NACE MR0175 Bolting: Complete Compliance Guide for Sour Service

Introduction

In oil and gas, chemical, and petrochemical industries, equipment and fasteners exposed to sour environments (containing H₂S) must resist sulfide stress cracking (SSC) and stress corrosion cracking (SCC). NACE MR0175/ISO 15156 is the globally recognized standard for materials used in H₂S-containing environments. This guide provides a deep dive into bolting compliance under NACE MR0175, covering scope, acceptable materials, hardness requirements, heat treatment, certification, and common mistakes. Whether you are a procurement engineer or project manager, understanding these requirements is critical to ensuring safety and reliability in sour service.

Scope of NACE MR0175/ISO 15156 for Bolting

NACE MR0175/ISO 15156 applies to all metallic materials used in equipment exposed to sour environments where H₂S is present in the produced fluids. For bolting, the standard covers stud bolts, hex bolts, nuts, and washers used in pressure-containing joints, flanges, valves, and other connections. The standard defines acceptable material compositions, heat treatment conditions, hardness limits, and testing requirements to prevent SSC and SCC. It is important to note that NACE MR0175 is not a design standard but a materials selection standard; it does not specify bolt dimensions or strength grades (those come from ASTM, ASME, or API standards).

Acceptable Bolting Materials per NACE MR0175

NACE MR0175/ISO 15156 Part 1 (general principles) and Part 2 (carbon and low-alloy steels) and Part 3 (corrosion-resistant alloys) list acceptable materials. For bolting, the most common categories are:

Carbon and Low-Alloy Steels

• ASTM A193 Grade B7 – Not acceptable in the standard quenched and tempered condition due to hardness > HRC 22. However, B7 can be used if hardness is limited to ≤ HRC 22 (or ≤ 248 HB) and the material is in the normalized and tempered condition (not Q&T). In practice, B7M (a modified B7 with controlled hardness) is the preferred choice.
• ASTM A193 Grade B7M – Specifically developed for sour service. Hardness ≤ HRC 22 (or ≤ 235 HB). Acceptable as-received.
• ASTM A193 Grade B16 – Not acceptable for sour service unless hardness is limited to ≤ HRC 22 (typically not standard). B16M is available but less common.
• ASTM A320 Grade L7 – Not acceptable for sour service due to high hardness (typically HRC 23-34). L7M (modified L7) with hardness ≤ HRC 22 is acceptable.
• ASTM A194 Grade 2H – Nuts for B7 bolts; not acceptable for sour service unless hardness ≤ HRC 22 (Grade 2HM).
• ASTM A194 Grade 2HM – Acceptable for sour service.

Stainless Steels

• ASTM A193 Grade B8 (Class 1 & 2) – Type 304 stainless steel. Acceptable in the solution-annealed condition (Class 1) or strain-hardened (Class 2) provided hardness ≤ HRC 22. However, strain-hardened B8 (Class 2) often exceeds HRC 22; thus Class 1 is preferred.
• ASTM A193 Grade B8M (Class 1 & 2) – Type 316 stainless steel. Similar to B8; Class 1 (solution-annealed) is acceptable with hardness ≤ HRC 22.
• ASTM A193 Grade B8MN – Nitrogen-strengthened 316N; acceptable with hardness ≤ HRC 22.

Nickel Alloys

• ASTM A193 Grade B8N (Type 304N) – Acceptable with hardness ≤ HRC 22.
• Nickel Alloy 625 (UNS N06625) – Acceptable in the solution-annealed condition; hardness typically ≤ HRC 22.
• Nickel Alloy 718 (UNS N07718) – Acceptable in the age-hardened condition with hardness ≤ HRC 40 (per NACE MR0175 Table A.25).
• Nickel Alloy 400 (Monel) – Acceptable in the annealed condition; hardness ≤ HRC 22.
• Nickel Alloy C-276 (UNS N10276) – Acceptable in the solution-annealed condition; hardness ≤ HRC 22.

