Inconel 718

Developed in the early 1960's, IN718 is still considered the material of choice for the majority of aircraft engine components with service temperatures below 1200°F (650°C). Inconel 718 is a precipitation-hardenable nickel-chromium alloy containing also significant amounts of iron, niobium, and molybdenum along with lesser amounts of aluminum and titanium. It combines corrosion resistance and high strength with outstanding weldability including resistance to post-weld cracking. The alloy has excellent creep-rupture strength at temperatures to 1300°F (700°C).

Standard Specification

Common Names:

• UNS N07718
• Alloy 718
• DIN W. Nr. 2.4668

Plate, Sheet, and Coil:

• ASME SB 670
• ASTM B 670
• B50TF14
• AMS 5596
• UNS N07718
  

Round Bar:

• AMS 5662
• B50809D
• UNS N07718
• B50TF15
• ASTM B 637

  Chemical Composition Inconel 718

Table  1 - Chemical Composition

  Applications of Inconel 718

• Jet engine
• Gas turbine operations
• Base plates
• Rotor bolts (Power generation)
• Burst discs
•  Springs
• Hold-down springs for nuclear components
• Seal rings
• Components for gas turbine
• Instrumentation parts
• Fasteners

 Physical Properties

718 Inconel is an alloy often used in engineering and aerospace applications requiring a combination of high strength and corrosion resistance. It has a yield strength greater than 90 ksi, making it a material preferred for components and parts requiring considerable strength. The Young’s modulus for Inconel 718 is around 28msi, resulting in good formability, ductility and weldability. Additionally, it has excellent creep-rupture properties at 1300°F and can tolerate temperature extremes ranging from -422°F to 1500°F depending on heat treatment condition. As such, Inconel 718 is often used in high-temperature engine and airframe components due to the robust physical properties that make it an ideal material for challenging engineering conditions.

 Table 2 - Physical Properties

 Mechanical Properties

This combination gives the alloy excellent mechanical properties like high tensile strength, yield strength, creep resistance, fatigue life, oxidation resistance, and corrosion resistance. The alloy also has good thermal stability, which means it can be used in temperatures up to 1300 degrees Fahrenheit without losing its mechanical or chemical properties.

Table 3 - Modulus of Elasticity 

 Table 4 - Thermal Properties

Work condition and Heat Resistance of Inconel 718

It also has excellent heat resistance, making it suitable for applications that involve very high temperatures, like jet engine components or combustion chambers in cars or motorcycles.

The alloy is used in jet engines and high-speed airframe parts such as wheels, buckets and spacers, and high-temperature bolts and fasteners. Inconel 718 is also used in the oil and gas drilling and production industries due to its high strength and resistance to chlorides, stress corrosion and sulfide stress cracking. Within these industries, the alloy has been used for valves, pump shafts and wellhead components. Inconel 718 Slit Coil, Inconel® 718 Sheets, Inconel Alloy 718 CR Plates, WNR. 2.4668 Sheets, Inconel 718 Sheets, UNS N07718 Plates, Alloy 718 Circle, Inconel Alloy 718 Plates Distributors. The addition of molybdenum and niobium makes the alloy stiffen thus providing high strength with no heat treatment. They can be readily welded and fabricated in precipitated or annealed hardened conditions. These superalloys are made available in various dimensions, specifications, and sizes that ultimately make is suitable for marine engineering, chemical processing, and other applications too.

Hot and Cold Forming

Because of its strength, Alloy 718 is more resistant than most materials to deformation during hot forming. Its relative resistance is shown by pressures developed in the roll gap at 20% reduction (Table 5). It is readily hot-worked if sufficiently powerful equipment is used. Hot forming is performed in the 1650°-2050°F temperature range. In the last operation, the metal should be worked uniformly with a gradually decreasing temperature, finishing with some light reduction in the 1650°-1750°F range. This procedure is necessary to ensure notch ductility in stress-rupture applications when the material has been annealed and aged. In heating for hot working, the material should be brought up to temperature, allowed to soak a short time to ensure uniformity, and withdrawn.

To avoid duplex grain structure Alloy 718 should be given uniform reductions. Final reductions of 20% minimum should be used for open-die work and 10% minimum for closed-die work. Parts should generally be air-cooled from the hot working temperature rather than water-quenched. Care should be taken to avoid overheating the metal by heat build-up due to working. Also, the piece should be reheated when any portion has cooled below 1650°F. Preheating tools and dies to 500°F is recommended. Any ruptures appearing on the surface of the workpiece must be removed at once.

