Inconel 738: Composition, Properties, and Applications

When it comes to materials that can withstand extreme temperatures and corrosive environments, Inconel 738 stands out as a titan in the field of superalloys. Renowned for its exceptional high-temperature strength and resistance to oxidation, Inconel 738 has become indispensable in the aerospace and gas turbine industries. But what exactly gives this superalloy its remarkable properties? This article delves deep into the intricate chemical composition of Inconel 738, breaking down the specific elements that contribute to its unique characteristics. We will also explore its mechanical and thermal properties, providing a comprehensive understanding of why Inconel 738 is the material of choice for high-stress applications. Ready to uncover the secrets behind this powerhouse alloy and its pivotal role in advanced engineering? Let’s dive in.

Introduction to Inconel 738

Overview

Inconel 738 is a nickel-based superalloy known for its exceptional strength at high temperatures, corrosion resistance, and thermal stability. This alloy is indispensable in various high-performance sectors, including aerospace, energy, and automotive industries, due to its ability to retain structural integrity under extreme conditions.

Composition

Inconel 738 is composed primarily of nickel, with significant amounts of chromium, cobalt, and aluminum, along with smaller quantities of molybdenum, tungsten, titanium, niobium, and tantalum. Trace elements such as carbon, manganese, silicon, phosphorus, sulfur, and boron are meticulously controlled to enhance its properties.

High Carbon Version (IN-738C)

The high carbon version (IN-738C) contains 0.15-0.20% carbon, while the low carbon version (IN-738LC) has 0.09-0.13% carbon.

Properties

Inconel 738 exhibits several key properties that make it suitable for high-temperature applications:

High-Temperature Strength

The alloy retains its mechanical strength at elevated temperatures, up to 750°C and beyond, making it ideal for high-stress environments.

Melting Point

With a melting point ranging from approximately 1,310°C to 1,335°C, Inconel 738 ensures structural integrity under extreme thermal conditions.

Density

The density of Inconel 738 typically ranges between 8.15 g/cm³ and 8.24 g/cm³, contributing to its robust composition and exceptional mechanical properties. This high-density alloy offers outstanding strength and durability, making it suitable for demanding applications such as turbine blades and other components in aerospace and power generation industries.

Thermal Conductivity

Inconel 738 has a thermal conductivity of about 11.2 W/m·K, making it suitable for managing heat in high-temperature environments.

Creep Resistance

The alloy exhibits superior resistance to creep, a critical property for applications involving prolonged exposure to high temperatures and stresses.

Corrosion and Oxidation Resistance

The high chromium and aluminum content in Inconel 738 forms a protective oxide layer, providing excellent resistance to corrosion and oxidation.

Applications

Inconel 738 is utilized in various applications across different industries due to its unique combination of properties:

Aerospace

In the aerospace industry, Inconel 738 is used for turbine blades, vanes, and other components that require high strength and oxidation resistance at elevated temperatures.

Energy Sector

In the energy sector, components for gas turbines, such as combustor cans and afterburner parts, benefit from its resistance to creep and rupture at high temperatures.

Automotive

High-performance exhaust systems and turbocharger components in the automotive industry leverage the thermal stability and corrosion resistance of Inconel 738.

Marine Environments

Inconel 738’s corrosion resistance makes it suitable for parts exposed to saltwater in high-performance naval vessels.

Manufacturing Processes

Inconel 738 is produced in various forms, including powder, sheet, plate, rod, and bar. Advanced manufacturing techniques such as additive manufacturing (Selective Laser Melting, Electron Beam Melting), Hot Isostatic Pressing, and Metal Injection Molding are employed to exploit its superior properties in complex geometries and high-performance applications.

Chemical Composition and Properties

Chemical Composition

Inconel 738 is a nickel-based superalloy designed for high-temperature applications. Its chemical composition is meticulously balanced to provide optimal performance under extreme conditions.

Elemental Breakdown

The primary constituents of Inconel 738 include Nickel (Ni), which makes up the bulk of the alloy and enhances stability and high-temperature performance, and Chromium (Cr) at 15-17%, which significantly contributes to corrosion and oxidation resistance by forming a protective oxide layer. Other important elements are Cobalt (Co) at 8-10%, Aluminum (Al) at 3-4%, Molybdenum (Mo) at 1.5-2.5%, Tungsten (W) at 2-3%, Titanium (Ti) at 0.7-1.2%, Niobium (Nb) at 0.1-0.6%, and Tantalum (Ta) at 0.5-1.5%. Trace amounts of Carbon (C), Boron (B), Manganese (Mn), Silicon (Si), Phosphorus (P), and Sulfur (S) are also present and meticulously controlled to ensure consistent properties and performance.

