# 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, copper, and titanium. This austenitic material is designed for excellent corrosion resistance in both oxidizing and reducing environments, making it a versatile choice for various industrial applications.

## Chemical Composition

The chemical composition of Alloy 825 contributes significantly to its performance characteristics:

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

## Mechanical Properties

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

– Tensile Strength: 586 MPa (85 ksi) min
– Yield Strength: 241 MPa (35 ksi) min
– Elongation: 30% min
– Hardness: Rb 90 max
– Density: 8.14 g/cm³ (0.294 lb/in³)

## Corrosion Resistance

The alloy’s corrosion resistance makes it particularly valuable in harsh environments:

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

## Heat Treatment

Alloy 825 can be heat treated to optimize its properties:

– Annealing temperature: 927-1038°C (1700-1900°F)
– Rapid cooling from annealing temperature is recommended
– Stress relieving can be performed at 538-649°C (1000-1200°F)

## Fabrication and Welding

The alloy offers good fabrication characteristics:

– Can be formed 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)

## 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
– Flare stacks
– Offshore platform components

### Power Generation

– Flue gas desulfurization systems
– Nuclear fuel reprocessing
– Heat exchangers in power plants

### Marine Applications

– Seawater handling systems
– Propeller shafts
– Pump components
– Ballast systems

## Advantages Over Other Alloys

Alloy 825 offers several advantages compared to similar materials:

– Better resistance to reducing acids than standard stainless steels
– Superior performance in chloride environments compared to 300 series stainless steels
– More cost-effective than some higher nickel alloys for certain applications
– Good combination of strength and corrosion resistance

## Limitations and Considerations

While highly versatile, Alloy 825 has some limitations:

– Not recommended for hydrofluoric acid service
– Limited resistance to strong oxidizing acids
– Higher cost than standard stainless steels
– Requires proper welding procedures to maintain corrosion resistance

## Conclusion

Alloy 825 UNS N08825 remains a critical material for industries requiring reliable performance in corrosive environments. Its balanced composition provides an excellent combination of corrosion resistance, mechanical properties, and fabricability, making it a preferred choice for demanding applications across multiple sectors.