What Is Hastelloy B2 (UNS N10665)?

Hastelloy B2 is a wrought nickel-molybdenum alloy introduced in the 1970s to replace the original alloy B. Its corrosion defense rests on 26-30% molybdenum dissolved in a nickel matrix, with chromium deliberately held at 1.0% max and iron at 2.0% max. That lean chemistry is what gives B2 its outstanding behavior in hydrochloric acid, where it survives all concentrations and most temperatures that industrial equipment sees. Equivalent designations include UNS N10665, W.Nr. 2.4617, EN NiMo28 and GB NS322.

The same lean chemistry creates its manufacturing constraint. When B2 spends time in the 550 to 850 C range during welding, hot forming or slow cooling, it orders into the brittle Ni4Mo intermetallic phase along grain boundaries. The kinetics are fast: significant precipitation starts within minutes near 750 C. A sensitized microstructure loses ductility and, more importantly for plant engineers, becomes susceptible to intergranular attack in acid service. This is why sound B2 fabrication practice includes controlled interpass temperatures and a full solution anneal at about 1065 C with rapid water quenching for welded vessels and heavy sections.

What Is Hastelloy B3 (UNS N10675)?

Hastelloy B3, UNS N10675, W.Nr. 2.4600, EN NiMo29Cr, is the redesigned B2. The molybdenum level rises slightly to 27-32%, and the stabilizing additions do the real work. Chromium is deliberately raised to a controlled 1.0-3.0% window, iron is balanced at 1.0-3.0%, carbon is halved to 0.01% max, and small amounts of aluminum and titanium tie up residual carbon. Together these shifts redirect the precipitation pathway away from fast-ordering Ni4Mo toward the far slower-growing Ni3Mo phase.

The practical result is a large thermal-stability margin. B3 tolerates hours of exposure near 650 C before any damaging phase reaches a harmful size, and it keeps a ductile, corrosion-resistant heat-affected zone after normal welding. Fabricators can complete complex vessels, coils and piping systems and place them in service as-welded, without the mandatory 1065 C anneal that B2 demands. B3 is today the default choice for new B-family designs, while B2 remains common in legacy equipment and drawings.

Chemical Composition Comparison

The table below lists composition limits per the ASTM B333 and B335 product specifications, which govern plate, sheet, strip, rod and bar in both grades.

The takeaway is simple: the corrosion-active element, molybdenum, stays high in both alloys, while the stabilizing elements around it change. Those secondary additions do not affect the acid resistance you buy the alloy for, but they completely change how the alloy behaves on the fabrication floor.

Thermal Stability: The Deciding Factor

For most buyers, thermal stability is the whole argument between B2 and B3. High-molybdenum Ni-Mo alloys are thermodynamically unstable in the mid-temperature band and want to precipitate intermetallic phases. The difference between the two grades is how quickly that happens.

In B2, the brittle Ni4Mo beta phase begins forming within minutes at 700 to 870 C, with the fastest kinetics near 750 C. Every weld pass, every slow cool and every furnace hold in that window adds sensitization. In B3, exposure near 650 C takes several hours before a deleterious second phase appears, and the phase that eventually forms is the slower-growing Ni3Mo. The engineering consequence is that B2 fabrication must be engineered around the thermal cycle, while B3 fabrication does not carry that burden to the same degree.

This also shows up in the workshop as process tolerance. B2 forgings, formed heads and weldments need tight control of interpass temperature and a mandatory solution anneal near 1065 C with water quench for most welded equipment. B3 forgives ordinary shop practice: normal interpass temperatures, ordinary cooling rates and as-welded delivery are all acceptable. For complex equipment with many weld passes, the reduced rework and heat-treatment cost often outweighs the higher B3 material price.

Corrosion Resistance in Reducing Acids

In pure hydrochloric acid, the two alloys are effectively interchangeable. Both deliver corrosion rates below 0.1 mm/year at any HCl concentration up to about 70 C, which is the core of the B-family reputation. Both also perform strongly in pure and impure reducing sulfuric acid, acetic acid, phosphoric acid and wet-process phosphoric media.

The separation appears in three harder cases. First, boiling acid: B3 holds a more stable passivity envelope as temperature climbs toward the boiling point. Second, welded construction: a B2 heat-affected zone that was held in the sensitization band corrodes faster than the parent metal, while a properly welded B3 HAZ stays close to parent-metal rates. Third, contaminated acid: B3 tolerates traces of ferric ions and other residual oxidants noticeably better than B2, although neither alloy is a real candidate once oxidant levels become significant.

One boundary applies to both grades without exception. Alloys with roughly 1% chromium cannot passivate against oxidizing species. Where the process contains nitric acid, wet chlorine gas, hypochlorite, chlorine dioxide or meaningful levels of ferric or cupric ions, specify a nickel-chromium-molybdenum alloy such as Hastelloy C22 or C276 instead. Using B2 or B3 in oxidizing service is the fastest way to destroy either alloy.

Welding Performance and Filler Metal Selection

Welding is where B2 and B3 behave most differently. B2 weldments carry two risks at once: the HAZ sits in the Ni4Mo precipitation band during every pass, and the weld metal itself can be sensitive to heat-input variation. Standard practice therefore calls for low heat input, controlled interpass temperature and a full solution anneal after welding for corrosion-critical equipment. Skip the anneal and the as-welded joint may work in cold dilute acid but fail early in hot or contaminated service.

