The Alloy Selection Problem Every Corrosion Engineer Faces
Hastelloy C4 (UNS N06455) is the best choice for welded structures that cannot be post-weld heat-treated and for high-temperature service above 649°C. Hastelloy C276 (UNS N10276) leads in reducing acid environments for machined, non-welded parts. Hastelloy C22 (UNS N06022) is the specialist for mixed oxidising/reducing acids, FGD systems, and nitric acid service.
Hastelloy C4, C276, and C22 share the same Ni-Cr-Mo foundation — but the differences between them determine whether your equipment lasts five years or twenty-five, and whether your maintenance team spends weekends re-welding heat-affected zones.
For procurement engineers and materials specialists, the choice among these three is rarely obvious from datasheets alone. All three carry the "Hastelloy" trade designation from Haynes International, Inc. All three resist acids that destroy stainless steel in weeks. The wrong choice, however, can mean grain-boundary sensitisation in a weld zone, carbide precipitation under thermal cycling, or a catastrophic stress-corrosion cracking failure in a chloride-rich sour service well.
This guide delivers side-by-side chemical compositions, corrosion rate benchmarks across ten environments, real weldability differences with total-cost implications, and a clear decision framework for oil and gas, chemical processing, nuclear, and FGD applications.
All data applies to forged product forms (bars, rings, custom open-die shapes) in the solution-annealed condition. Corrosion rate data is derived from ASTM G31 standard immersion testing and Haynes International published technical bulletins. Mechanical property data is typical for solution-annealed forged material at Jiangsu Liangyi, consistent with UNS material specifications.
Where the Real Differences Live: Alloy Chemistry Compared
The three alloys share a nickel-chromium-molybdenum base, but their alloying additions differ in ways that cascade into completely different performance profiles. The composition table below focuses specifically on the elements that govern corrosion behaviour, weldability, and thermal stability.
| Element | C4 · N06455 | C276 · N10276 | C22 · N06022 |
|---|---|---|---|
| Nickel (Ni) % | Balance ~65% | Balance ~57% | Balance ~56% |
| Chromium (Cr) % | 14 – 18 | 14.5 – 16.5 | 20 – 22.5 ← Highest |
| Molybdenum (Mo) % | 14 – 17 | 15 – 17 ← Highest | 12.5 – 14.5 |
| Tungsten (W) % | None | 3 – 4.5 | 2.5 – 3.5 |
| Carbon (C) max % | 0.015 ← Ultra-low | 0.010 | 0.010 |
| Silicon (Si) max % | 0.08 ← Ultra-low | 0.08 | 0.08 |
| Titanium (Ti) max % | 0.7 ← Stabiliser | None | None |
| Iron (Fe) max % | 3 | 4 – 7 | 3 |
| Cobalt (Co) max % | 2 | 2.5 | 2.5 |
What Each Key Difference Means for Performance
Chromium (Cr) — C22 leads: Chromium governs oxidation resistance and resistance to oxidising acids like nitric acid. C22's higher Cr (20–22.5% vs 14–18% in C4) gives it a decisive edge in mixed oxidising/reducing environments — the primary reason FGD systems and nitric-acid-adjacent processes specify C22 first.
Molybdenum (Mo) — C276 leads marginally: Molybdenum stabilises the passive film against dissolution by non-oxidising reducing acids like hydrochloric acid. C276's slightly higher Mo combined with tungsten gives it the broadest reducing-acid resistance of the three, particularly at elevated temperature and concentration.
Titanium (Ti) — C4's key engineering advantage: Hastelloy C4 is the only alloy of this trio with a titanium addition (up to 0.7%). Titanium acts as a carbide stabiliser: it preferentially bonds with carbon rather than chromium, preventing the formation of Cr₂₃C₆ carbides at grain boundaries during welding. This is why C4 maintains excellent corrosion resistance in the as-welded condition without any post-weld heat treatment — a capability neither C276 nor C22 replicates to the same degree.
Tungsten (W) — present in C276 and C22, absent in C4: Tungsten improves localised corrosion resistance (pitting and crevice) in the most aggressive reducing-acid environments. Its absence in C4 is a deliberate trade-off: by eliminating tungsten and keeping silicon ultra-low (≤ 0.08%), the alloy's as-welded microstructural stability is maximised.
