Alloy 925 UNS N09925: Properties and Applications

# Alloy 925 UNS N09925: Properties and Applications

## Introduction to Alloy 925

Alloy 925, also known as UNS N09925, is a nickel-iron-chromium alloy with additions of molybdenum, copper, and titanium. This precipitation-hardenable superalloy offers excellent mechanical properties and corrosion resistance, making it suitable for demanding applications in various industries.

## Chemical Composition

The chemical composition of Alloy 925 plays a crucial role in its performance characteristics:

– Nickel (Ni): 42-46%
– Iron (Fe): Balance
– Chromium (Cr): 19.5-22.5%
– Molybdenum (Mo): 2.5-3.5%
– Copper (Cu): 1.5-3.0%
– Titanium (Ti): 1.9-2.4%
– Aluminum (Al): 0.1-0.5%
– Carbon (C): ≤0.03%

## Mechanical Properties

Alloy 925 exhibits impressive mechanical properties, especially after precipitation hardening:

– Tensile Strength: 1100-1400 MPa (160-200 ksi)
– Yield Strength: 760-1100 MPa (110-160 ksi)
– Elongation: 15-25%
– Hardness: 30-40 HRC (after aging)
– Fatigue Strength: Excellent resistance to cyclic loading

## Physical Properties

The physical characteristics of Alloy 925 include:

– Density: 8.14 g/cm³ (0.294 lb/in³)
– Melting Point: 1350-1400°C (2460-2550°F)
– Thermal Expansion: 13.9 μm/m·°C (20-100°C)
– Electrical Resistivity: 0.98 μΩ·m
– Magnetic Permeability: Slightly magnetic in cold worked condition

## Corrosion Resistance

Alloy 925 offers outstanding corrosion resistance in various environments:

– Excellent resistance to chloride-ion stress corrosion cracking
– Good resistance to pitting and crevice corrosion
– Resistance to sour gas environments (H2S)
– Performs well in acidic and alkaline solutions
– Maintains properties in high-temperature environments

## Heat Treatment

The optimal performance of Alloy 925 is achieved through proper heat treatment:

– Solution Annealing: 980-1010°C (1800-1850°F), followed by rapid cooling
– Aging Treatment: 620-650°C (1150-1200°F) for 4 hours, air cooling
– Stress Relieving: 590-620°C (1100-1150°F) for 1-4 hours

## Applications of Alloy 925

Due to its exceptional properties, Alloy 925 finds applications in various demanding industries:

### Oil and Gas Industry

– Downhole components
– Valve bodies and stems
– Pump shafts and impellers
– Wellhead equipment
– Offshore platform components

### Chemical Processing

– Reactor vessels
– Heat exchangers
– Piping systems
– Fasteners and bolts
– Pressure vessels

### Marine Applications

– Propeller shafts
– Subsea equipment
– Desalination plants
– Seawater handling systems

### Aerospace Industry

– Landing gear components
– Engine mounts
– Fasteners
– Structural components

## Fabrication and Machining

Alloy 925 can be fabricated using standard methods, though it requires special considerations:

– Machining: Use rigid setups, sharp tools, and positive rake angles
– Welding: Recommended methods include GTAW and GMAW with matching filler metal
– Forming: Can be cold worked, but requires annealing between severe operations
– Heat Treatment: Essential for achieving optimal properties

## Advantages of Alloy 925

The key benefits of using Alloy 925 include