead hardware, two nickel-base alloys dominate the shortlist: Inconel 625 (UNS N06625) and Inconel 718 (UNS N07718). They are often assumed to be interchangeable because both carry the Inconel name and both resist sulfide environments, but they are built on different strengthening mechanisms and make completely different trade-offs between corrosion resistance and strength. Choosing wrongly means either over-paying for strength you do not need, or specifying a high-strength part that fails NACE hardness limits in sour service.
The direct answer: Inconel 625 is a solid-solution alloy chosen for corrosion resistance and good strength, ideal for sour downhole tubing and corrosive environments; Inconel 718 is a precipitation-hardened alloy chosen where very high strength and stiffness are required, such as wellhead valves, springs and fasteners — but its age-hardened condition must be controlled to stay within NACE MR0175 / ISO 15156 hardness limits for sour service. This comparison sets out the metallurgy, the sour-service rules, the application map and the selection logic.
We manufacture both as seamless tube, bar, forgings and wire, so this is written from the melt shop. Our production runs from virgin charge through VIM+ESR or VIM+VAR melting and scale removal to controlled working, heat treatment and finishing, with the melting route and heat number recorded for every lot. Our Inconel 625 guide, our Inconel 718 guide and our sour gas selection guide give the property baselines, and our NACE MR0175 / ISO 15156 guide explains the hardness and heat-treatment controls. For the procurement side, see our procurement pitfalls buyer’s guide.
The difference starts with chemistry. Inconel 625 is strengthened in the solid solution by molybdenum and niobium; there is no precipitation-hardening element package, so its strength comes from the lattice and stays stable through welding. Inconel 718 is strengthened by precipitates (γ″, niobium-rich, plus γ′ and delta) that form during aging, driven by titanium and aluminium additions and an iron-balanced matrix.
| Element / attribute | Inconel 625 (UNS N06625) | Inconel 718 (UNS N07718) | What it means |
|---|---|---|---|
| Nickel | 58.0 min | 50.0 – 55.0 | Both nickel-rich; 718 has more Fe |
| Chromium | 20.0 – 23.0 | 17.0 – 21.0 | 625 slightly better oxidizing resistance |
| Molybdenum | 8.0 – 10.0 | 2.8 – 3.3 | 625 stronger pitting/crevice resistance |
| Niobium (Cb) | 3.15 – 4.15 | 4.75 – 5.50 | Both use Nb; 718 also for γ″ |
| Titanium / Aluminium | low | 0.65 – 1.15 / 0.20 – 0.80 | 718 precipitation hardening |
| Iron | 5.0 max | balance (17 – 20) | 718 is iron-balanced |
| Strengthening | Solid solution | Precipitation (γ″ + γ′) | Different thermal sensitivity |
That chemistry gap is the root of every selection decision below: 625 keeps its corrosion resistance partly because molybdenum stays high, while 718 trades some molybdenum for the titanium-aluminium package that gives it roughly double the strength of annealed 625 once aged.
In the annealed condition both alloys are strong, but 718 is normally used age-hardened, and that is where the gap opens. Age-hardened 718 reaches ultimate tensile strengths around 1250–1400 MPa (180–200 ksi) depending on condition, while annealed 625 sits near 830–1000 MPa (120–145 ksi). For a given load, 718 lets you use a smaller cross-section or thinner wall.
| Property | Inconel 625 (annealed) | Inconel 718 (age-hardened) |
|---|---|---|
| Ultimate tensile strength | ~830 – 1000 MPa | ~1250 – 1400 MPa |
| Yield strength | ~415 – 620 MPa | ~1050 – 1200 MPa |
| Typical use of strength | Corrosion-critical tubing and hardware | High-load wellhead and rotating parts |
| Wall / section benefit | Moderate | Thinner wall, lighter component |
The trade: that strength in 718 only exists after aging, and aging is exactly the step that must be controlled for sour service. A 718 part that is over-aged or improperly solution-treated can lose strength or develop delta-phase networks; a 625 part simply does not have that sensitivity because it is not precipitation-strengthened.
In sour service the controlling document is normally NACE MR0175 / ISO 15156, which limits hardness and requires documented heat treatment and melting route for materials exposed to H₂S. Both alloys can qualify, but by different routes.
| Aspect | Inconel 625 | Inconel 718 |
|---|---|---|
| Sulfide stress cracking resistance | Excellent in annealed condition | Good, but hardness-limited in aged condition |
| NACE path | Generally accepted for sour service per the standard | Accepted with hardness and heat-treat controls |
| Hardness control | Typically not a limiting issue | Age-hardened condition must stay within limits |
| Heat-treat documentation | Solution anneal record | Solution + age record, tightly controlled |
| Pitting / crevice (PREN) | Very high (Mo + Nb rich) | High, but lower Mo than 625 |
The practical warning: specifying 718 for a sour wellhead without controlling the aging condition is the classic failure. If the aged hardness exceeds the permitted ceiling, the part is non-compliant regardless of how good the alloy is. For that reason many operators use 718 in the annealed or lightly hardened condition for sour duty, or move to age-hardenable variants such as Inconel 725 or Custom Age 625 Plus that are designed for the combination. 625, by contrast, is the corrosion-first default for sour downhole tubing.
