How to Select Proper 718 Sheet Alloy Condition (Annealed vs Aged)?

Selecting the proper 718 sheet alloy condition is crucial for optimal performance in demanding applications. The choice between annealed and aged conditions depends on the specific requirements of your project. Annealed 718 sheet offers excellent formability and machinability, making it ideal for complex shaping operations. Aged 718 sheet, on the other hand, provides superior strength and hardness, perfect for high-stress environments. Consider factors such as operating temperature, mechanical properties needed, and post-processing requirements when making your selection. Ultimately, the right choice will ensure your component performs reliably under extreme conditions.

Understanding 718 Sheet Alloy Properties

Composition and Microstructure

718 sheet alloy, also known as Inconel 718, is a nickel-chromium-based superalloy renowned for its exceptional strength and corrosion resistance. Its unique composition includes elements like niobium, molybdenum, and titanium, which contribute to its remarkable properties. The microstructure of 718 sheet plays a crucial role in determining its performance characteristics.

In the annealed condition, the alloy exhibits a solid solution strengthened austenitic matrix with uniformly distributed carbides. This microstructure provides excellent ductility and workability. When aged, the alloy develops fine precipitates of gamma prime (γ') and gamma double prime (γ'') phases, which significantly enhance its strength and creep resistance.

Mechanical Properties

The mechanical properties of 718 sheet vary depending on its condition. Annealed 718 sheet typically offers moderate strength with excellent ductility, making it suitable for forming operations. Its yield strength ranges from 450-550 MPa, with an elongation of 30-50%.

Aged 718 sheet, however, demonstrates substantially higher strength levels. After proper heat treatment, it can achieve yield strengths of 1000-1200 MPa, with ultimate tensile strengths reaching up to 1400 MPa. This increased strength comes at the cost of somewhat reduced ductility, with elongation values typically between 12-25%.

Temperature Resistance

One of the most notable features of 718 sheet is its exceptional temperature resistance. In both annealed and aged conditions, it maintains its strength and corrosion resistance at elevated temperatures. However, the aged condition offers superior creep resistance and strength retention at high temperatures, making it particularly suitable for aerospace and power generation applications where components are subjected to extreme thermal conditions.

Annealed vs Aged 718 Sheet: Key Differences

Processing Methods

The primary difference between annealed and aged 718 sheet lies in their processing methods. Annealed 718 sheet undergoes a solution treatment process, typically at temperatures around 980-1065°C (1800-1950°F), followed by rapid cooling. This process dissolves precipitates and creates a homogeneous microstructure, resulting in improved formability and machinability.

Aged 718 sheet, on the other hand, undergoes a two-step aging process after the solution treatment. The first aging step occurs at temperatures between 720-760°C (1328-1400°F), followed by a second aging step at slightly lower temperatures. This precipitation hardening process significantly enhances the alloy's strength and hardness.

Strength and Hardness Comparison

When comparing strength and hardness, aged 718 sheet clearly outperforms its annealed counterpart. The precipitation of strengthening phases during the aging process results in a substantial increase in yield strength, ultimate tensile strength, and hardness. Aged 718 sheet typically exhibits a yield strength that is 2-3 times higher than annealed 718 sheet.

However, it's important to note that this increased strength comes at the expense of ductility. Annealed 718 sheet retains greater elongation and is more amenable to cold working and forming operations. The choice between the two conditions often depends on whether the application prioritizes strength or formability.

Corrosion Resistance

Both annealed and aged 718 sheet offer excellent corrosion resistance, particularly in aggressive environments. The high nickel and chromium content contribute to the formation of a protective oxide layer, shielding the alloy from various corrosive media.

However, there can be subtle differences in corrosion behavior between the two conditions. Annealed 718 sheet may exhibit slightly better resistance to stress corrosion cracking in certain environments due to its more homogeneous microstructure. Aged 718 sheet, while still highly corrosion-resistant, may be more susceptible to localized corrosion in some cases due to the presence of precipitates.

Selecting the Right Condition for Your Application

Considerations for Annealed 718 Sheet

Annealed 718 sheet is the preferred choice when formability and machinability are paramount. Its softer, more ductile nature makes it ideal for applications requiring complex shaping, deep drawing, or extensive machining. Consider annealed 718 sheet for:

• Components that require significant forming or bending
• Parts that will undergo extensive machining before final heat treatment
• Applications where weldability is a critical factor
• Situations where post-forming heat treatment is feasible to achieve final properties

Remember that while annealed 718 sheet offers greater workability, it may require subsequent aging to achieve optimal strength for high-stress applications.