Data shown in Table 48 show the importance of a 1650°F-1750°F finish-forging temperature for the achievement of notch ductility in large forgings in stress-rupture applications. In these tests, a 0.75-in. square forged bar was cut into 12-in. lengths, heated to the rolling temperatures shown in Table 6 and 7, and given a 25% reduction in one pass. Following annealing and aging, specimens were ruptured tested at 1200°F and 100 ksi. Success in achieving notch rupture ductility with forgings of alloy 718 through this type of procedure has also been reported by others.

 Table 5 - Pressure, ksi, Developed in Roll Gap at 20% Reduction

Table 6 - Effect of Hot-Forging Temperature on Rupture Properties (1200°F, 100 ksi)

Table 7 - Effect of Cold Reduction on Properties of Sheet

INCONEL alloy 718 can be cold-formed by standard procedures used for steel and stainless steel. Figure 1 below shows its rate of work hardening in comparison with other materials. The effect of cold reduction on the tensile properties of the sheet in the cold-rolled and cold-rolled and aged conditions is shown in Table 7.

Figure 1 -  Effect of cold work on hardness.

 Corrosion Resistance of Inconel 718

The high chromium content of Inconel 718 makes it highly resistant to corrosion from strong acids like hydrochloric acid or sulfuric acid at room temperature or elevated temperatures up to 1000°C (1800°F).

Alloy 718 has excellent corrosion resistance to many media. This resistance, which is similar to that of other nickel-chromium alloys, is a function of its composition. Nickel contributes to corrosion resistance in many inorganic and organic, other than strongly oxidizing, compounds throughout wide ranges of acidity and alkalinity. It also is useful in combating chloride-ion stress-corrosion cracking. Chromium imparts an ability to withstand attack by oxidizing media and sulfur compounds. Molybdenum is known to contribute to resistance to pitting in many media.

Stress-Corrosion Cracking

Alloy 718 is generally considered a highly corrosion-resistant material but can still be susceptible to stress corrosion cracking (SCC). The combination of factors leading to SCC susceptibility in the alloy is not always clear enough. For more details click here

Fabrication

Inconel 718 alloy can be readily machined however the strength and work toughening features should be considered while choosing and using the precise apparatus alloys and design, processing speed and quenchers. When machining in the age-toughened form, the strip offers the enhanced finish, and chip performance on the chip breaker tools are enhanced. If annealed alloy is used, it provides more convenient machining and extended tool life.

 Machinability

Nickel & cobalt base corrosion, temperature and wear-resistant alloys, such as Inconel 718, are classified as moderate to difficult when machining, however, it should be emphasized that these alloys can be machined using conventional production methods at satisfactory rates. During machining these alloys work to harden rapidly, generate high heat during cutting, weld to the cutting tool surface and offer high resistance to metal removal because of their high shear strengths. The following are key points which should be considered during machining operations:

• CAPACITY - The machine should be rigid and overpowered as much as possible.
• RIGIDITY - The workpiece and tool should be held rigid. Minimize tool overhang.
• TOOL SHARPNESS - Make sure tools are sharp at all times. Change to sharpened tools at regular intervals rather than out of necessity. A 0.015-inch wear land is considered a dull tool.
• TOOLS - Use positive rake angle tools for most machining operations. Negative rake angle tools can be considered for intermittent cuts and heavy stock removal. Carbide-tipped tools are suggested for most applications. High-speed tools can be used, with lower production rates, and are often recommended for intermittent cuts.
•  POSITIVE CUTS - Use heavy, constant, feeds to maintain positive cutting action. If feed slows and the tool dwells in the cut, work hardening occurs, tool life deteriorates and close tolerances are impossible.
• LUBRICATION - Lubricants are desirable, and soluble oils are recommended especially when using carbide tooling. Detailed machining parameters are presented in Tables 8 and 9. General plasma cutting recommendations are presented in Table 10.

 Table 8 - Tool Type and Matching Conditions

 Table 9 - Tool Type and Matching Conditions

 Table 10 - General plasma cutting

Welding of Inconel 718

Welding can be done using several different welding processes like Gas Tungsten Arc Welding(GTAW), Plasma arc welding(PAW), Gas Metal Arc Welding(GMAW) etc., depending on the size/shape/location/thickness of the weld joint required for joining two pieces together using this material, e.g., GTAW would be preferred when welding thin sections. At the same time, GMAW would be suitable when welding thicker sections with larger weld beads, etc.