Hardness Requirements and Heat Treatment

Hardness is the primary control for SSC resistance. NACE MR0175 specifies maximum hardness values for bolting materials:

Heat treatment must be carefully controlled to achieve the required hardness while maintaining mechanical properties. For carbon and low-alloy steels, tempering temperature must be at least 620°C (1150°F) to ensure hardness below HRC 22. For stainless steels, solution annealing at 1040-1120°C followed by rapid cooling is required. Nickel alloys require specific solution annealing or age hardening cycles per ASTM specifications.

Certification and Documentation Requirements

NACE MR0175 compliance requires documented evidence. Typically, the following documents are required:

• Material Test Report (MTR) per EN 10204 3.1 or 3.2, showing chemical composition, mechanical properties, and heat treatment details.
• Hardness test report for each heat/lot, confirming maximum hardness per NACE MR0175.
• Heat treatment records including time and temperature.
• Statement of compliance to NACE MR0175/ISO 15156.
• Third-party certification may be required by end users (e.g., PED 2014/68/EU, API 20F).

LOKRON supplies all fasteners with full EN 10204 3.1 documentation, including hardness reports and NACE compliance statements.

Common Mistakes and How to Avoid Them

1. Using standard B7 bolts – B7 typically has hardness HRC 28-34, exceeding the HRC 22 limit. Always specify B7M or B7 with hardness ≤ HRC 22.
2. Ignoring nut hardness – Nuts must also comply; use Grade 2HM instead of 2H.
3. Assuming all stainless steel is acceptable – Strain-hardened B8 (Class 2) often exceeds HRC 22. Specify Class 1 (solution-annealed) and verify hardness.
4. Overlooking coating effects – Zinc plating or other coatings can cause hydrogen embrittlement. Use coatings approved per NACE MR0175 (e.g., zinc-nickel, PTFE) or avoid coating.
5. Inadequate documentation – Without proper MTR and hardness reports, compliance cannot be proven. Always request full traceability.

Supplier Selection Checklist

When sourcing NACE MR0175-compliant bolting, consider the following:

• Does the supplier have experience with sour service fasteners?
• Are materials sourced from approved mills with NACE compliance?
• Does the supplier provide EN 10204 3.1 certification with hardness data?
• Is heat treatment controlled and documented?
• Can the supplier provide third-party inspection (e.g., PED, API)?
• Are coatings (if any) NACE-approved?

LOKRON meets all these criteria, with 20+ years of experience supplying certified fasteners to the energy and chemical sectors.

LOKRON Solution

LOKRON (Suzhou Fulida) offers a full range of NACE MR0175-compliant bolting, including ASTM A193 B7M, L7M, B8 Class 1, B8M Class 1, and nickel alloy stud bolts with matching nuts (A194 2HM, 8, 8M). All products are supplied with EN 10204 3.1 documentation, hardness reports, and compliance statements. We also provide custom solutions for special sizes and materials. Contact us for a quote.

Frequently Asked Questions

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

Standard B7 is not acceptable due to hardness > HRC 22. However, B7 can be used if hardness is limited to ≤ HRC 22 (often designated B7M). Always specify B7M for sour service.

2. What is the maximum hardness for NACE MR0175 bolting?

For carbon and low-alloy steels and most stainless steels, the maximum hardness is HRC 22 (235 HB). Some nickel alloys (e.g., 718) allow up to HRC 40.

3. Do nuts also need to comply with NACE MR0175?

Yes, nuts must meet the same hardness requirements. Use ASTM A194 Grade 2HM (instead of 2H) for carbon steel nuts, and Grade 8M for stainless steel nuts.

4. What documentation is required for NACE compliance?

Typically, an EN 10204 3.1 material test report showing chemical composition, mechanical properties, hardness, and heat treatment details, plus a statement of compliance to NACE MR0175.

5. Can coated bolts be used in sour service?

Some coatings are acceptable if they do not cause hydrogen embrittlement. Zinc plating is generally not recommended; zinc-nickel, PTFE, or other approved coatings may be used. Always verify with the standard.

Summary

NACE MR0175/ISO 15156 compliance for bolting is essential for safety in sour service. Key points: use materials with hardness ≤ HRC 22 (except some nickel alloys), specify B7M or L7M for carbon steel, use solution-annealed stainless steels (Class 1), ensure nuts match, and obtain full documentation. LOKRON provides certified NACE-compliant fasteners with full traceability. Contact us for your sour service bolting needs.