Alloy 903 UNS N19903: Properties and Applications

# Alloy 903 UNS N19903: Properties and Applications

## Introduction to Alloy 903 UNS N19903

Alloy 903, also known as UNS N19903, is a high-performance nickel-iron-cobalt superalloy designed for applications requiring low thermal expansion and high strength at elevated temperatures. This alloy is widely used in aerospace, industrial gas turbines, and other high-temperature environments due to its unique combination of properties.

## Chemical Composition

The chemical composition of Alloy 903 plays a crucial role in its performance characteristics. The primary elements include:

– Nickel (Ni): 38-41%
– Iron (Fe): Balance
– Cobalt (Co): 14-16%
– Titanium (Ti): 1.3-1.7%
– Aluminum (Al): 0.6-1.0%

– Niobium (Nb): 2.8-3.2%

This carefully balanced composition contributes to the alloy’s exceptional thermal stability and mechanical properties.

## Physical Properties

Alloy 903 exhibits several notable physical properties:

– Density: 8.22 g/cm³
– Melting Range: 1315-1365°C (2400-2490°F)
– Thermal Expansion Coefficient: 6.5-8.5 μm/m·°C (20-400°C)
– Electrical Resistivity: 0.85 μΩ·m
– Magnetic Permeability: <1.02 (at 200 Oe)

## Mechanical Properties

The mechanical properties of Alloy 903 make it particularly suitable for demanding applications:

– Tensile Strength: 1100-1300 MPa (160-190 ksi)
– Yield Strength: 800-1000 MPa (115-145 ksi)
– Elongation: 10-15%
– Hardness: 30-35 HRC
– Fatigue Strength: Excellent at elevated temperatures

## Heat Treatment

Proper heat treatment is essential for optimizing the properties of Alloy 903. The standard heat treatment process includes:

1. Solution annealing at 980-1010°C (1800-1850°F)
2. Rapid cooling (water quenching or air cooling)
3. Aging at 720-760°C (1325-1400°F) for 8-16 hours
4. Air cooling

This treatment sequence ensures the development of optimal mechanical properties and dimensional stability.

## Applications of Alloy 903

Alloy 903 finds application in numerous industries due to its unique properties:

### Aerospace Industry

– Jet engine components
– Turbine seals
– Exhaust systems
– High-temperature fasteners

### Power Generation

– Gas turbine components
– Steam turbine parts
– Heat exchangers
– Power transmission systems

### Industrial Applications

– Precision instruments
– Optical systems
– Cryogenic equipment
– Nuclear reactors

## Advantages of Alloy 903

The key advantages that make Alloy 903 stand out include:

– Excellent dimensional stability over a wide temperature range
– High strength-to-weight ratio
– Good corrosion resistance in various environments
– Superior fatigue resistance
– Compatibility with other high-temperature alloys

## Fabrication and Machining

While Alloy 903 can be challenging to machine, proper techniques can yield excellent results:

– Use carbide or high-speed steel tools with positive rake angles
– Maintain sharp cutting edges
– Employ slow speeds and heavy feeds
– Use ample coolant to prevent work hardening
– Consider grinding for final finishing operations

## Welding Considerations

Alloy 903 can be welded using several methods, though precautions are necessary:

– Gas tungsten arc welding (GTAW) is preferred
– Use matching filler metal (Alloy 903)
– Preheating to 200-300°C (400-600°F) is recommended
– Post-weld heat treatment is essential
– Avoid excessive heat input to