Inconel 625 vs Hastelloy C276 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Engineers, procurement managers, and manufacturing planners frequently choose between Inconel 625 and Hastelloy C-276 when a design requires a corrosion-resistant, high-performance nickel alloy. The decision typically balances corrosion resistance (including resistance to pitting, crevice corrosion, and chemical attack) against strength, weldability, availability, and cost. In service environments ranging from chemical processing to marine and aerospace, the selection dilemma is often “maximum corrosion resistance vs. optimal mechanical performance and fabricability.”
The primary metallurgical distinction between these two alloys is their alloying strategy: one emphasizes nickel-based solid-solution strengthening with niobium additions (Inconel 625), while the other emphasizes a higher content of molybdenum and tungsten to extend corrosion resistance in aggressive chemical environments (Hastelloy C-276). Because that difference drives many downstream behaviors (mechanical properties, corrosion performance, fabrication characteristics), the two alloys are commonly compared for harsh-service components.
1. Standards and Designations
Major standards and common designations for each alloy include industry specifications and UNS numbers:
- Inconel 625
- UNS: N06625
- Common ASTM/ASME: ASTM B443 (for wrought products), ASTM B443/B444 for sheet/plate/strip/forgings; ASME boiler and pressure vessel code references as applicable.
- EN/JIS/GB equivalents: No direct 1:1 EN standard number; often cross-referenced in supplier datasheets.
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Category: Nickel-based alloy (commonly treated as a high-performance alloy rather than a stainless steel).
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Hastelloy C-276
- UNS: N10276
- Common ASTM/ASME: ASTM B622 (for corrosion-resistant wrought shapes and tubing), ASTM B619/B626 for other product forms.
- EN/JIS/GB equivalents: As with many proprietary Ni alloys, direct EN numbers are limited; used per material specification.
- Category: Nickel-based corrosion-resistant alloy (often grouped with high-performance corrosion alloys).
Both are nickel-based alloys rather than carbon steels, stainless steels, tool steels, or HSLA grades.
2. Chemical Composition and Alloying Strategy
Below is a compact view of typical composition ranges. Values are representative typical ranges (wt%) encountered in vendor datasheets and standards; always verify specific mill certification for procurement.
| Element | Inconel 625 (typical wt%) | Hastelloy C-276 (typical wt%) |
|---|---|---|
| C | ≤ 0.10 | ≤ 0.02 |
| Mn | ≤ 0.50 | ≤ 0.50 |
| Si | ≤ 0.50 | ≤ 0.08 |
| P | ≤ 0.015 | ≤ 0.03 |
| S | ≤ 0.015 | ≤ 0.02 |
| Cr | 20–23 | 15.5–17.5 |
| Ni | Balance (~58) | Balance (~57) |
| Mo | 8–10 | 15–17 |
| V | — | — |
| Nb (+Ta) | 3.0–4.2 | ≤ 0.5 |
| Ti | ≤ 0.40 | ≤ 0.2 |
| B | ≤ 0.006 | ≤ 0.01 |
| N | ≤ 0.05 | ≤ 0.05 |
| W | — | 3.5–4.5 |
| Fe | ≤ 5 | ~4–7 |
How alloying affects behaviour - Nickel (Ni) is the base matrix providing ductility and resistance to many corrosive media. - Chromium (Cr) provides general oxidation and passivation capability. - Molybdenum (Mo) and tungsten (W) dramatically improve resistance to localized corrosion (pitting, crevice) and resistance to reducing acids; higher Mo+W content in C-276 gives superior resistance in many aggressive chemical environments. - Niobium (Nb) and tantalum (Ta) in Inconel 625 contribute to strong solid-solution strengthening and stabilization against certain precipitates, enhancing high-temperature strength and creep resistance. - Low carbon in C-276 minimizes carbide precipitation and intergranular attack in welded or sensitized conditions.
