Hastelloy C276 vs C22 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Hastelloy C276 and Hastelloy C22 are two widely used nickel‑based corrosion‑resistant alloys that often appear as competing choices in chemical processing, pollution control, and offshore systems. Engineers and procurement professionals commonly balance corrosion performance, fabrication needs, and lifecycle cost when deciding between them. Typical decision contexts include selection for aggressive chloride-bearing environments, mixed oxidizing/reducing chemistries, high‑temperature service, or where welding and fabricability are critical.
The principal technical distinction between these alloys is their alloying strategy: C276 emphasizes molybdenum and tungsten to resist localized attack in reducing and mixed environments, while C22 emphasizes a higher chromium level (combined with molybdenum) to strengthen resistance in oxidizing and a broader range of corrosives. Because both are nickel‑based, their mechanical behavior is similar, but corrosion performance and cost considerations usually drive the choice.
1. Standards and Designations
- Common specifications and designations:
- ASTM/ASME: Often referenced via UNS numbers — C276 (UNS N10276), C‑22 (UNS N06022).
- EN / European: Less commonly used for these proprietary alloys; equivalents may be listed by supplier.
- JIS / GB: Not direct one‑to‑one equivalents; typically procured to UNS/ASTM or vendor datasheets.
- Classification:
- Both Hastelloy C276 and C22 are nickel‑based corrosion‑resistant alloys (commonly grouped with corrosion‑resistant alloys rather than traditional stainless steels). They are not carbon steels, tool steels, or HSLA grades.
2. Chemical Composition and Alloying Strategy
The following table shows representative, approximate typical alloying elements and their roles. Exact limits vary by specification and supplier; consult the manufacturer datasheet for certified compositions.
| Element | Typical composition (approx. wt%) — C276 | Typical composition (approx. wt%) — C22 |
|---|---|---|
| C | ≤ 0.02 (very low) | ≤ 0.02 (very low) |
| Mn | 0.2–1.0 (trace to low) | 0.2–1.0 (trace to low) |
| Si | ≤ 0.08 (deoxidizer) | ≤ 0.08 (deoxidizer) |
| P | ≤ 0.03 (impurity control) | ≤ 0.03 (impurity control) |
| S | ≤ 0.015 (impurity control) | ≤ 0.015 (impurity control) |
| Cr | ~15–17 (moderate) | ~20–22 (higher) |
| Ni | Balance (~50–60) | Balance (~50–60) |
| Mo | ~15–17 (high) | ~12–14 (substantial) |
| V | ≤ 0.35 (minor) | ≤ 0.35 (minor) |
| Nb | ≤ 0.4 (minor) | ≤ 0.4 (minor) |
| Ti | ≤ 0.4 (minor) | ≤ 0.4 (minor) |
| B | trace | trace |
| N | trace (very low) | trace (very low) |
| Fe | ~4–7 (residual) | ~3–6 (residual) |
| W (tungsten) | up to ~3 (enhances pitting resistance) | limited to low levels or absent |
Notes on alloying strategy: - Nickel: primary matrix former, provides corrosion resistance and toughness. - Chromium: promotes passivation and resistance to oxidizing conditions; C22 has a higher chromium fraction to broaden oxidizing corrosion resistance. - Molybdenum and tungsten: strengthen resistance to pitting, crevice corrosion, and corrosion in reducing environments; C276 is richer in Mo (and may include W) to improve performance in chloride‑bearing and reducing chemistries. - Low carbon, controlled impurities, and minor stabilizers (Nb, Ti) are used to minimize sensitization and improve high‑temperature stability.
3. Microstructure and Heat Treatment Response
Microstructure: - Both alloys are solid solution austenitic (face‑centered cubic nickel matrix) with carbide/precipitate control via low carbon and small additions of Nb/Ti where applicable. - They do not undergo martensitic transformations typical of carbon steels; microstructural variations are mainly from carbide or intermetallic precipitates that can form with improper thermal exposure.
Heat‑treatment and processing response: - Typical processing is solution anneal (high temperature soak followed by rapid cooling) to dissolve precipitates and restore corrosion resistance. For example, suppliers specify solution annealing in the austenitic range (consult datasheet for exact temperatures). - Neither alloy is heat‑treatable to significantly increase strength by precipitation hardening in the way that some stainless or precipitation‑hardening nickel alloys are. - Long exposure to intermediate temperatures can produce carbide/nitride precipitation or intermetallic phases that degrade corrosion resistance. Proper solution anneal and controlled cooling restore matrix homogeneity. - Thermo‑mechanical processing (cold work followed by solution anneal) is used to produce sheet, plate, and tubing with desired mechanical properties; significant cold work increases strength but may affect localized corrosion resistance if not solution treated.
4. Mechanical Properties
Mechanical performance is influenced by product form (plate, sheet, pipe), cold work, and thermal history. The following table provides a qualitative comparison suitable for design selection; for project design, use supplier data for exact numeric values by product form.
| Property | C276 | C22 |
|---|---|---|
| Tensile strength | Comparable; both moderate for nickel alloys (good for pressure equipment) | Comparable; similar tensile range to C276 |
| Yield strength | Similar; neither is designed for high yield strength without cold work | Similar; minor variations by product and heat treatment |
| Elongation (ductility) | Good ductility (allows forming/welding) | Good ductility, comparable to C276 |
| Impact toughness | Good at ambient and low temperatures; retains toughness due to nickel matrix | Good, comparable to C276 |
| Hardness | Moderate; can be increased by cold work | Moderate; similar behavior |
Interpretation: - In practical engineering terms, C276 and C22 have broadly similar mechanical properties. Differences in strength or toughness are typically small relative to variability from product form and cold working. Both alloys are selected primarily for corrosion resistance rather than mechanical strength.
