What is the difference between Incoloy 825 and 925?

• Incoloy 825 is a solid-solution strengthened alloy. It is used in its annealed state and is prized for its exceptional corrosion resistance.
• Incoloy 925 is a precipitation-hardened alloy. It is a derivative of 825 that has been chemically modified to be age-hardened, resulting in very high strength while retaining good corrosion resistance.

Here is a detailed breakdown of their differences:

At a Glance: Incoloy 825 vs. 925

Feature	Incoloy 825 (UNS N08825)	Incoloy 925 (UNS N09925)
Primary Purpose	Corrosion Resistance	High Strength + Corrosion Resistance
Strengthening Method	Solid-Solution (Annealed)	Precipitation Hardening (Aged)
Key Alloying Elements	Ni, Cr, Mo, Cu, Ti	Ni, Cr, Mo, Cu, Ti, Al
Key Characteristic	Excellent ductility and fabricability	High yield and tensile strength
Typical Yield Strength	~250 MPa (36 ksi) (Annealed)	~550 MPa (80 ksi) (Aged)
Corrosion Resistance	Superior in a wide range of media	Very Good, but slightly less than 825
NACE MR0175/ISO 15156	Approved for sour service	Approved for sour service

Detailed Comparison

1. Chemistry and Metallurgy

• Incoloy 825: Its composition is optimized for corrosion resistance. It contains Nickel (for chloride stress corrosion cracking resistance), Chromium (for oxidizing environments), Molybdenum (for reducing environments and pitting resistance), and Copper (for sulfuric acid resistance). Titanium (Ti) is added to stabilize the alloy against sensitization during welding.
• Incoloy 925: It has a very similar base composition to 825 but includes key additions of Aluminum (Al) and a higher Titanium (Ti) content. These elements allow it to form the gamma prime (ϒ') phase during a specific heat treatment (aging), which precipitates throughout the microstructure, dramatically increasing its strength.

2. Mechanical Properties (The Biggest Difference)

• Incoloy 825 has good, ductile mechanical properties similar to other austenitic alloys. It is not a high-strength alloy.
• Incoloy 925, after the aging process, achieves approximately double the yield strength and tensile strength of 825. This makes it capable of handling much higher mechanical loads and pressures.

3. Corrosion Resistance

• Incoloy 825 is considered the benchmark for corrosion resistance in this family. It excels in sulfuric and phosphoric acids, seawater, and chloride-containing environments. It is highly resistant to pitting, crevice corrosion, and stress corrosion cracking.
• Incoloy 925 offers very good corrosion resistance, very similar to 825, and is fully approved for sour service (NACE MR0175). However, due to its different microstructure from aging, its general corrosion resistance in some very aggressive media can be marginally lower than 825. It is still far superior to stainless steels.

Application Examples

• Chemical processing equipment (pickling tanks, heating coils, reactor vessels)
• Pollution control and acid production equipment
• Seawater cooling tubes and heat exchangers
• Components handling sulfuric acid and other corrosive chemicals.

• Downhole oil & gas equipment in sour (H₂S) wells: tubing hangers, valves, mandrels, perforated guns, connectors.
• Fasteners and high-strength bolts for offshore platforms and chemical reactors.
• Shafts and agitators that require both high strength and corrosion resistance.
• Any component where the mechanical load is too high for 825.