INCONEL alloy 718 is readily welded by the gas tungsten arc (TIG) process using INCONEL Filler Metal 718. The composition of this filler metal is shown in Table 11. The mechanical properties of its all-weld metal specimens are shown in Table 1 above. These test data show the effect of postwelding treatment on tensile properties. The highest room temperature ductility is obtained by annealing at 1950°F prior to ageing.

Table 11 - Effect of Post Welding Heat Treatment on Tensile Properties of INCONEL Filler Metal 718 All-Weld Metal (Manual Gas Tungsten-Arc Process between 0.5-in. Plates) (Average of 2 Tests)

Slow response of alloy 718 to age hardening enables parts to be welded and directly age-hardened without intermediate stress relief. Joint efficiencies very close to 100% were realized in the tests shown in Table 12; these specimens were from plate that had been annealed, then welded and aged. Better properties, however, are obtained by re-annealing after welding prior to ageing.

 Table 12 - Room Temperature Tensile Properties of Welds (Aged 1325°F/8 hr, F.C. to 1150°F, Held at 1150°F for Total Aging Time of 18 Hours)

Silver brazing compounds are known to cause stress cracking in nickel-based alloys. If alloy 718 is cold-worked and/or precipitation-hardened, silver brazing compounds should not be used. Also, brazing alloys containing cadmium are best avoided; while cadmium has not been shown to cause cracking by itself, it can aggravate cracking from other sources.

Weld Tensile Properties

Room-temperature properties of welds receiving the low-temperature anneal and/or age can be compared with the results of the high-temperature anneal and/or age in Tables 12 and 13

Additional properties of welds annealed at 1950°F/15 min. and aged at 1400°F/10 hr, F.C. to 1200°F, hold at 1200°F for a total ageing time of 20 hours are shown in Table 14. Welding was done by the manual gas tungsten-arc process, using Filler Metal 718. These welds were found satisfactory in bend tests and radiographic examination. Slightly better results were obtained when helium was used as the torch gas. The notch strength of the butt-welded sheet in both the heat-affected zone and weld is shown in Table 15. These welds were heat-treated by the low-temperature schedule.

Another laboratory using the high-temperature heat treatment has found that the notch toughness of the parent metal and that of the weld metal are quite consistent and exceed a notch-to-smooth bar tensile ratio of 1.30 throughout the test temperature range of -423°F to 1200°F. Test data are shown in Table 16. The weld joint efficiency is approximately 93% at -423°F and 95% at room temperature and 1200°F. For more details regarding the welding system click here.

Weld Fatigue Properties

Weldments were found to have a room-temperature fatigue strength (108 cycles) of approximately 62.5 ksi (tested in R.R. Moore rotating-beam apparatus). They were made from hot-rolled, annealed (per AMS 5596) 0.500-in. plate, joined with 0.125-in.-diameter INCONEL Filler Metal 718 by the gas tungsten arc process. Samples were aged 1325°F/8 hr, F.C. to 1150°F, hold at 1150°F for the total ageing time of 18 hours and tested as polished specimens. In incomparable tests, alloy 718 bar had a fatigue strength (108 cycles) of 89.0 ksi.

Weld Rupture Properties

Rupture strength at 1200° and 1300°F of heat-treated weldments in sheet is compared with parent metal in Table 17. In other tests, welds whose process steps were age, weld, and age had lives of 0.3 hr (1200°F, 100.0 ksi) and 4.9 hr (1300°F, 72.5 ksi); fractures were in the heat-affected zone. Notch-bar life at 1300°F and 75.0 ksi is shown in Table 18.

Table 17 - Rupture Strength of Butt Joints in Annealed Sheet ^a

Table 18 - Rupture Strength of Weldsa (Test Conditions: 1300°F and 75.0 ksi)

Conclusion

Inconel 718’s superior mechanical properties make it an indispensable material for many industries, including aerospace, automotive, medical, nuclear power plants, gas turbines, oil refineries, industrial valves, pressure vessels, fasteners, bolts, nuts etc. Its excellent heat resistance, combined with its corrosion resistance, makes it ideal for any application involving elevated temperatures, like jet engine components or combustion chambers. With correct tool selection & cutting parameters coupled with appropriate heat treatments applied according to desired outcomes from these processes, this material can easily be machined into complex shapes & welded together using various welding processes. Thus making this superalloy an integral part of any industry requiring materials that have superior performance qualities.