Incoloy 800 vs Incoloy 825 – Composition, Heat Treatment, Properties, and Applications

Introduction

Incoloy 800 and Incoloy 825 are two nickel‑iron‑chromium alloys widely used across chemical processing, power generation, and petroleum refining. Engineers, procurement managers, and manufacturing planners commonly weigh tradeoffs between corrosion performance, high‑temperature stability, weldability, and cost when selecting between them. Typical decision contexts include choosing a grade for high‑temperature, oxidizing environments versus selecting a material for severe acidic or chloride‑bearing process streams.

The principal functional difference is corrosion performance in aggressive aqueous environments: Incoloy 825 is alloyed for enhanced resistance to reducing acids and chloride‑assisted corrosion (thanks to additions of Mo and Cu), whereas Incoloy 800 is optimized for high‑temperature strength and oxidation/corrosion resistance as a solid‑solution austenitic alloy. This is why the two grades are often compared when designers must balance acid/chloride resistance against high‑temperature service and budget constraints.

1. Standards and Designations

Major specifications and common designations:

• ASTM/ASME:
• Incoloy 800: UNS N08800; common specs include ASTM B409/B409M (pipe), B407 (cold‑worked), and ASME SB-163 references.
• Incoloy 825: UNS N08825; commonly supplied per ASTM B407/B409, ASME SB-163, and B444.
• EN: equivalents are often referenced by chemical family and applications; check EN lists for specific product forms.
• JIS / GB: equivalents exist regionally but always verify chemical and mechanical requirements against UNS numbers.
• Classification: both are austenitic nickel‑iron‑chromium alloys (Ni‑Fe‑Cr), i.e., alloy/stainless family—neither is categorized as low‑alloy carbon, tool steel, or HSLA.

2. Chemical Composition and Alloying Strategy

Below are representative composition ranges (wt%) commonly encountered in mill specs and material datasheets. Values vary by product form and specification—confirm with product certificates for procurement.

Element	Incoloy 800 (typical range, wt%)	Incoloy 825 (typical range, wt%)
C	≤ 0.10	≤ 0.05
Mn	≤ 1.0	≤ 1.0
Si	≤ 0.5	≤ 0.5
P	≤ 0.02	≤ 0.02
S	≤ 0.015	≤ 0.015
Cr	19.0 – 23.0	19.5 – 23.5
Ni	30.0 – 35.0	38.0 – 46.0
Mo	— / trace	2.5 – 3.5
Cu	— / trace	1.0 – 2.0
Ti / Al (stabilizers)	small controlled additions (combined)	Ti ≤ 0.6 (control N/CB)
Nb, V, B	typically none	typically none
N	trace – low	trace – low
Fe	Balance	Balance

Alloying strategy and effects: - Ni stabilizes the austenitic matrix, conferring toughness and formability. Higher Ni in 825 increases overall corrosion resistance and ductility. - Cr provides oxidation and general corrosion resistance. Both grades have similar Cr, so base stainless behavior is comparable. - Mo and Cu in 825 substantially improve resistance to localized corrosion (pitting, crevice) and to reducing acid attack (sulfuric, phosphoric-like chemistries), and lower susceptibility to chloride‑induced stress corrosion cracking in many cases. - Incoloy 800 relies on Fe–Ni–Cr solid solution and controlled Al/Ti for stabilization at elevated temperatures (to limit carbide precipitation). It is not primarily alloyed for enhanced acid resistance.

3. Microstructure and Heat Treatment Response

Microstructure: - Both alloys are austenitic (face‑centered cubic). The as‑fabricated microstructure is a single‑phase austenite with a possible dispersion of stable nitrides or Ti/Al‑based phases if present in significant amounts. - Incoloy 800 variants (800H/800HT) are tailored for creep resistance by controlling carbon (800H higher C) and thermal stability—these variants resist grain growth and carbide precipitation better at high temperatures. - Incoloy 825 remains a stable austenite with solid‑solution strengthening from Ni, and secondary phases are uncommon unless exposed to extreme thermal cycles.

Heat treatment response: - Both alloys are typically supplied annealed/solution‑treated to dissolve precipitation and restore ductility. A solution anneal followed by rapid cooling minimizes carbide and intermetallic formation. - Thermal exposure at intermediate temperatures (400–850°C) can promote carbide or intermetallic precipitation (carbonitride or sigma phase depending on time and composition); 800 and 825 differ in sensitivity due to alloying content—825’s Mo can promote secondary phase formation under certain long‑term exposures. - Normalizing/quenching and tempering cycles used for carbon steels are not relevant for these austenitic nickel alloys; mechanical properties are adjusted by solution annealing and controlled cold work, and by selecting specific subgrades (800H/HT).

4. Mechanical Properties

Representative mechanical properties (annealed condition). Actual values depend on product form, thickness, and heat treatment—use mill certificates.

Property (annealed)	Incoloy 800 (typical)	Incoloy 825 (typical)
Tensile Strength (MPa)	~480 – 700	~480 – 700
Yield Strength 0.2% (MPa)	~200 – 350	~200 – 350
Elongation (%)	~30 – 50	~30 – 50
Toughness (impact)	Good, notch‑insensitive at RT	Good, notch‑insensitive at RT
Hardness (HB)	~140 – 220	~140 – 220

Interpretation: - Room‑temperature mechanical properties are broadly similar in annealed condition thanks to the austenitic matrix. Differences are more application‑specific: 800H/HT variants provide better creep strength at elevated temperatures due to controlled carbon and processing; 825 is not intended for high creep applications. - Toughness and ductility are generally comparable; nickel content helps maintain ductility at low temperatures and resistance to brittle fracture.

5. Weldability

Both alloys are considered weldable by standard fusion and resistance methods, but attention to procedure and post‑weld treatment is necessary for service performance.

