Alloy 738 UNS N06738: Properties and Applications

# Alloy 738 UNS N06738: Properties and Applications

## Introduction to Alloy 738 UNS N06738

Alloy 738, also known as UNS N06738, is a nickel-based superalloy renowned for its exceptional high-temperature strength and corrosion resistance. This material is widely used in demanding applications where extreme conditions are present, such as in gas turbines and aerospace components.

## Chemical Composition

The chemical composition of Alloy 738 plays a crucial role in its performance characteristics. The primary elements include:

– Nickel (Ni): 58-63%
– Chromium (Cr): 15-17%
– Cobalt (Co): 8-10%
– Molybdenum (Mo): 1.5-2.5%
– Aluminum (Al): 3.2-3.7%
– Titanium (Ti): 3.2-3.7%

This carefully balanced composition contributes to the alloy’s remarkable properties at elevated temperatures.

## Mechanical Properties

Alloy 738 exhibits outstanding mechanical properties that make it suitable for high-stress applications:

– Tensile Strength: 1100-1300 MPa at room temperature
– Yield Strength: 750-900 MPa at room temperature
– Elongation: 10-20%
– Hardness: 30-40 HRC

These properties remain relatively stable even at temperatures up to 850°C (1562°F), making it ideal for turbine blade applications.

## Thermal Properties

The thermal characteristics of Alloy 738 are particularly noteworthy:

– Melting Range: 1290-1345°C (2354-2453°F)
– Thermal Expansion Coefficient: 13.5 × 10⁻⁶/°C (20-1000°C)
– Thermal Conductivity: 11.4 W/m·K at 100°C

These properties ensure dimensional stability and thermal fatigue resistance in high-temperature environments.

## Corrosion Resistance

Alloy 738 demonstrates excellent resistance to various forms of corrosion:

– Oxidation resistance up to 1000°C (1832°F)
– Good resistance to sulfidation
– Resistance to hot corrosion in gas turbine environments
– Moderate resistance to chloride-induced stress corrosion cracking

This corrosion resistance is particularly valuable in power generation applications where combustion products can be highly corrosive.

## Fabrication and Processing

Working with Alloy 738 requires specialized techniques due to its high strength and work hardening characteristics:

– Machining: Requires rigid setups and carbide tools
– Welding: Can be welded using gas tungsten arc welding (GTAW) with matching filler metal
– Heat Treatment: Typically solution treated at 1120-1150°C followed by aging
– Forming: Hot forming is preferred due to the alloy’s high strength at room temperature

Proper processing is essential to maintain the alloy’s desirable properties in the final product.

## Applications of Alloy 738

Alloy 738 finds extensive use in several critical industries:

### Gas Turbine Components

– Turbine blades and vanes
– Combustion chambers
– Afterburner components

### Aerospace Industry

– Jet engine components
– Rocket engine parts
– High-temperature fasteners

### Power Generation

– Gas turbine power plants
– Combined cycle power systems
– Cogeneration systems

### Industrial Applications

– High-temperature furnace components
– Chemical processing equipment
– Nuclear reactor components

## Advantages Over Other Superalloys

Alloy 738 offers several advantages compared to similar nickel-based superalloys:

– Superior creep resistance at high temperatures
– Better microstructural stability during long-term exposure
– Improved thermal fatigue resistance
– Good balance between strength and ductility
– Excellent oxidation resistance in turbine environments

These characteristics make it a preferred choice for critical rotating components in gas turbines.

## Limitations and Considerations

While Alloy 738 is an excellent material for high-temperature applications, there are some limitations to