The heat treatment and performance of high-temperature superalloys, and Inconel® 718 in particular, has always been of interest to The Doctor. With greater availability and higher demand for this alloy, understanding its heat treatment will be of great value to the heat treater. Let’s learn more. 

 

What is a superalloy?

The term “superalloy” was coined just after World War II to describe that group of alloys developed specifically to extend the life of aircraft turbine engines and turbosuperchargers so as to offer better performance life at elevated service temperatures.[1,2]

Today, superalloys (aka high-performance alloys) are those materials engineered for extreme-duty environments where high levels of mechanical strength and creep resistance at high temperatures are necessary in combination with excellent corrosion and oxidation resistance. The surface stability (of oxide particles and the microstructure) of these alloys helps them perform in these extreme-duty applications. They typically have an austenitic face-centered-cubic crystal structure with base alloying elements of either nickel, cobalt or iron, hence their classification as nickel-based, cobalt-based or iron-based superalloys (Fig. 1).

When one thinks of Inconel® 718 and other superalloys in this family (e.g., Hastelloy® C-276, Hastelloy® X and Alloy 20), the aerospace industry (turbine blades, ducting systems, engine exhaust systems) comes immediately to mind since approximately 50% of these materials are used in the manufacturing of aircraft and rocket engines. However, these materials are also found in chemical and petrochemical plants (vessels, pumps, valves, piping), power plants (industrial gas turbines), submarines (propeller blades, quick-disconnect fittings, auxiliary propulsion motors), nuclear reactors (heat-exchanger tubing, fittings), and in the oil and gas industry (downhole tubulars, well-head hardware, flare booms).[5]

Nickel-based superalloys such as Inconel® 718 are available in sheet, plate, bar, pipe and tube (welded and seamless) and wire. The alloy can be in wrought or cast form. Alloying elements such as chromium, aluminum, titanium, molybdenum, tungsten, niobium, tantalum and cobalt provide a variety of benefits. For example, nickel enhances corrosion resistance up to around 950˚C (1740˚F), resistance to chloride-induced stress corrosion cracking and attack by most organic and inorganic compounds, while chromium prevents corrosion in oxidizing media and sulfur-bearing environments. Molybdenum improves resistance to pitting attack.

 

Heat-Treat Process[1,5,6,8,9]

Inconel® 718 is typically purchased in the mill- or solution-annealed condition. In some instances, it has a stress-relief operation performed on it prior to fabrication and heat treatment. The solution anneal is followed by a precipitation (age) hardening step. Precipitation of secondary phases (e.g., gamma prime and gamma double-prime) into the metal matrix hardens the material. The precipitation of nickel-aluminum, nickel-titanium and nickel-niobium phases is triggered by aging in the temperature range of 600-815˚C (1100-1500˚F). The key to the heat-treatment process is to be sure these age-hardening constituents are fully in solution at high temperature (i.e., fully dissolved in the matrix), otherwise precipitation will not result in full strength.

Two heat treatments are common (although slight temperature variations of these recipes are often employed).

  • Solution anneal at 925-1010˚C (1700-1850˚F) followed by rapid cooling (usually in water). This is followed by precipitation hardening at 720˚C (1325˚F) for 8 hours, furnace cooling to 620˚C (1150˚F) and holding for 10 hours followed by air cooling.
  • Solution anneal at 1040-1065˚C (1900-1950˚F) followed by rapid cooling (usually in water). This is followed by precipitation hardening at 760˚C (1400˚F) for 10 hours, furnace cool to 650˚C (1200˚F) and holding for 10 hours followed by air cooling.

Solution heat treatments are typically done in low-dew-point argon or vacuum furnaces, often with all-metal hot zones to avoid discoloration. The effect of annealing for 30 minutes on the grain size (Fig. 2) of sheet varies with temperature. Also, aging response of Inconel® 718 is rather slow in comparison with that of aluminum-titanium-hardened alloys. Thus, in most sizes, the alloy can be heated and cooled through the aging temperature range at normal speeds yet retain softness and ductility. The effect of aging time and temperature on the hardness of annealed sheet (Fig. 3) reflects this.