Physical Properties

Inconel 738 exhibits several key physical properties that make it suitable for high-stress and high-temperature applications.

Density

The density of Inconel 738 ranges from 8.11 g/cm³ to 8.24 g/cm³, contributing to its robust structural composition and making it ideal for components subjected to high mechanical stresses.

Melting Range

The melting range of Inconel 738 is approximately 1230-1315°C, ensuring that the alloy maintains structural integrity under extreme thermal conditions, making it suitable for applications like gas turbine blades.

Thermal Conductivity

With a thermal conductivity of about 11.2 W/m·K, Inconel 738 effectively manages heat dissipation in high-temperature environments, enhancing the performance and longevity of components.

Thermal Expansion

Inconel 738 has a thermal expansion coefficient of 6.45 x 10^-6 per °F (between 70-200°F), which helps maintain its shape and reduce stress when temperatures change.

Specific Heat

The specific heat of Inconel 738 is approximately 0.10 Btu/lb/°F at 70°F, aiding in its ability to absorb and dissipate heat efficiently.

Mechanical Properties

Inconel 738 is renowned for its exceptional mechanical properties, which include:

Tensile Strength

At 70°F, Inconel 738 has a tensile strength of approximately 159,000 psi, making it ideal for components that must withstand significant mechanical loads without deforming.

Creep Resistance

Inconel 738 has superior creep resistance, making it suitable for applications involving prolonged exposure to high temperatures and stresses. This property is vital for maintaining the structural integrity of components over time.

Hardness and Ductility

The alloy balances hardness and ductility, allowing it to resist wear and deformation while maintaining the ability to withstand mechanical stresses without fracturing.

Corrosion and Oxidation Resistance

The high chromium and aluminum content in Inconel 738 forms a protective oxide layer at high temperatures, ensuring excellent resistance to corrosion and oxidation in harsh environments.

High-Temperature Performance

Inconel 738 is specifically designed to perform exceptionally well in high-temperature environments. Its combination of high melting point, thermal stability, and resistance to thermal creep and oxidation makes it a preferred choice for components in gas turbines, aerospace engines, and other demanding applications.

Applications

In gas turbines, Inconel 738 is essential for its high-temperature strength and oxidation resistance. Components like turbine blades, vanes, and combustor cans benefit from the alloy’s ability to maintain mechanical integrity at temperatures exceeding 750°C. The high chromium content forms a protective oxide layer, enhancing resistance to oxidation and corrosion in the hot sections of gas turbines. This ensures prolonged component life and reliability, even under cyclic thermal stresses and high mechanical loads.

In aerospace engineering, Inconel 738 is critical for manufacturing various engine components. The alloy’s superior creep resistance and high tensile strength make it ideal for turbine blades and vanes operating in the hot gas path of jet engines. Its ability to withstand extreme temperatures and corrosive environments ensures that these components endure the rigorous conditions of high-altitude flight. Additionally, Inconel 738’s excellent fatigue resistance is crucial for the long-term durability of aircraft engines, contributing to safety and performance.

In the automotive industry, Inconel 738 is employed in high-performance and racing applications. Turbocharger components and exhaust systems benefit from the alloy’s ability to handle high temperatures and resist corrosion. The high melting point and excellent mechanical properties allow these parts to operate efficiently at elevated temperatures, improving engine performance and longevity. The alloy’s resistance to thermal fatigue and oxidation ensures that automotive parts maintain their structural integrity under the harsh conditions of high-speed operation.

In the energy sector, Inconel 738 is used in constructing components for gas turbines and other high-temperature equipment. The alloy’s robust creep resistance and high-temperature strength make it suitable for critical parts such as turbine blades, vanes, and combustor liners. These components must withstand prolonged exposure to high temperatures and stresses, and Inconel 738’s properties ensure reliable performance and extended service life. Its resistance to oxidation and thermal fatigue further enhances its suitability for energy applications, reducing maintenance costs and downtime.

