Alloy C4
Alloy C4 (UNS N06455) is a nickel-chromium-molybdenum alloy that removes the tungsten and iron additions found in C276. This gives it a cleaner composition and better thermal stability. Its resistance to sensitization and grain boundary precipitation makes it the preferred C-family alloy for use in the as-welded condition.
1. Chemical Composition
The table below shows the elemental composition of Alloy C4. The absence of tungsten and tightly controlled iron content set it apart from C276 and give it better microstructural stability after thermal exposure.
| Grade | Ni | Cr | Mo | Fe | Ti | Co | Mn | C | Si | P | S |
| N06455 (C-4) | Balance (≈65%) | 14.00 – 18.00 | 14.00 – 17.00 | ≤ 3.00 | ≤ 0.70 | ≤ 2.00 | ≤ 1.00 | ≤ 0.015 | ≤ 0.08 | 0.025 | 0.015 |
2. Mechanical Properties
The following minimum mechanical values apply to Alloy C4 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 |
| N06455 | Low C Ni-Mo-Cr (C-4) | ≥ 100 (690) | ≥ 40 (276) | ≥ 40 % | 25,000 |
3. Equivalent Grade
The table below lists the international equivalents of Alloy C4. This grade has fewer regional equivalents than C276 because of its more specialized market position.
| STANDARD | WERKSTOFF NR. | UNS | AFNOR |
| Alloy C4 | 2.4610 | N06455 | NC 20 FeMo |
4. Key Technical Advantages
- Thermal Stability and As-Welded Corrosion Resistance: The low iron content (<=3%) and absence of tungsten in C4 greatly reduce the formation of harmful intermetallic phases (mu phase, P phase) during fabrication and heat treatment at 650-1040 degrees C. As a result, C4 weldments perform reliably without mandatory post-weld solution annealing in most chemical service.
- Resistance in Reducing Acid Media: With 14-17% Mo, Alloy C4 matches C276 in hydrochloric acid and reducing sulfuric acid environments. It is a practical alternative in pure reducing service where thermal processing is part of fabrication.
- Minimal Sensitization Risk: The very low carbon (<=0.010%) combined with the thermally stable matrix minimizes intergranular corrosion risk in heat-affected zones. This matters most for complex welded assemblies such as shell-and-tube heat exchangers with multiple weld seams.
Technical Note: Alloy C4 is not the best choice for environments with strong oxidizing agents (ferric or cupric chlorides, concentrated nitric acid), where the higher chromium of C22 performs better. C4 is best suited for thermally processed equipment in reducing acid service where as-welded corrosion performance must be guaranteed without post-weld heat treatment.
5. Common Manufacturing Standards
ASTM B622: Standard specification for seamless nickel and nickel-cobalt alloy pipe and tube, covering UNS N06455 (Alloy C4) in seamless pipe and tube form for corrosive service.
ASTM B619: Standard specification for welded nickel and nickel-cobalt alloy pipe, applicable to Alloy C4 in welded pipe form.
ASTM B626: Standard specification for welded nickel and nickel-cobalt alloy tube, covering C4 in welded tube form for condenser and heat exchanger service.
DIN 17751: European standard for nickel alloy tubes; C4 is designated 2.4610 under EN material numbering.
JIS H4552: Japanese Industrial Standard for nickel and nickel alloy tubes, covering NW6455 designation for Alloy C4 tubular products.
Manufacturing Standards Comparison Table:
| Standard | GB | EN/DIN | JIS | GOST |
| ASTM B622 (Seamless Pipe/Tube) | - | DIN 2.4610 | JIS H4552 (NW6455) | - |
| ASTM B619 (Welded Pipe) | - | DIN 2.4610 | - | - |
6. Primary Applications
- Chemical Plant Heat Exchangers: Shell-and-tube heat exchangers in multipurpose chemical plants where weldments contact reducing acid service and post-weld solution annealing of fully assembled units is impractical.
- Formic and Acetic Acid Production: Piping, condensers, and reboilers in acetic acid and formic acid plants where reducing organic acids at elevated temperatures attack standard nickel-chromium alloys, but C4's molybdenum content provides adequate resistance.
- Hydrochloric Acid Handling: Piping, absorption towers, and storage tank connections in HCl production and handling facilities where reducing conditions prevail and thermal stability of fabricated assemblies is needed.
- Sulfuric Acid Processing: Heat exchanger tubes and piping in sulfuric acid concentration and dilution systems operating in the reducing concentration range (below approximately 70% H2SO4 at ambient temperature).
- Thermally Cycled Process Equipment: Reactors and vessel liners in batch chemical manufacturing where the equipment undergoes frequent thermal cycles across the sensitization temperature range. These applications need an alloy whose microstructure stays stable without repeated annealing.