Inconel 625 vs Incoloy 825 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Inconel 625 and Incoloy 825 are two widely used nickel‑base alloys that frequently appear as alternatives during material selection for corrosive or high‑temperature service. Engineers, procurement managers, and manufacturing planners commonly weigh tradeoffs between corrosion resistance, strength, weldability, and cost when choosing between them. Typical decision contexts include offshore and subsea systems, chemical process equipment, heat exchangers, and high‑temperature gas or flue environments.
The decisive contrast between the two alloys lies in their alloying strategies: Inconel 625 is a high‑strength, Ni‑Cr‑Mo‑Nb alloy designed for solid‑solution strengthening and exceptional resistance to pitting and crevice corrosion, whereas Incoloy 825 is a Ni‑Fe‑Cr alloy with deliberate Cu and Ti additions that deliver balanced corrosion resistance in reducing and mixed acid environments with good ductility and fabricability. This difference in alloy system underpins their mechanical behavior, corrosion performance, and fabrication responses, which is why they are often compared.
1. Standards and Designations
- Inconel 625
- Common designation: UNS N06625.
- Covered by numerous wrought and fabricated product specifications under ASTM/ASME for nickel alloys (wrought bars, plate, sheet, pipe and forgings), and by international equivalents (EN, JIS, GB) for Ni‑Cr‑Mo alloys.
- Classification: Nickel‑base alloy (not a carbon steel, tool steel, or HSLA).
- Incoloy 825
- Common designation: UNS N08825.
- Available under ASTM/ASME wrought product specifications and equivalent EN/JIS/GB listings for Ni‑Fe‑Cr alloys.
- Classification: Nickel‑iron‑chromium alloy (nickel‑based but Fe‑rich relative to high‑Ni alloys).
Note: Specific ASTM/ASME part numbers differ by product form (sheet, bar, pipe). For procurement, confirm the exact spec (e.g., wrought vs cast, and applicable ASME code for pressure components).
2. Chemical Composition and Alloying Strategy
Typical composition ranges (wt%). Values shown are indicative of common wrought, annealed product specifications and published datasheets; verify the exact range for the supplier and specification you intend to purchase.
| Element | Inconel 625 (typical wt%) | Incoloy 825 (typical wt%) |
|---|---|---|
| C | ≤ 0.10 (usually ≤0.10) | ≤ 0.05 |
| Mn | ≤ 0.50–1.00 | ≤ 1.00 |
| Si | ≤ 0.50 | ≤ 0.50 |
| P | ≤ 0.015–0.03 | ≤ 0.035 |
| S | ≤ 0.015 | ≤ 0.03 |
| Cr | 20–23 | 19.5–23.5 |
| Ni | Balance (~58) | ~38–46 |
| Mo | 8–10 | 2–3 |
| V | trace | trace |
| Nb (±Ta) | 3.15–4.15 (Nb main) | trace |
| Ti | trace / ≤0.40 | 0.6–1.2 |
| B | trace (ppm) | trace (ppm) |
| N | trace | trace |
| Cu | trace | 1.0–2.0 |
| Fe | ~5–10 (balance remainder) | remainder (major Fe fraction) |
How the alloying affects properties - Inconel 625: High Ni gives a corrosion‑resistant matrix; Cr provides oxidation and pitting resistance; Mo and Nb (niobium) enable strong solid‑solution strengthening and resistance to localized corrosion (Mo) and stabilization against carbide precipitation (Nb). The Nb and Mo content is key for high strength at both ambient and elevated temperatures. - Incoloy 825: The Ni–Fe balance gives a less expensive matrix than very high Ni alloys while maintaining good corrosion resistance. Cr provides passivity; Cu improves resistance to reducing acids and stress corrosion cracking in some environments; Ti stabilizes against intergranular attack and helps maintain ductility. Mo is modest, so pitting resistance is lower than high‑Mo alloys like 625.
