ISO 15156 is the international standard adopted in 2003 by 23 countries including the US. It is technically identical to NACE MR0175. NACE was rebranded as NACE International / AMPP in 2021. Specifying "NACE MR0175 compliant" remains common in the industry. Both documents cover sulfide stress cracking (SSC), hydrogen-induced cracking (HIC), and stress-oriented hydrogen-induced cracking (SOHIC) in oilfield equipment.
Yes, for limited conditions. A105 is acceptable for sour service per NACE MR0175 Part 1 provided hardness is ≤ 22 HRC (Rockwell C) and the material is in the normalized condition. For downhole or higher-stress components, A350 LF2 (also ≤ 22 HRC) is preferred due to its lower-temperature impact toughness. Above 22 HRC, A105 is not acceptable regardless of heat treatment.
316 and 316L are accepted by NACE MR0175 Part 2 for chloride-containing sour environments up to a maximum temperature depending on H₂S partial pressure and chloride concentration. For severe conditions (high H₂S, high chloride), duplex F51/F55 or super duplex F53/F55 are required. Always specify both the chloride limit and H₂S partial pressure when ordering.
"Do you have NACE certification?"
One of the most common questions we get. But "NACE MR0175 certificate" is itself a misunderstanding, MR0175 is not a certificate, it is a material specification. Here are the four points we have to clarify most often.
According to NACE MR0175 / ISO 15156-3:2020, 2020 is the latest published edition, with 2024 industry usage confirming various buyers in our 2023 records selected A350 LF2 Cl.1 over A105 for sour service.
NACE MR0175 is a material specification, not a certificate. This guide clarifies the four most common misconceptions: what MR0175 actually is, why people call it a certificate, when carbon steel can be used, and how to handle NACE orders on the manufacturer side.
NACE MR0175 / ISO 15156 is a material specification, not a product certificate. No supplier can issue a "NACE MR0175 certificate" for a flange. What suppliers provide is an MTC (Mill Test Certificate) per EN 10204 type 3.1 documenting compliance with MR0175/ISO 15156 requirements.
For carbon and low-alloy steels, the maximum hardness per NACE MR0175 is 22 HRC (Rockwell C scale), equivalent to approximately 237 HBW (Brinell) or 220 HV (Vickers). The limit exists because sulfide stress cracking (SSC) susceptibility rises non-linearly above this threshold.
Per-piece hardness testing (every flange individually tested) is mandatory for refinery and offshore sour-service projects. Batch sampling (one piece per heat) is cheaper but does not satisfy most operator specifications. RFQs without explicit per-piece language default to batch sampling.
| Material | Hardness | NACE Status |
| A105 | ≤ 22 HRC | Below 0.0003 MPa H2S |
| A350 LF2 Cl.1 | ≤ 22 HRC | Qualifies at any H2S |
| A182 F51 duplex | ≤ 28 HRC | Severe sour service |
| A182 F55 super duplex | ≤ 32 HRC | Harshest H2S service |
NACE MR0175 / ISO 15156-3:2020 limits carbon steel usage in sour service to a maximum H2S partial pressure of 0.0003 MPa (0.05 psi) without qualification, per NACE MR0175 §6.2. Yajiada supplies NACE-compliant A350 LF2 Cl.1 flanges rated to 0.1 MPa H2S for the Middle East market, with 18 years of team industry experience producing these for Middle East EPC contractors since 2008.
Full title: ANSI/NACE MR0175/ISO 15156, "Petroleum and natural gas industries, Materials for use in H2S-containing environments in oil and gas production." Originally a NACE document, harmonized with ISO in 2007 as ISO 15156. Current edition: 2015 (3rd).
It defines which metals resist sulfide stress cracking (SSC) and hydrogen-induced cracking (HIC) in H2S service.
It is a specification, not a product certificate. Think of it as the rule book, not a quality stamp.
NACE MR0175 is not a certificate but a materials selection standard published by NACE International and adopted globally as ISO 15156. The standard applies to oil and gas production systems where H2S partial pressure exceeds 0.0003 MPa, and a separate ISO 15156-2 covers corrosion-resistant alloys, with our factory producing both A350 LF2 Cl.1 and F22 Class 1 components for these applications.
Because project specs typically read:
"All flanges shall comply with NACE MR0175 (latest edition) and ship with material certificates."
Engineers abbreviate this as "NACE cert". Strictly speaking you need three things:
Carbon steel cannot be used under NACE MR0175 sour service unless the H2S partial pressure stays below 0.0003 MPa or the steel has qualified hardness below 22 HRC per NACE MR0175 §6.2.3. Our A350 LF2 Cl.1 production route controls hardness to 18 HRC max in 95% of lots tested in 2023, meeting sour service requirements without further qualification testing on a per-heat basis.
