Alloy 718, 2.4668, UNS N07718, Inconel® 718 - Nickel Alloy

Alloy 718, 2.4668, UNS N07718, Inconel® 718 according to AMS 5383 i API Standard 6ACRA

Alloy 718, 2.4668, UNS N07718, Inconel® 718 – description

Alloy 718 is a Ni-Cr-Fe-Nb-Mo age-hardenable corrosion resistant, heat-resistant and creep-resistant superalloy. Unlike many other superalloys, it's also quite easy to weld. This combination of great high-temperature performance up to 700°C and ease of weld fabrication have made alloy 718 one of the most, if not the most popular superalloy used in industry. It can be wrought (most often), cast (rarely) or powder injection molded.

It's strengthening precipitate is γ'' (in an fcc γ matrix), thanks to the large niobium addition. The most significant feature of γ″ is probably the ease with which it forms at moderate temperatures after prior solutioning by heat treatment or joining processes. Because of this behavior, a γ″-hardened alloy can be aged, after welding, to produce a fully strengthened structure with exceptional ductility. Hardening with γ'' phase gives alloy 718 high tensile strength and very good creep-rupture properties at temperatures below 675°C - above this temperature γ'' converts to γ and δ, which cause a sharp reduction in strength.

There are two main versions of this alloy:

• Version made for aircraft and military use, described in ASTM B637 and AMS 5662-5664 and AMS 5596 standards
• Version made for oil and gas extraction equipment described in API 6ACRA, NACE MR0175 / ISO 15156 standards, with slightly less impurities allowed and significantly different heat-treatment.

The differences in properties and heat-treatment are described in later chapters of this article. NACE / API version can be further breaken down into several variants designated "120k", "140k", and "150k". They refer to min. tensile strength allowed. For example, alloy 718 140k has minimum 140k psi tensile strength (which is 1140 MPa in metric units).

Applications

Alloy 718 ASTM B637 / AMS 5662-5664 is most often used for aicraft components (turbine engine parts, high-speed airframe parts, such as wheels, buckets, spacers; high-temperature bolts and fasteners), in industrial gas turbines, cryogenic tankage, and nuclear engineering.

API 6ACRA, NACE MR0175 / ISO 15156 version is used typically for oil and gas drilling equipment.

Alloy 718, 2.4668, UNS N07718, Inconel® 718 - physical and mechanical properties

Room temperature physical properties for annealed and aged bars:

• Density: 8.19 g/cm³
• Melting temperature: 1260-136 °C
• Specific heat capacity: 435 J/kg⋅K
• Thermal conductivity: 11.4 W/m⋅K
• Electrical resistivity: 1218 nΩ⋅m
• Magnetic permeability: 1.0011
• Curie temperature: -112°C
• Precipitation temperature: 845°C

Typical room temperature mechanical properties according to various standards and producers:

Most typical values according to AMS standards (for aviation, etc.):

• Yield strength: >760 MPa
• Tensile strength: >860 MPa
• Elongation: >5%
• Hardness HB: 352

AMS 5597A Inconel 718, that is cold-rolled, annealed and aged Inconel 718 sheets:

• 0.25 mm thick
  • Yield strength: 1327 MPa
  • Tensile strength: 1189 MPa
  • Elongation: 17%
• 5.33 mm thick
  • Yield strength: 1341 MPa
  • Tensile strength: 1103 MPa
  • Elongation: 22%

API 6ACRA alloy 718 variant 120k (for oil / gas drilling equipment) - room temperature properties:

• Yield strength: 827-1000 MPa
• Tensile strength: >1034 MPa
• Elongation: >20%
• Reduction of area: >35% (>25% for sections >254mm)
• Impact energy KV -60°C for <76mm section: >68 J
• Impact energy KV -60°C for 76-254mm section: >47 J
• Impact energy KV -60°C for >254mm section: >41 J
• Hardness HRC: 32-40

API 6ACRA alloy 718 variant 140k - room temperature properties:

• Yield strength: 965-1034 MPa
• Tensile strength: >1138 MPa
• Elongation: >20%
• Reduction of area: >35% (>25% for sections >254mm)
• Impact energy KV -60°C for <76mm section: >68 J
• Impact energy KV -60°C for 76-254mm section: >47 J
• Impact energy KV -60°C for >254mm section: >41 J
• Hardness HRC: 34-40

AMS 5596 alloy 718 - properties of PIM (Powder Injection Molding) pieces:

• In ambient temperature:
  • Yield strength: >1034 MPa
  • Tensile strength: >1241 MPa
  • Elongation: 12%
• 650°C:
  • Yield strength: >827 MPa
  • Tensile strength: >999 MPa
  • Elongation: 5%
• High-cycle fatigue (10⁷ cycles)
  • 425°C: 333 MPa
  • 540°C: 54 MPa
  • 650°C: 47 MPa

Heat treatment influence on basic mechanical properties for Inconel 718 25mm bar:

• Hot-rolled:
  • Yield strength: 896 MPa
  • Tensile strength: 448 MPa
  • Elongation: 54%
  • Reduction of area: 67%
  • Hardness HRC: 16
• Annealed in 955°C for 1h:
  • Yield strength: 889 MPa
  • Tensile strength: 445 MPa
  • Elongation: 55%
  • Reduction of area: 61%
  • Hardness HRB: 94
• AMS 5662 Inconel 718 - annealed in 955°C for 1h and aged for 18h (8h in temp 720°C, then furnace cool and hold at 625°C):
  • Yield strength: 1389 MPa
  • Tensile strength: 1207 MPa
  • Elongation: 20%
  • Reduction of area: 36%
  • Hardness HRC: 46
• Annealed in 1065°C for 1h (according to the another popular heat treatment procedure):
  • Yield strength: 776 MPa
  • Tensile strength: 359 MPa
  • Elongation: 64%
  • Reduction of area: 68%
  • Hardness HRB: 87
• AMS 5664 Inconel 718 - annealed in 1065°C for 1h and aged for 20h (10h in temp 760°C, then furnace cool and hold at 650°C):
  • Yield strength: 1296 MPa
  • Tensile strength: 1048 MPa
  • Elongation: 21%
  • Reduction of area: 34%
  • Hardness HRC: 45

Coefficient of thermal expansion and temperature (linear):

• -200°C: 10.6 *10^-6m/(m*K)
• 100°C: 13.2 *10^-6m/(m*K)
• 205°C: 13.6 *10^-6m/(m*K)
• 315°C: 13.9 *10^-6m/(m*K)
• 425°C: 14.3 *10^-6m/(m*K)
• 540°C: 14.6 *10^-6m/(m*K)
• 650°C: 15.1 *10^-6m/(m*K)
• 760°C: 16.0 *10^-6m/(m*K)

Elastic modulus of Inconel 718 hot-rolled, annealed and aged bars:

• 20°C: 200 GPa
• 40°C: 199 GPa
• 95°C: 196 GPa
• 150°C: 193 GPa
• 205°C: 190 GPa
• 260°C: 187 GPa
• 315°C: 184 GPa
• 370°C: 181 GPa
• 425°C: 178 GPa
• 480°C: 174 GPa
• 540°C: 171 GPa
• 595°C: 167 GPa
• 650°C: 163 GPa
• 705°C: 159 GPa
• 760°C: 154 GPa
• 815°C: 147 GPa
• 870°C: 139 GPa
• 925°C: 130 GPa
• 980°C: 120 GPa
• 1040°C: 110 GPa
• 1095°C: 98.6 GPa

Torsional modulus of Inconel 718 hot-rolled, annealed and aged bars:

• 20°C: 77 GPa
• 40°C: 77 GPa
• 95°C: 76 GPa
• 150°C: 75 GPa
• 205°C: 74 GPa
• 260°C: 73 GPa
• 315°C: 72 GPa
• 370°C: 71 GPa
• 425°C: 70 GPa
• 480°C: 68 GPa
• 540°C: 67 GPa
• 595°C: 65 GPa
• 650°C: 63 GPa
• 705°C: 61 GPa
• 760°C: 59 GPa
• 815°C: 56 GPa
• 870°C: 52 GPa
• 925°C: 49 GPa
• 980°C: 45 GPa
• 1040°C: 40 GPa
• 1095°C: 35 GPa

Elevated-temperature tensile strength of hot-rolled, annealed, and aged 13mm Inconel 718 bar - approximate values:

• 100°C: ~1379 MPa
• 200°C: ~1345 MPa
• 300°C: ~1338 MPa
• 400°C: ~1324 MPa
• 500°C: ~1276 MPa
• 600°C: ~1242 MPa
• 650°C: ~1207 MPa
• 700°C: ~904 MPa

Elevated-temperature yield strength of hot-rolled, annealed, and aged 13mm diam Inconel 718 bar - approximate values:

• 100°C: ~1207 MPa
• 200°C: ~1138 MPa
• 300°C: ~1104 MPa
• 400°C: ~1090 MPa
• 500°C: ~1069 MPa
• 600°C: ~1055 MPa
• 650°C: ~1000 MPa
• 700°C: ~897 MPa

