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
Monel 400 Sour Service - NACE MR0175 H2S Oil & Gas Applications
Monel 400 sour service bolting and components are specified for upstream oil and gas hardware in wet hydrogen sulphide. Monel 400 H2S hardware is listed in NACE MR0175 / ISO 15156-3 Table A.4 as a solid-solution nickel alloy resistant to sulphide stress cracking within the qualified hardness, chloride and temperature envelope. Sour gas Monel bolting carries a 35 HRC cap per the NACE listing; ASTM F468 fastener-grade Monel 400 holds 25 HRC max in the annealed condition. Typical service: wellhead trim, downhole tools, sour-gas flange bolting, refinery hydroprocess fasteners above the NACE 0.05 psi (0.3 kPa) H2S threshold. Monel® is a registered trademark of Special Metals Corporation; TorqBolt is not authorized by, affiliated with or endorsed by Special Metals.
What Sour Service Means per NACE MR0175 / ISO 15156-3
Sour service is the wetted exposure of carbon steel and CRA components to fluids carrying hydrogen sulphide. The triggering threshold per NACE MR0175 / ISO 15156-1 Clause 8 is H2S partial pressure at or above 0.05 psi (0.3 kPa) in the gas phase, with liquid water present. Severity is set by four co-acting variables: H2S partial pressure, in-situ pH driven by CO2 and bicarbonate buffering, chloride concentration, and operating temperature. ISO 15156-2 covers carbon and low-alloy steels; ISO 15156-3 covers corrosion-resistant alloys and is the document that lists UNS N04400.
Monel 400 Qualification per ISO 15156-3 Table A.4
ISO 15156-3 Annex A, Table A.4 lists solid-solution nickel alloys qualified for sour service without prequalification testing, subject to per-alloy environmental restrictions. Monel 400 is listed with a 35 HRC hardness ceiling in the wrought hot-worked annealed condition. The qualified envelope sets bounds on H2S partial pressure, chloride and temperature; specific limits are read off the Annex A.4 cells for the procurement file. Cold-worked Monel 400 is restricted by the same table; re-qualification per Annex B is required if the chemistry, product form, condition or environment falls outside the listed cells. The cross-reference standard is NACE MR0175; both documents are technically equivalent.
Where Monel 400 is Specified in H2S Oil & Gas Service
Monel 400 sour-service hardware lands on four classes of upstream and refinery equipment:
| Service class | Typical Monel 400 hardware | Reason vs alternatives |
|---|---|---|
| Wellhead and Christmas tree trim | Valve stems, gate inserts, seat rings, control-line fittings | Nickel-copper holds in wet H2S plus chloride brine where chromium passivation pits. |
| Downhole tools and packer hardware | Springs (cold-worked R-405 variant), bolting, snap rings | Non-magnetic above Curie point, resists wet H2S, strength to 480 deg C. |
| Sour-gas flange and pipework bolting | Stud bolts per F468, heavy hex nuts per F467 | F468 25 HRC cap clears the NACE 35 HRC ceiling automatically. |
| Refinery hydroprocess | Heat-exchanger tube and tubesheet bolting, sour-water stripper internals | Resists wet H2S, ammonium bisulphide and dilute caustic. |
Stud Bolt and Heavy Hex Nut Requirements per ASTM F468 and F467
Monel 400 stud bolts for NACE sour service are supplied to ASTM F468 Grade N04400, hot-worked annealed. Mechanical envelope per F468 Table 1: tensile 70 ksi (480 MPa) min, yield 25 ksi (170 MPa) min, elongation 35 percent min, hardness 25 HRC max. Monel 400 heavy hex nuts are supplied to ASTM F467 Grade N04400 at matched hardness. The hardness cap makes F468 / F467 Monel 400 sour-service compliant by certificate without per-lot SSC testing. EN 10204 type 3.2 mill test certificate is the procurement-class default, adding third-party inspector sign-off (Lloyd's, BV, DNV or end-user nominated) on heat chemistry, mechanical, hardness map and marking.