In the nuclear and chemical industries, Inconel 738’s excellent corrosion resistance and high-temperature performance make it a preferred material. The alloy is used in reactor vessels and heat exchangers that require durability in corrosive environments and under high thermal loads. Its ability to maintain mechanical strength and resist oxidation at elevated temperatures ensures the safe and efficient operation of nuclear reactors and chemical processing equipment. Additionally, the alloy’s stability under irradiation and resistance to stress corrosion cracking are beneficial in these demanding fields.

In marine environments, Inconel 738 is well-suited for applications requiring outstanding corrosion resistance, particularly in saltwater conditions. Components such as naval vessel exhaust systems and offshore oil rig equipment benefit from the alloy’s ability to resist corrosion and maintain mechanical integrity. The high chromium and aluminum content in Inconel 738 forms a protective oxide layer, preventing degradation and extending the service life of marine components.

Manufacturing Processes

Casting Processes

Vacuum Investment Casting

Vacuum investment casting is a primary method used to manufacture large and complex Inconel 738 components. The process involves creating a wax model of the desired part, which is then surrounded by a ceramic shell. Once the ceramic hardens, the wax is melted away, leaving a cavity into which the molten Inconel 738 is poured under vacuum conditions. This process minimizes contamination and ensures high-quality castings with excellent mechanical properties.

Directional Solidification and Single Crystal Casting

Directional solidification and single crystal casting are advanced techniques that enhance the high-temperature performance of Inconel 738 components. Directional solidification involves controlling the solidification front to create a columnar grain structure, improving the alloy’s creep resistance. Single crystal casting eliminates grain boundaries entirely, resulting in superior mechanical properties and longer service life in high-temperature environments.

Forging

Forging is another critical manufacturing process for Inconel 738, used to produce high-strength components with refined grain structures. The process involves heating the alloy to its recrystallization temperature and then shaping it using compressive forces. This method enhances the mechanical properties of Inconel 738, making it ideal for components that require superior strength and durability, such as turbine blades and disks.

Powder Metallurgy

Direct Metal Laser Sintering (DMLS)

Direct Metal Laser Sintering (DMLS) is an additive manufacturing technique that uses a laser to fuse powdered Inconel 738 layer by layer, creating precise and complex shapes. This method is ideal for producing intricate components that are difficult to make with traditional techniques. The resulting parts have excellent mechanical properties and can be further enhanced through treatments like Hot Isostatic Pressing (HIP).

Laser Engineered Net Shaping (LENS)

LENS is another additive manufacturing process that involves depositing powdered Inconel 738 using a laser. This technique allows for the repair of existing components as well as the creation of new parts. LENS is advantageous for its ability to produce near-net-shape parts with minimal material waste and high mechanical integrity.

Hot Isostatic Pressing (HIP)

HIP is a critical post-processing technique used to improve the mechanical properties of Inconel 738 parts produced by casting or additive manufacturing. The process involves subjecting the components to high pressure and temperature in an inert gas atmosphere, which eliminates internal porosity and improves density. HIP enhances the overall strength, ductility, and fatigue resistance of the material, making it more suitable for high-stress applications such as turbine blades in aerospace and power generation industries. 

Metal Injection Molding (MIM)

Metal Injection Molding (MIM) is a process for making small, complex Inconel 738 parts with tight tolerances. It involves mixing metal powders with a binder to form a feedstock, which is then injected into a mold. After molding, the binder is removed, and the parts are sintered to achieve full density. MIM combines the flexibility of plastic injection molding with the material properties of Inconel 738, making it ideal for producing intricate parts in large quantities.

Compliance with Standards

Inconel 738 components must meet strict industry standards to ensure reliability and performance. Key standards include ISO and ASTM specifications, which define the chemical composition, mechanical properties, and manufacturing processes for Inconel 738. Adhering to these standards ensures that the components meet the required quality for use in demanding environments like aerospace and energy sectors.

Comparative Analysis with Other Superalloys

Composition

Inconel 738 is a nickel-based superalloy composed of nickel, chromium, cobalt, aluminum, and other elements such as molybdenum, tungsten, titanium, niobium, and tantalum. Inconel 738, available in high and low carbon versions, exhibits several key properties, including improved castability and stability in large sections when using the low carbon version (IN-738LC).

Comparison with Other Superalloys:

• Inconel 625: Contains a higher concentration of molybdenum compared to Inconel 738, providing superior resistance in high-chloride environments but less thermal stability.
• Haynes 230: Comprises chromium, molybdenum, tungsten, and other minor elements, offering excellent high-temperature resistance and oxidation resistance, though it is not as versatile as Inconel 738 in terms of applications.
• Waspaloy: A nickel-based alloy with a composition similar to Inconel 738 but with a higher gamma prime content, enhancing its high-temperature strength and creep resistance.

