IN718values describe: 718 aged
Fusion, privateBig science (accelerators, light sources, public fusion)Vacuum equipment & components
Why it wins
Keeps strength to 700 C, corrosion resistant, weldable, non-magnetic. The ARC fusion concept baselines a double-wall Inconel 718 vacuum vessel immersed in molten salt.
Why not the alternative
Stainless yields at these temperatures and loads; titanium creeps.
Watch out
Expensive, slow to machine; Nb content activates under neutrons.
Properties
The values below are candidate: compiled from the sources named, not yet individually validated.
Provisional provenance: ASM Handbook: properties, corrosion, heat treatment; Electron tube and ceramic-to-metal sealing practice.
Wet cleaning
| Recipe | alkaline degrease, DI rinse; pickle only with a nitric-HF qualified for the alloy |
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| Forbidden | over-pickling attacks the aged microstructure |
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| Limit | ageing oxide must come off before brazing |
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Vacuum and outgassing
| Outgassing, unbaked (10 h) | 2e-9 mbar·L/s/cm² |
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| Outgassing, baked | 1e-11 mbar·L/s/cm² |
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| Vapour pressure | negligible |
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Temperature
| Bake, assembled | 450 °C |
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| Vacuum degas | 1000 °C |
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| Braze / H2 firing | 1100 °C |
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| Metallurgical limit | solution 980–1065 °C then age 720/620 °C |
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Thermal
| CTE | 13 ppm/K |
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| Thermal conductivity | 11.4 W/m·K |
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| Specific heat | 435 J/kg·K |
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| Emissivity | 0.2–0.3 |
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| Melting / softening | 1260–1336 °C |
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Mechanical
| Strength | 1030 ys aged MPa |
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| Tensile | 1240–1400 MPa |
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| Elongation | 12 % |
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| Young's modulus | 200 GPa |
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| Hardness | 380–450 HV |
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| Density | 8.19 g/cm3 |
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Electrical and magnetic
| Relative permeability | ~1.001–1.02 |
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| Resistivity | 125 µΩ·cm |
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Engineering
| Corrosion | excellent, including hot |
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| Joining | weldable with strain-age cracking risk; standard braze cycles |
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| Process notes | machining is slow and tool-hungry |
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| Availability and cost | stock, expensive |
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Grades
| 718 (2.4668) | age hardened vessels and bolting |
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| 625 (2.4856) | weldable, corrosion duty, bellows |
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| X-750 | high temperature springs in vacuum |
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Sources · 1
- Sorbom et al., ARC design, arXiv 1409.3540
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