Hiduron Galling Resistance Test Data

Galling is a cold-weld phenomenon: under sliding load, asperity contact between two metal surfaces creates micro-welds that tear from one surface and adhere to the other, accumulating until the joint seizes. Austenitic stainless steels are notoriously prone to galling because the chromium-oxide passive film is brittle and breaks under sliding contact, exposing fresh metal that cold-welds readily. Hiduron carries a cupric-oxide passive film that is mechanically robust and self-replenishing; the asperity contact does not expose fresh metal because the cupronickel oxide regenerates faster than it breaks. The result is a galling threshold stress that sits two orders of magnitude above austenitic stainless and above most other engineering alloys.

The threshold galling stress (TGS) values above place Hiduron in the upper bracket of engineering alloys. The Hiduron-against-Hiduron pairing sits roughly 8 times above 304 SS and 2 times above Inconel 718. In practice the Hiduron pairing is treated as galling-immune at any practical assembly torque, with no need for special lubrication or thread coatings.

Standard practice on Hiduron bolting is to apply a thin film of marine-grade molybdenum disulphide (MoS2) grease to the thread valleys during make-up. The grease is not needed for anti-galling protection (the Hiduron pairing clears galling at any practical assembly torque without lubrication); the grease serves three other purposes. First, it reduces torque scatter between bolts in the same make-up by smoothing the friction coefficient. Second, it protects the thread valleys during transport and shore-side storage from cosmetic surface marking. Third, it eases disassembly inspection workflow at the 5, 10 and 15 year planned recovery campaigns. Specify the grease in the procurement order if the bolting will ship to a long-storage shore base before deployment.

Recommended assembly procedure on Hiduron stud/Hiduron nut subsea make-up: (1) inspect thread valleys at 10x magnification for nicks or burrs; reject and re-tap any stud showing damage; (2) apply a thin MoS2 grease film with a clean brush across the entire engaged thread length; (3) hand-tighten the nut to first contact and verify free running of the last 3 to 5 turns; (4) torque to 60 percent of final value in a star pattern across the bolt set; (5) torque to 100 percent of final value in the same star pattern; (6) wait 10 minutes and re-torque any bolt showing more than 5 percent relaxation. Disassembly inspection at planned 5 to 7 year intervals follows the reverse pattern: (1) loosen in star pattern, (2) record breakaway torque per bolt as a preload-retention measurement, (3) remove and inspect for thread valley damage, (4) re-grease and re-install if the bolt and nut clear the visual inspection. Galling is not a failure mode encountered on the Hiduron pairing in field practice; the inspection focus is preload retention and external corrosion rather than thread damage.

Q. What is the ASTM G98 button-on-block galling test result for Hiduron against itself?
Hiduron against Hiduron typically returns a galling threshold stress above 200 MPa on the ASTM G98 button-on-block test, compared with austenitic stainless 304 against 304 which galls below 30 MPa. The two-order-of-magnitude difference comes from the cupronickel surface oxide which acts as a self-lubricating barrier preventing the cold-weld asperity contact that drives galling on austenitic stainless.

Q. Is Hiduron stud and Hiduron nut a safe make-up combination, or should the nut be a different alloy?
Safe and recommended. Hiduron stud / Hiduron nut is the standard subsea and naval make-up because the Cu-Ni-Al on Cu-Ni-Al contact is the inherently anti-galling pairing. Mixing the stud and nut alloys is unnecessary on Hiduron and introduces a galvanic couple. The single exception is when galvanic isolation is needed for a specific design reason; in that case use a PEEK washer or thread insert rather than a mixed-alloy bolting set.

Q. How does Hiduron compare to Hidurel 5 on galling resistance?
Hidurel 5 (Cu-Ni-Mn-Al-Co-Fe) is a different Langley alloy with a different microstructure; it is not interchangeable with Hiduron but galling behaviour is broadly similar because both alloys carry the cupronickel surface oxide that suppresses cold-weld asperity contact. See Hidurel 5 vs Hiduron for the full disambiguation.

Q. Does thread lubrication change the galling result?
Yes, lubrication shifts the galling threshold up by 50 to 100 percent on most alloy pairings. Hiduron against Hiduron starts so far above the galling threshold that lubrication is not required for assembly. Standard practice on subsea and naval bolting is to apply a thin film of marine-grade molybdenum disulphide grease for torque consistency and to protect the thread valleys during transport, rather than for galling resistance.

Q. What surface finish is needed on Hiduron threads for the anti-galling claim?
Anti-galling on Hiduron is robust across the practical surface-finish band (Ra 1.6 to 6.3 micron). Cut threads, rolled threads and ground threads all clear the galling test. The behaviour is driven by the bulk alloy chemistry and the surface oxide rather than by surface topography, so machinist-level workshop finish is sufficient. Mirror-polished surfaces are not necessary and provide no additional benefit.

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).

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