Properties

Inconel 738 exhibits several key properties:

• High-Temperature Strength: It retains its mechanical properties at temperatures up to 750°C, and even beyond 1000°C in certain conditions.
• Thermal Conductivity: Approximately 11.2 W/m·K, adequate for thermal management in high-temperature environments.
• Density: Ranges from 8.15 g/cm³ to 8.24 g/cm³, contributing to its robust composition.
• Creep Resistance: Superior due to its composition and microstructure, crucial for applications like turbine blades.

Comparison with Other Superalloys:

• Inconel 625: Has a lower melting point and less creep resistance than Inconel 738 but excels in marine environments due to its resistance to chloride-induced corrosion.
• Haynes 230: Offers superior oxidation resistance but is less strong at high temperatures compared to Inconel 738.
• Waspaloy: Exhibits higher high-temperature strength and creep resistance than Inconel 738 due to its gamma prime precipitation hardening.

Applications

Inconel 738 is widely used in:

• Aerospace: Components like turbine blades, vanes, and other parts in jet engines due to its high-temperature strength and resistance to oxidation and corrosion.
• Energy Production: Gas turbine blades and other components where high creep resistance is essential.
• Automotive: High-performance exhaust systems and turbocharger components take advantage of its thermal stability.

Comparison with Other Superalloys:

• Inconel 625: Used in marine and chemical processing industries where high-chloride resistance is necessary.
• Haynes 230: Often used in high-temperature furnace components due to its excellent oxidation resistance.
• Waspaloy: Utilized in aerospace applications requiring high strength at elevated temperatures, such as gas turbine components.

Manufacturing Processes

Inconel 738 can be manufactured through various methods, including additive manufacturing (e.g., Selective Laser Melting, Electron Beam Melting), hot isostatic pressing (HIP), and metal injection molding (MIM). These techniques enable the production of complex and high-density components, essential for aerospace and energy sectors.

Frequently Asked Questions

Below are answers to some frequently asked questions:

What is the chemical composition of Inconel 738?

Inconel 738 is a nickel-based superalloy known for its exceptional high-temperature strength and resistance to oxidation and corrosion. Its primary chemical composition includes:

• Nickel (Ni): Balance of the alloy
• Chromium (Cr): 15.70-16.30%
• Cobalt (Co): 8.00-9.00%
• Aluminum (Al): 3.20-3.70%
• Titanium (Ti): 3.20-3.70%
• Molybdenum (Mo): 1.50-2.00%
• Tungsten (W): 2.40-2.80%
• Tantalum (Ta): 1.50-2.00%
• Niobium (Nb): 0.60-1.10%
• Boron (B): 0.005-0.015%
• Zirconium (Zr): 0.05-0.15% (0.03-0.08% in IN-738LC)
• Carbon (C): 0.15-0.20% in IN-738C, 0.09-0.13% in IN-738LC
• Iron (Fe), Manganese (Mn), Silicon (Si), Sulfur (S): Present in smaller quantities within specified limits (e.g., Fe ≤ 0.05%, Mn ≤ 0.02%, Si ≤ 0.30%, S ≤ 0.015%)

This precise combination of elements gives Inconel 738 its robust mechanical properties, making it ideal for high-temperature applications such as gas turbines and aerospace components.

How is Inconel 738 used in industrial applications?

Inconel 738 is extensively utilized in industrial applications that demand high performance under extreme conditions due to its exceptional high-temperature strength, corrosion resistance, and thermal stability. In the aerospace industry, it is used for manufacturing turbine blades, vanes, combustion cans, and afterburner parts in gas turbine engines, where it ensures durability and resistance to oxidation at elevated temperatures.

In the energy sector, Inconel 738 is employed in gas turbines for components such as combustor cans and turbine blades, providing reliable performance under high thermal and mechanical stresses. The automotive industry benefits from its use in high-performance exhaust systems and turbocharger components, enhancing engine efficiency and longevity. In the oil and gas industry, it is utilized in downhole safety valves and wellhead components, where its strength and corrosion resistance are crucial in harsh environments. The chemical processing industry leverages Inconel 738 for reactor internals, scrubbers, and cyclone components due to its stability and resistance to aggressive chemicals.

