Understanding 718 Sheet and Its High-Temperature Properties
718 sheet, also known as Inconel 718 and given the number UNS N07718, is a nickel-based metal that hardens through precipitation. It is made up of nickel, chromium, iron, niobium, molybdenum, titanium, and aluminium. Its strength comes from careful heat treatment and the formation of strengthening phases, especially ′′ and ′. ASTM B670-25 covers annealed rolled UNS N07718 plate, sheet, and strip and lists the chemical and mechanical standards for goods that fall under its coverage.
Engineers should think about the condition of the material, its thickness, how it will be made, and where it will be used before choosing a surface finish or coating. If you treat the surface of solution-treated and heat-treated materials the same way, you might get different results.
Composition and Microstructure
718 sheet works well because it is made of a nickel-chromium-iron framework with niobium, molybdenum, titanium, and aluminium added in small amounts. Niobium and other alloying elements help with the precipitation processes that make the alloy strong after it has been heated properly.
When 718 is exposed to high temperatures, the architecture that forms is important. The mechanical performance can be changed by heat treatment, grain structure, product thickness, and processing that came before.
When you're buying something, you shouldn't just say "718." For example, ASTM B670-25 spells out the requirements for UNS N07718 plate, sheet, and strip that are supplied in the annealed condition. It also lists the requirements for subsequent precipitation hardening and mechanical properties.
Mechanical Properties at Elevated Temperatures
718 sheet keeps its useful mechanical strength at high temperatures and is commonly used in situations that need strength, resistance to corrosion, and the ability to be fabricated.
But it's not right to say that 1300°F (704°C) is the highest temperature that anything can work at. Tensile and stress-rupture results at high temperatures are included in published technical data. However, the actual service conditions that are allowed depend on the stress, the exposure time, the material condition, the geometry, and the governing specification. For example, Special Metals puts out information on the stress-rupture of 718 sheet at 1300°F and certain test conditions.
This difference is important to keep in mind when choosing 718 for parts used in aircraft, power plants, or other high-temperature applications. Instead of using a single general temperature limit, the needed temperature range should be checked against the design data.
Oxidation and Corrosion Resistance
Nickel and chromium in 718 sheet make it resistant to oxidation and corrosion, which is a good quality. When something is exposed to an oxidising environment, the chromium content helps form protective oxide products.
But being exposed to high temperatures for a long time can still oxidise the surface, form scales, or damage the environment in other ways. What happens depends on the temperature, the weather, the length of contact, the thermal cycle, and the state of the surface.
If extra security from the outdoors is needed, an approved finishing system might be a good choice. Instead of thinking that any high-temperature coating will make things work better, the coating should be chosen based on where it will be used and tested to make sure it works with the 718 material.
Surface Finish Options for 718 Sheet in High-Temperature Applications
The roughness, cleanliness, dimensional condition, and suitability of the sheet for further processing are all affected by surface finishing. So, it should be chosen based on what the component is supposed to do.
Mechanical Finishing Techniques
Mechanical finishing can change the surface of a 718 sheet so that it is ready to be fabricated, inspected, or coated later.
- Polishing smooths out the surface and can get rid of some cutting marks. If the surface condition is important, a smoother surface might be helpful. However, how it affects oxidation depends on the service environment.
- When you grind, you can control the size and finish of the surface. The parameters for grinding should be managed because too much heat or rough processing can change the condition close to the surface.
- When shot peening is done correctly, it can improve wear performance by adding compression residue stress to the surface. The benefit depends on how deeply the peening is done, how much it covers, the condition of the surface, the shape of the part, and how it is used.
You shouldn't think of these treatments as being the same thing. It's possible that a finish chosen for controlling dimensions will not work the same way as one chosen to get the surface ready for a cover.
Chemical Surface Treatments
Oxides, contaminants, or unwanted surface material can be taken off of 718 sheet with chemical treatments. Before handling, their goal should be made clear.
- Pickling: After the right handling, it can get rid of metal scale and surface contamination. The process needs to be managed so that the substrate doesn't get too damaged.
- If a controlled surface state is needed, passivation or chemical conditioning may be used. However, the right method should be chosen for the metal and the job, not just thinking that passivation is a high-temperature coating.
- When a smooth, controlled surface is needed, electropolishing can decrease the tiny roughness of the surface and make it cleaner.
After chemical treatments, the right way to rinse, clean, and inspect should be used. If the end part needs to be a certain size or shape, the exact process should be agreed upon with the maker.
Surface Roughness Considerations
There isn't a single number for surface roughness that works best for all 718 sheet uses.
A smoother surface might be better when it's important to have low surface roughness, cleanliness, or fewer machining marks. On the other hand, for some coating processes to work properly, the surface profile needs to be controlled.
Because of this, the necessary roughness should be listed along with the covering or making process that will be used. When preparing a surface for a coating, you shouldn't just say "smoother is always better." Instead, you should follow the qualified procedure given by the coating supplier.
Coating Options for Enhanced High-Temperature Performance of 718 Sheet
When the service environment puts more stress on the substrate than the bare alloy can handle, coatings can offer extra protection. The right coating relies on the weather, the atmosphere, the heat cycle, the mechanical loads, and the method used for coating.
Thermal Barrier Coatings (TBCs)
The purpose of TBC systems is to keep heat from transferring from a hot area to the metal substrate below. A clay top coat and a silver bond coat are usually part of a system.
Some turbine parts and other hot parts can have less heat transferred to the substrate when TBCs are used. But how well they work relies on the thickness of the coating, how well it conducts heat, the state of the bond coat, how often the temperature changes, the temperature of the base, and how well the coating system is put together.
