NACE MR0175 Hiduron 191

NACE MR0175 / ISO 15156 is the global materials qualification standard for the sour-service oil and gas envelope where hydrogen sulphide is present at partial pressures and concentrations that can drive sulphide-stress cracking, stress-corrosion cracking and hydrogen-induced cracking. Hiduron 191 (UNS C72420) qualifies as a Cu-Ni-Al precipitation-hardened cupronickel for sour-service bolting at the 286 HBW (28 HRC) maximum hardness ceiling, with the body-centred-cubic ferrite phase absent from the gamma-prime strengthened copper matrix giving the alloy native hydrogen-embrittlement immunity that bcc steels cannot match.

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Scope of NACE MR0175 for Cu-Ni-Al Bolting

NACE MR0175 / ISO 15156 specifies materials for the oil and gas H2S service envelope, capturing acceptance conditions, hardness ceilings, environment limits and qualification testing methods for steels and alloys exposed to H2S. The Cu-Ni-Al precipitation-hardened cupronickel family carrying Hiduron 191 (UNS C72420) is qualified for sour-service bolting at the 286 HBW (28 HRC) maximum hardness ceiling, allowing the alloy to substitute Monel K-500 in service conditions where K-500 hardness limits exclude it.

Latest Revision and Sister Standards

NACE MR0175 is harmonised internationally with ISO 15156 covering parts 1 (general principles), 2 (cracking-resistant carbon and low-alloy steels), and 3 (cracking-resistant CRAs and other alloys). The Cu-Ni-Al family acceptance for Hiduron 191 is found in the ISO 15156-3 Annex on copper-base alloys. The standard is maintained jointly by AMPP (the successor body to NACE International) and ISO TC 67. Sour-service procurement typically calls out NACE MR0175 and ISO 15156 together with the controlling product specification (NES 835, DEF STAN 02-835 or DOD-C-24676 for Hiduron 191).

Hiduron Grade Acceptance Under NACE MR0175

Chemistry Acceptance Under NACE MR0175

The NACE acceptance for Hiduron 191 rests on the parent chemistry (nickel 14.0 to 15.5 percent, manganese 4.0 to 5.0 percent, aluminium 1.5 to 2.5 percent, iron 0.5 to 1.5 percent, balance copper) plus the solution-anneal plus age heat treatment cycle that lands the hardness within the 286 HBW envelope. The matrix is face-centred-cubic copper-rich solid solution with gamma-prime Ni3Al precipitates; there is no body-centred-cubic ferrite phase to trap hydrogen, which is the source of the alloy native hydrogen-embrittlement immunity.

Mechanical-Property Floor Under NACE MR0175

Hiduron 191 mechanical floor for NACE-compliant sour-service supply: tensile strength 820 to 1000 MPa, 0.2 percent proof stress at least 580 MPa, elongation A5 at least 15 percent, Charpy V impact at least 40 J at room temperature. Hardness controlled below 286 HBW (28 HRC) at every measurement point on the bolt, including thread root, body and head, with the hardness reading captured on the mill test certificate as the NACE compliance evidence. The detailed property table is in the shared Mechanicals block below.

Inspection Categories to EN 10204 (NACE Documentation)

NACE MR0175 supply follows EN 10204 inspection-document categories. The TorqBolt default for NACE-compliant Hiduron 191 supply is EN 10204 type 3.1 mill test certificate carrying heat number, chemistry, solution-anneal plus age cycle, mechanical results, hardness map showing every reading is below 286 HBW, and a NACE MR0175 / ISO 15156 compliance statement signed against the heat. Type 3.2 with Lloyd's Register, DNV, BV, SGS or TUV witness inspection is supplied on call-out and is standard practice for sour-service offshore platform and subsea procurement.

Cross-References to Hiduron Procurement Standards

NACE MR0175 acceptance for Hiduron 191 commonly travels on the same purchase order as the controlling product specification: NES 835 (UK Naval Engineering Standard), DEF STAN 02-835 (UK Ministry of Defence) or DOD-C-24676 (US Department of Defense). For sour-service bolting the design path is: pick the product specification for naval traceability, add the NACE MR0175 hardness certificate for sour-service traceability, and supply both documents on the EN 10204 type 3.1 or 3.2 certificate.

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