Specifications
Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175/ISO 15156-2
- NORSOK M-650
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
NACE MR0175 / ISO 15156-3 ASTM A193 Stud Bolts for Sour Service
ASTM A193 stud bolt grades qualified to NACE MR0175 / ISO 15156-3 for hydrogen-sulfide-containing oil and gas production: hardness ceilings, grade selection, certification, and procurement.
Sour service bolting in upstream oil & gas, refining, and downstream petrochemical facilities requires hydrogen-sulfide-resistant material qualified to NACE MR0175 / ISO 15156-3. ASTM A193 covers several stud bolt grades whose chemistry, heat treatment, and hardness fall within the limits the standard defines for sulfide stress cracking (SSC) resistance. This page lists which A193 grades qualify under MR0175, the hardness ceilings that govern their use, and the certification trail buyers should specify on every sour-service order. For specific service envelopes (H2S partial pressure, chloride, in situ pH, temperature), consult your engineering team and the applicable annexes of ISO 15156-2 (carbon and low-alloy steels) and ISO 15156-3 (CRAs).
What NACE MR0175 / ISO 15156-3 covers
NACE MR0175, jointly issued with ISO 15156, sets requirements for selection and qualification of metallic materials in oil and gas production equipment whose failure from H2S-related cracking can pose a risk to safety or the environment. Part 1 covers general principles. Part 2 covers carbon and low-alloy steels (the path qualifying ASTM A193 Grade B7M). Part 3 covers cracking-resistant CRAs (qualifying A193 austenitic stainless grades B8, B8M, B8C, B8T in Class 1 solution-annealed condition). MR0175 addresses SSC, stress corrosion cracking, hydrogen-induced cracking, stepwise cracking, soft-zone cracking, and galvanically induced hydrogen stress cracking. General or localised corrosion is outside scope. Refining and downstream equipment is covered by the sister standard NACE MR0103 / ISO 17945.
Why hardness ceilings matter: sulfide stress cracking mechanism
Sulfide stress cracking is a form of hydrogen embrittlement. When wet H2S contacts a metallic surface it releases atomic hydrogen at the corrosion interface. Sulfide ions poison the hydrogen-recombination reaction, so atomic hydrogen diffuses into the steel lattice instead of recombining to harmless H2 gas. The trapped hydrogen accumulates at lattice defects, grain boundaries, and inclusions. Under residual or applied tensile stress, these hydrogen atoms reduce the cohesive strength of the lattice and initiate brittle cracks that propagate without warning.
Resistance to SSC is linked to strength and hardness. High-hardness microstructures (untempered martensite, heavily cold-worked austenite) trap more hydrogen and crack more readily. NACE MR0175 sets maximum hardness per material class. For carbon and low-alloy steel bolting (ISO 15156-2 path) the limit is 22 HRC. For austenitic stainless bolting (ISO 15156-3 path, Annex A.2) the limit is also 22 HRC, with the additional requirement that the material be solution-annealed and free of cold work intended to enhance mechanical properties.
A193 stud bolt grades qualified under NACE MR0175
The following ASTM A193 grades are acceptable as sour-service stud bolting when supplied in the heat-treatment condition and hardness range NACE MR0175 / ISO 15156 requires. Verify the service envelope against the applicable annex tables before specifying.
| A193 Grade | UNS | Material type | NACE path | Max hardness | Typical sour-service role |
|---|---|---|---|---|---|
| B7M | G41400 | Cr-Mo low-alloy 4140, quenched and tempered | ISO 15156-2 Annex A, Table A.4 | 22 HRC (235 HBW) | Low-alloy bolting where chloride is not a co-factor. |
| B8 Class 1 | S30400 | Type 304 austenitic stainless, solution-annealed | ISO 15156-3 Annex A.2 | 22 HRC | Atmospheric or low-chloride sour service. |
| B8M Class 1 | S31600 | Type 316 Mo-bearing austenitic stainless, solution-annealed | ISO 15156-3 Annex A.2 (S31603 envelope) | 22 HRC | Preferred where chloride co-exists with H2S. |
| B8C Class 1 | S34700 | Type 347 niobium-stabilised austenitic stainless | ISO 15156-3 Annex A.2 | 22 HRC | Higher-temperature sour service requiring stabilised chemistry. |
| B8T Class 1 | S32100 | Type 321 titanium-stabilised austenitic stainless | ISO 15156-3 Annex A.2 | 22 HRC | Stabilised-grade alternative to B8C. |
| B8A / B8MA / B8RA / B8SA / B8CA / B8TA (Class 1A) | S30400 / S31600 / S20910 / S31254 / S34700 / S32100 | Solution-treated in the finished condition (no post-thread cold work) | ISO 15156-3 Annex A.2 / A.3 | 22 HRC (S20910 up to 35 HRC per A.2 note c) | Class 1A variants where threading must not impart cold work. |
The Class 1A suffix on B8A, B8MA, B8RA, B8SA, B8CA, B8TA designates material solution-treated after thread rolling, so residual cold work from bolt-making is removed. Specify Class 1A when ISO 15156-3 Annex A.2 cold-work limits are the governing constraint.
