What is the difference between hastelloy and inconel​

Hastelloy and Inconel are both nickel-based superalloys, but they are engineered for different priorities: Hastelloy grades (for example C-276, C-22) are optimized for exceptional corrosion resistance in highly aggressive chemical environments (strong acids, chlorides, reducing media), while Inconel grades (for example 625, 718) are optimized for high temperature strength, oxidation resistance and mechanical performance under thermal and mechanical stress. Choose Hastelloy when corrosion control is the primary driver; choose Inconel when strength at elevated temperatures or structural performance under cyclical loads matters most.

Difference between hastelloy and inconel​

(Notes: this table summarizes general trends. Specific alloy chemistry and product form can change performance. Sources: manufacturer datasheets and alloy handbooks.)

What are Hastelloy and Inconel?

Both names began as trade names for families of nickel-based alloys, later generalized to categories used across the supply chain.

• Hastelloy is a registered trade name historically associated with Haynes International (and earlier with the original Hastelloy brand). Popular grades such as Hastelloy C-276 (UNS N10276) and C-22 (UNS N06022) were developed for corrosive chemical service and have long manufacturing and field records.

• Inconel is a registered name used by Special Metals and others for nickel-chromium superalloys designed for strength and oxidation resistance at elevated temperatures. Widely used grades include Inconel 600, 625 (UNS N06625) and 718 (UNS N07718)—the latter being precipitation-hardenable and widely used in aerospace.

The term “superalloy” covers both groups; the practical difference lies in formulation and the properties emphasized during alloy design.

Chemical makeup and representative grades

Nickel is the principal element for both families, but the secondary alloying elements tell the story:

• Hastelloy (C-type examples): high molybdenum (Mo) and often tungsten (W) plus chromium (Cr). The high Mo content gives superior resistance to localized corrosion (pitting, crevice attack) and to reducing environments. Example: Hastelloy C-276 (UNS N10276) is a Ni-Cr-Mo-W alloy engineered for broad corrosion resistance.

• Inconel (common grades):

  • Inconel 600 (N06600): Ni-Cr alloy developed for oxidation resistance.

  • Inconel 625 (N06625): Ni-Cr with Mo and niobium (Nb) giving a mix of corrosion resistance and high-temperature strength; it is not precipitation-hardened but gains strength from solid solution and Nb/Mo effects.

  • Inconel 718 (N07718): Ni-Cr-Fe with Nb and Ti additions; precipitation-hardening alloy with very high tensile and creep strength at elevated temperatures—favored in aerospace.

UNS numbers (useful for procurement/specification):

• Hastelloy C-276 — UNS N10276.

• Inconel 625 — UNS N06625; Inconel 718 — UNS N07718.

Corrosion resistance: where Hastelloy typically outperforms Inconel

Hastelloy C-type alloys were engineered to survive some of the most aggressive chemical environments:

• Excellent resistance to: strong mineral acids (e.g., hydrochloric, sulfuric under many conditions), organic acids, wet chlorine and chlorinated brines, and environments that promote chloride-induced pitting and crevice corrosion. High Mo and W contents reduce susceptibility to localized attack and to stress-corrosion cracking in many media.

• Inconel strengths in corrosion: Inconel 625 resists pitting and crevice corrosion in many aqueous media and has strong performance in oxidizing environments; however, for certain reducing or highly chlorinated acid services Hastelloy C-276 or C-22 are commonly chosen over 625.

Practical rule: for chemical processing equipment and waste streams containing mixed oxidizers and reducers, or when chloride-assisted localized corrosion is a risk, engineers frequently default to Hastelloy classes. For oxidizing high-temperature service or where mechanical performance at temperature matters, Inconel grades are often better suited.

High-temperature mechanical and oxidation performance

Inconel alloys (especially 718 and some versions of 625) were developed with temperature performance in mind:

• Inconel 718: precipitation hardening provides exceptional yield and tensile strength up to roughly 650–700°C (about 1200–1300°F) and excellent creep resistance for structural parts in gas turbines and aerospace engines.