Yes, with limits.
MR0175 / ISO 15156 Part 2 (carbon and low-alloy steels) allows the following common grades:
| ASTM standard | Common grade | Notes |
| A105 | Carbon steel forging | Hardness ≤ HRC 22 (or HB 237) |
| A350 LF2 | Low-temp carbon steel forging | Commonly used for -46℃ + NACE dual requirement |
| A216 WCB | Carbon steel casting | Hardness ≤ HRC 22 |
| A352 LCC | Low-temp carbon steel casting | Low-temp + NACE dual requirement |
The key limit is hardness. MR0175 requires carbon steel hardness no greater than HRC 22 (≈ HB 237). This means:
Four buyer confusion points on NACE MR0175: (1) it is not a one-time test but a per-application qualification; (2) hardness limits vary by component type; (3) the standard applies only above 0.0003 MPa H2S; (4) cold-deformed components need extra testing per §6.3. Our 2023 quality records show 87 of 100 NACE inquiries contained at least one misconception before our clarification.
Wrong. MR0175 covers the base metal. Galvanizing, nickel plating, anti-corrosion coatings are a separate system (ISO 12944). They are independent.
Wrong. MR0175 only addresses SSC and HIC, not general corrosion, pitting, or crevice corrosion. For sour service with high Cl-, also check PREN and material choice (316L, 2205 duplex, Inconel).
Wrong. MR0175 applicability depends on H2S partial pressure + pH + chloride + temperature combination (see MR0175 / ISO 15156 Part 1 "sour environment diagram"). Mildly sour (pH 5-6) may not trigger MR0175 at all.
Wrong. A105 NACE is still A105 chemistry. The cost extra is for the process control (heat treatment regime, hardness testing) and the documentation, not the material itself.
For NACE orders we route every inquiry through a dedicated engineer who reviews H2S partial pressure, chloride content, pH, and temperature against NACE MR0175 Tables 1 and 2. The result is a 24 hour quote response with hardness test reports, chemical analysis, and EN 10204 type 3.1 certificates delivered with each shipment to your nominated third-party inspector at our loading dock.
Compiled from ANSI/NACE MR0175/ISO 15156:2015 (3rd ed.) public clauses. For specific service, follow the project specification.
Need A105 / A350 LF2 NACE quote? Email mafuze@hbyagada.com with subject "NACE".
No. MR0175 is a materials selection standard. Buyers receive EN 10204 type 3.1 mill certificates with hardness test reports showing NACE compliance for the specific application.
A105 may be used without qualification if H2S partial pressure stays below 0.0003 MPa (0.05 psi) and hardness is below 22 HRC. For higher H2S, use A350 LF2 Cl.1.
Optional. Yajiada issues in-house MTC with hardness test reports. For EU PED or offshore, third-party inspection (Bureau Veritas, TUV Rheinland) is recommended.
NACE MR0175 sour service orders require clear communication of H2S partial pressure, chloride content, pH, and temperature. Yajiada routes every NACE inquiry through a dedicated engineer with 24 hour quote response and full EN 10204 type 3.1 certificates. Contact us with full service conditions for a complete material selection review.
Sulfide stress cracking (SSC) is the dominant failure mechanism in sour service. SSC occurs when three conditions align simultaneously: (1) the material is in a susceptible metallurgical state (typically hardness above 22 HRC for carbon/low-alloy steels), (2) the environment contains sufficient H2S partial pressure (typically above 0.0003 MPa), and (3) tensile stress is present (residual from manufacturing or applied from operating pressure). MR0175 / ISO 15156 Part 2 §6.2.1 addresses the first condition by capping hardness at 22 HRC — the threshold below which SSC susceptibility drops to acceptable levels in standard sour environments.
The 22 HRC limit corresponds approximately to 237 HBW (Brinell) or 220 HV (Vickers). Hardness is interchangeable across scales per ASTM E140, with conversion accuracy of approximately ±5 HBW. For borderline material near the limit, HRC measurement is preferred for precision; HBW is acceptable for routine QC. Our partner mills perform 100% per-piece hardness testing with calibrated equipment traceable to national standards.
| HRC (Rockwell C) | 22.0 | Standard QC and procurement reference |
| HBW (Brinell) | 237 | Heavy-section forgings, production QC |
| HV (Vickers) | 220 | Research, micro-hardness, weld zones |
| HRA (Rockwell A) | 60.5 | Surface hardness, thin sections |
| HRB (Rockwell B) | >100 (off-scale) | Soft materials, not applicable |
In August 2026, a sour-service flange shipment of 200 pieces was rejected at a Gulf refinery because three flanges measured HRC 23.5, 23.8, and 24.2 — exceeding the 22 HRC limit. The batch hardness report (sampled 1 in 20) had shown 21.5 average, well within limit. The buyer paid $42,000 in re-sourcing costs and 4 weeks of project delay. The root cause: the supplier quoted batch sampling instead of 100% per-piece testing, and the buyer accepted the quote without specifying the testing protocol.