B7M vs B8M Class 1: which to choose for sour service
B7M and B8M Class 1 are the two most commonly specified sour-service A193 grades, sitting on opposite sides of the cost/corrosion-resistance trade-off.
- ASTM A193 B7M (UNS G41400, 4140 Cr-Mo): low-alloy steel quenched and tempered to 22 HRC (235 HBW) max. Cost-effective, widely available, strong enough for most flanged joints. Qualified under ISO 15156-2 Annex A, Table A.4. B7M is not chloride-resistant; chloride is a separate risk it does not mitigate.
- ASTM A193 B8M Class 1 (UNS S31600, Type 316 austenitic stainless): Mo-bearing stainless qualified under ISO 15156-3 Annex A.2 for combined chloride + sour environments at the envelopes the table defines. Higher cost, lower allowable stress than B7M (solution-annealed not quenched and tempered), but adds chloride pitting resistance.
Use B7M where service is sour but chloride is not a meaningful co-factor (dry-gas trim, hydrocarbon-dominated flanges, no aqueous chloride phase). Use B8M Class 1 where chloride and H2S co-exist (sub-sea wet-gas, produced water, chloride-bearing brines). Confirm actual H2S partial pressure, chloride, in situ pH, and operating temperature against the ISO 15156 annex table; some combinations fall outside the qualified envelope for either grade.
Certification and testing requirements per NACE MR0175
Sour-service procurement carries documentary requirements above and beyond a standard ASTM A193 mill test certificate. The minimum items buyers should specify on every NACE MR0175 stud-bolting PO:
| Document / Test | Requirement | Reference |
|---|---|---|
| Material Test Certificate (MTC) | EN 10204 Type 3.1 minimum; Type 3.2 (third-party witnessed) recommended for critical service. | EN 10204; MR0175 traceability |
| Hardness testing | Lot-by-lot testing on every stud and nut lot. Rockwell C (HRC) or Brinell (HBW) per ISO 6508-1 / ISO 6506-1. Average within limit; no individual reading more than 2 HRC above the specified maximum. | NACE MR0175; ISO 15156-2 / -3 |
| Heat-treatment record | Documented solution-anneal (austenitic grades) or quench and temper (B7M) condition. Free of cold work intended to enhance mechanical properties. | ISO 15156-3 Annex A.2; ISO 15156-2 Annex A |
| Chemical composition | Full heat analysis matched to the UNS number. PMI (positive material identification) recommended on every stud for critical sour service. | ASTM A193 / A193M; MR0175 |
| Marking and traceability | Grade symbol plus heat number on each stud. Documented chain: Heat to Forge to Heat-treat to Thread to Inspection to Pack. | ASTM A193; MR0175 traceability clause |
Service environment limits: when NACE MR0175 applies
NACE MR0175 / ISO 15156 applies to oil and gas production equipment whose service environment is classified as sour. The most widely cited screening threshold is a hydrogen-sulfide partial pressure of approximately 0.05 psia (0.3 kPa) in the gas phase; above that level the system is generally considered to fall within scope. ISO 15156-1 requires the user to define and document the actual H2S partial pressure, in situ pH, chloride concentration, presence of elemental sulfur, temperature, mechanical stress, and exposure time to liquid water. Treat 0.05 psia as a screening trigger, not a substitute for service-envelope evaluation by your engineering team.
Companion nut requirements: A194 2HM for B7M, A194 8M for B8M
A sour-service stud is only as compliant as the nuts paired with it. ISO 15156-2 Annex A, Table A.4 lists A194 Grade 2HM (or 7M) as the matched nut for A193 B7M; these are the controlled-hardness NACE variants of standard 2H. For austenitic A193 grades, the corresponding A194 austenitic nut grades apply.
| A193 Stud Grade | Acceptable A194 Nut Grade(s) | Hardness ceiling on nut |
|---|---|---|
| B7M (G41400) | A194 2HM, A194 7M | 22 HRC (235 HBW) |
| B8 Class 1 (S30400) | A194 Grade 8 (solution-treated) | 22 HRC per Annex A.2 |
| B8M Class 1 (S31600) | A194 Grade 8M | 22 HRC per Annex A.2 |
| B8C Class 1 (S34700) | A194 Grade 8C | 22 HRC per Annex A.2 |
| B8T Class 1 (S32100) | A194 Grade 8T | 22 HRC per Annex A.2 |
A common procurement error: specifying standard A194 2H (no M suffix) against a B7M stud. The 2H nut's uncontrolled hardness can exceed the NACE limit even when the stud is compliant. Order matched sets.