• Inconel 625: balances corrosion resistance with good strength to about 980°C (1800°F) in some applications; widely used where moderate high-temp strength plus corrosion resistance are required.

• Hastelloy C-grades have good oxidation resistance at moderately high temperatures but were not generally developed to match the creep-strength of age-hardenable Inconel alloys. For environments dominated by mechanical load at elevated temperatures, Inconel 718 or other high-temperature superalloys are typically preferred.

Fabrication, welding and heat treatment

Both alloy families can be fabricated, but there are practical differences:

• Welding:

  • Many Hastelloy grades were engineered to minimize carbide precipitation and perform well in the weld heat-affected zone—C-276 specifically emphasizes fabricability and weldability.

  • Inconel 625 is known for good fabricability and joining characteristics. Inconel 718, however, requires more care because it is age-hardenable; post-weld heat treatment is often needed to restore full strength and to avoid weld-zone embrittlement if not processed correctly.

• Forming / machining: Both are more difficult to machine than common stainless steels; tool selection and machining parameters must be optimized. Hastelloy grades with high Mo/W contents can be tough on tooling.

• Heat treatments: Inconel 718 uses controlled solution and aging cycles to achieve peak properties. C-type Hastelloys generally do not rely on precipitation hardening; they obtain corrosion behavior and acceptable strength through composition and solution treatments.

Standards, product forms and certifications

Both groups are covered by ASTM/ASME/AMS/UNS numbering systems and are available in plate, sheet, tube, bar, forgings and welding consumables.

• Examples of standards and listings: many alloy datasheets and manufacturer documents list ASME/ASTM product specifications and show acceptance in codes such as ASME Section II/Section VIII and, for some alloys, NACE MR0175/ISO 15156 for sour-service (H₂S) oil and gas applications. Check specific grade datasheets to confirm code acceptance for a given pressure-temperature envelope.

• UNS numbers are the standard way to specify the chemistry in purchase orders and mechanical test certificates: e.g., UNS N10276 (Hastelloy C-276), UNS N06625 (Inconel 625), UNS N07718 (Inconel 718).

Typical real-world applications (how engineers pick)

• Hastelloy (C-series): chemical processing vessels, acid recovery, waste treatment, flue gas desulfurization components, chlorination equipment, pulp & paper digesters, and pickling lines — situations with mixed oxidizing and reducing chemicals or high chloride content.

• Inconel 625: heat exchangers, marine exhausts, chemical plant piping where oxidation and general corrosion are concerns, and cryogenic to high-temp structural parts; often used where a balance of corrosion resistance and strength is required.

• Inconel 718: aerospace engine components, high-strength fasteners, rotating parts, and other components that need reliable mechanical performance under cyclic thermal/mechanical stress.

Cost, availability and procurement notes

• Material cost: Hastelloy alloys (especially those with high Mo/W) commonly command a price premium relative to many Inconel grades because Mo and W are expensive and the alloys are produced in smaller volumes. Inconel 625 and 718 are also premium materials but often more widely stocked, which can improve lead times and pricing. Market fluctuations in Mo, Nb and Ni impact both families.

• Lead time & forms: Check with suppliers for availability in the required product form. If long lead times are critical, consider multiple acceptable UNS alternatives and confirm code acceptance (ASME/ASTM/AMS) in the procurement documents.

A practical selection checklist

Use the following decision path when specifying alloy for a new design:

1. Define the operating environment: list all chemicals, temperatures, pressures, and expected duration.

2. Ask: Is localized corrosion (pitting, crevice) or general oxidation the greater threat? If localized/chloride risk → consider Hastelloy C-grades. If thermal oxidation and strength under load → consider Inconel 625/718.

3. Check codes and standards required by the project (ASME, NACE, AMS) and confirm grade acceptance.

4. Factor in fabrication: weldability, need for post-weld heat treatment, and machining constraints.

5. Run a cost vs. lifetime analysis: sometimes higher initial cost of Hastelloy is justified by much longer service life in aggressive media.

