Hastelloy C-4 vs C-276: Thermal Stability in Elevated-Temperature Chemical Service
Hastelloy C-276 is the most widely specified corrosion-resistant nickel alloy — but it has a thermal stability limitation above 650°C. The tungsten content that gives C-276 excellent reducing acid resistance also promotes precipitation of mu phase and other intermetallics during prolonged elevated-temperature exposure. Hastelloy C-4 (UNS N06455) was developed to solve this problem, sacrificing some reducing-acid performance to gain thermally stable microstructure at elevated temperatures.
Composition Comparison
| Element | Hastelloy C-276 (N10276) | Hastelloy C-4 (N06455) |
|---|---|---|
| Ni | Balance (~57%) | Balance (~65%) |
| Cr | 14.5–16.5% | 14–18% |
| Mo | 15–17% | 14–17% |
| W | 3–4.5% | <0.35% (essentially none) |
| Fe | 4–7% | <3% |
| Ti | — | 0.7% (stabiliser) |
| EN designation | 2.4819 (NiMo16Cr15W) | 2.4610 (NiMo16Cr16Ti) |
| ASME P-No. | P-No. 44 | P-No. 44 |
| Welding filler | ERNiCrMo-4 | ERNiCrMo-7 |
The Thermal Stability Difference
Tungsten in C-276 (3–4.5%) increases PRE and reducing acid resistance but also promotes the precipitation of intermetallic phases (mu phase, P phase) in the temperature range 600–1050°C during prolonged exposure. These phases preferentially nucleate at grain boundaries, causing embrittlement and sensitisation. This means C-276 cannot be used in the as-welded condition for service temperatures above ~370°C without risk of degradation during service — any extended time in the precipitation range causes progressive property loss.
C-4 eliminates tungsten and adds 0.7% Ti (like B-3, Ti suppresses grain boundary sensitisation). The result is a thermally stable microstructure that can sustain long-term exposure to 650°C without significant embrittlement or loss of corrosion resistance. C-4 can be used in the as-welded condition in most elevated-temperature applications without post-weld annealing.
Corrosion Performance Comparison
| Environment | C-276 | C-4 |
|---|---|---|
| HCl, ambient | Excellent | Excellent (similar) |
| H₂SO₄ reducing | Excellent | Good (lower W = slightly lower performance) |
| FGD scrubber (hot H₂SO₄/HCl) | Excellent | Excellent |
| Mixed oxidising/reducing acids | Excellent | Good |
| Service at 500–650°C (as-welded) | Risk of embrittlement | Stable — C-4 preferred |
| Chlorinated solvents, seawater | Excellent | Excellent |
When to Choose C-4 Over C-276
Choose C-4 when the service temperature exceeds 370°C for extended periods and the pipe system will operate in the as-welded condition without post-weld solution annealing. Typical applications: chemical reactor jacketing, heat exchangers where the shell-side reaches 400–600°C, and systems where field welding (and therefore no factory re-anneal) is unavoidable. For ambient-to-300°C aqueous corrosion service with no elevated-temperature exposure, C-276 is typically specified because its slightly superior reducing-acid performance and the broader engineering data available for it.
Filler Rods — Not Interchangeable
C-276 uses ERNiCrMo-4 (AWS A5.14); C-4 uses ERNiCrMo-7. These have different tungsten contents and are not interchangeable. Using ERNiCrMo-4 (C-276 filler) to weld C-4 fittings deposits a weld with the thermal instability of C-276 — negating the reason for choosing C-4 in the first place. Always confirm filler specification when welding C-4 joints.