Hastelloy B2
Bolts
UNS N10665
Why B2 Bolts Outperform C276 in Pure HCl
Engineers selecting hex bolts for reducing acid equipment — HCl synthesis lines, reducing H₂SO₄ process connections, acetic acid reactor covers, and phosphoric acid plant — face a fundamental choice between Hastelloy B2 (UNS N10665) and Hastelloy C276 (UNS N10276). For mixed or undefined acid media, C276 is the correct default. For identified reducing acid service where HCl, reducing H₂SO₄, acetic acid, or phosphoric acid is the specific corrosive species, B2 is the technically correct choice and delivers corrosion rates that C276 cannot match.
The chemistry: Hastelloy C276 has 15–17% Mo with 14.5–16.5% Cr. The chromium is essential for C276’s oxidising acid and chloride resistance — but in reducing acid environments, Cr adds no benefit and the effective Mo available for reducing acid resistance is diluted by the alloy budget allocated to Cr. Hastelloy B2 allocates the full alloy budget — 26–30% — to molybdenum because there is no chromium. The result is near-zero corrosion rates in HCl at all concentrations from dilute to near-anhydrous. C276’s 16% Mo provides excellent reducing acid resistance but cannot equal B2’s 28% Mo in pure HCl service.
B2 hex bolts are specified for chemical equipment cover bolts, reactor body bolting, acid plant structural connections, and all non-pressure-flange bolted connections in reducing acid service. For ASME B16.5 pressure flanges, B2 stud bolts are the correct form. For new project design, the upgraded alloy is Hastelloy B3 (UNS N10675) — which eliminates B2’s HAZ sensitisation while maintaining equal reducing acid performance.
| Element | Min % | Max % | Typical |
|---|---|---|---|
| Nickel (Ni) | Balance | — | ~62% |
| Molybdenum (Mo) | 26.0 | 30.0 | ~28% |
| Iron (Fe) | — | 2.0 | ~1% |
| Cobalt (Co) | — | 1.0 | — |
| Chromium (Cr) | — | 1.0 | <0.5% |
| Manganese (Mn) | — | 1.0 | — |
| Carbon (C) | — | 0.02 | — |
| Silicon (Si) | — | 0.10 | — |
Hastelloy B2 Bolt Types
Full Thread Hex Bolts — DIN 933 / ISO 4017
Thread runs the complete length from head to tip. The standard B2 hex bolt for reducing acid chemical equipment cover bolts, agitated vessel body connections, and structural bolting in HCl synthesis equipment, acetic acid process vessels, and phosphoric acid plant where full thread engagement is required across the joint. M6–M100. Available in coarse pitch and fine pitch on request.
Most common B2 bolt form for equipment maintenance replacement. When replacing existing B2 fasteners in reducing acid equipment, use B2 full thread hex bolts (UNS N10665) — not C276 — to maintain galvanic compatibility with adjacent B2 fasteners and B2 flanges. Matched B2 hex nuts from same ASTM B335 certified heat available.
Partial Thread Hex Bolts — DIN 931 / ISO 4014
Unthreaded plain shank between the head and the threaded section — the shank spans the reducing acid flange or equipment wall and carries shear load across the joint. Used in reducing acid chemical plant structural connections, HCl synthesis equipment skid bases, acid plant structural bolting, and reactor support connections where shear capacity must be combined with reducing acid immunity.
The unthreaded shank also prevents thread galling in the body bore — important in B2 bolts because the solution-annealed Ni-Mo alloy has a higher galling tendency than Ni-Cr-Mo alloys at the shank-bore interface. Fine pitch available for vibrating equipment applications. M6–M100.
Heavy Hex Bolts — ASME B18.2.3.6M
Wider across-flats (AF) and greater head height than DIN 933 standard hex — allows higher torque to be applied without head deformation. Used for large-diameter reducing acid equipment covers, HCl reactor manways, high-pressure acid vessel closures, and any bolted connection requiring maximum wrench engagement in B2 Ni-Mo alloy. M12–M100. Matched heavy hex nuts from same certified B2 heat available.
For ASME B16.5 flanged pressure joints, see B2 stud bolts with heavy hex nuts — the correct configuration for reducing acid pressure flanges to Class 150–2500. Heavy hex bolts are for equipment covers and structural connections rather than pipe flanges.
Socket Cap Screws — DIN 912 / ISO 4762
Cylindrical hex-socket head for confined-access bolting in reducing acid plant — chemical reactor internals, agitator baffle attachment bolts, heat exchanger internal baffle screws, HCl scrubber internal fixtures, and acid-resistant instrument connections where standard hex head clearance is unavailable. Allen key drive. M4–M36.
In B2 solution-annealed condition, socket cap screws should be lubricated on assembly to prevent galling at the hex socket drive — a standard precaution with Ni-Mo alloy fasteners that is more critical than with stainless steel socket screws. B2 socket screws from same ASTM B335 certified heat as hex bolts and stud bolts.
