NACE MR0175 / ISO 15156 Explained for Welders and Engineers

By WeldFabWorld June 22, 2026 36 min read
NACE MR0175 / ISO 15156 — Sour Service Standard Explained | WeldFabWorld

NACE MR0175 / ISO 15156 Explained for Welders and Engineers

By WeldFabWorld · Published: July 2025 · Updated: June 2026 · 30 min read

Pressure vessel and piping in sour service oil and gas environment requiring NACE MR0175 ISO 15156 compliance
Fig. 1 — Pressure vessels and piping in sour oil and gas service must comply with NACE MR0175 / ISO 15156 for material selection, welding, and hardness control.

Key Takeaways

  • NACE MR0175 and ISO 15156 are the same standard — NACE MR0175 was adopted by ISO in 2003 and redesignated ISO 15156.
  • The standard applies when H2S partial pressure in the gas phase exceeds 0.05 psia (0.0003 MPa) — defining “sour service.”
  • Maximum hardness for carbon and low-alloy steels is 22 HRC (248 HV) in base metal, weld metal, and HAZ.
  • PWHT is the primary means of reducing HAZ hardness to within this limit after welding.
  • Welding procedure qualifications must include a hardness survey of the weld cross-section — this applies to both carbon steel and CRA alloys.
  • HIC resistance is qualified by NACE TM0284; SSC resistance is qualified by NACE TM0177.
  • The standard is published in three parts: Part 1 (General), Part 2 (Carbon and Low-Alloy Steels), Part 3 (CRAs and other alloys).

NACE MR0175 / ISO 15156 is the international standard that governs material selection, qualification, and fabrication requirements for metallic components used in hydrogen sulfide (H2S) containing environments in oil and gas production. If you are a welder, welding inspector, CWI, fabrication engineer, or QA/QC professional working on oil and gas, petrochemical, or refinery equipment, understanding this standard is not optional — it defines the specific controls on material chemistry, heat treatment, hardness, welding procedures, and testing that must be applied whenever H2S is present in the process fluid.

The term “sour service” describes any operating environment where H2S is present above the defined threshold. H2S is highly corrosive, extremely toxic, and drives two distinct cracking mechanisms in steel — Sulfide Stress Cracking (SSC) and Hydrogen-Induced Cracking (HIC) — both of which can cause sudden, catastrophic failure with no visible warning. These failure modes have caused major accidents in the oil and gas industry, which is why material and fabrication controls under NACE MR0175 / ISO 15156 are non-negotiable in sour service design.

This article explains every key element of NACE MR0175 / ISO 15156 that a welder or engineer needs to understand: what the standard requires, why each requirement exists metallurgically, what the hardness limits mean in practice, how welding procedures must be qualified, and what the material testing requirements are. It expands substantially on the WeldFabWorld sour service overview to give you the full technical depth the standard demands.

Contents

  1. Background and History of the Standard
  2. The Three Parts of NACE MR0175 / ISO 15156
  3. Defining Sour Service — The Threshold Conditions
  4. Cracking Mechanisms: SSC, HIC, SOHIC, and SZC
  5. Part 2: Carbon and Low-Alloy Steel Requirements
  6. Hardness Limits and Why They Matter
  7. Welding Requirements Under NACE MR0175
  8. PWHT for Sour Service Fabrication
  9. Part 3: Corrosion-Resistant Alloys (CRAs)
  10. Material Testing: TM0284 and TM0177
  11. Practical Material Selection Guide
  12. Recommended References
  13. Frequently Asked Questions

Background and History of the Standard

The history of NACE MR0175 begins in 1975, when the National Association of Corrosion Engineers (now AMPP — Association for Materials Protection and Performance) published the first edition in response to catastrophic failures of oil and gas equipment caused by sulfide stress cracking. These failures occurred in high-strength steel components — wellheads, valves, tubing, and pressure vessels — that had been designed to meet mechanical strength requirements but not sour service cracking resistance requirements.

From 1975 through multiple revisions, NACE MR0175 evolved into the primary industry reference for sour service material qualification. In 2003, the International Organization for Standardization (ISO) adopted the standard and published it as ISO 15156, creating a three-part structure that has been maintained through subsequent revisions. The most significant revision was the 2015 edition, which introduced a more nuanced environmental severity framework based on H2S partial pressure, pH, and temperature rather than a single threshold criterion.

NACE MR0175 vs ISO 15156 — Are They the Same? Yes — NACE MR0175 and ISO 15156 are the same standard published by two organisations. The current version is NACE MR0175 / ISO 15156:2015. In practice, project specifications may reference either designation; both refer to the same requirements. The standard is now maintained jointly by AMPP (formerly NACE) and ISO, with the 2015 text being the current active edition.

