Alloy B3
Alloy B3 (UNS N10675) is the third-generation nickel-molybdenum alloy, developed to solve the thermal instability problems of B2 while keeping the same level of resistance to reducing acids. Its modified composition, with controlled iron, chromium, and cobalt additions, gives it better microstructural stability across the sensitization temperature range encountered during fabrication.
1. Chemical Composition
The table below lists the elemental composition of Alloy B3. The carefully controlled iron, chromium, and cobalt additions beyond the B2 baseline are what provide the thermal stability improvements in this third-generation alloy.
| Grade | C | Mn | Si | S | Cr | Mo | Fe | W | Co | Other |
| N10675 (B-3) | 0.01 | 3.0 | 0.10 | 0.03 | 1.0–3.0 | 27.0–32.0 | 1.0–3.0 | 3.0 | 3.0 | Al 0.50 |
2. Mechanical Properties
The following minimum values apply to Alloy B3 in the solution-annealed condition for seamless pipe and tube per ASTM B622.
| Grade (UNS) | Alloy Family | Tensile Min, ksi (MPa) | Yield Min, ksi (MPa) | Elong. Min, % | Fiber Stress, psi |
| N10675 | Ni-Mo (B-3) | 110 (760) | 51 (350) | 40 | 27,500 |
3. Equivalent Grade
The table below maps Alloy B3 to international equivalents. As a third-generation proprietary development, it has fewer equivalent designations than older B-series alloys.
| GRADE | UNS | GB | JIS | ISO | DIN/EN | GOST | |
| ISC | NEW | ||||||
| Alloy B3 | N10675 | — | — | — | — | 2.4600 | — |
4. Key Technical Advantages
- Thermal Stability at Fabrication Temperatures: B3's controlled Fe (1–3%), Cr (1–3%), and Zr (<=0.10%) additions suppress precipitation of Ni4Mo and other ordered phases during slow cooling through 700–950°C. This is the main improvement over B2: weldments and thermally processed assemblies keep their corrosion resistance without mandatory rapid quenching or post-weld solution annealing.
- Equivalent Reducing Acid Performance to B2: Despite the microstructural improvements, B3 matches B2's corrosion resistance to hydrochloric acid at all concentrations. Corrosion rates typically stay below 0.13 mm/year in boiling HCl across the full concentration range. This makes it a direct functional replacement.
- Improved Tolerance to Low-Level Oxidizing Contaminants: The 1–3% Cr addition in B3 (compared to <=1% in B2) provides a modest but real improvement in tolerance to trace ferric ions and dissolved oxygen in the HCl process stream. This is not a substitute for full oxidizing acid resistance, but it widens the operating window beyond B2.
Technical Note: B3 is the recommended choice for all new HCl service equipment and as a direct replacement for existing B2 installations at equipment renewal. The thermal stability advantage matters most for complex welded assemblies (heat exchanger bundles, column internals) where post-weld annealing is logistically difficult or too costly. B3 does not replace C-family alloys for oxidizing or mixed acid service.
5. Common Manufacturing Standards
ASTM B622: Standard specification for seamless nickel and nickel-cobalt alloy pipe and tube, covering UNS N10675 (Alloy B3) in seamless pipe and tube form for corrosive chemical process service.
ASTM B619: Standard specification for welded nickel and nickel-cobalt alloy pipe, applicable to Alloy B3 in welded pipe configurations.
ASTM B626: Standard specification for welded nickel and nickel-cobalt alloy tube, covering B3 in welded tube form. B3's thermal stability advantage over B2 is most practically relevant in this product form.
DIN EN ISO 6207 / DIN 17752: European standards for nickel alloy tubes and bars. B3 is designated 2.4600 under EN material numbering.
Manufacturing Standards Comparison Table:
| Standard | GB | EN/DIN | JIS | GOST |
| ASTM B622 (Seamless Pipe/Tube) | - | DIN 2.4600 | - | - |
| ASTM B619 (Welded Pipe) | - | DIN 2.4600 | - | - |
| ASTM B626 (Welded Tube) | - | DIN 2.4600 | - | - |
6. Primary Applications
- HCl Synthesis and Purification: Synthesis reactor tubes, condenser tubes, and column piping in hydrochloric acid production plants where welded assemblies need reliable as-welded corrosion performance without post-weld heat treatment.
- Hydrochloric Acid Regeneration Systems: Piping and heat exchangers in spent HCl acid regeneration units associated with steel pickling operations. Thermal cycling during production repeatedly crosses the B2 sensitization range, making B3 the better choice.
- Acid Concentration and Distillation: Column internals, reboiler tubes, and overhead condensers in HCl concentration and azeotropic distillation systems where both reducing acid conditions and elevated temperatures are present.
- Organic Acid Reactors: Reactor vessels and piping in acetic acid, formic acid, and other organic acid synthesis processes. Where confirmed reducing conditions allow it, B-series alloys can be specified over higher-cost C-series grades.
- Chemical Weapons Demilitarization: B3 has been used in specialized reactor and piping systems for chemical agent hydrolysis. This is a highly controlled, high-purity reducing acid environment where B3's better thermal stability over B2 adds engineering confidence.