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Nickel 718 Double End Stud
QS Fastener: Nickel 718 Double End Stud
Name: Nickel 718 Double End Stud
Standard: ASME
Material: Inconel 718
Size: 3/4 inch
Lenght: 150 mm
Temperature: -250 to 700℃
Surface: Natural color
Standard: ASME
Material: Inconel 718
Size: 3/4 inch
Lenght: 150 mm
Temperature: -250 to 700℃
Surface: Natural color
Quantity
Nickel 718 Double End Stud, 3/4 in × 150 mm, is made of Inconel 718 super-alloy and meets ASME standards. It retains high strength and fatigue resistance from –250 °C to 700 °C and needs no coating for corrosion protection, making it a reliable choice for double-ended fastening in petrochemical plants, gas turbines and cryogenic vessels.
What is 718?
718 is the short name for Inconel 718, a Ni-Cr base precipitation-hardening super-alloy containing 50-55 % Ni, 17-21 % Cr and additions of Nb, Mo, Ti and Al. After solution treatment plus double ageing it precipitates γ″ (Ni₃Nb) and γ′ (Ni₃Al,Ti) inside the grains, keeping toughness at –250 °C and giving ≥ 965 MPa tensile strength and ≥ 550 MPa yield strength at 700 °C. Its fatigue, creep and stress-corrosion resistance rank among the best of wrought super-alloys. The alloy is readily welded and machined and is non-magnetic. It is widely used for turbine disks/blades, aerospace fasteners, reactor parts, down-hole tools and cryogenic vessels; among these, the Nickel 718 Double End Stud is the first-choice standard fastener for liquid-hydrogen piping and high-temperature flanges.
718 is the short name for Inconel 718, a Ni-Cr base precipitation-hardening super-alloy containing 50-55 % Ni, 17-21 % Cr and additions of Nb, Mo, Ti and Al. After solution treatment plus double ageing it precipitates γ″ (Ni₃Nb) and γ′ (Ni₃Al,Ti) inside the grains, keeping toughness at –250 °C and giving ≥ 965 MPa tensile strength and ≥ 550 MPa yield strength at 700 °C. Its fatigue, creep and stress-corrosion resistance rank among the best of wrought super-alloys. The alloy is readily welded and machined and is non-magnetic. It is widely used for turbine disks/blades, aerospace fasteners, reactor parts, down-hole tools and cryogenic vessels; among these, the Nickel 718 Double End Stud is the first-choice standard fastener for liquid-hydrogen piping and high-temperature flanges.
Influence of Ni content in 718 on fastener performance
In Inconel 718 fasteners the Ni level (50–55 %) is the “performance pivot”. It fixes the matrix structure and controls how strengthening phases precipitate, so it simultaneously affects strength, toughness, fatigue and corrosion resistance. The following discussion uses the Nickel 718 Double End Stud to show what happens when Ni varies.
In Inconel 718 fasteners the Ni level (50–55 %) is the “performance pivot”. It fixes the matrix structure and controls how strengthening phases precipitate, so it simultaneously affects strength, toughness, fatigue and corrosion resistance. The following discussion uses the Nickel 718 Double End Stud to show what happens when Ni varies.
1. Effect on matrix structure and strength-toughness balance
a. Austenite stabilisation
> 50 % Ni keeps the FCC structure down to –250 °C, prevents BCC phases (α′, σ) and gives Charpy impact > 60 J, so liquid-hydrogen tanks sealed with Nickel 718 Double End Stud will not fail by cold-brittle fracture.
a. Austenite stabilisation
> 50 % Ni keeps the FCC structure down to –250 °C, prevents BCC phases (α′, σ) and gives Charpy impact > 60 J, so liquid-hydrogen tanks sealed with Nickel 718 Double End Stud will not fail by cold-brittle fracture.
b. Stacking-fault-energy control
Ni raises SFE, eases cross-slip, gives 22 % elongation at room temperature, and lets thin γ″ (Ni₃Nb) plates precipitate uniformly. After ageing, UTS reaches 1 275 MPa and yield 1 035 MPa, 2.5× higher than 42CrMo bolts of the same size, allowing the Nickel 718 Double End Stud to be pre-loaded to higher stresses safely.
Ni raises SFE, eases cross-slip, gives 22 % elongation at room temperature, and lets thin γ″ (Ni₃Nb) plates precipitate uniformly. After ageing, UTS reaches 1 275 MPa and yield 1 035 MPa, 2.5× higher than 42CrMo bolts of the same size, allowing the Nickel 718 Double End Stud to be pre-loaded to higher stresses safely.
c. Suppression of δ-phase over-ageing
At the lower Ni limit (50 %) Nb solubility drops and δ (Ni₃Nb) easily strings along grain boundaries at 950–1 000 °C, raising the 650 °C/200 MPa notch-sensitivity factor by 40 %. Raising Ni to 54 % cuts δ volume to < 1 % and increases stress-rupture life from 120 h to 210 h—vital for long-service Nickel 718 Double End Stud.