The Three Parts of NACE MR0175 / ISO 15156

Part 1
General Principles, Cracking Mechanisms, and Laboratory Test Methods
Defines sour service conditions, cracking mechanism terminology, general requirements, environmental severity levels, and the framework for material qualification. Applies to all material types.
Part 2
Cracking-Resistant Carbon and Low-Alloy Steels, and Use of Cast Irons
Covers carbon steel, low-alloy steel (including Cr-Mo grades), and cast iron. Defines hardness limits, heat treatment requirements, weld procedure qualification, and HIC/SSC testing requirements for these materials.
Part 3
Cracking-Resistant Corrosion-Resistant Alloys (CRAs) and Other Alloys
Covers austenitic and duplex stainless steels, nickel-based alloys, titanium alloys, and other CRAs. Provides H2S threshold limits, temperature and chloride constraints, and welding requirements for each alloy family.

Understanding which part applies to your material is the first step. For most pressure vessel and piping fabrication in carbon steel or low-alloy steel, Part 2 is the operative document. For equipment using stainless steel or nickel alloy components — heat exchangers, valve trim, instrumentation, CRA-clad vessels — Part 3 applies. Part 1 provides the foundation that governs both.

NACE MR0175 / ISO 15156 — Standard Structure PART 1 — General Principles & Laboratory Test Methods Sour service definitions  |  Cracking mechanism terminology  |  Environmental severity levels (Regions 0–3) General qualification requirements  |  Test method references (TM0284, TM0177) PART 2 Carbon & Low-Alloy Steels Max 22 HRC hardness PWHT required for welds HIC + SSC testing PART 3 Corrosion-Resistant Alloys Austenitic SS, Duplex SS Nickel alloys, Titanium H2S threshold tables Material scope: CS & LAS — P1, P4, P5 Material scope: SS, Ni alloys, CRAs Both Part 2 and Part 3 rely on Part 1 for general requirements, test methods, and sour service definitions
Fig. 2 — Structure of NACE MR0175 / ISO 15156. Part 1 provides the common foundation; Part 2 covers carbon and low-alloy steels; Part 3 covers corrosion-resistant alloys. All three parts apply together.

Defining Sour Service — The Threshold Conditions

Not every environment containing H2S requires full NACE MR0175 compliance. The standard defines “sour service” based on specific threshold conditions. Understanding this threshold is essential for engineers deciding whether NACE MR0175 applies to a given system.

The Classical 0.05 psia Threshold

Historically, the threshold most commonly referenced in the industry is an H2S partial pressure in the gas phase exceeding 0.05 psia (0.0003 MPa absolute). Below this partial pressure, the risk of SSC and HIC is considered negligible for carbon steel at typical yield strengths, and NACE MR0175 does not apply. Above it, sour service controls are required. This threshold remains the most commonly cited in project specifications and engineering standards that reference NACE MR0175.

The 2015 Revision — Environmental Severity Regions

The 2015 revision of ISO 15156 introduced a more rigorous framework for determining environmental severity, based on the combination of H2S partial pressure, in-situ pH, and temperature. This system defines four severity regions — Region 0 through Region 3 — where Region 0 represents conditions below the sour threshold (no NACE requirements apply) and Region 3 represents the most severe sour conditions, requiring the most stringent material and hardness controls.

Environmental Severity Determination (Simplified) H2S partial pressure (pH2S) = H2S mole fraction x total system pressure Units: pH2S in psia or kPa absolute Threshold for sour service (classical criterion): pH2S > 0.05 psia (0.0003 MPa) = sour service applies pH2S ≤ 0.05 psia = sweet service — NACE MR0175 not required Example: Natural gas stream at 70 bar total pressure containing 500 ppm H2S pH2S = 500 / 1,000,000 x 70 bar = 0.035 bar = 0.507 psia Result: 0.507 psia > 0.05 psia threshold => SOUR SERVICE — NACE MR0175 applies
Important: Always Use Actual Operating Conditions, Not Design Conditions The partial pressure calculation must be based on actual reservoir or stream composition at in-situ conditions, not design basis or nominal values. Gas chromatograph analysis of actual well fluids should be used. If any operating scenario — including upsets, start-up, or emergency cases — results in sour conditions, NACE MR0175 requirements apply to the entire system.

Cracking Mechanisms: SSC, HIC, SOHIC, and SZC

NACE MR0175 / ISO 15156 is designed to prevent specific cracking damage mechanisms that occur in steel exposed to H2S environments. Understanding each mechanism is essential for understanding why each requirement in the standard exists.