3. Microstructure and Heat Treatment Response
Microstructure (as-fabricated and after typical thermal processing) - Both alloys are essentially single-phase nickel-based solid solutions in the annealed condition. They do not rely on martensitic transformation like steels; instead their properties come from solid-solution strengthening and, for Inconel 625, controlled clustering/precipitation of Nb-rich phases if aged. - Inconel 625: predominantly face-centered cubic (FCC) Ni matrix with Cr, Mo, and Nb in solid solution. Under certain aging/thermal exposure, small Nb-rich (γ″/δ-like) or carbides can form if exposed to high temperatures for long times; however, N06625 is formulated to resist deleterious phase precipitation and is normally used in the annealed condition. - Hastelloy C-276: FCC Ni matrix with higher Mo and W in solution. It is highly resistant to intermetallic precipitation and carbide formation when used in standard annealed condition; however, prolonged high-temperature exposure can still cause grain boundary phases in extremes.
Heat treatment response and typical processing routes - Neither alloy benefits from conventional quench-and-temper cycles as steels do. Solution annealing (e.g., heating to a specified homogenization temperature and rapid cooling) is used to dissolve any precipitates and restore corrosion resistance after fabrication/welding. - Thermo-mechanical processing (hot working) controls grain size and influences toughness; both alloys respond to controlled hot-working followed by annealing. - Normalizing is not applicable in steel sense. For precipitation-hardenable Ni alloys, specialized aging is used; Inconel 625 is not primarily a precipitation-hardening alloy but can show some age-related strengthening — typically it is supplied in annealed form for best corrosion resistance.
4. Mechanical Properties
Typical room-temperature mechanical behavior (annealed condition). Provide these as qualitative/typical ranges — confirm mill certificates for design.
| Property | Inconel 625 (annealed, typical) | Hastelloy C-276 (annealed, typical) |
|---|---|---|
| Tensile strength (UTS) | Moderate to high (commonly in the upper MPa-range for Ni alloys) | Moderate (generally similar order but often slightly lower than 625) |
| Yield strength (0.2% offset) | Relatively high for a corrosion alloy (solid-solution + Nb) | Lower than 625 in annealed condition |
| Elongation (%) | Good ductility (typically high elongation) | Good to excellent ductility |
| Impact toughness | Good toughness at ambient; retains ductility | Good toughness and ductility; sensitive to fabrication defects |
| Hardness | Moderate (soft relative to hardened steels) | Moderate |
Interpretation - Inconel 625 commonly exhibits higher yield/tensile strength relative to Hastelloy C-276 because its alloying (particularly Nb/Ta and the solid-solution design) emphasizes strength while retaining corrosion performance. - Hastelloy C-276 emphasizes maximum corrosion resistance (Mo + W) and therefore tends to be slightly less strong in the annealed condition, but still ductile and tough for typical fabrication operations.
5. Weldability
Weldability is excellent for both alloys compared with many stainless steels, but there are important distinctions.
Factors: - Low carbon in C-276 reduces carbide precipitation and intergranular corrosion risk in weld HAZ. - Inconel 625 is renowned for its excellent weldability, minimal need for post-weld heat treatment, and robustness against weld cracking due to its high nickel content and solid-solution strengthening.
Useful weldability indices (qualitative use) - Carbon equivalent formula for steels (example) can be used to estimate hardenability: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - A more comprehensive parameter: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation (qualitative) - These formulas are tailored to steels and are not directly quantitative for Ni alloys, but they illustrate that elements such as Nb and Mo raise the metric for susceptibility to weld-induced cracking or hardening in steels. For Ni alloys, the high Ni content and inherently low hardenability mean both alloys are highly forgiving in welding. - Best practice: use matched filler metals, control heat input, and follow supplier recommendations. Inconel 625 typically can be welded with no post-weld anneal; C-276 also welds well but attention to filler selection and avoiding contamination is important.
6. Corrosion and Surface Protection
- For non-stainless steels, surface protection such as galvanizing, painting, or plating is standard; for Ni-based alloys, intrinsic corrosion resistance is the design point and surface coatings are less commonly used unless for cost or cosmetic reasons.
- PREN (Pitting Resistance Equivalent Number) is mainly used for stainless steels: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This index is not directly applicable to nickel-base alloys; use it only as a stainless-steel comparison metric.