Flanged Hex Bolts — DIN 6921 / ISO 4162
Integrated flange (washer face) beneath the hex head — distributes bolt load over greater bearing area without a separate washer, and eliminates the washer-flange face crevice where stagnant reducing acid accelerates corrosion. Used in FRP-lined HCl vessel nozzle connections, rubber-lined reducing acid equipment covers, composite substrate connections in phosphoric acid plant, and any reducing acid application where a standard washer is impractical or creates an additional crevice corrosion site. M6–M64.
DIN 931 / DIN 933 Hastelloy B2 Hex Bolt Dimensions
Standard metric hex bolt dimensions per DIN 931/933 in mm. Fine pitch, SAE sizes, and non-standard lengths on request to sales@akbarfasteners.in.
| Size (M) | Pitch | AF (s) mm | Head Ht (k) mm | Full Thread (DIN 933) | Part Thread b (DIN 931) | Heavy Hex AF mm | Condition |
|---|---|---|---|---|---|---|---|
| METRIC HEX BOLTS — UNS N10665 · ASTM B335 · SOLUTION ANNEALED · Mo 28% · NACE MR0175 | |||||||
| M6 | 1.0 mm | 10.0 | 4.0 | 10–100 mm | 18 mm | — | Sol. Ann. |
| M8 | 1.25 mm | 13.0 | 5.3 | 12–200 mm | 22 mm | — | Sol. Ann. |
| M10 | 1.5 mm | 17.0 | 6.4 | 16–200 mm | 26 mm | — | Sol. Ann. |
| M12 | 1.75 mm | 19.0 | 7.5 | 20–300 mm | 30 mm | 21.0 | Sol. Ann. |
| M16 | 2.0 mm | 24.0 | 10.0 | 30–300 mm | 38 mm | 26.9 | Sol. Ann. |
| M20 | 2.5 mm | 30.0 | 12.5 | 40–300 mm | 46 mm | 33.5 | Sol. Ann. |
| M24 | 3.0 mm | 36.0 | 15.0 | 50–300 mm | 54 mm | 40.4 | Sol. Ann. |
| M30 | 3.5 mm | 46.0 | 18.7 | 60–300 mm | 66 mm | 50.8 | Sol. Ann. |
| M36 | 4.0 mm | 55.0 | 22.5 | 70–300 mm | 78 mm | 60.3 | Sol. Ann. |
| M42 | 4.5 mm | 65.0 | 26.0 | On order | On order | 71.0 | Sol. Ann. |
| M48 | 5.0 mm | 75.0 | 30.0 | On order | On order | 82.6 | Sol. Ann. |
| M56–M100 | Custom | Custom | Custom | On order | On order | Custom | Sol. Ann. |
* Fine pitch, inch UNC/UNF, non-standard — sales@akbarfasteners.in
Standards We Manufacture & Test To
Same quality system as our Hastelloy B3 stud bolts and complete B2 fastener range. Full heat traceability from mill certificate to dispatch.
Industry Applications
Reducing acid environments where hex bolt connections — equipment covers, reactor body bolting, structural connections — require corrosion resistance that stainless steels and even Ni-Cr-Mo alloys cannot provide.
HCl Gas Plant
hydrochloric acid vessels
Chemical Processing
acid process equipment covers
HF Acid Systems
alkylation unit cover bolts
Acetic Acid Plant
anhydride process covers
Phosphoric Acid
fertiliser acid vessel covers
Acid Recovery
spent acid equipment covers
Pharmaceutical
reducing acid process bolts
Oil Refining
acid unit structural bolting
Complete Documentation Package
Heat & Lot Traceable
Conformance
100% Lots Verified
Tensile · HB Hardness
TÜV · BV · Lloyds
Related B2 & Hastelloy Fasteners
B2 hex bolts for equipment covers. B2 stud bolts for ASME flanges. Matched B2 nuts and washers. Or B3 for new design.
Hastelloy B2 Stud Bolts
B2 fully threaded stud bolts for ASME B16.5 pressure flanges in reducing acid service. Class 150–2500, M12–M100. Matched heavy hex nuts from same certified heat for single MTC coverage.
View Page →Hastelloy B2 Washers
Plain flat DIN 125, spring DIN 127, and large OD washers in B2 for matched assembly — zero galvanic potential at the washer-nut face in HCl, reducing H₂SO₄, and acetic acid environments.
View Page →Hastelloy B3 Stud Bolts
Current-generation Ni-Mo alloy preferred for new reducing acid projects. Eliminates B2’s HAZ sensitisation risk while maintaining equal HCl and reducing acid corrosion performance.
View Page →Hastelloy B2 Fasteners — Full Range
Complete B2 fastener range — stud bolts, hex bolts, nuts, washers, threaded rods, socket screws. All forms from same certified ASTM B335 heat for matched assembly supply.
View Range →Need Hastelloy B2
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