Sour Service Cracking Mechanisms — Schematic Cross-Section MnS Inclusions (elongated by rolling) HIC Hydrogen pressure builds at inclusion SOHIC Weld Metal HAZ HAZ SSC Tensile stress + high hardness HAZ SZC Soft zone cracking at HAZ / base edge H+ atoms from H2S corrosion SSC Sulfide Stress Cracking — tensile stress + H in high-hardness steel HIC Hydrogen-Induced Cracking — H pressure at inclusions, no stress required SOHIC Stress-Oriented HIC — HIC steps linked by stress into through-wall crack SZC Soft Zone Cracking — cracking in under-matched soft zone at edge of HAZ
Fig. 3 — Sour service cracking mechanisms shown in a schematic plate cross-section. SSC occurs in high-hardness HAZ under tensile stress; HIC occurs at elongated inclusions with no applied stress required; SOHIC links HIC steps into a through-wall path; SZC occurs in the soft zone at the outer edge of the HAZ.

Sulfide Stress Cracking (SSC)

SSC is a form of hydrogen embrittlement and stress corrosion cracking in which atomic hydrogen — generated by the cathodic reaction of H2S corrosion on the steel surface — diffuses into the steel microstructure and causes brittle cracking under the combined action of tensile stress (applied or residual) and susceptible microstructure. SSC is most severe in high-hardness zones: the weld heat-affected zone (HAZ) in as-welded condition is the primary site because martensitic transformation produces high local hardness, and residual welding stresses are tensile. SSC requires both tensile stress and absorbed hydrogen — eliminating either prevents SSC. This is why NACE MR0175 targets both hardness (which controls susceptibility) and stress relief (PWHT, which reduces residual stress).

Hydrogen-Induced Cracking (HIC)

HIC does not require applied tensile stress. It occurs when atomic hydrogen diffuses into the steel and accumulates at traps — primarily elongated non-metallic inclusions such as manganese sulphide (MnS) stringers and elongated silicate inclusions formed during rolling. Hydrogen atoms recombine to molecular H2 at these sites, generating internal gas pressure that eventually exceeds the local fracture stress of the steel. Internal “blister” cracks form parallel to the plate surface. HIC is controlled by specifying low-sulphur content steel (typically S ≤ 0.002% for HIC-resistant grades), calcium treatment to modify MnS inclusion morphology into globular form, and HIC testing per NACE TM0284 on the actual plate heat.

Stress-Oriented HIC (SOHIC)

SOHIC is a more severe damage mode in which individual HIC crack “steps” are linked by small through-thickness cracks driven by stress concentration at the tips of HIC damage. The result is a staircase crack path that can propagate through the full wall thickness of the pressure vessel or pipe. SOHIC requires both the microstructural conditions that allow HIC and tensile stress (from applied loading, residual stress, or stress concentration). It is most common in the HAZ region of welds in plate material. The standard’s advice to consider SOHIC when evaluating carbon steels and welded products is contained in Part 2, Annex B.

Soft Zone Cracking (SZC)

SZC occurs in a narrow, low-hardness band at the outer edge of the HAZ of welds in quenched and tempered (Q&T) or thermomechanically processed steels. In this zone, the base material’s original tempered microstructure is softened by the weld thermal cycle, creating a region of under-matched yield strength. Under applied and residual stress in the presence of H2S, this soft zone can crack. SZC is most relevant for high-strength pipeline steels (X65 and above) and Q&T pressure vessel steels.

Part 2: Carbon and Low-Alloy Steel Requirements

Part 2 of NACE MR0175 / ISO 15156 covers the most commonly used materials in oil and gas fabrication: carbon steels and low-alloy steels. It defines which grades are acceptable, what conditions they must meet, and what controls must be applied during fabrication.

Acceptable Material Conditions

Part 2 lists specific ASTM material specifications and delivery conditions that are acceptable for sour service. The key material conditions acceptable under Part 2 include:

Material / Grade ASTM Spec Acceptable Condition Hardness Limit Notes
Carbon steel plate A516 Gr. 60, 65, 70 Normalized or N&T; HIC-tested 22 HRC max Must specify HIC per TM0284. Low S (≤0.002%) required for HIC-resistant grade.
Carbon steel pipe A106 Gr. B, API 5L Seamless; as-produced or N 22 HRC max Sulphur content per purchase order. HIC test per project spec.
Carbon steel forgings A105 As-forged (NPS ≤ 4 Cl. 300) or N/Q&T 22 HRC / 187 HBW max Specific exemption from heat treatment for small forgings per Table A.1 of Part 2.
Cr-Mo alloy steel A335 P11, P22 Normalized and tempered 22 HRC max Subject to concentration limits and temperature range per Part 2 tables.
P91 / Grade 91 A335 P91 Normalized and tempered 22 HRC max Restricted by temperature and partial pressure conditions. See Part 2 Table A.2.
High-strength steel API 5L X52–X65 TMCP or Q&T 22 HRC max SOHIC risk increases with strength grade. HIC and SSC testing required.
The 22 HRC Hardness Limit is Absolute The 22 HRC maximum applies everywhere — base metal, weld deposit, HAZ, and heat-affected region. There are no exceptions to this limit for carbon and low-alloy steel in sour service. A single hardness reading above 22 HRC anywhere in the weld cross-section is a non-conformance that must be addressed by additional PWHT before the weld can be accepted for sour service use.

Hardness Limits and Why They Matter

The 22 HRC hardness limit (248 HV, 237 HBW) is the most frequently cited and most practically important requirement in NACE MR0175 / ISO 15156 Part 2. Understanding why this number was chosen and what it means in practice is essential for every welder and fabricator working in sour service.

Metallurgical Basis for 22 HRC

The susceptibility of carbon steel to SSC increases dramatically with hardness — or more precisely, with yield strength, since hardness correlates strongly with yield strength. The 22 HRC limit corresponds approximately to a yield strength of 620 MPa (90 ksi). At and below this yield strength, carbon and low-alloy steels are generally considered to have adequate resistance to SSC under typical sour service conditions without requiring any special metallurgical treatment. Above this hardness, the risk of SSC rises sharply, and even brief exposure to sour conditions can cause rapid crack initiation and propagation.

The HAZ of a weld in carbon steel commonly reaches hardness values of 30–45 HRC in as-welded condition due to martensitic transformation during rapid cooling from the weld thermal cycle. This is exactly where SSC initiates in inadequately heat-treated sour service welds. PWHT — by tempering the martensite and relieving residual stresses — reduces this HAZ hardness to below 22 HRC, making the weld suitable for sour service.

Hardness Equivalents — NACE MR0175 Part 2 Limit 22 HRC = 248 HV = 237 HBW = approximately 620 MPa yield strength Hardness conversion context: HRC (Rockwell C) — preferred scale for NACE hardness surveys on cross-sections HV (Vickers) — required for weld cross-section surveys per ISO 15156 (HV10 or HV5) HBW (Brinell) — used for bulk component acceptance (e.g., A105 flanges: 187 HBW max) Typical as-welded HAZ hardness (before PWHT): Carbon steel (A516-70): HAZ = 300–380 HV = 31–40 HRC — EXCEEDS NACE LIMIT After PWHT at 620 degC / 1 hr per 25mm: HAZ = 180–210 HV = ~15–19 HRC Post-PWHT result: below 22 HRC / 248 HV — NACE COMPLIANT

Hardness Test Locations and Survey Requirements

NACE MR0175 / ISO 15156 specifies that hardness testing of the weld cross-section must be performed using Vickers HV10 or HV5 (or Rockwell 15N, converted to HV), with specific indentation patterns covering the weld metal, fusion line, HAZ, and base metal. The exact indentation locations are shown in Annex A of Part 2 and depend on whether PWHT was performed:

  • Without PWHT: Three rows of Vickers indentations — one at mid-thickness, one near each surface — with indentations at 1 mm intervals through the HAZ.
  • With PWHT: Reduced survey required, but must still include the HAZ and confirm all readings are below 248 HV (22 HRC).
Inspector Tip — Hardness Survey Documentation Always record the full hardness survey map, not just the highest and lowest values. The survey map should show every individual indentation result against its location in the weld cross-section. Regulators and TPI agencies have rejected NACE-qualified welds where the documentation showed only a pass/fail summary without the individual location-referenced data. The mechanical testing guide on WeldFabWorld covers Vickers hardness test procedure in detail.

Welding Requirements Under NACE MR0175

NACE MR0175 / ISO 15156 Part 2 imposes specific requirements on welding procedure qualification for sour service. These requirements go beyond what ASME Section IX or AWS D1.1 require for standard weld procedure qualification, and must be incorporated into the WPS/PQR documentation for any sour service application.