3. Microstructure and Heat Treatment Response
- Inconel 625
- Typical microstructure: a single‑phase face‑centered cubic (FCC) austenitic matrix with Nb and Mo in solid solution; in some conditions fine Nb‑rich precipitates (e.g., Ni3Nb) can form with prolonged exposure at intermediate temperatures (sensitization is less of an issue than for stainless steels).
- Heat treatment response: solution annealing (e.g., 1000–1150 °C depending on product) followed by rapid cooling restores the single‑phase structure and optimum toughness. Inconel 625 is not hardened by conventional quench‑and‑temper cycles (no martensitic transformation); strength is primarily solid‑solution and precipitation contributions.
- Incoloy 825
- Typical microstructure: a stable austenitic (FCC) matrix with distributed stabilizers (Ti) and small amounts of carbides or intermetallics possible after improper heat exposure.
- Heat treatment response: standard practice is annealing to dissolve unwanted precipitates and restore ductility. Incoloy 825 is not age‑hardening; normalizing/quenching approaches used for steels do not apply. Thermo‑mechanical processing affects grain size and thus toughness and formability.
Both alloys are designed to remain stable in common annealed conditions; neither achieves high strength by martensitic hardening—thermal aging can, however, produce embrittling phases if components are held for long periods in certain temperature ranges.
4. Mechanical Properties
Typical room‑temperature properties depend strongly on product form (sheet, plate, bar) and temper (annealed, cold‑worked). Values below are representative annealed ranges; verify supplier mill certificates for exact values.
| Property (annealed, typical) | Inconel 625 | Incoloy 825 |
|---|---|---|
| Tensile Strength (MPa) | ~760–930 | ~550–750 |
| Yield Strength (0.2% proof, MPa) | ~310–550 | ~200–450 |
| Elongation (%) | ~30–50 | ~35–50 |
| Impact Toughness (Charpy or equivalent) | Good; retains toughness at low temp | Good; generally more ductile |
| Hardness (HV) | ~200–300 (varies by product) | ~150–250 (annealed) |
Interpretation - Strength: Inconel 625 typically has higher tensile and yield strength thanks to Mo and Nb solid solution and subtle precipitation strengthening. - Ductility & toughness: Both alloys are ductile and tough in the annealed condition; Incoloy 825 often exhibits marginally higher ductility at comparable hardness because of lower solid‑solution strength. - Practical implication: For applications demanding higher static or creep strength, or where thin sections must resist deformation at elevated temperature, Inconel 625 is commonly preferred.
5. Weldability
Weldability of both alloys is generally excellent relative to many steels, but different factors must be considered: - Carbon and hardenability: Both alloys have low carbon and are austenitic; risk of weld‑zone martensite/hardening is negligible. Intergranular sensitization is less problematic than with some stainless steels because of stabilization (Inconel 625 by Nb; Incoloy 825 by Ti). - Microalloy effects: High Mo and Nb in 625 increase hot‑cracking susceptibility if inappropriate consumables or joint designs are used, but matched filler metals are available and common practice yields reliable welds. - Welding formulas (qualitative use) - Carbon equivalent for steels can be informative for carbon steels but is less applicable to Ni‑based alloys. For context, one common weldability metric for steels is: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ This formula shows how Ni and Mo increase hardenability in steels; in Ni alloys the interpretation differs because the base matrix is austenitic and Ni is the principal element. - A more detailed steel‑focused formula: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$ Again, these formulas highlight how elements such as Nb and Ti influence weld cracking propensity in steels—useful only as qualitative guidance when thinking about alloying effects. - Practical guidance: - Use matched or recommended filler metals; follow preheat/postheat and joint design recommendations from supplier datasheets. - Inconel 625 has a wide welding record in demanding services (subsea, chemical plants); Incoloy 825 welds readily but filler selection is important when service involves strong reducing acids or chloride environments.
6. Corrosion and Surface Protection
- Inconel 625
- Excellent resistance to pitting, crevice corrosion, and stress‑corrosion cracking in chloride environments due to high Mo and Ni content and Nb stabilization. Also resistant to oxidizing and most reducing media and to sea water and high‑temperature corrosion.