Virtually no hardening occurs during the first 2-3 minutes of exposure. This is ample time to permit air cooling after welding or annealing. In comparison, aluminum-titanium-hardened alloys having sufficient hardening alloys to approach the strength level of Inconel® 718 would develop almost full hardness in this same period of time.

The solution anneal at 925-1010˚C (1700-1850˚F) with its corresponding aging treatment is considered the optimum heat treatment for Inconel® 718 where a combination of rupture life, notch rupture life and rupture ductility is of greatest concern. The highest room-temperature tensile and yield strengths are also associated with this treatment. In addition, because of the fine grain developed, it produces the highest fatigue strength.

By contrast, the solution anneal at 1040-1065˚C (1900-1950˚F) with its corresponding aging treatment is preferred in tensile-limited applications because it produces the best transverse ductility in heavy sections, impact strength and low-temperature notch tensile strength. However, this treatment has a tendency to produce notch brittleness in stress rupture.[6]

 

Tweaking the Cycles[1,5,6,8,9]

Optimum properties are not always achieved by the first solution annealing and aging heat treatments. Changes to the annealing or aging times and temperatures and the addition of an intermediate (stabilizing) treatment or a third age-hardening treatment are common. For example, to achieve the best combination of high tensile strength, high fatigue and stress-rupture life for use in aerospace applications (often referred to as high-strength 718) such as rotational parts, turbine blades, bearings and fasteners, a somewhat more complex heat-treatment cycle is recommended.

  • Soak one hour at 955-980˚C (1750-1800˚F) followed by air cooling, reheating and holding for 8 hours at 720˚C (1325˚F) followed by cooling at 56˚C/hour (100˚F/hour) to 620˚C (1150˚F) then holding for 8 hours with a final air cool to room temperature.

To achieve better impact strength, low-temperature notch tensile strength and lower (40 HRC) hardness (Table 1) for use in oil-patch applications (often referred to as API 6A 718) for parts such as gate valves, choke stems, fasteners and tubing hangers, a different heat treatment is required.

  • Soak for 1-2 hours at 1065˚C (1950˚F) followed by air cooling, then reheating and holding 8 hours at 720˚C (1325˚F) and cool at 56˚C/hour (100˚F/hour) to 620˚C (1150˚F), hold for 8 hours and air cool to room temperature.

 

Summary

With the demand for Inconel® 718 and other superalloys increasing over a broader range of application uses, the heat treater must first understand the desired performance characteristics of the application and then be prepared to devise recipes and heat-treatment strategies to maximize the alloy’s response to these treatments.

 

 

References

  1. Donachie, Matthew J., and Stephen J. Donachie, Superalloys: A Technical Guide, 2nd Edition, ASM International, 2002
  2. Mega Mex (www.megamex.com) 
  3. Basuki, Eddy Agus, Djoko Hadi Prajitno and Fadhli Muhammad, “Alloys Developed for High Temperature Applications,” AIP Conference Proceedings 1805, 020003 (2017); 20 January 2017 (https://doi.org/10.1063/1.4974409)
  4. “Inconel 718 for Aerospace Engine Applications”, Heanjia Super Metals Co., Ltd. (https://super-metals.com/news/inconel-718-for-aerospace-engine-applications/)
  5. Specialty Metals Inconel 718 Datasheet (www.specialmetals.com)
  6. Maher Alloy 718 Data Sheet Vs1.0 (www.maher.com)
  7. “Inconel Alloys | Uses and Characteristics,” Ferralloy (https://www.ferralloy.com/inconel-alloys-uses-characteristics/)
  8. Herring, Daniel H., Vacuum Heat Treatment, BNP Media, 2012
  9. Heat Treaters Guide: Practices and Procedures for Ferrous Alloys, Harry Chandler (Ed.), ASM International, 1995