Weldability factors: - Low carbon content reduces susceptibility to hardening in the heat‑affected zone; residual elements (Mo, Cu) affect solidification cracking behavior and weld metal composition. - Because both are low‑hardenability austenitic alloys, preheat is generally unnecessary; however, weld procedure qualification is needed for critical services.

Useful carbon‑equivalent and weldability indices (interpret qualitatively): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ and $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

• Interpretations: Higher $CE_{IIW}$ and $P_{cm}$ imply greater cold‑cracking risk and need for controlled preheat/post‑weld heat treatment. Both alloys typically plot in a region indicating good weldability but 825’s higher Ni and presence of Mo/Cu modestly raise indices relative to 800.
• Practical note: Use matched or over‑matching filler metal recommended by suppliers for corrosion and mechanical property compatibility; control interpass temperature and avoid excessive dilution when welding in corrosive service.

6. Corrosion and Surface Protection

• For non‑stainless treatments: not applicable—both are corrosion‑resistant alloys (stainless/HP Ni alloys). Surface coatings (painting, thermal spray, cladding) can be used for additional mechanical or aesthetic protection but do not substitute for alloy selection in corrosive media.
• Localized corrosion resistance can be approximated for stainless alloys using PREN: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Application of PREN: It’s most useful for austenitic stainless steels where Mo and N strongly boost pitting resistance. Incoloy 825, with measurable Mo, will show a higher PREN than Incoloy 800 (which has negligible Mo), indicating better resistance to pitting and crevice corrosion in chloride‑containing environments.
• Qualitative corrosion summary:
• Incoloy 825: superior resistance to reducing acids (sulfuric, phosphoric), good resistance to chloride‑induced stress corrosion cracking in many process conditions, and improved pitting/crevice resistance due to Mo + Cu.
• Incoloy 800: excellent oxidation and general corrosion resistance at high temperatures and good resistance to carburizing/oxidizing atmospheres; less capable than 825 in aggressive acidic or localized corrosion regimes.

7. Fabrication, Machinability, and Formability

• Forming: Both alloys form and bend reasonably well in the annealed state; require larger bend radii than mild steel due to lower modulus and higher springback. Cold forming increases work hardening; intermediate anneal may be required for severe forming.
• Machinability: Nickel alloys are more difficult to machine than ferritic steels—lower thermal conductivity, higher strength, and work‑hardening demand robust tooling, reduced cutting speeds, good chip control, and coolant. Incoloy 825 (higher Ni) is often slightly more machinable than some higher‑nickel alloys but still challenging compared with carbon steels; Incoloy 800 is comparable.
• Finishing: Surface grinding, polishing, and welding‑related finishing follow standard nickel‑alloy practice; passivation is less relevant than stainless steels but controlled cleaning and pickling may be required to restore corrosion performance after fabrication.

8. Typical Applications

Incoloy 800 (common uses)	Incoloy 825 (common uses)
High‑temperature furnace components, heat exchangers, and steam generator tubing (oxidizing atmospheres)	Chemical process equipment handling sulfuric, phosphoric, and organic acids
Tubing and piping in power plants (moderate corrosion, high temp)	Piping, vessels, and fittings in acid pickling, fertilizer, and offshore systems
Elements in petrochemical reformers, radiant tubes	Heat exchangers, flanges, and weld overlay where chloride/pitting resistance needed
Components requiring stability against carburization and oxidation	Wet chemical process environments, where reducing conditions and localized corrosion are concerns

Selection rationale: - Choose Incoloy 800 when high‑temperature strength, oxidation resistance, and cost efficiency are priorities and the process fluids are not aggressively reducing or chloride‑rich. - Choose Incoloy 825 when aqueous corrosion in reducing acids, chloride presence, or localized corrosion resistance is a primary requirement.

9. Cost and Availability

• Relative cost: Incoloy 825 is typically more expensive than Incoloy 800 due to higher Ni and the addition of Mo and Cu. Market prices fluctuate with nickel and molybdenum markets.
• Availability: Both grades are commonly available worldwide in plate, sheet, coil, pipe, tube, and bar. Incoloy 800 (and its high‑temperature variants) is widespread in power and petrochemical supply chains; 825 is a standard selection in chemical and offshore supply chains. Lead times may be longer for specialty product forms and large diameters or thicknesses.

10. Summary and Recommendation

Summary table (qualitative):

Attribute	Incoloy 800	Incoloy 825
Weldability	Very good (standard procedures)	Very good (procedural controls recommended)
Strength–Toughness	High thermal stability (esp. 800H/HT)	Good toughness; not for high creep
Corrosion (acid/chloride)	Moderate (general corrosion/oxidation)	Superior (reducing acids, pitting/crevice resistance)
Cost	Lower	Higher

Recommendations: - Choose Incoloy 800 if: - Service temperature or oxidation resistance is a primary driver (e.g., high‑temperature heat exchangers, radiant tubes). - The process environment is not strongly reducing or chloride‑bearing and budget pressures favor a lower‑cost nickel‑iron‑chromium alloy. - Creep resistance is required (select 800H/800HT variants).

• Choose Incoloy 825 if:
• The service contains reducing acids (sulfuric, phosphoric) or significant chloride concentrations where localized corrosion or SCC is a concern.
• Superior resistance to pitting, crevice corrosion, and acid attack is required, and the premium cost is justified by reduced maintenance and longer life.
• Application involves mixed oxidizing and reducing chemistries where Mo and Cu additions provide clear advantages.

Final note: Both alloys are robust engineering choices; the correct selection depends on a careful review of process chemistry, temperature, mechanical loading (including creep), fabrication constraints, and life‑cycle cost. Always confirm exact chemical and mechanical requirements with mill certificates and consult corrosion specialists for critical or novel process environments.