In Hastelloy C4, titanium bonds preferentially to carbon (TiC), leaving the chromium in solid solution throughout the grain boundary region. This preserves the passive chromium-oxide film in the heat-affected zone of every weld — without any post-weld heat treatment required. Neither C276 nor C22 offers this mechanism, because neither contains titanium.
Benchmark Corrosion Rate Data Across Ten Environments
Performance ratings are based on ASTM G31 standard immersion testing and Haynes International published data. Excellent = <0.5 mm/yr · Very Good = 0.5–1.0 mm/yr · Moderate = >1.0 mm/yr.
| Corrosive Medium & Condition | C4 · N06455 | C276 · N10276 | C22 · N06022 |
|---|---|---|---|
| HCl 10% — Boiling | Excellent <0.5 mm/yr | Excellent <0.5 mm/yr | Very Good <1.0 mm/yr |
| H₂SO₄ 20% — 80°C | Excellent <0.3 mm/yr | Excellent <0.3 mm/yr | Excellent <0.4 mm/yr |
| H₂SO₄ 70% — 80°C | Excellent <0.5 mm/yr | Excellent <0.5 mm/yr | Very Good ~0.6 mm/yr |
| HNO₃ 65% — Boiling (oxidising) | Very Good ~0.8 mm/yr | Moderate >1.5 mm/yr | Excellent <0.3 mm/yr * |
| H₃PO₄ 85% — Boiling | Excellent <0.3 mm/yr | Excellent <0.3 mm/yr | Excellent <0.3 mm/yr |
| Acetic Acid — Glacial, Boiling | Excellent <0.1 mm/yr | Excellent <0.1 mm/yr | Excellent <0.1 mm/yr |
| Saturated NaCl — Boiling | Excellent — no pitting | Excellent — no pitting | Excellent — no pitting |
| Sour Service H₂S+CO₂+Cl⁻ (NACE MR0175) | ✓ Fully Compliant | ✓ Fully Compliant | ✓ Fully Compliant |
| Mixed Acid (HNO₃ + HCl — Aqua Regia) | Very Good | Moderate | Excellent ★* |
| As-Welded HAZ Corrosion Resistance | Excellent — Ti-stabilised | Moderate — PWHT advised | Very Good — low-C |
* C22's highest Cr content delivers dominant performance in oxidising acids. ★* C22 is universally preferred wherever both nitric and hydrochloric acids are simultaneously present.
Stress Corrosion Cracking (SCC) and Pitting Resistance
All three alloys pass ASTM G48 Method A (6% FeCl₃, 22°C) without pitting. In aggressive seawater above 80°C, C22 shows a marginal advantage due to higher chromium. For standard offshore sour service below 232°C, all three are considered equivalent under NACE MR0175/ISO 15156. Long-term field data from Middle East sour gas wells at 15,000 psi confirms over eight years of corrosion-free service for Hastelloy C4 forged wellhead components supplied by Jiangsu Liangyi to Saudi Arabia and UAE clients. For full product specifications, size range, and available forms, see our Hastelloy C4 forged parts product page.
Weldability: The Costly Difference Most Buyers Overlook
Of all performance parameters that separate these three alloys, as-welded corrosion resistance has the largest real-world cost impact — and it is the parameter most often omitted from standard material datasheets.
As-welded HAZ corrosion resistance compared to parent metal (longer bar = superior weld zone integrity without PWHT):
Why Hastelloy C4 Does Not Require Post-Weld Heat Treatment
When any Ni-Cr-Mo alloy is welded, the heat-affected zone (HAZ) is exposed to 500–900°C — the sensitisation temperature range where carbon reacts with chromium to form Cr₂₃C₆ carbides at grain boundaries. These carbides deplete the surrounding matrix of chromium, creating a chromium-depleted band that is highly susceptible to intergranular corrosion attack in service.
Hastelloy C276 requires post-weld heat treatment (solution annealing at 1121°C + rapid quench) to dissolve these carbides and restore chromium distribution. For large fabricated chemical reactors or field-welded structures, PWHT adds significant cost, schedule delay, and logistical complexity — and is sometimes physically impossible for large assembled structures.