- PREN (Pitting Resistance Equivalent Number) is typically used for stainless steels; for Ni‑based alloys PREN is less routinely applied, but the formula is: $$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$ For Ni alloys with significant Mo, a high PREN suggests strong pitting resistance.
- Incoloy 825
- Good resistance to a wide range of acid environments (sulfuric, phosphoric) and to stress‑corrosion cracking in many chloride environments, particularly where Cu provides improved resistance to reducing acids. Less effective than 625 for severe oxidizing chloride/pitting conditions because Mo is lower.
- Surface protection
- For non‑stainless steels, galvanizing or organic coatings are common; for Ni alloys, surface protection is often unnecessary as the alloys are intrinsically corrosion resistant. If coatings are used for abrasion or aesthetic reasons, choose systems compatible with Ni alloys.
7. Fabrication, Machinability, and Formability
- Machinability
- Inconel 625: moderate to poor machinability relative to carbon steels; work hardening and high strength require robust tool materials (carbide/CBN), rigid setups, and conservative cutting parameters.
- Incoloy 825: somewhat easier to machine than 625 due to lower strength; still tougher to machine than common stainless steels.
- Formability and bending
- Both alloys form and bend in the annealed condition, but springback and tooling wear are considerations. Inconel 625’s higher strength can make deep drawing or tight bends more challenging.
- Finishing
- Both take standard surface finishes (grinding, polishing); care with temperature build‑up to avoid surface oxidation or precipitation.
8. Typical Applications
| Inconel 625 | Incoloy 825 |
|---|---|
| Offshore risers, subsea components, and marine fasteners (excellent chloride/pitting resistance) | Chemical process piping and vessels handling reducing acids (sulfuric, phosphoric) |
| High‑temperature exhausts, combustion cans, turbine parts (temperature strength) | Heat exchanger tubing and fittings where acid resistance and ductility are prioritized |
| Chemical processing equipment exposed to fluorides, chlorides, and sour gas | Nuclear chemical plants, pollution control scrubbers |
| Weld overlay and cladding for corrosion‑resistant surfaces | General fabrication where high ductility and weldability are needed at a lower cost than high‑Mo alloys |
Selection rationale - Choose Inconel 625 when localized corrosion (pitting/crevice), high temperature strength, or exposure to highly oxidizing or chloride‑rich environments are primary concerns. - Choose Incoloy 825 when resistance to reducing acids, economical material cost, good ductility, and ease of fabrication are the key drivers.
9. Cost and Availability
- Relative cost: Inconel 625 is typically more expensive than Incoloy 825 because of higher nickel, molybdenum, and niobium content. Material cost is a function of global Ni/Mo/Nb markets.
- Availability: Both alloys are widely available worldwide in common forms (tubing, pipe, plate, sheet, bar, wire, forgings), but lead times can vary by product form and market conditions. Long lead times and minimum order quantities are more often an issue with thick or specialty forms of Inconel 625.
10. Summary and Recommendation
| Metric | Inconel 625 | Incoloy 825 |
|---|---|---|
| Weldability | Excellent with appropriate procedures; attention to hot‑cracking risk due to Nb/Mo | Excellent; generally forgiving and easy to weld |
| Strength–Toughness | Higher strength, excellent high‑temp strength; good toughness | Good toughness and ductility; lower strength than 625 |
| Cost | Higher (Ni, Mo, Nb content) | Lower (less Mo/Nb; more Fe) |
Recommendation - Choose Inconel 625 if you need superior pitting and crevice corrosion resistance, higher static and elevated‑temperature strength, and the best performance in aggressive chloride or oxidizing environments—even at higher material cost. - Choose Incoloy 825 if your service involves reducing acids or mixed acid environments, you need very good ductility and ease of fabrication, and cost or availability favors a lower‑Mo, Ni‑Fe‑Cr chemistry.
Final note: Always confirm final selection with environment‑specific corrosion data, mechanical property requirements, welding procedure specifications, and supplier mill certificates. For pressure‑containing or safety‑critical parts, ensure compliance with the applicable ASME/EN codes and verify product form acceptance by the fabricator and certifying authorities.