NACE MR0175 Compliant Materials List for Fasteners: Selection Guide

Introduction to NACE MR0175/ISO 15156

NACE MR0175/ISO 15156 is the international standard for materials used in H₂S-containing environments in oil and gas production, refining, and chemical processing. It provides requirements and recommendations for the selection and qualification of metallic materials to resist sulfide stress cracking (SSC) and stress corrosion cracking (SCC). For fasteners, compliance is critical to prevent catastrophic failures in sour service.

LOKRON supplies a range of fasteners certified to NACE MR0175, with full traceability and documentation. This article provides a detailed list of compliant materials, hardness requirements, and selection guidance.

List of Compliant Materials for Fasteners

The standard categorizes materials into groups based on composition and heat treatment. Below is a list of commonly used fastener materials that are acceptable under NACE MR0175/ISO 15156 for use in sour environments.

Carbon and Low-Alloy Steels

Carbon and low-alloy steels are widely used for stud bolts and nuts in less severe sour service. They must be heat-treated to achieve a maximum hardness of HRC 22 (or 248 HV) and meet specified mechanical properties.

• ASTM A193 Grade B7 (AISI 4140/4142) – Quenched and tempered, max HRC 22. Commonly used for general sour service up to certain H₂S partial pressures.
• ASTM A193 Grade B7M – Modified B7 with lower hardness (max HRC 22) specifically for sour service. Often supplied with additional testing.
• ASTM A320 Grade L7 (AISI 4140/4142) – Low-temperature carbon steel, quenched and tempered, max HRC 22. Suitable for low-temperature sour service.
• ASTM A194 Grade 2H – Heavy hex nuts, quenched and tempered, max HRC 22 (or 248 HV). Commonly paired with B7M studs.

Stainless Steels

Stainless steels offer improved corrosion resistance and are used in more aggressive sour environments. However, they must be in the solution-annealed condition and free from sensitization.

• ASTM A193 Grade B8 (Type 304) – Solution-annealed, max HRC 22. Suitable for low H₂S concentrations.
• ASTM A193 Grade B8M (Type 316) – Solution-annealed, max HRC 22. Better pitting resistance than 304.
• ASTM A193 Grade B8 Class 2 (Type 304) – Strain-hardened, but hardness must be controlled to max HRC 22 for sour service.
• ASTM A193 Grade B8M Class 2 (Type 316) – Strain-hardened, max HRC 22.
• Duplex stainless steels (e.g., UNS S31803, S32205) – Acceptable in solution-annealed condition with hardness ≤ HRC 28 (or 290 HV) per NACE MR0175/ISO 15156 Table A.31.
• Super duplex stainless steels (e.g., UNS S32750, S32760) – Hardness ≤ HRC 32 (or 330 HV) per standard.

Nickel Alloys

Nickel alloys are used in the most severe sour environments, including high H₂S partial pressures, high chlorides, and low pH. They offer excellent resistance to SSC and SCC.

• Alloy 625 (UNS N06625) – Solution-annealed, max HRC 35 (or 350 HV). Used for extreme conditions.
• Alloy 825 (UNS N08825) – Solution-annealed, max HRC 35.
• Alloy C-276 (UNS N10276) – Solution-annealed, max HRC 35.
• Alloy 400 (UNS N04400) – Solution-annealed, max HRC 35. Not recommended for high H₂S.
• Alloy K-500 (UNS N05500) – Age-hardened, but must be used with caution; hardness limits apply.

Hardness Requirements and Testing

Hardness is a key factor in SSC resistance. NACE MR0175/ISO 15156 specifies maximum hardness values for each material group. For carbon and low-alloy steels, the limit is typically HRC 22 (or 248 HV). For stainless steels, limits vary: austenitic stainless steels (solution-annealed) max HRC 22, duplex max HRC 28, super duplex max HRC 32. Nickel alloys generally have higher limits (HRC 35).

Hardness testing must be performed on the finished fastener (or representative sample) using Rockwell C or Vickers methods. For nuts, hardness is often measured on the bearing face or side. LOKRON conducts 100% hardness testing on all NACE-compliant fasteners and provides reports per EN 10204 3.1.

Material Selection for Different Sour Service Conditions

The selection of fastener material depends on several factors: H₂S partial pressure, pH, chloride concentration, temperature, and the presence of elemental sulfur. The standard provides a decision matrix in Annex A (informative).

It is important to consult the latest edition of NACE MR0175/ISO 15156 and the specific equipment user requirements. LOKRON’s engineering team can assist in material selection based on your service conditions.

Certification and Documentation

For NACE MR0175 compliance, fasteners must be supplied with documentation that includes material test reports (MTRs) showing chemical composition, mechanical properties, hardness values, and heat treatment details. LOKRON provides EN 10204 Type 3.1 certificates (inspection certificate issued by the manufacturer) or Type 3.2 (certificate issued by an independent third party) upon request.

Additional testing may include SSC testing per NACE TM0177 (Method A) for critical applications. LOKRON can coordinate third-party testing and certification to meet project specifications.

LOKRON Solution

LOKRON (Suzhou Fulida) has over 20 years of experience manufacturing high-strength fasteners for sour service. Our NACE MR0175-compliant product range includes:

• Stud bolts: ASTM A193 B7M, B8M, B8, L7, and custom grades.
• Heavy hex nuts: ASTM A194 2H, 8, 8M, and 7M.
• Custom fasteners in duplex, super duplex, and nickel alloys.
• Full documentation: EN 10204 3.1/3.2, hardness reports, and traceability.

All fasteners are manufactured under ISO 9001 and IATF 16949 quality systems, with PED 2014/68/EU certification for pressure equipment applications.

Frequently Asked Questions

1. What is the difference between NACE MR0175 and ISO 15156?

NACE MR0175 and ISO 15156 are technically identical; the standard is jointly published. ISO 15156 is the international version, while NACE MR0175 is the North American edition. Both are recognized globally.

2. Can ASTM A193 B7 be used in sour service?

Standard B7 (hardness up to HRC 35) is not acceptable for sour service. Only B7M (max HRC 22) is compliant. However, some operators allow B7 if hardness is controlled to ≤ HRC 22 and additional testing is performed.

3. What hardness test method is required for NACE compliance?

Rockwell C (HRC) or Vickers (HV) are accepted. For thin sections, Vickers is preferred. The test must be performed on the finished product or a representative sample.

4. Are stainless steel fasteners always NACE compliant?

No. Only solution-annealed austenitic stainless steels with hardness ≤ HRC 22 are compliant. Strain-hardened grades (Class 2) must also meet hardness limits. Duplex and super duplex have higher limits but must be in the solution-annealed condition.

5. Does LOKRON provide third-party certification for NACE fasteners?

Yes. LOKRON can supply EN 10204 Type 3.2 certificates issued by an independent inspection body, along with any required testing (e.g., SSC per NACE TM0177).

Summary

Selecting the correct fastener material for sour service is critical to safety and reliability. NACE MR0175/ISO 15156 provides a clear framework for material qualification, with hardness limits and material restrictions. LOKRON offers a comprehensive range of compliant fasteners with full documentation and traceability. Contact our team for assistance with your specific application.