Hastelloy C4 eliminates this problem through two simultaneous mechanisms: ultra-low carbon (≤ 0.015%) leaves very little carbon available for carbide formation; titanium (up to 0.7%) preferentially bonds with whatever carbon is present, forming stable TiC particles that do not deplete chromium. The result: the HAZ of a C4 weld retains essentially the same corrosion resistance as the base metal — confirmed by ASTM G28 Method A as-welded specimen testing. Jiangsu Liangyi supplies UNS N06455 forgings in solution-annealed condition with full EN 10204 3.1 material test certificates.
If your fabricated component cannot be placed in a solution-annealing furnace after welding — or if PWHT creates a project schedule bottleneck — specifying C276 forces a corrosion performance compromise. Hastelloy C4 eliminates this constraint entirely. Across the full project lifetime, C4 is frequently the lower total-cost alloy even when its per-kilogram price is comparable to or marginally higher than C276.
Recommended Welding Filler Metals
| Base Metal | Recommended Filler Metal | AWS Spec | PWHT Required? |
|---|---|---|---|
| C4 (N06455) | ERNiCrMo-7 (matching chemistry) | AWS A5.14 | No — as-welded service |
| C276 (N10276) | ERNiCrMo-4 | AWS A5.14 | Recommended for corrosion-critical HAZ |
| C22 (N06022) | ERNiCrMo-10 (matching chemistry) | AWS A5.14 | Usually not required |
| C4 → C276 dissimilar | ERNiCrMo-10 (C22 filler — neutral) | AWS A5.14 | No — conservative choice |
High-Temperature Stability: Critical Differences Above 600°C
High-temperature service introduces a failure mode absent from room-temperature tests: long-term microstructural instability. All Ni-Cr-Mo alloys can precipitate deleterious phases — mu-phase, sigma-phase, P-phase, or Cr-carbides — during prolonged exposure to 649–1038°C. These phases embrittle the alloy and sharply reduce corrosion resistance.
| Test Temperature | C4 Tensile Strength (MPa) | C4 Yield Strength (MPa) | C4 Elongation (%) |
|---|---|---|---|
| Room Temp — 21°C | ≥ 738 | ≥ 492 | ≥ 42 |
| 200°C (392°F) | ~690 | ~430 | ~44 |
| 400°C (752°F) | ~650 | ~380 | ~46 |
| 600°C (1112°F) | ~600 | ~330 | ~50 |
| 800°C (1472°F) | ~420 | ~240 | ~60 |
| 1000°C (1832°F) | ~150 | ~100 | ~75 |
Source: Solution-annealed forged material. Jiangsu Liangyi production data, consistent with Haynes International N06455 technical bulletin values.
C276's relatively higher carbon and silicon content causes it to be more susceptible to secondary phase precipitation during prolonged 700–900°C exposure, resulting in measurable loss of ductility and corrosion resistance. Hastelloy C4's ultra-low C and Si content, combined with titanium stabilisation, dramatically reduces the driving force for these precipitates. After 1,000 hours at 870°C, C4 retains a significantly higher fraction of original ductility and impact toughness compared to C276 — a critical difference for nuclear reactor components, chemical process reactors, and turbine system parts with continuous high-temperature duty cycles.
All 15 Performance Criteria — Side by Side
| Criterion | C4 · N06455 | C276 · N10276 | C22 · N06022 |
|---|---|---|---|
| Alloy Family | Ni-Cr-Mo + Ti stabilised | Ni-Cr-Mo-W | Ni-Cr-Mo-W (high Cr) |
| Carbon Max (%) | 0.015 | 0.010 | 0.010 |
| Reducing Acid Resistance | |||
| Oxidising Acid Resistance | |||
| As-Welded Corrosion Resistance | |||
| PWHT Requirement | None (most applications) | Recommended | Usually not required |
| High-Temp Stability 649–1038°C | |||
| Pitting / Crevice Resistance | |||
| SCC Resistance (Chloride) | |||
| NACE MR0175 / ISO 15156 | ✓ Compliant | ✓ Compliant | ✓ Compliant |
| Nuclear Power Suitability | |||
| FGD System Suitability | |||
| Forgeability (complex shapes) | |||
| Relative Material Cost | $$$ Moderate-High | $$$ Moderate-High | $$$$ High |
| Primary UNS / EN Designation | N06455 / 2.4610 | N10276 | N06022 |
Which Alloy to Specify: Scenario-by-Scenario Decision Guide
The decision cards below map the most common industrial scenarios to the recommended alloy. Use this as a starting framework — your specific process chemistry, temperature profile, and fabrication constraints may adjust the recommendation.