Additionally, the nuclear industry uses this alloy for components that require high temperature and corrosion resistance, maintaining stability under radiation and extreme conditions. These applications highlight Inconel 738’s versatility and reliability in demanding industrial environments.

What are the key properties of Inconel 738?

Inconel 738 is a nickel-based superalloy known for its remarkable high-temperature strength, corrosion and oxidation resistance, and creep resistance. Its chemical composition includes significant amounts of nickel, chromium, and cobalt, along with aluminum, molybdenum, tungsten, titanium, niobium, and tantalum. Key properties of Inconel 738 include:

• High-Temperature Strength: Maintains mechanical properties at elevated temperatures, ideal for gas turbine blades.
• Corrosion and Oxidation Resistance: Chromium and aluminum provide excellent resistance in harsh environments.
• Creep Resistance: Superior resistance to deformation under constant stress at high temperatures.
• Thermal Conductivity: Approximately 11.2 W/m·K, aiding in effective heat dissipation.
• Density and Melting Point: Density ranges from 8.15 to 8.24 g/cm³, with a melting point between 1310°C and 1335°C.
• Tensile Strength: Ranges from 1050 MPa to 1300 MPa, ensuring it can withstand significant mechanical stresses.

These properties make Inconel 738 a preferred material for high-performance applications in aerospace and energy sectors.

How does Inconel 738 compare with other nickel-based superalloys?

Inconel 738 stands out among nickel-based superalloys due to its superior high-temperature strength, corrosion resistance, and thermal stability. When compared to other nickel-based superalloys such as Inconel 718, Hastelloy X, and Waspaloy, Inconel 738 demonstrates exceptional creep resistance and maintains its mechanical properties at elevated temperatures, making it particularly suitable for demanding applications like turbine blades and combustors in gas turbines and aerospace engines.

In contrast, Inconel 718 is known for its excellent mechanical properties and weldability but is less suitable for the highest temperature applications where Inconel 738 excels. Hastelloy X offers good oxidation resistance and high-temperature strength but does not match Inconel 738’s performance in extremely high-stress environments. Waspaloy, another high-temperature superalloy, has comparable high-temperature strength but Inconel 738 often provides better creep resistance and is preferred in applications where long-term stability at extreme temperatures is crucial.

Thus, Inconel 738 is preferred in applications where prolonged exposure to high temperatures and mechanical stress is critical.

What are the common manufacturing challenges with Inconel 738?

Inconel 738, a nickel-based superalloy, presents several manufacturing challenges primarily due to its high alloying content and complex composition. Key challenges include:

1. Fabricability and Machinability: The high content of elements like chromium, cobalt, and molybdenum enhances its mechanical properties but makes machining and welding difficult. The alloy is prone to strain-age cracking during welding and requires precise control of welding parameters and techniques.

2. Additive Manufacturing (AM) Issues: Techniques like Electron Beam Melting (EBM) and Selective Laser Melting (SLM) face difficulties such as inconsistent powder spreading, ineffective temperature monitoring, and deterioration of reused powder. Optimizing parameters like beam current and scan strategy is essential to produce dense, crack-free components.

3. High Material Costs: The cost of Inconel 738 powder is significantly higher compared to other metals, impacting its economic feasibility for widespread use.

4. Hydrogen Embrittlement: This alloy, like other nickel-based superalloys, is susceptible to hydrogen embrittlement, which can compromise its mechanical integrity.

Addressing these challenges requires ongoing research and optimization of manufacturing processes, including advanced powder metallurgy techniques and improved AM process controls.

What standards must Inconel 738 comply with?

Inconel 738 must comply with several technical standards to ensure its quality and performance in various high-temperature applications. Key standards include:

• ASTM B446: This standard specification covers nickel-chromium-molybdenum-columbium alloys, which, while not specific to Inconel 738, is often referenced in related documentation.
• AMS 5666, AMS 5663, and AMS 5599: These Aerospace Material Specifications (AMS) are relevant for Inconel alloys used in aerospace applications, including Inconel 738.
• DIN EN 2.4856 and BS 3076: These are European (DIN EN) and British (BS) standards ensuring compliance with regional regulations and industry requirements for Inconel 738.
• UNS N06738: This Unified Numbering System (UNS) designation is specific to Inconel 738, defining its chemical composition and properties.

These standards ensure that Inconel 738 meets the stringent requirements for high-temperature performance, mechanical strength, and corrosion resistance necessary for its applications in gas turbines, aerospace, and other high-stress environments.