The selection follows the duty, not the name.
| Component | Preferred alloy | Reason |
|---|---|---|
| Sour downhole tubing / flowlines | Inconel 625 | Corrosion resistance and SSC resistance with good strength |
| Wellhead christmas-tree bodies / valves | Inconel 718 (controlled age) | High strength and stiffness at pressure boundaries |
| Valve stems, springs, fasteners | Inconel 718 | High strength-to-size ratio |
| Hangers, packer components | Either, by strength need | 625 for corrosion; 718 where load dominates |
| Corrosive, moderate-load hardware | Inconel 625 | Best all-round corrosion resistance |
| High-strength sour fasteners | Inconel 718 (hardness-controlled) | Strength with documented NACE compliance |
For subsea and deepwater variants, our Inconel 718M marine grade and the broader offshore and subsea guide cover the seawater-plus-sour combination.
A useful way to frame the two is by failure mode. A 625 tubing string that is chemically correct but marginally undersized fails by corrosion or by losing wall to pitting; it does not fail by being too weak. A 718 wellhead component that is the wrong age condition fails by exceeding the NACE hardness ceiling and becoming sulfide-stress-crack susceptible, even though its strength on paper looks excellent. In other words, 625 risk sits in the chemistry and cleanliness, while 718 risk sits in the heat-treatment window. Both are fully manageable with the right purchase-order controls, which is why naming the condition and the standard edition matters more for 718 than for almost any other nickel alloy in oil and gas.
Our seamless tube and pipe programme supplies 625 sour-service tubing, and our round bar and forging programme supplies 718 for wellhead parts, each with the heat-treatment and NACE documentation the duty requires.
| Product form | Inconel 625 | Inconel 718 |
|---|---|---|
| Bar and rod | ASTM B446 | ASTM B637 |
| Seamless pipe & tube | ASTM B444, B705 (welded), B443 (plate) | ASTM B983 (tube), B637 |
| Forgings | ASTM B564 | ASTM B637 / B564 family |
| Sour-service control | NACE MR0175 / ISO 15156 | NACE MR0175 / ISO 15156 with hardness limit |
| Welding | ERNiCrMo-3 (625) | ERNiFeCr-2 (718) per procedure |
Our test reports and traceability process shows the document set you should expect for sour-service material, including melting route, heat treatment and NDT coverage.
Inconel is a registered trademark of Special Metals Corporation; the grades are defined by their UNS numbers (N06625, N07718) and ASTM product specifications. This article is guidance for specifiers, not a substitute for the current edition of NACE MR0175 / ISO 15156 or the applicable ASTM specification.
Not automatically. Inconel 625 is the corrosion-first choice for sour downhole tubing because of its high molybdenum and niobium content and its freedom from precipitation-hardening sensitivity. Inconel 718 is better only where very high strength is needed, such as wellhead valves, springs and fasteners, and then its age-hardened condition must be controlled to NACE hardness limits.
Yes, but only with documented controls. NACE MR0175 / ISO 15156 limits hardness and requires the heat-treatment route to be recorded; an aged 718 part that exceeds the hardness ceiling is non-compliant regardless of alloy quality. Many operators use 718 in the annealed or lightly hardened condition for sour duty.
Because it combines excellent resistance to sulfide stress cracking with high pitting and crevice resistance from its molybdenum and niobium, and it is not precipitation-hardened, so it avoids the aging-sensitivity that complicates 718 in sour service. It gives good strength without a hardness-control burden.
For general and chloride corrosion, 625 is generally superior because it carries roughly three times the molybdenum of 718. 718 wins on strength, not on corrosion. Where both corrosion and very high strength are needed, age-hardenable variants such as 725 or Custom Age 625 Plus are often considered.
Both are weldable with the correct procedures and fillers (ERNiCrMo-3 for 625, ERNiFeCr-2 for 718). 625 is less heat-treatment-sensitive after welding because it is not precipitation-strengthened; 718 weldments need careful post-weld heat treatment to recover properties and must respect the sour-service hardness limits.
Name the UNS number, the ASTM product specification and edition, the required condition (annealed for 625; solution plus age condition for 718), the NACE MR0175 / ISO 15156 compliance, the melting route and heat number, and the required NDT and hardness documentation. Avoid vague “Inconel” wording that lets a mill choose.
If you are specifying Inconel 625 or 718 for sour-gas tubing, wellhead parts or fasteners, send us the H₂S partial pressure, chloride level, pressure, temperature and the component duty. We will confirm the right UNS and condition, advise the NACE MR0175 / ISO 15156 controls you must write into the order, and quote our own virgin-material, double-melted production with full heat-to-piece traceability and the heat-treatment records the service requires. Typical response time is ten minutes, and we ship worldwide from Shanghai.
Email: serena@hangboalloy.com
Phone: +86 166 2109 2936
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Send us the H2S partial pressure, chloride level, pressure, temperature and the component duty. We will confirm the right UNS and condition, advise the NACE MR0175 / ISO 15156 controls to write into the order, and quote our own virgin-material, double-melted production with full heat-to-piece traceability and the heat-treatment records the service requires.