- Welded assembly cannot undergo PWHT after fabrication
- Continuous service above 649°C with long-term stability required
- Nuclear reactor coolant systems or radiation environments
- Sour service (H₂S + CO₂) with welded nozzles or flanges
- Chemical reactors at 700–1038°C continuous service
- Eliminating PWHT cost outweighs any alloy premium
- Primary medium is non-oxidising reducing acid (HCl, H₂SO₄) at moderate temperature
- PWHT is feasible and already in the fabrication workflow
- Broadest-spectrum alloy needed for variable or undefined chemistry
- Component is a solid machined part — no welding involved
- Replacement parts for an existing C276 material system
- Process contains both oxidising and reducing acids simultaneously
- Flue gas desulphurisation (FGD) with high-chloride condensate
- Nitric acid or aqua-regia-adjacent environments
- Pharmaceutical manufacturing with mixed solvent streams
- Maximum oxidation resistance outweighs higher cost
1. Will the part be welded and enter service without full PWHT? If yes → Hastelloy C4 is strongly preferred.
2. Is nitric acid or a strongly oxidising acid a major component of the process stream? If yes → Hastelloy C22 leads.
3. Is the primary threat a non-oxidising reducing acid (HCl), with no fabrication constraints? If yes → Hastelloy C276 is the proven, cost-effective choice.
Hastelloy C4 vs C276 vs C22 — Common Engineering Questions
The main difference is Hastelloy C4 (UNS N06455) contains a titanium addition (up to 0.7%) that stabilises the alloy against carbide precipitation in the weld heat-affected zone. This gives C4 excellent corrosion resistance in the as-welded condition without requiring post-weld heat treatment, whereas Hastelloy C276 (UNS N10276) typically requires PWHT to maintain its corrosion resistance after welding. C4 also offers superior long-term thermal stability above 649°C compared to C276.
No. Hastelloy C4 (UNS N06455) does not require post-weld heat treatment in most applications. Its ultra-low carbon content (max 0.015%) and titanium stabilisation prevent grain-boundary carbide precipitation in the weld heat-affected zone, maintaining excellent corrosion resistance in the as-welded condition. This is confirmed by ASTM G28 Method A testing on as-welded specimens. Eliminating PWHT is C4's primary engineering and economic advantage over Hastelloy C276.
Hastelloy C22 (UNS N06022) is superior in oxidising acid environments due to its significantly higher chromium content (20–22.5% vs 14–18% in C4). In concentrated nitric acid (65%, boiling), C22 achieves a corrosion rate below 0.3 mm/yr (Excellent), while C4 reaches approximately 0.8 mm/yr (Very Good) and C276 exceeds 1.5 mm/yr (Moderate). For mixed oxidising and reducing acid environments — or wherever FGD service is involved — C22 is the preferred selection.
All three alloys — Hastelloy C4 (N06455), C276 (N10276), and C22 (N06022) — are fully compliant with NACE MR0175 / ISO 15156 for sour service (H₂S + CO₂ + Cl⁻) environments. For welded wellhead components such as tubing heads, casing hangers, and valve bodies, Hastelloy C4 is particularly preferred because it maintains excellent as-welded corrosion resistance without requiring PWHT. Jiangsu Liangyi has supplied C4 forged wellhead components with over 8 years of corrosion-free service in 15,000 psi sour gas wells in Saudi Arabia and UAE.
The UNS designation for Hastelloy C4 is UNS N06455. The European Werkstoff number is 2.4610. The alloy is also known as Alloy C-4. It is covered by ASTM B564 (forgings), ASTM B574 (rod), and ASTM B575 (plate). Our UNS N06455 forgings are produced and tested to conform with both UNS N06455 (US) and Werkstoff 2.4610 (European) material specifications, with test reports documenting compliance upon request.
Hastelloy C22 (UNS N06022) is the preferred choice for FGD systems. Its higher chromium content (20–22.5%) provides superior resistance to the mixed oxidising and reducing acid conditions typical of FGD scrubbers, which often contain sulphuric acid and hydrochloric acid at elevated temperatures with high chloride concentrations. Hastelloy C276 is also widely used in FGD and is an acceptable alternative, particularly for machined, non-welded components.