5. Weldability
Weldability considerations focus on low carbon content, nickel base, and alloying elements that can promote hot cracking or sensitization.
- General weldability: Both C276 and C22 are considered weldable with standard nickel‑based filler metals and appropriate procedures. Preheating is generally not required; post‑weld solution anneal is recommended in critical corrosion applications to recover corrosion resistance.
- Carbon and hardenability: Very low carbon content reduces risk of sensitization and hardening‑related cracking.
- Microalloying effects: Nb and Ti can form stable carbides/nitrides; control of filler metal and heat input minimizes undesirable precipitates.
Useful indices for qualitative interpretation (no numeric evaluation here): - IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Preventive index $P_{cm}$: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation: - These formulas indicate that although Ni itself reduces "CE" concerns relative to steels, the presence of Cr, Mo, and Nb influences weldability indices. In practice, both alloys weld readily with recommended nickel‑based filler metals; qualified procedures and post‑weld heat treatment are used for corrosion‑critical fabrications.
6. Corrosion and Surface Protection
- Applicability of PREN: The pitting resistance equivalent number (PREN) is widely used for austenitic stainless steels and can be indicative for nickel alloys in evaluating pitting propensity: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ However, PREN was developed for Cr–Mo–N stainless steels and is only a rough indicator for nickel‑based alloys; direct corrosion testing is preferred.
Corrosion behavior — practical comparison: - C276: Designed to resist pitting, crevice corrosion, and stress‑corrosion cracking in reducing and mixed oxidizing/reducing environments with chlorides and sulfur species. High Mo (and W where present) enhances resistance in localized and reducing chemistries. - C22: Higher chromium content provides robust passivation and superior resistance to oxidizing conditions (e.g., nitric acid environments) while maintaining strong resistance to a wide range of non‑oxidizing acids due to significant Mo content. C22 is often preferred where oxidizing agents are present alongside reducing species.
Surface protection: - For carbon steels one would consider galvanizing, painting, or coatings. For C276 and C22 (both corrosion alloys), coatings are generally unnecessary unless specific wear, fouling, or abrasion concerns exist. Mechanical or chemical polishing, passivation, or cathodic protection may be applied depending on service.
7. Fabrication, Machinability, and Formability
- Machinability: Nickel‑based alloys are generally more difficult to machine than carbon steels; they work‑harden and have lower thermal conductivity. C276 and C22 have similar machinability characteristics; careful tool selection, speeds, feeds, coolant, and chip breaking are required.
- Formability: Good ductility allows bending and drawing operations. Where tight bending radii or extensive forming are required, intermediate solution annealing may be used to restore ductility.
- Finishing: Surface finish and polishing are feasible; both respond well to chemical cleaning and electropolishing if required for corrosion control.
8. Typical Applications
| Hastelloy C276 (typical uses) | Hastelloy C22 (typical uses) |
|---|---|
| Chemical process equipment handling chloride‑bearing and reducing acids, flue gas desulfurization (FGD) components, valves and fittings in mixed environments | Chemical process equipment exposed to oxidizing acids (e.g., nitric acid), pilot plants and scrubbing systems with mixed oxidizers, high‑reliability connectors in harsh environments |
| Wastewater treatment systems with mixed chemistries | Applications requiring strong passivation in oxidizing and reducing cycles |
| Heat exchangers and piping handling brines and organic acids | Vessels and piping where combined oxidizing and reducing attacks may occur and where high Cr offers advantage |
Selection rationale: - Choose C276 where localized attack from chlorides or reducing agents is the main concern; its higher Mo/W content is advantageous. - Choose C22 where oxidizing conditions or cyclic oxidizing/reducing environments are present and higher Cr provides more robust passivation.
9. Cost and Availability
- Relative cost: Both alloys are premium nickel alloys and cost significantly more than common stainless steels. In many markets C22 can be priced slightly higher than C276 due to composition and demand patterns, but market fluctuations affect pricing. Cost differences are project‑specific.
- Availability: Both are produced by major specialty alloy suppliers and are available in common product forms (plate, sheet, pipe, tube, bar, forgings, weld filler). Lead times vary by form, size, and market demand. Fabricated consumables (filler rods, electrodes) are also commercially available.
10. Summary and Recommendation
Summary table (qualitative)
| Characteristic | Hastelloy C276 | Hastelloy C22 |
|---|---|---|
| Weldability | Very good with Ni‑based fillers; similar to C22 | Very good with Ni‑based fillers; similar to C276 |
| Strength–Toughness | Moderate strength; good toughness and ductility | Comparable strength and toughness |
| Corrosion resistance (localized/reducing) | Excellent (higher Mo/W) | Very good (slightly lower Mo than C276) |
| Corrosion resistance (oxidizing/passive) | Very good | Excellent (higher Cr) |
| Cost (relative) | High (premium alloy) | High (often comparable or slightly higher) |
Recommendation (practical guidance): - Choose Hastelloy C276 if you need the best overall resistance to localized corrosion, crevice attack, and reducing or mixed environments where chlorides and sulfide species are present. C276 is frequently specified for flue gas desulfurization, chloride‑bearing process streams, and general duty where localized corrosion is the primary risk. - Choose Hastelloy C22 if your process includes strong oxidizers, cyclic oxidizing/reducing conditions, or you require particularly robust passivation in the presence of oxidizing acids. C22’s higher chromium content gives it an edge in oxidizing environments and in applications where both oxidizing and reducing chemistries are encountered intermittently.
Final note: For any critical procurement or design decision, request current manufacturer datasheets, corrosion testing data for the specific service, and qualified welding/fabrication procedures. Corrosion performance is application dependent; laboratory and field testing in the actual process media is the most reliable guide.