Corrosion behavior - Hastelloy C-276 provides superior resistance to localized corrosion (pitting, crevice corrosion) and general corrosion in reducing acidic and chloride-bearing environments due to its higher Mo and W content. C-276 is often specified for mixed-oxidizing/reducing chemical environments, wet chlorine, and aggressive organic acids. - Inconel 625 offers excellent resistance to a broad range of corrosive medias including seawater, hydrofluoric acid at certain concentrations, and many other industrial media. Its compromise of strong mechanical properties with very good corrosion resistance makes it a popular general-purpose Ni alloy. - In applications where galvanic coupling matters, consider electrochemical potential differences; Ni alloys are nobler than steels and may accelerate corrosion of less-noble metals if coupled without insulation.
7. Fabrication, Machinability, and Formability
- Machinability: Both alloys are more difficult to machine than carbon steels and stainless steels. They are work-hardening and generate high heat; tooling recommendations include rigid setups, sharp carbide tooling, controlled feeds, and high coolant flow. Inconel 625 is considered challenging but manageable; C-276 often machines similarly or slightly worse due to high Mo/W content.
- Formability: Both have good ductility for forming and bending when annealed. Hot forming is common for heavy sections. Springback may be more pronounced than in softer steels.
- Surface finishing and grinding require appropriate abrasives and cooling; both alloys respond well to electrolytic polishing for corrosion-critical surfaces.
8. Typical Applications
| Inconel 625 – Typical Uses | Hastelloy C-276 – Typical Uses |
|---|---|
| Aerospace components (engine frames, afterburner parts) and rocket motor cases where strength + corrosion resistance are required | Chemical process equipment handling mixed corrosives, acid recovery, chlorination systems |
| Marine components: seawater systems, piping, and flanges | Flue gas desulfurization scrubbers, pollution control, chemical reactors with reducing acids |
| Heat exchangers and pressure vessels exposed to high temperature and corrosive atmospheres | Waste treatment, electrochemical process equipment, paper/pulp corrosive streams |
| Fasteners, springs, and high-stress welded fabrications benefiting from high strength and weldability | Tank linings, valves, and fittings for aggressive chemical processing |
Selection rationale - Choose Inconel 625 where mechanical load, temperature, and fabrication ease (weldability) are critical while still demanding broad corrosion resistance. - Choose Hastelloy C-276 when the primary driver is corrosion resistance to chlorides, mixed oxidants/reductants, or highly acidic/reducing chemical environments.
9. Cost and Availability
- Cost: Hastelloy C-276 typically costs more than Inconel 625 per kilogram because of the higher Mo and W content and the associated material cost. Market prices vary with elemental commodity prices (Mo, W, Ni).
- Availability: Inconel 625 is produced in a wide range of product forms (bar, plate, pipe, welded constructions) and is highly available worldwide. Hastelloy C-276 is widely available but may be offered in fewer standard mill forms and in some regions may have longer lead times for large or specialized orders.
- Procurement tip: specify UNS number, product form, and required mill test report (MTR) to avoid substitution confusion; price variation can be substantial depending on market for Mo and Ni.
10. Summary and Recommendation
| Criterion | Inconel 625 | Hastelloy C-276 |
|---|---|---|
| Weldability | Excellent; forgiving; minimal PWHT typically required | Excellent; requires matching filler and care to avoid contamination |
| Strength–Toughness | Higher yield/tensile in annealed condition; very good toughness | Good toughness; generally somewhat lower strength in annealed condition |
| Cost | Lower (relative) and more widely stocked | Higher cost due to Mo/W; sometimes longer lead time |
Recommendation - Choose Inconel 625 if you need a robust combination of high-strength, excellent weldability, and very good corrosion resistance across a broad set of environments — typical for high-stress, elevated-temperature, or welded assemblies where fabrication ease and mechanical performance are priorities. - Choose Hastelloy C-276 if the service environment is dominated by highly aggressive chemical attack (chlorides, mixed oxidizing and reducing agents, organic acids) and maximum corrosion resistance is the dominant requirement, even at higher material cost and potential fabrication constraints.
Final procurement note: always confirm the specific alloy lot/heat with mill test reports, verify the required product form and heat treatment state, and consider lifetime cost analysis (initial material cost vs. maintenance/repair and service life) when selecting between these two high-performance nickel alloys.