B3 was engineered so that this burden disappears. Its heat-affected zone retains near-parent corrosion resistance after normal welding, so qualified fabricators deliver complex vessels, spools and coils as-welded. This is the single largest cost and schedule advantage of B3 over B2 in fabrication projects.

Filler metal selection follows AWS A5.14. B2 uses ERNiMo-7 and B3 uses ERNiMo-10, both nominally 28% Mo deposits with deliberately over-alloyed molybdenum to offset segregation in the weld pool. Matching electrodes are ENiMo-7 and ENiMo-10 per AWS A5.11. Whichever grade you weld, keep sulfur, phosphorus, lead and low-melting metals away from the joint surface, because nickel alloy welds crack catastrophically when contaminated.

Mechanical and Physical Properties

The table below compares typical room-temperature values for annealed product. Always verify final acceptance against the current ASTM B333 or B335 specification and your project data sheet.

In service, both alloys keep useful toughness down to cryogenic temperature and are non-magnetic. Neither is intended for high-temperature structural duty: without chromium they lack oxidation resistance above about 750 C, and the embrittlement behavior described earlier dominates any design near that range.

ASTM and ASME Standards for B2 and B3

Both grades sit inside the ASME code system with matching SB specifications and allowable stresses in ASME Section II-D, and both are usable in pressure equipment built to ASME Section VIII and process piping built to ASME B31.3. The table below maps product forms to the governing ASTM and ASME specifications.

When ordering flanges and butt-welding fittings to ASME B16.5 and ASME B16.9 dimensions, state the UNS number explicitly with the product specification, for example B564 N10665 or SB-366 N10675. Intergranular corrosion testing per ASTM G28 Method A is commonly added as a supplementary requirement for B-family material, because it directly detects the harmful second-phase precipitation these alloys are sensitive to.

Product Forms Supplied

HT PIPE supplies both grades across the full product chain for chemical process equipment:

Hastelloy plate, sheet and coil to B333/SB-333 for reactor shells, tank linings, baffles and tube sheets; seamless pipe and tube to B622/SB-622 for HCl transfer lines, heat exchanger tubing and reactor coils; welded product to B619 and B626 for larger bore lines; bar and rod to B335/SB-335 for valve trim, pump shafts and fasteners; forgings to B564/SB-564 and fittings to B366/SB-366 for flanged and welded systems. Every mill certificate carries heat-number traceability, and PMI verification of molybdenum content is strongly recommended on this family, because a lean Ni-Mo substitute can look identical on a warehouse rack.

Industrial Applications

B-family alloys earn their place wherever a clean reducing acid is the process medium. Typical service includes HCl synthesis and absorption units, HCl coolers, reboilers and evaporators, alkylation unit internals in sulfuric acid service, acetic acid and wet-process phosphoric acid plants, HCl pickling and metal regeneration loops, and fine chemical and pharmaceutical batch reactors handling reducing acid chemistries. B2 carries decades of service history in these systems, and B3 now takes the majority of new orders because fabrication risk is lower.

Where the same equipment also sees oxidizing conditions, for example mixed acid with nitric content or acid streams carrying ferric chloride, the correct family changes rather than the grade, and nickel-chromium-molybdenum alloys such as C276 or C22 become the candidates.

Selection Guidelines: When to Use Each Alloy

As a procurement rule: if the equipment is a new welded fabrication, B3 wins on total installed cost despite the 10-25% raw-material premium. B2 remains relevant for replacement parts in existing plants built around its heat-treatment practice and for plate or bar components that never see a welding torch in anger.

Frequently Asked Questions

1. What is the main difference between Hastelloy B2 and B3?

B3 is the thermally stabilized successor to B2. Both are nickel-molybdenum alloys with excellent reducing-acid resistance, but B3 keeps chromium at 1.0-3.0%, rebalances iron to 1.0-3.0% and holds carbon at 0.01% max, which delays the precipitation of brittle second phases. B2 forms the Ni4Mo phase within minutes when held between 700 and 870 C, while B3 tolerates hours of exposure in the 550-850 C range before any damaging phase grows.

2. Which alloy is better for hydrochloric acid service?

Both alloys resist all concentrations of pure hydrochloric acid up to about 70 C at corrosion rates below 0.1 mm/year. B3 performs better in boiling acid, in welded equipment and in acid contaminated with traces of ferric ions, so it is the safer default for welded HCl systems.

3. Do Hastelloy B2 and B3 need post-weld solution annealing?

B2 welded equipment is usually solution annealed at about 1065 C followed by a rapid water quench to remove heat-affected-zone precipitation risk. B3 is normally placed in service in the as-welded condition, which is one of the main reasons fabricators prefer it for complex weldments.

4. Can Hastelloy B2 or B3 handle oxidizing media such as nitric acid or ferric ions?

No. Both alloys have almost no chromium and fail rapidly in the presence of oxidizers. Ferric or cupric ions, nitric acid, wet chlorine gas and hypochlorite require a nickel-chromium-molybdenum alloy such as Hastelloy C276 or C22 instead.

5. Which filler metal is used for welding Hastelloy B2 and B3?

Per AWS A5.14, B2 is welded with ERNiMo-7 and B3 with ERNiMo-10. Both deposits are over-alloyed in molybdenum to compensate for molybdenum segregation in the weld metal and preserve reducing-acid corrosion resistance.