Sulphide Stress Cracking Mechanism and Why Monel 400 Resists
Sulphide stress cracking (SSC) is brittle fracture under tensile stress in aqueous H2S. The mechanism is hydrogen embrittlement: cathodic atomic hydrogen at the corroding surface diffuses into the lattice, accumulates at trap sites near hard martensite or carbide phases, and drops fracture toughness below applied stress. Susceptibility scales with hardness and microstructure; BCC steels above 22 HRC, martensitic stainless and precipitation-hardened nickel alloys above their qualified hardness are the high-risk families. Monel 400 is a single-phase FCC nickel-copper solid solution with no martensite, no chromium carbide network and no precipitation-hardening phases. The FCC lattice diffuses atomic hydrogen roughly two orders of magnitude slower than BCC iron, and the soft matrix lacks the hard-particle trap sites that nucleate SSC. Combined with the 25 HRC F468 cap, Monel 400 bolting clears the Table A.4 listing without prequalification.
Inspection and Certification for Sour-Service Monel 400
The procurement-class inspection package for NACE-controlled Monel 400 bolting carries five mandatory items: chemistry verification per ASTM B164 or F468 spectrometric; tensile and yield test per F606; hardness map at head, mid-shank and thread-run-out per ASTM E18; PMI by handheld XRF or OES on every lot; ultrasonic testing of bar stock per ASTM A388. Dimensional per ASME B18.2.1 / B18.2.2. Marking includes heat number, F468 / F467 grade, manufacturer ID and the NACE / ISO 15156-3 endorsement.
Companion Alloys When Sour Service Exceeds the Monel 400 Envelope
Three conditions push past the Annex A.4 envelope and route the specification to a chromium-bearing nickel-base CRA:
| Service condition | Specified alloy | Rationale |
|---|---|---|
| Free elemental sulphur in produced fluid | Hastelloy C-276 (N10276), Inconel 625 (N06625) | Cr-Mo passivation holds; Monel 400 attacked by elemental sulphur films. |
| High chloride above Monel 400 cell limit plus oxidising contaminants | Inconel 625, Incoloy 825 (N08825) | Mo resists chloride pitting; Cr resists oxidising acid attack. |
| Temperature above Monel 400 wet H2S envelope | Inconel 718 (N07718, controlled hardness), Hastelloy C-276 | Higher service temperature with SSC resistance. |
Compare on the vs Hastelloy C-276 page. Monel K-500 is precipitation-hardened and qualified separately at controlled hardness for downhole springs and shafts.
Frequently Asked Questions
What alloys qualify for sour service under NACE MR0175 / ISO 15156-3? ISO 15156-3 lists solid-solution nickel CRAs in Annex A and precipitation-hardened nickel alloys in Annex C at controlled hardness. Monel 400 (UNS N04400) is in Table A.4 at 35 HRC max. Other listed CRAs: 316L stainless (restricted), duplex S31803 / S32750, Inconel 625, Inconel 718, Incoloy 825, Hastelloy C-276.
Monel 400 vs other CRAs for sour service? Monel 400 has no chromium and resists SSC via the nickel-copper FCC matrix. Preferred where chloride plus wet H2S would attack the chromium-oxide film on stainless or duplex. With elemental sulphur, high CO2 plus chloride, or temperatures above the Monel 400 envelope, harsher-tier CRAs (Hastelloy C-276, Inconel 625) are routed instead.
What is the hardness cap for Monel 400 fasteners in sour service? ISO 15156-3 Table A.4 caps hardness at 35 HRC for solid-solution nickel alloys. ASTM F468 Monel 400 is supplied at 25 HRC max in the annealed condition, ten HRC below the NACE ceiling. ASTM F467 nuts match.
Is Monel K-500 acceptable for sour service? Yes at controlled hardness per ISO 15156-3 Annex A or by per-application qualification per Annex B. K-500 is precipitation-hardened and qualified separately. Typical landing: downhole shafts, packer springs, snap rings. See Monel 400 vs Monel K-500.
Related Pages
Standards: NACE MR0175 / ISO 15156-3, ASTM F468, ASTM F467, ASTM B164, ASTM B564, UNS N04400. Bolting: stud bolts, heavy hex nuts, heavy hex bolts, fasteners. Mill forms: round bar, seamless pipe, flanges, forgings. Alloy selection: vs stainless steel, vs Hastelloy C-276, vs Monel K-500. Companion services: HF acid resistance, seawater corrosion, marine applications, corrosion resistance.
Request a Quote
For Monel 400 sour-service bolting, fittings or mill stock, send H2S partial pressure, chloride concentration (ppm), operating temperature, product form and quantity to info@torqbolt.com or WhatsApp +91-22-66157017. TorqBolt QA-QC returns the ISO 15156-3 Annex A.4 envelope check, EN 10204 type 3.2 MTC scope, mill source and Ex-Mumbai price within one working day. Export through JNPT. Full Monel 400 alloy reference on the main reference.
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