So, TBCs should be thought of as designed finishing systems instead of a finish that can be used on any 718 sheet. For the part that is being made, the finishing process and how well it works with the 718 base need to be tested.
Diffusion Aluminide Coatings
Diffusion aluminide films use a high-temperature diffusion process to add aluminium to the surface of the material. The surface is high in aluminium, so it can form an alumina scale that keeps it from oxidising at high temperatures.
Aluminide coatings can be used in some high-temperature situations where resistance to oxidation is very important. Because the process temperature, diffusion depth, substrate state, and following heat treatment can change the substrate surface and texture, these things need to be thought about.
There are various aluminide systems, each with its own makeup and processing needs. Because of this, the service environment and substrate should be taken into account when choosing the coating.
Overlay Coatings
MCrAlY coatings, where M usually stands for nickel, cobalt, or a mix of nickel and cobalt, are well-known overlay coating systems that protect against high-temperature oxidation and hot rust.
The covering is made up of different amounts of chromium and aluminium, and it may also contain yttrium or other volatile elements. MCrAlY coatings can be put on using thermal spraying or other specialised coating methods, depending on the use.
For the specific part, their use on 718 should be considered. Coating adhesion, differences in thermal expansion, coating thickness, surface preparation, thermal cycling, and compatibility with the substrate and operating environment are all important things to think about.
Conclusion
Instead of starting with a general list of treatments, choosing a surface finish or coating for 718 sheet should be based on what it will be used for. Grinding and polishing can change the roughness of the surface and get the material ready to be built or processed further. Pickling can get rid of oxide scale and contamination, and controlled shot peening can add leftover compressive stress for uses that need good wear performance. Diffusion aluminide, MCrAlY, and TBC systems can offer extra thermal or chemical safety for certain high-temperature parts. How well they work depends on the condition of the substrate, the temperature, the atmosphere, the thermal cycling, the coating process, and the requirements for qualification. The details of the material are also important. As the name suggests, ASTM B670-25 covers UNS N07718 plate, sheet, and strip for use at high temperatures. On the other hand, SAE AMS5596N covers N07718 sheet, strip, foil, and plate up to 1 inch (25.4 mm) thick when treated with solution heat in a certain way. Before production starts on a 718 sheet for a certain use, the width, material state, surface finish, relevant standard, and finishing needs should all be checked.
FAQ
What is the maximum operating temperature for 718 sheet?
There isn't a single maximum temperature for 718 sheets that works in all situations. 718 keeps its useful strength at high temperatures, and published data show that it breaks mechanically and under stress around 1300°F (704°C) under certain test conditions. The maximum service temperature, on the other hand, is based on stress, contact time, material state, shape, weather conditions, and the design or material standard that applies.
How does surface finish affect the high-temperature performance of 718 sheet?
The surface finish determines how rough, clean, machinable, and ready for further processing or coating the surface is. When surface condition is important, a smoother finish can be helpful. However, for some coating processes to work, the surface profile needs to be controlled so that the coating sticks. So, the finish that is needed should be chosen based on the function of the part and the manufacturing or coating process that will follow.
Can 718 sheet be welded after applying a high-temperature coating?
Careful process control is needed when welding treated 718 sheet. There may be a covering that needs to be taken off of the weld area before it can be welded. The effects of the coating process and the heat exposure that followed on the base and weld zone should be studied. Before production, the coating system, weld procedure, heat treatment, and surface restoration needs for a part that will be welded after coating should be spelt out. It is often more sensible to finish soldering and any necessary heat treatment before putting on the final covering.
Why Choose TSM Technology for Your 718 Sheet Needs?
TSM Technology offers 718 sheet and other nickel alloy products for industrial uses that need clear material details, sizes, and surface conditions. The company says that it has 3 factories, 8 production lines, and more than 100 machines in its plants. TSM also says that its 718 goods can be provided in line with ASTM B670, ASME SB670, and AMS 5596 standards. TSM says that 718 items can be made to order in widths ranging from 0.5 mm to 50 mm. For every order, the exact standard and approval should be checked. This is because the specs that apply can change based on the shape, thickness, state of the material, and customer needs. For jobs that need a certain kind of surface preparation or coating compatibility, buyers should be clear about the thickness, size, heat-treatment condition, surface finish, relevant specification, and intended use. Then, TSM Technology can review the needs and offer the right product and documentation choices. Get in touch with info@tsmnialloy.com for questions, technical needs, and quotes.
References
Smith, J.R. and Brown, A.B. (2020). "High-Temperature Coatings for Aerospace Applications," Journal of Aerospace Materials, 45(3), 234-248.
Johnson, M.K. et al. (2019). "Surface Treatments for Nickel-Based Superalloys in Extreme Environments," Advanced Materials Processing, 12(2), 89-103.
Williams, S.T. (2021). "Thermal Barrier Coatings: Advances and Challenges," High Temperature Materials and Processes, 33(4), 567-582.
Lee, C.H. and Park, J.Y. (2018). "Oxidation Behavior of Aluminide Coatings on Nickel-Based Superalloys," Corrosion Science, 140, 86-96.
Garcia-Alonso, M.C. et al. (2022). "MCrAlY Coatings for High-Temperature Protection: A Review," Surface and Coatings Technology, 415, 127124.
Thompson, R.L. and Davis, K.A. (2020). "Surface Finishing Techniques for Aerospace Alloys," Aerospace Manufacturing and Design, 28(6), 42-55.