NACE MR0175 Hiduron 191 Qualification Evidence

NACE MR0175 paragraph A.7 governs the acceptance of copper-base and nickel-base alloys for sour service. The headline number is 35 HRC absolute maximum hardness. The working acceptance applied across qualified mills on Cu-Ni-Al is tighter: 286 HBW (Brinell), which converts to roughly 28 HRC and gives a 7-point HRC safety margin against the absolute ceiling. The tighter working acceptance accounts for the heat-to-heat scatter on age-hardened cupronickel and the conservative approach to sour-service qualification. Every Hiduron 191 mill certificate carries a hardness column with heat-by-heat readings, and the NACE acceptance gate refuses any heat that reads above 286 HBW on any test piece.

The representative readings above are pulled from five typical Hiduron 191 heats across the standard product form factors. Hardness scatter across heats and across section thicknesses sits in the 238 to 275 HBW band, well inside the 286 HBW NACE working ceiling with a 11 to 48 HBW margin. The heat-to-heat consistency reflects the disciplined solution-anneal plus age cycle: solution treat at 815 to 845 deg C, water quench, age at 480 to 510 deg C for 4 hours, air cool.

HIC cracking does not initiate on the FCC copper-nickel matrix because the laminar hydrogen-blister mechanism that drives HIC in carbon and low-alloy steels requires elongated MnS inclusions and a body-centred-cubic matrix; Hiduron has neither. Most qualified mills now waive HIC testing on Cu-Ni-Al by inspection; where the procurement spec retains the call-out, the test passes uniformly with zero cracking on all three planes.

NACE call-outs on Hiduron 191 are typically witnessed by one of: Lloyd's Register, DNV, Bureau Veritas, SGS, TUV Rheinland or TUV SUD. The witness scope covers the chemistry verification, the hardness reading across every test piece, the SSC and HIC test sample preparation and post-test inspection, and the endorsement of the EN 10204 type 3.2 mill test certificate. Witness selection follows the client's pre-qualified vendor list; the cost differential between witness houses is small (typically under 5 percent of the test package cost). For Royal Navy and DEF STAN DEF STAN 02-835 bolting the witness scope adds magnetic permeability per naval permeability spec.

Q. What is the NACE MR0175 hardness ceiling on Hiduron 191?
NACE MR0175 paragraph A.7 sets the hardness ceiling for nickel-base and copper-base alloys at 35 HRC for general acceptance. For copper-based alloys including Cu-Ni-Al, the working acceptance applied by qualified mills is 286 HBW (equivalent to roughly 28 HRC), which gives a safety margin against the 35 HRC absolute ceiling. Hiduron 191 in the standard solution-anneal plus age cycle reads 240 to 280 HBW heat-by-heat, sitting comfortably below 286.

Q. Does Hiduron 130 also qualify for NACE MR0175?
No. Hiduron 130 in the fully aged condition reads 290 to 320 HBW heat-by-heat, which exceeds the 286 HBW working ceiling applied to the Cu-Ni-Al family. For NACE sour service the qualified grade is Hiduron 191. The two grades come from the same Langley Cu-Ni-Al family but the manganese addition and lower aluminium content on 191 keep the aged hardness inside the NACE window.

Q. What SSC (sulfide stress cracking) test data exists on Hiduron 191?
Standard SSC qualification follows NACE TM0177 method A (tensile) and method D (DCB). Method A on Hiduron 191 at 80 percent of actual yield in solution A (5 percent NaCl + 0.5 percent acetic acid, H2S saturated, 24 deg C) typically runs to 720 hours without failure on the gauge length. Method D DCB crack-arrest K_ISSC values typically read above 33 MPa-root-m. Both results clear the standard NACE acceptance for sour-service bolting.

Q. What HIC (hydrogen-induced cracking) result is expected on Hiduron 191?
NACE TM0284 HIC testing on Hiduron 191 returns zero cracking on all three planes (CLR = 0, CSR = 0, CTR = 0) because the FCC copper-nickel matrix does not support the laminar hydrogen-blister mechanism that drives HIC in carbon and low-alloy steels. HIC qualification is typically waived on Cu-Ni-Al by mutual agreement; where the spec retains the call-out, the test passes by inspection.

Q. Which third-party laboratories does TorqBolt use for NACE witness?
Lloyd's Register, DNV (formerly DNV GL), Bureau Veritas, SGS, TUV Rheinland and TUV SUD are the regular witness houses for NACE call-outs. The choice typically follows the client's pre-qualified vendor list and the discipline (subsea EPC usually picks LR or DNV; oil-and-gas operator usually picks BV or SGS). Witness scope is the chemistry, the hardness, the SSC and HIC tests, and the 3.2 endorsement on the mill certificate.

TorqBolt supplies Hiduron 130 (UNS C72400, DTD 900/4805) and Hiduron 191 (UNS C72420, NES 835, DEF STAN 02-835, DOD-C-24676) in round bar, stud bolts, hex bolts, heavy hex bolts, nuts, washers, forgings and machined components. Standard fastener lead time is 4 to 8 weeks from order, subsea machined components quote project-specific lead time. Send an enquiry through TorqBolt Contact with the controlling specification, the form factor, the size envelope and the certification level (3.1 default, 3.2 on call-out, NACE on call-out).

Request a Quote on Hiduron 130 and Hiduron 191 Bolting Stock and Fasteners