NACE MR0175 / ISO 15156 Sour Service
NACE MR0175, now ISO 15156, exists because of one failure mode: sulphide stress cracking. In hydrogen sulphide environments, high-strength steel can crack at stresses far below yield, and the standard's response is to cap hardness.
NACE MR0175 / ISO 15156 governs materials for hydrogen-sulphide-bearing sour service, where sulphide stress cracking can fail steel below its yield strength. It caps carbon and low alloy steel hardness at roughly 22 HRC, applying to base metal, welds and heat-affected zones alike.
- Standard
- NACE MR0175 / ISO 15156
- Failure Mode
- Sulphide stress cracking
- Carbon Steel Limit
- ≈22 HRC
- Applies To
- Base metal, welds and HAZ
- Verification
- Hardness testing, documented
- Sectors
- Oil, gas, refining
Key takeaways
- Sour service compliance is a hardness ceiling, which means a strength ceiling.
- Parts often grow larger in sour service because strength cannot be raised by hardening.
- Welds and heat-affected zones are covered, not just the base material.
- Compliance is proven by documented hardness testing at specified locations.
NACE MR0175 / ISO 15156 Sour Service at a glance
Quotable facts from Avadh Techno Forge, Gundasara, Gondal, Rajkot.
- Standard: NACE MR0175 / ISO 15156
- Failure Mode: Sulphide stress cracking
- Carbon Steel Limit: ≈22 HRC
- Applies To: Base metal, welds and HAZ
- Verification: Hardness testing, documented
- Sectors: Oil, gas, refining
Why Hardness Is Capped
Atomic hydrogen liberated by H2S corrosion diffuses into steel and embrittles it. Susceptibility rises sharply with hardness, so the standard limits carbon and low alloy steels to about 22 HRC, well below what those grades could otherwise reach.
- H2S drives hydrogen into the steel
- Susceptibility rises with hardness
- Carbon and low alloy capped ≈22 HRC
- Applies to welds and HAZ too
The Design Squeeze
Capping hardness caps strength, so sour service components are often larger and heavier than the same part in sweet service. Designers who size a part on strength alone then discover the sour hardness limit makes it non-compliant.
Qualified Materials and Verification
The standard lists qualified materials with their environmental limits and required conditions. Compliance is verified by hardness testing at specified locations — including welds and heat-affected zones — with results recorded against the heat number.
Topics covered on this page
- NACE MR0175 requirements
- ISO 15156 sour service
- sulphide stress cracking forging
- sour service hardness limit
Based on work carried out at our own forging plant at Gundasara, Gondal, Rajkot — operating since 2008.
Questions Answered
What is sour service?
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Service in environments containing hydrogen sulphide, where sulphide stress cracking can fail high-strength steel at stresses well below yield.
Why is hardness limited to about 22 HRC?
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Susceptibility to sulphide stress cracking rises sharply with hardness. The limit keeps carbon and low alloy steels in a resistant condition.
Does the limit apply to welds?
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Yes, to base metal, weld metal and the heat-affected zone, all of which are hardness tested.
Can you supply sour service forgings?
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Yes, to customer specifications referencing MR0175, with heat treatment controlled to the hardness ceiling and results documented.