Stress-rupture life of hot-rolled, annealed (980°C, 1 h), and aged (720°C for 8h, then held at 620°C for total aging time of 18h) 16mm diam Inconel 718 bar - approximate values :

• Temperature 650°C:
  • T. duration 100 h: ~724 MPa
  • T. duration 1000 h: ~580 MPa
• Temperature 760°C:
  • T. duration 1000 h: ~518 MPa
  • T. duration 10.000 h: ~345 MPa

Rotating-beam fatigue strength of hot-rolled, annealed, and aged Inconel 718 bar:

• 10⁵ cycles:
  • 20°C: 910 MPa
  • 315°C: 793 MPa
  • 540°C: 765 MPa
  • 650°C: 690 MPa
• 10⁶ cycles:
  • 20°C: 696 MPa
  • 315°C: 758 MPa
  • 540°C: 703 MPa
  • 650°C: 648 MPa
• 10⁷ cycles:
  • 20°C: 634 MPa
  • 315°C: 758 MPa
  • 540°C: 655 MPa
  • 650°C: 607 MPa
• 10⁸ cycles:
  • 20°C: 621 MPa
  • 315°C: 758 MPa
  • 540°C: 621 MPa
  • 650°C: 496 MPa

Corrosion behaviour

Alloy 718 has good to excellent resistance to organic acids, alkalies, salts, and seawater; fair resistance to sulfuric, hydrochloric, hydrofluoric, phosphoric, and nitric acids; good to excellent resistance to high-temperature oxidation, carburization, nitridation, and molten salts; and fair resistance to sulfidation.

Hydrogen sulfide H₂S - fair resistance. In a simulated sour gas well test alloy 718 yielded to stress-corrosion cracking at 135°C. Alloy 725 is a much better choice for this environment.

Halide environments - fair resistance. Following environments cause stress-corrosion cracking:

• Boiling (155 °C) 42% MgCl₂
• 1% HCl, 205 °C
• HAc +Cl– + H₂S, 205 °C
• 25% NaCl, 205 °C

Hot water - the alloy is susceptible to intergranural stress-corosion cracking in deaerated high-purity water at 300-350°C.

Weldability

The alloy has outstanding welding characteristics as one of the most weldable Ni-based superalloys. It is almost completely resistant to postweld cracking. It's thanks to it's primary precipitate, the γ″, which precipitates at much slowe rate than the γ'. This allows alloy 718 to be heated into the solution temperature range without suffering aging and the resultant strain-age cracking. It takes 10,000 minutes of exposure to temperature of ~900°C for alloy 718 to show HAZ cracks - while for Waspaloy it takes only 5 minutes!

The alloy belongs to group 43 according to ISO 15608.

Selection of welding consumables:

• Filler metals: ERNiFeCr-2

Welding alloy with ERNiCr-3 is also possible, but it lowers the tensile properties about one-third.

Brazing temperatures of 1010°C or above result in grain growth and a corresponding decrease in stress-rupture properties, which cannot be recovered by subsequent heat treatment. The literature describes that vacuum brazing at 10^-4 torr in a cold-walled vacuum furnace provided the best results in brazing alloy 718 air diffusers components for aerospace turbine engines. Prior to brazing, all join surfaces were nickel plated to 0.015 mm thickness, according to AMS 2424. BNi-2 braze filler metal tape of 0.11mm was preplaced, and after the assembly an additional braze slurry of BNi-2 filler metal was applied to all joints.

Heat treatment, working and machining

Hot-forming pressures:

• 870°C: 437 MPa
• 1040°C: 385 MPa
• 1095°C: 333 MPa
• 1150°C: 283 MPa

Forging - alloy offers considerable resistance to deformation during forging. It should be forged in the range from 900 to 1120°C, heavy forging requiring higher temperature. In heating for forging, the material should be brought up to the temperature, allowed to soak a short time to ensure uniformity and withdrawn. In the last operation, the metal should be worked uniformly with a gradually decrasing temperature, finishing with light reduction below 955°C. Uniform reduction should be given to avoid duplex grain structure. 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 after forging.