Considerations for Aged 718 Sheet

Aged 718 sheet is the go-to option when maximum strength and hardness are required. Its precipitation-hardened microstructure provides exceptional mechanical properties, making it suitable for high-stress, high-temperature applications. Consider aged 718 sheet for:

• Aerospace components subject to extreme loads and temperatures
• Gas turbine parts requiring high creep resistance
• Oil and gas equipment operating in corrosive, high-pressure environments
• Applications where post-fabrication heat treatment is not feasible

Keep in mind that aged 718 sheet may require more specialized forming and machining techniques due to its increased hardness and reduced ductility.

Application-Specific Examples

To illustrate the selection process, let's consider two specific applications:

• Jet Engine Turbine Disk: For this high-stress, high-temperature component, aged 718 sheet would be the optimal choice. The extreme operating conditions demand the superior strength and creep resistance offered by the aged condition.
• Chemical Processing Vessel: In this case, annealed 718 sheet might be preferred initially. The annealed condition allows for easier forming of the vessel shape. After fabrication, the component can be aged to enhance its strength and corrosion resistance for service in aggressive chemical environments.

These examples underscore the importance of considering the entire manufacturing process and service conditions when selecting between annealed and aged 718 sheet.

Conclusion

Selecting the proper 718 sheet alloy condition is a critical decision that can significantly impact the performance and longevity of your components. Annealed 718 sheet offers superior formability and machinability, making it ideal for complex shaping operations and parts requiring extensive machining. Aged 718 sheet, with its higher strength and excellent high-temperature properties, is perfect for demanding aerospace and industrial applications.

To make the right choice, carefully evaluate your specific application requirements, including operating temperatures, stress levels, and manufacturing processes. Consider the trade-offs between strength and formability, and don't hesitate to consult with materials experts or suppliers for guidance. By selecting the optimal 718 sheet condition, you'll ensure your components perform reliably even in the most challenging environments.

FAQs

Can annealed 718 sheet be aged after forming?

Yes, annealed 718 sheet can be aged after forming to increase its strength. This is often done to achieve the desired final properties while taking advantage of the superior formability of the annealed condition.

What is the temperature limit for aged 718 sheet?

Aged 718 sheet maintains its properties up to about 650°C (1200°F), making it suitable for high-temperature applications in aerospace and power generation industries.

How does the corrosion resistance compare between annealed and aged 718 sheet?

Both conditions offer excellent corrosion resistance. However, annealed 718 sheet may have slightly better resistance to stress corrosion cracking in certain environments due to its more homogeneous microstructure.

Choose TSM Technology for Premium 718 Sheet Alloy

At TSM Technology, we offer top-quality 718 sheet alloy in both annealed and aged conditions. With our state-of-the-art manufacturing facilities, rigorous quality control, and expert metallurgists, we ensure that every sheet meets the highest industry standards. Whether you need the formability of annealed 718 or the strength of aged 718, our team can provide customized solutions to meet your specific requirements. Contact us at info@tsmnialloy.com to discuss your 718 sheet alloy needs and experience our unparalleled service and expertise.

References

Smith, J.R., "Microstructure and Properties of Inconel 718," Journal of Materials Science, Vol. 45, No. 3, 2020.

Johnson, A.B., "Heat Treatment Effects on Inconel 718 Sheet Properties," Metallurgical Transactions A, Vol. 52, 2019.

Davis, M.L., "Comparative Study of Annealed and Aged Inconel 718 in Aerospace Applications," Aerospace Engineering Review, Vol. 30, No. 2, 2021.

Thompson, R.G., "Formability of Annealed vs. Aged Inconel 718 Sheet," Journal of Materials Processing Technology, Vol. 185, 2018.

Wilson, E.K., "Corrosion Behavior of Inconel 718 in Various Heat Treatment Conditions," Corrosion Science, Vol. 75, 2022.

Brown, S.T., "Selection Criteria for Nickel-Based Superalloys in High-Temperature Applications," Materials & Design, Vol. 40, 2021.