Welding Procedure Qualification — Additional Requirements

  1. Hardness survey included in PQR: Every weld procedure qualification test for sour service must include a full hardness survey of the production weld cross-section test coupon. The survey must be performed after all PWHT has been applied. Results must be documented in the PQR. This requirement applies to all material groups including austenitic stainless steels.
  2. Low-hydrogen consumables mandatory: For carbon and low-alloy steel sour service welding, only low-hydrogen welding consumables may be used. For SMAW, this means hydrogen-controlled (H4 or H8 designation) electrodes stored and dried per the manufacturer’s requirements. High-hydrogen rutile electrodes are not permitted for sour service root or fill passes on carbon steel.
  3. Preheat and interpass temperature controls: Preheat must be calculated from the Carbon Equivalent (CE) and applied before any welding begins. Maximum interpass temperature must be controlled to prevent excessive softening of previously deposited beads. Both minimum preheat and maximum interpass temperature must be specified in the WPS and verified during welding.
  4. PWHT parameters in WPS: Where PWHT is required, the WPS must specify the exact PWHT temperature range, minimum hold time per unit thickness, heating and cooling rates, and the extent of coverage. PWHT must be performed before hardness testing. The furnace chart must be retained as part of the quality record.
  5. Filler material restrictions: Some filler material classifications are specifically excluded from sour service use because their deposited weld metal chemistry results in hardness above 22 HRC even after PWHT. The WPS must specify a filler material type that has been demonstrated to produce compliant hardness after the specified PWHT.
Code Reference — NACE MR0175 / ISO 15156, Part 2, Section A.2.3 Section A.2.3 of Part 2 states that welding processes and consumables shall be selected in accordance with good practice and to achieve the required results, including hardness limits. All procedure qualifications are required to include hardness surveys per the patterns described in Annex A, whether or not PWHT is performed. This requirement extends to all weld procedure qualifications including repair welds and tack welds that will remain in the production weld.

Welding Process Considerations

Welding Process Sour Service Use Key Requirement Hydrogen Risk
SMAW (Stick) Acceptable Must use low-H electrodes (E7018-H4R or H8R). Proper storage & drying mandatory. High risk if rutile electrodes used. Controlled with correct electrode type.
GTAW / TIG Preferred Inherently low hydrogen. Ideal for root passes. Inert gas shielding essential. Very low — process does not introduce hydrogen.
GMAW / MIG Acceptable Solid wire (not flux-cored) preferred. Short-circuit transfer caution — unmelted flux-cored hydrogen possible. Low with solid wire. Moderate with flux-cored wire.
SAW Acceptable Dry flux mandatory. Flux moisture specification must be confirmed from MTC. Flux storage humidity control required. Low with properly dried flux. High risk with wet flux — must be controlled.
FCAW (Flux-Cored) Use with care Only self-shielded FCAW where specifically qualified. Gas-shielded FCAW preferred with controlled wire hydrogen designation. Moderate — flux core ingredients contribute hydrogen. Strict moisture control required.

PWHT for Sour Service Fabrication

Post-Weld Heat Treatment is the single most critical fabrication step for ensuring carbon steel sour service welds comply with NACE MR0175. PWHT serves two functions simultaneously: it tempers the martensitic HAZ to reduce hardness below 22 HRC, and it reduces residual welding stresses that would otherwise contribute to SSC driving force.

PWHT Parameters for Common P-Number Groups

P-Number Group Material Min PWHT Temp Hold Time Cooling
P-No. 1 Carbon steel (A516-70, A106) 595°C (1100°F) 1 hr / 25mm min (1 hr min) Air or controlled furnace
P-No. 4 P11 (1.25Cr-0.5Mo) 677°C (1250°F) 1 hr / 25mm (30 min min) Controlled cooling < 315°C/hr
P-No. 5A P22 (2.25Cr-1Mo) 677°C (1250°F) 1 hr / 25mm (30 min min) Controlled cooling < 315°C/hr
P-No. 5B Gr. 1 P91 (9Cr-1Mo-V) 730°C (1345°F) 1 hr / 25mm (2 hr min) Controlled < 85°C/hr above 425°C

PWHT parameters shown above are per ASME Section IX / ASME B31.3 minimum requirements. The WPS may specify higher temperatures or longer hold times if hardness survey results from procedure qualification indicate this is necessary to consistently achieve the 22 HRC maximum. For P91 alloy steel, PWHT temperature selection is particularly critical — insufficient PWHT temperature fails to adequately temper the martensite, while excessive temperature risks overtempering and reduction of creep strength.

Local PWHT (Induction or Flame) for Sour Service Where furnace PWHT of the complete vessel or spool is not practicable, local PWHT using induction heating, electrical resistance heating, or controlled flame heating may be used. For sour service, local PWHT must be qualified under the same PQR requirements and must demonstrate by hardness survey that the entire HAZ cross-section is below 22 HRC / 248 HV. The heated band width must be sufficient to ensure the HAZ is fully within the soak zone. API 582 and ASME Section I / VIII provide guidance on local PWHT band widths.

