Hangbo Alloy Group
Shanghai · Est. 2012
NACE MR0175 / ISO 15156 sets sulfide stress cracking limits for nickel alloys in sour H2S; learn hardness caps and which grades qualify for oil & gas.
Downhole and sour-processing equipment lives in a hostile mix of H2S, chloride, and water where ordinary stainless steel can fail by sulfide stress cracking (SSC) within weeks. NACE MR0175 / ISO 15156 is the standard that tells engineers which materials are allowed, at what hardness, and in which severity environment. This guide walks through what the standard actually controls, the hardness and heat-treatment limits that decide pass or fail, and the nickel alloys that are routinely qualified for sour service so you can specify with confidence.
NACE MR0175 (now harmonized with ISO 15156) limits materials for equipment in H2S-containing, aqueous environments. It targets three failure modes:
The standard sorts service into severity levels using partial pressure of H2S (pH2S), pH, and chloride content. A higher severity level shrinks the list of acceptable alloys and tightens hardness limits. Nickel alloys earn their place because their low hardness, high toughness, and corrosion resistance keep them out of the crack-sensitive zone even at meaningful H2S levels.
For most qualified nickel alloys the rule is simple: keep hardness below the cap and avoid microstructures that trap hydrogen. Typical limits from the standard:
Weld procedure matters: the standard expects welds and heat-affected zones to stay within the same hardness envelope, which is why post-weld heat treatment is mandatory for the age-hardened grades in sour duty.
The grades most often named for sour service, by application:
Always confirm the specific severity level against the current edition of the standard and your project's material specification — the allowable list tightens as pH2S and chloride rise.
Start from the environment, not the alloy. Define pH2S, pH, and chloride, then read the standard's tables to find the permitted material class. For low-to-moderate sour duty a solid-solution grade such as 825 or 625 is usually enough and cheaper to weld. For high-strength downhole parts where yield strength drives the design, move to an age-hardened grade like 725 or 718 but budget for qualified post-weld heat treatment. When acid gas meets high chloride, step up to the Ni-Cr-Mo grades (C-276, G-3, 28). Matching the alloy to the severity level — rather than over-specifying — keeps both safety margin and cost under control.
No. The standard qualifies specific alloys at specific hardness and heat-treatment conditions for defined severity levels. Solid-solution grades such as 625, 825, and C-276 are broadly accepted, while age-hardened grades such as 718 and 725 are allowed only within hardness caps and qualified heat treatments. Always check the current edition and your project spec, and see our sour gas alloy selection guide for a grade-by-grade view.
Inconel 718 and Inconel 725 are typically capped near 40 HRC maximum in sour-qualified condition, with the weld and heat-affected zone held to the same limit. The alloys must be supplied in a qualified age-hardened heat treatment, and a post-weld heat treatment is normally required so the HAZ does not exceed the hardness ceiling.
Yes. Hastelloy C-276 is a solid-solution Ni-Cr-Mo alloy with very high general and localized corrosion resistance, and it is accepted for sour service in the solution-annealed condition within the standard's hardness limits. It is commonly the choice when high chloride is combined with acid gas, though it is not age-hardenable so it is selected for corrosion resistance rather than ultra-high strength.
As a dedicated nickel alloy manufacturer serving oil & gas sectors, Hangbo Alloy Group supplies the grades discussed here across all standard product forms with full ASTM/ASME documentation and EN 10204 3.1 certification. Our material is specified for applications where corrosion resistance, high-temperature strength, and traceable quality are non-negotiable. See our production & testing facilities for more on how every heat is verified.