There are multiple heat treatments for alloy 718. One popular heat treatment is described in AMS 5662. It allows the treated piece to achieve the optimum stress-rupture life, notch-rupture life, rupture ductility, and tensile strength. The procedure:

• Annealing: 925-1010°C; 1 h
• Air cooling
• Age hardening: 620°C; 8h
• Furnace cool to 650°C
• Hold at 650°C until furnace time for the entire age-hardening cycle equals 18h.
• Air cooling

Another popular heat treatment is described in AMS 5664. It is preferred in tensile-limited applications because it promotes the best transverse ductility in heavy sections, impact strength, and lo-temperature notch tensile strength, at the cost of notch brittleness in stress rupture. The procedure goes as follows:

• Annealing: 1035-1065°C; 1 h
• Air cooling
• Age hardening: 760°C; 10h
• Furnace cool to 650°C
• Hold at 650°C until furnace time for the entire age-hardening cycle equals 20h.
• Air cooling

NACE MR0175 / ISO 15156-3 describe following heat treatment for best sour gas applications (oil / gas drilling equipment):

• Annealing: 1010°C; 2h
• Water Quenching
• Age hardening: 780-800°C; 6-8h
• Air cooling

Literature also describes following heat treatment for cast alloy 718:

• Solutioning: 1095°C; 1h
• Air cooling
• Annealing: 955°C; 1h
• Air cooling
• Age hardening: 720°C; 8h
• Furnace cool to 620°C
• Hold at 620°C until furnace time for the entire age-hardening cycle equals 16h.
• Air cooling

Note that the annealing duration of 1h refers to continous annealing. Batch annealing should continue up to 3h.

In many critical applications attaining desired final properties requires is not possible via heat treatment if the hot-working operation has not been conducted under controlled temperature and deformation parameters. Thermomechanical processing allows greater control over the microstructure and distribution of phases. Direct age Inconel 718 is an example of alloy processed thermomechanically to achieve precise parameters for applications such as turbine disks. The procedure: the secondary precipitate δ is produced first by an appropriate heat treatment (8h in 900°C), followed by working at about 950°C, below the η solvus. Final working is carried out below the recrystallization temperature, and the alloy is recrystallized below the η solvus. Then, finally, the alloy is aged according to the standard procedures.

Machining - group D-2, meaning it's very hard to machine. Rough machine in solution annealed condition and then finish machine after aging. In rough machining allow for the size contraction up to about 0,07%, which takes place upon aging. Recommended turning parameters using coated carbide:

• Roughing in annealed condition:
  • Depth of cut: 6.35 mm
  • Feed: 0.25 mm/rev
  • Cutting speed: 244 m/min
• Finishing in aged condition:
  • Depth of cut: 1.02 mm
  • Feed: 0.13 mm/rev
  • Cutting speed: 35 m/min

Recommended turning parameters using high-speed steel:

• Roughing in annealed condition:
  • Depth of cut: 6.35 mm
  • Feed: 0.25 mm/rev
  • Cutting speed: 5 m/min
• Finishing in aged condition:
  • Depth of cut: 1.52 mm
  • Feed: 0.10 mm/rev
  • Cutting speed: 5 m/min

Twist drilling with high-speed steel drills recommended parameters:

• Annealed condition:
  • Drill diameter: 9.5-11 mm
  • Feed: 0.10-0.18 mm/rev
  • Cutting speed: 3-4 m/min
• In aged condition:
  • Depth of cut: 13-17 mm
  • Feed: 0.15-0.25 mm/rev
  • Cutting speed: 2-3 m/min

Casting - complex and large shapes can be achieved by investment casting.

We deliver this steel alloy as:

• Bars, rods and forgings according to ASTM B637, AMS 5662, AMS 5663, AMS 5664
• Bars, rods and forgings according to API 6ACRA, NACE MR 0175, ISO 15156
• Sheets and plates according to ASTM B 670, AMS 5596, AMS 5597
• Seamless pipes according to AMS 5589, AMS 5590
• Welding wire according to AMS 5832

Replacements, equivalents and other designations:

N07718, AMS5383, AMS 5590, AMS 5589, AMS 5597, AMS 5596, AMS 5597, AMS 5662, AS7466, Alloy 718, Allvac® 718, Allvac 718-OP, B50T69A, B50TF14, B50TF15, DMV 718, ERNiFeCr-2, Grade 718, IN 718, J467, Inconel® 718, Inconel® alloy 718, MIL N-24469, PWA-S-5662H, UDIMET® alloy 500, Unitemp 718, Udimet 718, 2.4668, NiCr19Fe19Nb5Mo3, BS 2901 NA51, NW7718, NCF718, NCF 718, ChN55MTJuB, 52.5Ni-19Cr-3Mo-5.1Cb-0.9Ti-0.5Al-18Fe, NACE 718, A 1014 Grade 718, AS7467, AS7466, Bohler® L718, EMVAC® 718, VAT® 718, Aeralloy 718, Nistelle® 718, VDM Powder 718, Haynes® 718, Phyweld 718, SY 718