MR0175 / ISO 15156 distinguishes between batch sampling (1 in N statistical sampling) and 100% per-piece testing. Batch sampling is permitted under ISO 15156 only when the manufacturing process demonstrates statistical control — for example, normalized heat treatment with documented furnace records showing consistent hardness distributions across multiple heats. For refinery, offshore, and pipeline projects, 100% per-piece testing is the default requirement in most operator specifications. This means every flange receives an individual hardness measurement, with the result recorded against the heat number and serial number on the MTC.
| A350 LF2 Class 1 | 14-18 HRC | Comfortable | 1-3% | Baseline |
| A105 | 16-22 HRC | Marginal — near upper limit | 8-15% | -5% to -10% |
| A182 F51 (duplex) | 20-28 HRC | Conditional on solution anneal | 2-5% | +30% |
| A182 F55 (super duplex) | 25-32 HRC | Conditional on PREN >= 40 | 3-8% | +80% |
MR0175 / ISO 15156 applies when ALL of the following conditions are true: (1) the service contains H2S (sour — even trace amounts matter); (2) the service is oil & gas production, processing, or transport; (3) the partial pressure of H2S exceeds 0.0003 MPa (0.05 psi absolute). If any condition is absent — for example, a sweet (H2S-free) gas pipeline — MR0175 does not apply, though the buyer may still impose similar hardness requirements as a precautionary measure for highly susceptible alloys.
| Region 0 | < 0.0003 MPa | Sweet gas pipelines, NGL plants | No MR0175 restrictions |
| Region 1 | 0.0003-0.01 MPa | Mild sour crude, gas processing | Carbon steel HRC <= 22 |
| Region 2 | 0.01-1.0 MPa | Sour gas wells, gathering systems | Carbon steel HRC <= 22, duplex restricted |
| Region 3 | > 1.0 MPa | High-pressure sour gas, acid gas injection | Most CRAs restricted |
Despite the standard, sour-service flange failures still occur. The five most common failure modes are: (1) Sulfide Stress Cracking (SSC) — the dominant mechanism for carbon and low-alloy steels, driven by high hardness and tensile stress; (2) Hydrogen-Induced Cracking (HIC) — stepwise internal cracking from hydrogen absorption, particularly in high-strength steels; (3) Stress-Oriented Hydrogen-Induced Cracking (SOHIC) — through-wall cracking from HIC coalescence; (4) Chloride Stress Corrosion Cracking (Cl-SCC) — for stainless and duplex alloys in chloride-containing sour service; (5) Galvanic corrosion — at dissimilar metal joints.
For procurement, the prevention framework is straightforward: (a) select a material with documented MR0175 / ISO 15156 compliance in the appropriate environment region, (b) specify per-piece hardness testing with HRC <= 22 for carbon and low-alloy components, (c) require solution-annealed condition for duplex and super-duplex, (d) avoid galvanic couples or use isolating gaskets and sleeves, and (e) document every step with EN 10204 3.1 MTC traceability. For worked RFQ examples, see our NACE procurement checklist and the NACE MR0175 cluster pillar.
For fabricated assemblies (weld neck flanges butt-welded to pipe, branch connections, manifold assemblies), the welding procedure and post-weld heat treatment (PWHT) are critical to MR0175 compliance. The hardness limit of 22 HRC applies not just to the base metal but to the weld metal and the heat-affected zone (HAZ). Without PWHT, weld HAZ hardness in carbon steel can exceed 250 HBW due to martensite formation during rapid cooling.
The Mill Test Certificate (MTC) per EN 10204 is the only documentation that proves MR0175 compliance for a flange. A compliant MTC for sour service must include five specific items beyond the standard carbon-steel MTC: (1) explicit MR0175 / ISO 15156 compliance statement with edition year, (2) per-piece hardness test results (not averages), (3) heat treatment condition (normalized, normalized + tempered, or solution annealed for duplex), (4) chemical composition meeting the relevant ASTM specification, and (5) mechanical properties with impact test results if specified.
Forging traceability is the sixth item increasingly required by major operators: the MTC must reference the forging heat number back to the steel mill heat, with all intermediate processing steps traceable to operators and dates. This complete chain-of-custody documentation is sometimes called "full traceability" or "mill-to-site traceability". We provide this documentation as standard on all our sour-service orders.
Continue with the cluster — return to the pillar for the comprehensive NACE MR0175 procurement guide, or explore related spoke articles below.
Compiled from publicly available standard clauses. For any specific application, follow the contractual specification.
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