# Alloy 908 UNS N09908: Properties and Applications

## Introduction to Alloy 908 UNS N09908

Alloy 908, also known as UNS N09908, is a nickel-iron-based superalloy specifically designed for cryogenic applications. This material was developed to address the unique challenges posed by extremely low-temperature environments while maintaining excellent mechanical properties.

## Chemical Composition

The chemical composition of Alloy 908 plays a crucial role in its performance characteristics:

– Nickel (Ni): 48-50%
– Iron (Fe): Balance
– Niobium (Nb): 4.0-4.5%
– Titanium (Ti): 1.3-1.8%
– Aluminum (Al): 0.75-1.25%
– Carbon (C): ≤0.03%
– Manganese (Mn): ≤0.50%
– Silicon (Si): ≤0.50%
– Phosphorus (P): ≤0.015%
– Sulfur (S): ≤0.015%

## Physical Properties

Alloy 908 exhibits remarkable physical properties that make it suitable for demanding applications:

– Density: 8.19 g/cm³
– Melting Range: 1315-1365°C (2400-2490°F)
– Thermal Expansion Coefficient: 9.5 × 10⁻⁶/°C (20-100°C)
– Electrical Resistivity: 0.85 μΩ·m at 20°C
– Magnetic Permeability: <1.02 (at cryogenic temperatures)

## Mechanical Properties

The mechanical properties of Alloy 908 are particularly impressive at cryogenic temperatures:

– Tensile Strength: 1100-1300 MPa (160-190 ksi) at room temperature
– Yield Strength: 800-1000 MPa (116-145 ksi) at room temperature
– Elongation: 15-25% at room temperature
– Fracture Toughness: Excellent at cryogenic temperatures
– Fatigue Strength: Superior performance in cyclic loading conditions

## Key Characteristics

Alloy 908 offers several unique characteristics that set it apart from other materials:

– Exceptional strength and toughness at cryogenic temperatures
– Low thermal expansion coefficient
– Good weldability with proper procedures
– Excellent resistance to hydrogen embrittlement
– Maintains mechanical properties after prolonged exposure to cryogenic conditions
– Good fabricability despite its high strength

## Heat Treatment

Proper heat treatment is essential for optimizing the properties of Alloy 908:

– Solution Annealing: Typically performed at 980-1010°C (1800-1850°F)
– Aging: Usually conducted at 720-760°C (1330-1400°F) for 8 hours
– Stress Relieving: Recommended after welding operations

## Applications of Alloy 908

Alloy 908 finds its primary use in cryogenic applications, particularly in:

### Superconducting Magnet Systems

The material is extensively used in the construction of superconducting magnets for:

– Magnetic Resonance Imaging (MRI) systems
– Nuclear Magnetic Resonance (NMR) spectrometers
– Particle accelerators
– Fusion research devices

### Cryogenic Storage and Transportation

Alloy 908 is employed in components for:

– Liquid hydrogen storage tanks
– Cryogenic fuel systems for aerospace applications
– LNG (Liquefied Natural Gas) transportation vessels

### Aerospace Industry

The aerospace sector utilizes Alloy 908 for:

– Cryogenic fuel lines and components
– Spacecraft structural elements
– Rocket propulsion systems

### Energy Sector

Applications in the energy industry include:

– Superconducting power transmission cables
– Energy storage systems
– Components for fusion reactors

## Fabrication Considerations

When working with Alloy 908, several fabrication aspects should be considered:

### Welding

Alloy 908 can be welded using:

– Gas