Superalloys, such as Inconel® 718, have garnered significant interest due to their heat treatment and performance in high-temperature environments. As the demand for this alloy continues to rise, understanding its heat treatment process becomes crucial for heat treaters.
In this article, we will delve into the fascinating world of superalloys, explore the properties of Inconel® 718, and uncover the secrets behind its heat treatment.
What are Superalloys?
Superalloys, also known as high-performance alloys, were originally developed after World War II to enhance the longevity and performance of aircraft turbine engines and turbosuperchargers operating at elevated service temperatures.
Today, these alloys are engineered to withstand extreme-duty environments, combining exceptional mechanical strength and creep resistance at high temperatures with superior corrosion and oxidation resistance. Superalloys, including Inconel® 718, or Inconel 625 exhibit an austenitic face-centered-cubic crystal structure and are classified as nickel-based, cobalt-based, or iron-based superalloys, depending on the base alloying elements.
Applications of Inconel® 718
Inconel® 718, a nickel-based superalloy, finds extensive usage in various industries. While it is primarily associated with aerospace applications, such as turbine blades, ducting systems, and engine exhaust systems, its versatility extends to chemical and petrochemical plants, power plants, submarines, nuclear reactors, and the oil and gas industry. Inconel® 718 is available in different forms, including sheet, plate, bar, pipe, tube, and wire, and can be obtained in wrought or cast form.
The Role of Alloying Elements
Inconel® 718 derives its exceptional properties from a combination of alloying elements. Chromium, aluminum, titanium, molybdenum, tungsten, niobium, tantalum, and cobalt contribute to its unique characteristics. Nickel enhances corrosion resistance, while chromium prevents corrosion in oxidizing and sulfur-bearing environments. Molybdenum improves resistance to pitting attack.
The chemical composition of Inconel® 718 is as follows:
- Nickel (Ni): Approximately 50-55%
- Chromium (Cr): Approximately 17-21%
- Iron (Fe): Approximately 17-21%
- Niobium (Nb): Approximately 4.75-5.5%
- Molybdenum (Mo): Approximately 2.8-3.3%
- Titanium (Ti): Approximately 0.65-1.15%
- Aluminum (Al): Approximately 0.2-0.8%
- Carbon (C): Maximum of 0.08%
- Manganese (Mn): Maximum of 0.35%
- Silicon (Si): Maximum of 0.35%
- Sulfur (S): Maximum of 0.015%
- Phosphorus (P): Maximum of 0.015%
- Cobalt (Co): Maximum of 1%
- Copper (Cu): Maximum of 0.3%
Heat Treatment Process
The heat treatment process of Inconel® 718 involves a series of steps to achieve the desired properties. The alloy is typically purchased in the mill- or solution-annealed condition. A stress-relief operation may be performed prior to fabrication and heat treatment.
The process includes a solution anneal followed by precipitation (age) hardening. Precipitation of secondary phases into the metal matrix strengthens the material. The heat treatment is critical to ensure the complete dissolution of age-hardening constituents at high temperatures. Two common heat treatment cycles are employed, involving specific annealing and aging temperatures.
In most cases, when specifying INCONEL alloy 718, it is recommended to undergo a solution annealing and precipitation hardening process. This process, also known as precipitation heat treatment or age hardening, involves the precipitation of secondary phases, such as gamma prime and gamma double-prime, into the metal matrix to achieve hardening.
The precipitation of these phases, which contain nickel along with aluminum, titanium, and niobium, occurs during heat treatment within the temperature range of 1100 to 1500°F. To ensure the proper occurrence of this metallurgical reaction, it is essential for the aging constituents (aluminum, titanium, niobium) to be dissolved in the matrix, rather than being precipitated as a different phase or combined in another form. Incorrect precipitation or combination of these constituents will result in the alloy not achieving its full strength potential.
To initiate this process, the material must first undergo solution heat treatment, where it is heated to a specific temperature to dissolve the aging constituents within the matrix.
Inconel 718 Heat treatment
There are two commonly employed heat treatments for INCONEL alloy 718:
- Solution Anneal and Precipitation Hardening:
- The alloy is subjected to a solution annealing treatment at temperatures between 1700-1850°F. This process involves heating the material to dissolve any precipitated phases and promote homogeneity.
- After the solution annealing, rapid cooling, usually in water, is performed to quench the material.
- The next step is precipitation hardening at 1325°F for a duration of 8 hours. This promotes the precipitation of secondary phases, enhancing the strength of the alloy.
- Following the precipitation hardening, the material is furnace cooled to 1150°F and held at this temperature for a total aging time of 18 hours.
- Finally, the material is air cooled to room temperature.
- Alternative Solution Anneal and Precipitation Hardening:
- In this heat treatment, the alloy undergoes solution annealing at higher temperatures of 1900-1950°F, followed by rapid cooling.
- Similar to the previous process, precipitation hardening is carried out, but at a higher temperature of 1400°F for a duration of 10 hours.
- The material is then furnace cooled to 1200°F and held at this temperature for a total aging time of 20 hours.
- The final step involves air cooling the material to room temperature.
When INCONEL alloy 718 is intended for machining, forming, or welding purposes, it is typically acquired in the mill annealed or stress-relieved condition. This initial condition allows for easier manipulation of the material during fabrication.
Optimizing Heat Treatment
While the standard heat treatment cycles are effective, fine-tuning the annealing and aging times and temperatures can further enhance the properties of Inconel® 718. Additional treatments, such as a stabilizing treatment or a third age-hardening treatment, may be introduced to achieve specific combinations of tensile strength, fatigue life, and stress-rupture life. These customized cycles are particularly relevant for aerospace applications that demand high-strength and reliable performance.
To achieve optimal performance in aerospace applications(e.g., rotational parts, turbine blades, bearings, and fasteners) where a combination of high tensile strength, fatigue resistance, and stress-rupture life is required, following heat treatment cycle is recommended:-
- Soaking the Inconel 718 for one hour at temperatures ranging from 955-980˚C (1750-1800˚F), followed by air cooling. Subsequently, reheating it and held at 720˚C (1325˚F) for a duration of 8 hours. It is then slowly cooled at a rate of 56˚C/hour (100˚F/hour) until it reaches 620˚C (1150˚F), and held at this temperature for another 8 hours. Finally, the material is air cooled to room temperature.
On the other hand, for applications in the oil & gas industry, such as gate valves, choke stems, fasteners, and tubing hangers, where improved impact strength, low-temperature notch tensile strength, and lower hardness (40 HRC) are desired (commonly referred to as API 6A 718), a different heat treatment approach is necessary.
- Soak the Inconel 718 for 1-2 hours at 1065˚C (1950˚F) and then air cooled. It is subsequently reheated and held at 720˚C (1325˚F) for 8 hours, followed by a gradual cooling rate of 56˚C/hour (100˚F/hour) until it reaches 620˚C (1150˚F). The material is then held at this temperature for another 8 hours before being air cooled to room temperature.