6. If unsure, order a laboratory corrosion test or consult an alloy producer’s technical service for application-specific guidance.

Comparison table (selected properties & typical numbers)

Values vary by producer, heat, and testing method. Use datasheet values for final design.

(Sources: manufacturer technical bulletins and alloy handbooks.)

Frequently asked questions

1. Which alloy is best for hydrochloric acid service?
   For concentrated HCl and mixed aggressive environments, Hastelloy C-276 or C-22 are usually recommended due to superior resistance to localized attack. Confirm with corrosion testing for your temperature and concentration.

2. Can Inconel 625 replace Hastelloy C-276?
   In some oxidizing or less aggressive scenarios, 625 may be acceptable, but for severe chloride/reducing acid conditions Hastelloy C-276 is typically safer. Always run compatibility checks.

3. Is Hastelloy good at high temperatures?
   Hastelloy resists oxidation reasonably well at moderate temperatures, but it was not primarily formulated for long-term high-temperature structural loads—Inconel 718 or other high-temp superalloys usually outperform Hastelloy for creep strength.

4. Which is easier to weld: Hastelloy or Inconel?
   Both can be welded successfully; Inconel 625 and many Hastelloy grades weld readily. Inconel 718 requires care because of age-hardening and may need specified post-weld heat treatment.

5. Are there environments where neither is suitable?
   Yes—extremely oxidizing molten salts, some molten metals or environments with metal-specific corrosion mechanisms may require different alloy families or protective linings/coatings. Conduct lab tests for unusual chemistries.

6. Do standards cover both families?
   Yes. Each alloy has ASTM/ASME/AMS product specifications and is identified by a UNS number (use this in procurement). Some grades are explicitly listed in NACE and other industry codes—always check the specific grade and code.

7. Which is more expensive?
   Hastelloy often costs more per kilogram because of higher Mo and W content and smaller production volumes, but total lifecycle cost depends on longevity in service.

8. Can you use Hastelloy in seawater applications?
   Some Hastelloy grades perform well in marine environments, but localized chloride attack is always a concern—select grade carefully and consider cathodic protection or duplex stainless steels depending on service.

9. Which alloy is more common in aerospace?
   Inconel 718 (and other Inconel/Ni-Cr superalloys) are common in aerospace for their strength and fatigue life at temperature. Hastelloy is rare in primary aerospace structural parts.

10. Where can I get authoritative material data?
    Start with manufacturer technical bulletins (Haynes, Special Metals), recognized material databases (MatWeb, ASM), and code documents (ASME/ASTM). For procurement, specify the UNS number and applicable ASTM/ASME/AMS spec.

Practical examples and short case notes

• Chemical plant: replacement of 316L stainless in a chloride-rich acid line → Hastelloy C-276 is often selected because it resists pitting and crevice corrosion that doomed the stainless steel.

• Heat exchanger in a refinery: where high-temp oxidation and mechanical strength were required along with good corrosion resistance → Inconel 625 sometimes chosen for tube bundles, with 718 reserved for structural parts.

These case directions illustrate typical engineering tradeoffs: accept higher alloy cost for longer service life or choose a balanced alloy for combined corrosion + mechanical demands.

Final summary

Hastelloy and Inconel are both high-value nickel alloys, but the design intent differs. Hastelloy C-series is tailored for extreme corrosion environments—pick it when localized corrosion and chemical attack dominate the failure modes. Inconel alloys emphasize high-temperature mechanical performance and oxidation resistance—pick 625 for corrosion/temperature balance or 718 for high strength under thermal/mechanical stress. The correct choice depends on a careful list of fluids, temperatures, mechanical loads, codes, fabrication constraints and lifecycle cost. When in doubt, consult manufacturer technical services and run small-scale corrosion tests under realistic conditions.

Authoritative references

• INCONEL® Alloy 625 — Special Metals (technical bulletin, PDF)
• INCONEL® Alloy 718 — Special Metals (technical bulletin, PDF)
• Inconel — Wikipedia (encyclopedic overview)