Part 3: Corrosion-Resistant Alloys (CRAs)

NACE MR0175 / ISO 15156 Part 3 covers the qualification and use requirements for corrosion-resistant alloys in sour service. Unlike Part 2, which primarily controls hardness, Part 3 controls material selection through H2S threshold tables that specify the maximum allowable H2S partial pressure, temperature, chloride content, and pH for each alloy family.

Austenitic Stainless Steels (304, 316, 321, 347)

Austenitic stainless steels in the solution-annealed condition (no cold work, hardness below 22 HRC) are generally acceptable in sour service under Part 3 subject to chloride and temperature constraints. The primary concern for austenitic SS in sour service is Stress Corrosion Cracking (SCC) driven by chlorides at elevated temperature — not SSC in the same sense as carbon steel. Part 3 Table A.2 provides the temperature-chloride-H2S combinations within which each austenitic grade is acceptable. For sensitisation prevention, L-grade or stabilised grades (321, 347) are specified where welding in corrosive service is required.

Duplex Stainless Steels

Duplex stainless steels are frequently used in sour service because their high yield strength, chloride SCC resistance, and generally better SSC resistance compared to austenitic grades at equivalent strength levels. Part 3 permits duplex grades (2205, 2507, 2304) within specific H2S partial pressure, temperature, and chloride limits. The HAZ hardness for duplex SS is limited to a maximum of 36 HRC / 350 HV under Part 3. The ferrite-austenite balance must be maintained within the range 30–65% ferrite to avoid embrittlement. The PREN (Pitting Resistance Equivalent Number) must be verified from the material certificate to confirm adequate pitting resistance.

Nickel-Based Alloys

Nickel-based alloys such as Alloy 625 (UNS N06625), Alloy C-276 (UNS N10276), and Alloy 825 (UNS N08825) offer high resistance to SSC and general corrosion in aggressive sour environments. Part 3 provides H2S threshold limits and temperature-chloride envelopes for each alloy. These alloys are typically used in the most severe sour service conditions — deepwater wells, high-chloride formations, and systems with high CO2 partial pressure in addition to H2S. For valve internals and wellhead components in severe sour service, nickel alloys are often the only acceptable choice.

Material Testing: TM0284 and TM0177

Two NACE test methods are fundamental to qualifying materials for sour service. Both are referenced in NACE MR0175 / ISO 15156 and are required by project specifications when the standard is invoked.

NACE TM0284 — HIC Testing

TM0284 (Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-Induced Cracking) tests plates and pipes for their resistance to HIC by immersing unstressed specimens in a NACE test solution containing H2S for 96 hours. After exposure, specimens are cross-sectioned at multiple locations and metallographically examined for internal cracking. Results are quantified as:

TM0284 HIC Acceptance Criteria (typical project requirements) CLR (Crack Length Ratio) = (sum of crack lengths) / (width of specimen) x 100% CTR (Crack Thickness Ratio) = (sum of crack thicknesses) / (specimen thickness) x 100% CSR (Crack Sensitivity Ratio) = (CLR x CTR) / 100 Typical acceptance limits (per NACE TM0284 and most project specs): CLR ≤ 15% CTR ≤ 5% CSR ≤ 2% Any individual section exceeding these limits = FAIL — heat does not qualify for sour service HIC resistance

HIC testing must be performed on a heat-specific basis — each plate heat must be tested, not just the generic grade. The HIC test results must be reported on the Material Test Certificate and are part of the material acceptance criteria at goods receipt.

NACE TM0177 — SSC Testing

TM0177 (Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H2S Environments) qualifies materials for SSC resistance by applying a specified tensile or bending stress to the test specimen while immersed in H2S test solution. Multiple test methods are defined in TM0177 (Methods A through D), with Method A (smooth tensile bar) being the most commonly used for base material qualification at a defined percentage of yield strength (typically 80% SMYS) for a specified test duration (typically 720 hours).

Practical Material Selection Guide

The following table summarises the practical material selection considerations for common sour service applications, combining the requirements of both Part 2 and Part 3.

