Alloy 718 （UNS N07718） |
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Summary | A precipitation-hardenable nickel-chromium alloy also containing significant amount 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℉(700℃). Used in gas turbines, rocket motors, apacecraft, nuclear reactors, pumps, and tooling. |
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Standard Product Forms | Pipe, tube, sheet, strip, plate, round bar, flat bar, forging stock, hexagon and wire. |
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Chemical Composition Wt,% |
| Min | Max. |
| Min. | Max. |
| Min. | Max. |
Ni | 50.0 | 55.0 | Cu |
| 0.3 | C |
| 0.08 |
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Cr | 17.0 | 21.0 | Co |
| 1.0 | Si |
| 0.35 |
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Fe | Remainder | Al | 0.2 | 0.8 | P |
| 0.015 |
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Mo | 2.8 | 3.3 | Ti | 0.65 | 1.15 | S |
| 0.015 |
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Nb | 4.75 | 5.5 | Mn |
| 0.35 | N |
| 0.006 |
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Physical Constants | Density,g/ 8.19 |
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Melting Range,℃ 1260-1336 |
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Typical Mechanical Properties | (Solution Annealed) (1000h) Rupture Strength (1000h) ksi Mpa 1100℉/595℃ 110 760 1200℉/650℃ 86 590 1300℉/705℃ 53 370 1400℉/760℃ 24 170 |
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Microstructure
Alloy 718 is an age-hardenable austenitic material. Strength is largely dependent on the precipit prime phase during heat treatment.
Characters
Good fabricability in the annealed condition;
Good tensile, fatigue and creep-rupture strengths; at up to 700℃;
High-temperature strength up to 700℃;
Good oxidation resistance up to 1000℃;
Excellent mechanical properties in cryogenic environments;
Good weldability by arc and without susceptibility to post-weld cracking.
Corrosion Resistance
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.
Applications
Due to its high-temperature strength up to 700℃, excellent corrosion resistance and ease of fabrication Alloy 718 finds applications in many field. Initially it was used as a turbine disk material in aircraft jet engines. Here resistance to creep and stress rupture eas most important. Because of its properties, fabricability and cost effectiveness it has since gained wider acceptance. Today applications vary from highly stressed rotating and static components in gas turbines and rocket engines to high-strength bolting, spring and fasteners, components in nuclear reactors and space vehicles as well as high-temperature tooling for extrusion, of for example copper, and shearing. Another more recent important application involves pump shafts and other highly stressed well head and downhole components in offshore Particularly useful is the alloy for drilling equipment in sour (containing H2S, CO2 and chlorides) oil and gas wells.
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