Alloy 825 UNS N08825: Properties and Applications

# Alloy 825 UNS N08825: Properties and Applications

## Introduction to Alloy 825

Alloy 825, also known as UNS N08825, is a nickel-iron-chromium alloy with additions of molybdenum and copper. This material is designed for excellent corrosion resistance in both oxidizing and reducing environments. Its unique composition makes it particularly suitable for applications in aggressive chemical processing environments.

## Chemical Composition

The typical chemical composition of Alloy 825 includes:

– Nickel (Ni): 38-46%
– Chromium (Cr): 19.5-23.5%
– Iron (Fe): 22% minimum
– Molybdenum (Mo): 2.5-3.5%
– Copper (Cu): 1.5-3.0%
– Titanium (Ti): 0.6-1.2%
– Carbon (C): 0.05% maximum

This balanced composition provides the alloy with its distinctive properties and performance characteristics.

## Mechanical Properties

Alloy 825 offers excellent mechanical properties across a wide temperature range:

– Tensile Strength: 586 MPa (85 ksi) minimum
– Yield Strength: 241 MPa (35 ksi) minimum
– Elongation: 30% minimum
– Hardness: Rockwell B 90 maximum

These properties remain stable even at elevated temperatures, making Alloy 825 suitable for high-temperature applications.

## Corrosion Resistance

One of the most notable features of Alloy 825 is its exceptional corrosion resistance:

– Excellent resistance to sulfuric and phosphoric acids
– Good resistance to chloride stress corrosion cracking
– Performs well in both oxidizing and reducing environments
– Resistant to pitting and crevice corrosion in chloride-containing solutions

The addition of molybdenum enhances resistance to reducing environments, while chromium provides protection against oxidizing conditions.

## Heat Treatment

Alloy 825 is typically supplied in the annealed condition. The standard heat treatment involves:

– Heating to 927-1038°C (1700-1900°F)
– Rapid cooling (water quenching or rapid air cooling)
– Stress relieving at 593-649°C (1100-1200°F) when required

Proper heat treatment is crucial for achieving optimal corrosion resistance and mechanical properties.

## Fabrication and Welding

Alloy 825 can be readily fabricated using standard techniques:

– Hot working should be performed at 1038-1232°C (1900-2250°F)
– Cold working requires intermediate annealing
– Welding can be performed using matching filler metals (ERNiCrMo-3)
– Post-weld heat treatment is generally not required

The alloy maintains good ductility and workability despite its high strength.

## Applications of Alloy 825

Due to its unique combination of properties, Alloy 825 finds applications in numerous industries:

### Chemical Processing

– Acid production equipment
– Reactor vessels
– Heat exchangers
– Piping systems

### Oil and Gas Industry

– Downhole components
– Wellhead equipment
– Seawater handling systems
– Flue gas desulfurization systems

### Power Generation

– Nuclear fuel reprocessing
– Waste incineration systems
– Flue gas cleaning systems

### Other Applications

– Marine engineering components
– Pollution control equipment
– Pickling equipment
– Phosphoric acid evaporators

## Comparison with Similar Alloys

Alloy 825 is often compared to other nickel-based alloys:

– More resistant to sulfuric acid than Alloy 600
– Better chloride resistance than 316 stainless steel
– Superior to Alloy 800 in reducing environments
– More economical than Hastelloy alloys for many applications

## Conclusion

Alloy 825 UNS N08825 offers an excellent balance of corrosion resistance, mechanical properties, and fabricability. Its versatility makes it a preferred material for demanding applications across multiple industries, particularly where