Application Typical Material Key NACE Requirement Critical Check
Pressure vessel shell (mild sour) A516-70 HIC-tested Part 2 HIC per TM0284; ≤22 HRC base + HAZ after PWHT S ≤ 0.002%, Ca-treated, TM0284 on MTC
Process piping (sour gas) A106 Gr. B or API 5L X52 Part 2 ≤22 HRC; PWHT on all welds; low-H consumables Hardness survey on all production welds
Flanges and forgings A105 (small bore) / A694 (line pipe) Part 2 187 HBW max for A105; N or Q&T for larger sizes Verify delivery condition on MTC
Heat exchanger tubes (moderate sour) TP316L, TP321 (SA-213) Part 3 Solution-annealed; chloride and temperature limits Confirm temperature-chloride within Part 3 envelope
Valve body / trim (severe sour) Alloy 625, C-276 Part 3 Within H2S threshold for the alloy per Part 3 tables Verify alloy designation and hardness on MTC
Offshore riser / flowline (H2S + CO2) Duplex 2205 or CRA clad Part 3 ≤350 HV HAZ; ferrite 30–65%; PREN verification PREN from actual MTC chemistry; ferrite number test
CRA cladding on CS vessel Alloy 825 or 625 overlay weld Part 2 + Part 3 CS substrate per Part 2; overlay per Part 3 Hardness survey of CS HAZ; overlay chemistry by PMI

Recommended References

NACE MR0175 / ISO 15156 — Materials for H2S Environments
The standard itself. All three parts covering general principles, carbon steels, and CRAs for sour service in oil and gas production. The definitive technical reference for every sour service project.
View on Amazon
Hydrogen Embrittlement: Prevention and Control
Detailed coverage of hydrogen damage mechanisms in steels — HIC, SSC, SOHIC — including the metallurgical basis, test methods, and engineering controls. Essential background reading for NACE MR0175 compliance.
View on Amazon
Corrosion Engineering — Fontana
The classic reference on corrosion mechanisms including stress corrosion cracking, hydrogen embrittlement, and galvanic corrosion. Provides the metallurgical foundation for understanding NACE MR0175 requirements.
View on Amazon
Oil and Gas Pipeline Design, Maintenance and Repair
Practical engineering reference covering sour service pipeline design, material selection per NACE MR0175, and inspection and repair procedures for pipelines in H2S environments.
View on Amazon