Sour-service procurement checklist for ASTM A193 / A194 sets
- Reference NACE MR0175 / ISO 15156-3 (or 15156-2 for B7M) on the PO, with the annex table and service envelope your engineering team defined.
- Specify the A193 grade and class explicitly (e.g. "ASTM A193 B8M Class 1, UNS S31600, ISO 15156-3 Annex A.2 compliant").
- Specify the matched A194 nut grade (2HM, 7M, 8M, 8C, 8T). Do not accept standard 2H against a B7M stud.
- Require EN 10204 Type 3.1 MTC minimum; Type 3.2 (third-party witnessed) for critical service.
- Require lot-by-lot hardness testing on the MTC with method, locations, and individual readings traceable to the lot.
- Require PMI on a defined sample frequency, or 100 percent for critical service.
- Require grade symbol + heat number marking on each stud, grade symbol on each nut.
- Verify supplier heat-treatment records show the correct solution-anneal or quench-and-temper cycle.
- For PTFE, Xylan, zinc-nickel, or Geomet coatings, confirm the coating process stays inside MR0175 surface-treatment limits.
Related A193 sour-service grade pages
Each grade page below gives chemistry, mechanical properties, dimensional standards, and lead time:
- A193 Grade B7M (UNS G41400) sour-service low-alloy
- A193 Grade B8M Class 1 (UNS S31600) chloride + sour
- A193 Grade B8 Class 1 (UNS S30400) austenitic sour
- A193 Grade B8C Class 1 (UNS S34700) high-temperature sour
- A193 Grade B8M Class 1D (UNS S31600) lower-temperature variant
- A193 Grade B7 (UNS G41400) standard 4140 (not NACE)
- ASTM A194 Companion Nuts (2HM / 7M / 8M)
- Flange Bolt Chart ASME B16.5
References
- NACE MR0175 / ISO 15156-1:2020. General principles for selection of cracking-resistant materials.
- NACE MR0175 / ISO 15156-2:2020. Cracking-resistant carbon and low-alloy steels. Annex A, Table A.4 lists acceptable bolting (A193 B7M, A320 L7M; A194 2HM / 7M nuts).
- NACE MR0175 / ISO 15156-3:2020. Cracking-resistant CRAs and other alloys. Annex A.2 covers austenitic stainless steels (S30400, S31603, S34700, S32100) behind A193 B8, B8M, B8C, B8T Class 1.
- ASTM A193 / A193M. Bolting for High Temperature or High Pressure Service.
- ASTM A194 / A194M. Nuts for Bolts for High Pressure or High Temperature Service.
- NACE MR0103 / ISO 17945:2015. Refining and downstream sister standard.
- ISO 6508-1. Rockwell hardness test. ISO 6506-1. Brinell hardness test.
- NACE TM0177. SSC laboratory testing in H2S environments.
- EN 10204. Inspection document types (3.1 / 3.2).
Frequently Asked Questions
- What hardness limit does NACE MR0175 set for A193 stud bolts?
- NACE MR0175 / ISO 15156-3 limits low-alloy steel bolting (B7M) to a maximum 22 HRC, equivalent to 235 HBW. Austenitic stainless steels (B8 Class 1, B8M Class 1, B8C, B8T in solution-treated condition) are accepted up to the same 22 HRC ceiling. Higher-strength variants like B8R (Nitronic 50) carry a specific 35 HRC exception per Annex A.2 Note c. Always cross-reference the current NACE MR0175 / ISO 15156-3 revision and consult your engineering team for service envelope verification.
- Is ASTM A193 B7 acceptable for sour service?
- No. Standard ASTM A193 B7 is quenched and tempered to a hardness range that typically exceeds the 22 HRC NACE MR0175 ceiling. Use ASTM A193 B7M — the NACE-qualified variant of B7 with the same Cr-Mo chemistry but controlled-hardness tempering — for sour service applications. Pair B7M studs with A194 2HM nuts for end-to-end NACE compliance.
- What is the difference between NACE MR0175 and NACE MR0103?
- NACE MR0175 / ISO 15156-3 applies to UPSTREAM oil and gas production environments where wet H2S is present. NACE MR0103 / ISO 17945 applies to DOWNSTREAM refining and petrochemical service. The two standards have different qualification routes and acceptance lists. MR0175 is more conservative because upstream service often involves higher H2S partial pressures, chlorides, and dynamic loading. Always specify which standard the joint must comply with on the purchase order.
Satisfaction Survey