At the lower Ni limit (50 %) Nb solubility drops and δ (Ni₃Nb) easily strings along grain boundaries at 950–1 000 °C, raising the 650 °C/200 MPa notch-sensitivity factor by 40 %. Raising Ni to 54 % cuts δ volume to < 1 % and increases stress-rupture life from 120 h to 210 h—vital for long-service Nickel 718 Double End Stud.
2. Effect on stress-corrosion-cracking (SCC) threshold
In-reactor tests show:
- Ni ≤ 25 % → transgranular SCC (TGSCC), crack-growth rate 10⁻⁶ mm/s;
- Ni ≥ 65 % → intergranular SCC (IGSCC), rate 10⁻⁷ mm/s;
- Ni 50–55 % → mixed, least sensitive, rate only 10⁻⁸ mm/s.
In-reactor tests show:
- Ni ≤ 25 % → transgranular SCC (TGSCC), crack-growth rate 10⁻⁶ mm/s;
- Ni ≥ 65 % → intergranular SCC (IGSCC), rate 10⁻⁷ mm/s;
- Ni 50–55 % → mixed, least sensitive, rate only 10⁻⁸ mm/s.
A 3/4"-10UNC Nickel 718 Double End Stud in PWR water (340 °C, 1 200 ppm B, 2 ppm Li) ran 8 000 h with no crack > 0.5 mm, whereas an early heat with 48 % Ni showed IG micro-cracks after 2 000 h at the same load.
3. Effect on high-temperature fatigue life
Gas-turbine hold-down Nickel 718 Double End Stud (M16×180 mm, 700 °C, σa = 450 MPa, R = 0.1):
- 50 % Ni lot, 14 % γ″, 0.2 mm corner crack at 10⁴ cycles;
Gas-turbine hold-down Nickel 718 Double End Stud (M16×180 mm, 700 °C, σa = 450 MPa, R = 0.1):
- 50 % Ni lot, 14 % γ″, 0.2 mm corner crack at 10⁴ cycles;
- 54 % Ni lot, 18 % finer γ″, fatigue life 3.2×10⁴ cycles (3× longer); fracture fully transgranular, initiation area 40 % smaller.
4. Effect on cryogenic sealability
5/8"-18UNF Nickel 718 Double End Studs on a rocket H₂/O₂ engine flange cycled 50 times in liquid hydrogen (–253 °C):
- 55 % Ni bolts retained 92 % of preload;
- 50 % Ni bolts lost 28 % preload because local σ-phase reduced toughness and the O-ring leaked hydrogen.
5/8"-18UNF Nickel 718 Double End Studs on a rocket H₂/O₂ engine flange cycled 50 times in liquid hydrogen (–253 °C):
- 55 % Ni bolts retained 92 % of preload;
- 50 % Ni bolts lost 28 % preload because local σ-phase reduced toughness and the O-ring leaked hydrogen.
5. Composition-process window in production
a. VIM + ESR double melting keeps Ni within ±0.3 %, holding the scatter band of 700 °C/550 MPa stress-rupture life ≤ 1.5× and giving the best lot-to-lot stability for Nickel 718 Double End Stud.
a. VIM + ESR double melting keeps Ni within ±0.3 %, holding the scatter band of 700 °C/550 MPa stress-rupture life ≤ 1.5× and giving the best lot-to-lot stability for Nickel 718 Double End Stud.
b. For cryogenic fasteners use 52–55 % Ni plus 950 °C×1 h solution + 720 °C×8 h + 620 °C×8 h double ageing to raise –196 °C Charpy energy above 75 J while keeping hardness HRC 40, meeting the high cryogenic toughness demand of Nickel 718 Double End Stud.
c. For bolts operating > 650 °C continuously, raise Ni above 54 % and limit Nb to 5.15–5.35 % to suppress coarse δ-phase and guarantee ≤ 2 % relaxation after 10 000 h, ensuring the Nickel 718 Double End Stud stays tight throughout the entire overhaul cycle.
Conclusion
Within the 50–55 % window every 1 % increase in Ni raises the 718 fastener’s stress-rupture life by ~15 %, boosts cryogenic Charpy energy by 8–10 J and halves the SCC crack-growth rate; but > 57 % Ni lowers yield strength and raises cost. Therefore standards such as ASTM B637 and AMS 5663 rigidly limit Ni to 50–55 %—the optimum compromise that makes the Nickel 718 Double End Stud the core guarantee of joint safety under extreme conditions.
Within the 50–55 % window every 1 % increase in Ni raises the 718 fastener’s stress-rupture life by ~15 %, boosts cryogenic Charpy energy by 8–10 J and halves the SCC crack-growth rate; but > 57 % Ni lowers yield strength and raises cost. Therefore standards such as ASTM B637 and AMS 5663 rigidly limit Ni to 50–55 %—the optimum compromise that makes the Nickel 718 Double End Stud the core guarantee of joint safety under extreme conditions.
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