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Frequently Asked Questions

What is NACE MR0175 / ISO 15156 and what does it cover?
NACE MR0175 / ISO 15156 is the international standard governing material selection and qualification for metallic components used in hydrogen sulfide (H2S) containing environments in oil and gas production. It is published in three parts: Part 1 covers general principles and test methods; Part 2 covers carbon and low-alloy steels; Part 3 covers corrosion-resistant alloys. The standard defines acceptable materials, hardness limits, heat treatment requirements, welding procedure qualification requirements, and testing procedures to prevent sulfide stress cracking (SSC), hydrogen-induced cracking (HIC), and related damage mechanisms. For a broader overview of sour service engineering requirements, see the WeldFabWorld sour service guide.
What is the maximum hardness allowed under NACE MR0175 for carbon steel in sour service?
For carbon and low-alloy steels, NACE MR0175 / ISO 15156 Part 2 limits the maximum hardness to 22 HRC (248 HV or 237 HBW) for the base material, weld metal, and heat-affected zone (HAZ). This limit applies everywhere — no exceptions. Post-weld heat treatment (PWHT) is the primary means of reducing HAZ hardness to within this limit after welding. For flanges and forgings such as ASTM A105, the standard allows the as-forged condition provided the 187 HBW (equivalent to 22 HRC) maximum is met. Any component exceeding this hardness limit does not comply with NACE MR0175 and cannot be used in sour service without further heat treatment or formal disposition.
What is the difference between SSC and HIC in sour service?
Sulfide Stress Cracking (SSC) is caused by the combined action of tensile stress and atomic hydrogen absorbed from H2S corrosion in high-hardness steels. It occurs preferentially in the weld HAZ and in high-strength steels under yield-level stresses. Hydrogen-Induced Cracking (HIC) is an internal cracking mechanism where atomic hydrogen accumulates at non-metallic inclusions (MnS stringers), creating internal gas pressure that fractures the surrounding metal. HIC does not require applied stress — it is driven by hydrogen pressure and occurs in high-sulphur steels with elongated inclusion morphology. SOHIC (Stress-Oriented HIC) is a variant where stress aligns HIC cracks into a through-wall crack path, creating a more serious failure risk than isolated HIC blisters. Controlling hardness prevents SSC; using low-sulphur, calcium-treated steel and HIC testing per TM0284 prevents HIC.
What welding procedure requirements does NACE MR0175 / ISO 15156 impose?
NACE MR0175 / ISO 15156 requires weld procedure qualifications to include a full hardness survey of the weld cross-section (weld metal, fusion line, HAZ, and base metal) using HV10 or HV5 Vickers testing per the indentation patterns in Part 2 Annex A. PWHT is required for most carbon and low-alloy steel welds to reduce HAZ hardness below 22 HRC / 248 HV. Low-hydrogen welding consumables are mandatory. Preheat (from Carbon Equivalent) and interpass temperature controls must be specified in the WPS. All hardness results must be documented in the PQR. This requirement applies to both carbon steel and CRA alloy welds — including austenitic stainless steels, per the 2015 revision.
Does NACE MR0175 apply to stainless steel?
Yes. NACE MR0175 / ISO 15156 Part 3 covers corrosion-resistant alloys including austenitic stainless steels (304, 316, 321, 347), duplex stainless steels (2205, 2507), ferritic and martensitic stainless steels, nickel-based alloys, and titanium alloys. For austenitic stainless steels in solution-annealed condition, the standard generally permits use in sour service without hardness restrictions, but places limits on temperature, chloride content, and H2S partial pressure. Duplex stainless steels are permitted with a HAZ hardness limit of 350 HV maximum and requirements for maintaining correct ferrite-austenite balance. CRA weld procedure qualifications must also include hardness surveys per the standard.
What is the sour service threshold — at what H2S level does NACE MR0175 apply?
The classical industry threshold is an H2S partial pressure in the gas phase exceeding 0.05 psia (0.0003 MPa absolute). Below this, the risk of SSC and HIC is considered negligible for carbon steel at typical yield strengths, and NACE MR0175 does not apply. The 2015 revision introduced a more nuanced environmental severity framework with Regions 0 through 3 based on H2S partial pressure, in-situ pH, and temperature. Region 0 is below the sour threshold; Regions 1 through 3 represent increasing severity requiring progressively more stringent controls. Partial pressure is calculated as the H2S mole fraction multiplied by the total system pressure and must account for actual operating and upset conditions — not just design basis.
What tests are required to qualify materials for sour service under NACE MR0175?
Two primary test methods qualify materials for sour service: NACE TM0284 for HIC resistance (immersion of unstressed specimens in H2S test solution for 96 hours, followed by metallographic cross-section examination with CLR ≤ 15%, CTR ≤ 5%, and CSR ≤ 2% acceptance criteria), and NACE TM0177 for SSC resistance (stressed specimens in H2S test solution, typically at 80% of SMYS for 720 hours). HIC testing must be performed on a heat-specific basis with results on the material test certificate. Hardness testing of weld cross-sections per Annex A of Part 2 is also mandatory for all welding procedure qualifications. See the WeldFabWorld MTC guide for how to verify these test results on the material certificate.
What is PWHT temperature and hold time for carbon steel in sour service?
For P-No. 1 carbon steels (A516-70, A106) in sour service, PWHT per ASME Section IX / B31.3 requirements is 595°C (1100°F) minimum, with a hold time of 1 hour per 25 mm of weld thickness (1 hour minimum). The PWHT must be sufficient to reduce HAZ hardness to 22 HRC / 248 HV maximum throughout the weld cross-section, confirmed by a full hardness survey after heat treatment. For P-No. 4 and P-No. 5 chrome-moly alloys, higher PWHT temperatures apply (677°C minimum). Exceeding the maximum interpass temperature during welding or inadequate PWHT temperature and time are the two most common causes of HAZ hardness failures in sour service fabrication. The P-Number guide on WeldFabWorld explains how material P-Numbers determine the applicable PWHT parameters.

Further Reading on WeldFabWorld

Sour Service Overview Introduction to sour service engineering, HIC and SSC fundamentals, and the role of NACE MR0175 in pressure equipment design. How to Read an MTC Field-by-field guide to interpreting every section of a Mill Test Certificate — including HIC test results, NACE compliance fields, and hardness values. Carbon Equivalent Calculator Calculate CE and Pcm from MTC chemistry to determine preheat requirements for sour service carbon steel welds. Duplex Stainless Steel Guide Complete guide to duplex and super-duplex SS chemistry, PREN, ferrite balance, and NACE Part 3 application criteria for sour environments. ASTM G48 Corrosion Testing Ferric chloride pitting and crevice corrosion testing for stainless and nickel alloys — often specified alongside NACE testing for CRA selection. P91 Material Requirements Welding, PWHT, and inspection requirements for Grade 91 chrome-moly steel — including hardness control for sour service applications. Mechanical Testing Guide Vickers hardness testing, Charpy impact testing, and tensile testing methods used to verify NACE MR0175 compliance. PREN Calculator Calculate Pitting Resistance Equivalent Number from MTC chemistry to verify CRA suitability for sour and chloride-containing environments.