Hastelloy C4 (UNS N06455) achieves a corrosion rate below 0.5 mm/yr in 10% HCl at boiling temperature, rated Excellent per ASTM G31 immersion testing. This performance is equivalent to Hastelloy C276 under the same test conditions. Both C4 and C276 significantly outperform Hastelloy C22 in HCl service, where C22 reaches approximately 0.8–1.0 mm/yr (Very Good) due to its lower molybdenum content.
Hastelloy C4 (UNS N06455) outperforms C276 in nuclear applications primarily due to its superior long-term high-temperature stability. C4's titanium stabilisation and ultra-low silicon content prevent the precipitation of deleterious intermetallic phases (mu-phase, sigma-phase) during prolonged service at 649–1038°C — a temperature range critical for nuclear reactor coolant systems. After 1,000 hours at 870°C, C4 retains significantly higher ductility and impact toughness than C276. Jiangsu Liangyi has supplied UNS N06455 forgings for nuclear power projects in China, Vietnam, and Thailand with full nuclear-grade quality documentation.
Summary: The Definitive Hastelloy C4 vs C276 vs C22 Verdict
Hastelloy C4: Best for welded assemblies (no PWHT), nuclear service, and continuous high-temperature operation above 649°C.
Hastelloy C276: Best for non-oxidising reducing acid environments with machined parts or when PWHT is feasible.
Hastelloy C22: Best for mixed oxidising/reducing acids, FGD systems, and nitric acid-dominant service.
The table below summarises the recommended choice by scenario. If Hastelloy C4 fits your requirements, the Alloy C-4 forging parts page covers available product forms, size ranges, delivery standards, and how to request a quotation.
| Application Scenario | Recommended Alloy | Primary Engineering Reason |
|---|---|---|
| Welded chemical reactor — no PWHT possible | C4 | Ti stabilisation → HAZ performs like base metal as-welded |
| Nuclear reactor coolant system components | C4 | No brittle phase at 649–1038°C; highest thermal stability |
| HCl or H₂SO₄ — solid machined parts only | C276 | Highest Mo + W content; PWHT is irrelevant for machined parts |
| FGD scrubber / mixed oxidising-reducing acid | C22 | Highest Cr content; dominant performance in mixed acid |
| Offshore sour service wellhead forgings | C4 | NACE MR0175 compliant; 8+ yr field record Middle East sour wells |
| Replacement part — existing C276 system | C276 | Material matching; widest global stock availability |
| Pharmaceutical — organic + nitric acid streams | C22 | Best oxidation resistance in mixed acid environments |
Need Hastelloy C4, C276, or C22 Forged Parts?
Jiangsu Liangyi Co., Limited manufactures custom open die forgings and seamless rolled rings in all three alloys. ISO 9001:2015 certified quality management system. EN 10204 3.1 MTCs supplied as standard; 3.2 available via client-nominated inspectors. Export to 50+ countries. Formal quotation within 48 hours.
Trademark Notice: "Hastelloy" is a registered trademark of Haynes International, Inc. Jiangsu Liangyi Co., Limited is not affiliated with, sponsored by, or endorsed by Haynes International, Inc. Use of the designation "Hastelloy C4 / C276 / C22" on this page refers solely to the alloy compositions defined by the corresponding UNS specifications (N06455 / N10276 / N06022), which are open industry standards. Jiangsu Liangyi forges materials to these published UNS specifications. Corrosion rate data cited on this page is derived from ASTM G31 standard immersion testing protocols and publicly available Haynes International technical bulletins.
Certification & Conformance Notice: Jiangsu Liangyi Co., Limited holds ISO 9001:2015 certification for its Quality Management System (QMS) — issued by an accredited third-party certification body. This is a QMS certification covering our manufacturing processes; it is not a product certification for any specific alloy grade. References on this page to API 6A, ASME BPVC, NACE MR0175/ISO 15156, ASTM, DIN, EN, and JIS indicate that our forgings are produced and tested to conform with the material requirements of these standards — not that Jiangsu Liangyi holds independent certification status under any of these bodies. EN 10204 3.1 mill test certificates are issued by Jiangsu Liangyi as manufacturer. EN 10204 3.2 certificates require witness testing and co-signature by a client-nominated accredited third-party inspection agency (such as TÜV, Bureau Veritas, DNV, SGS, or Intertek) — Jiangsu Liangyi coordinates this process but does not issue 3.2 certificates independently.