UNS N07718 (Alloy 718) is often seen as the workhorse of the nickel-chromium alloy range, with many applications in Aerospace, General Engineering and the Oil & Gas industry. The grade is one of a type that is precipitation-hardenable, and which, with its addition of niobium, molybdenum, aluminium and titanium, provides a metal that is relatively high strength, and has excellent creep-stress rupture strength and good corrosion resistance. It is easy to weld and durable at high temperatures, although has a spread of operating temperatures of between -200°C and +550°C.
Alloy 718 is well suited for use in fasteners, pump and valves, turbine parts, motor components, springs, cryogenic tanks, and down-hole applications and tools.
However, as with most grades, there are different variations rather than just one “Alloy 718”.
This article is one of a series that will explain each variant and pin-point differences:
This first article detailed the “Oil & Gas” API 6A CRA properties. This second article will concentrate on AMS5662. A third will address AMS5663 properties, and finally the fourth will discuss the general engineering ASTM B637 standard requirements.
I will compare and contrast API 6A 718 and AMS 5662 below. However, be aware that many end users of Alloy 718 may write their own specifications, usually extending certain requirements of the standards detailed below, and such specifications must always be checked to ensure material conforms with the requirements.
Our AMS5662 stock range stretches between 38 mm to 177 mm in diameter.
Alloy 718 To AMS5662
For bars, forgings, flash welded rings and stock for the same. Typically used for parts requiring high resistance to creep and stress rupture up to 704o C and oxidation resistance up to 902o C. Parts are typically formed or machined, then precipitation heat treated to develop their required properties. AMS5662 material therefore comes with “capability” test results, where the test piece has been aged at the required times and temperatures, then tested to show what results would be achieved with that ageing treatment.
Standard properties:
| Alloy 718 to AMS 5662 | API 6A 718 | ||
| Material Designation: | |||
| 120K (Yield strength 120 ksi minimum) | Same | ||
| Chemistry: | |||
| Element | Min | Max | Same |
| C | - | 0.08 | 0 - 0.045 max |
| Mn | - | 0.35 | Same |
| Si | - | 0.35 | Same |
| P | - | 0.015 | 0 – 0.010 max |
| S | - | 0.015 | 0 – 0.010 max |
| Cr | 17.00 | 21.00 | Same |
| Ni | 50.00 | 55.00 | Same |
| Mo | 2.80 | 3.30 | Same |
| Nb | 4.75 | 5.50 | Reported with Ta Range: 4.87 – 5.20 |
| Ti | 0.65 | 1.15 | 0.80 – 1.15 |
| Al | 0.20 | 0.80 | 0.40 – 0.60 |
| Co | - | 1.00 | Same |
| B | - | 0.006 | Same |
| Cu | - | 0.30 | 0 – 0.23 |
| Pb | - | 0.0005 | 0 – 0.0010 |
| Bi | - | 0.00003 | 0 – 0.00005 |
| Fe | Bal | Same | |
| Se | Not required | 0 – 0.0005 | |
| Ca | Not required | Reported if intentionally added. Range 0 – 0.0030 |
|
| Mg | Not required | Reported if intentionally added. Range 0 – 0.0060 |
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| Melt Practice: | |||
| Multiple melted using consumable electrode practice in the remelt cycle or shall be induction melted under vacuum. | Dual melted – usually VIM (Vacuum Induction Melted) & VAR (Vacuum Arc Remelted) or ESR (ElectroSlag Remelted). | ||
| Forge & Hot Working Ratio: | |||
| Not formally required | Greater than 4.0:1 | ||
| Heat Treatment Equipment control: |
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| Equipment and controls used are to be qualified as per API 6A or to a similar standard such as AMS 2750. | Same | ||
| Temperature Monitoring: | |||
| Temperatures are only monitored by either a contact surface thermocouple or heat sink. Heat treatment monitoring charts are provided. | Same | ||
| Heat Treatment: | |||
| Solution Anneal - 941o C to 1010o C for time commensurate with cross-sectional thickness and cool in air or faster (liquid). | Solution Anneal - 1021o C to 1052o C for 1 hour to 2.5 hours and usually cool in liquid | ||
| Age Hardening – only required for capability tests for conformance to AMS5663 | Age Hardening - 774o C to 802o C for 6 hours to 8 hours and air cool | ||
| Material Testing Requirements: Macroetch examination: |
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| Not formally required | |||
| Microstructural Analysis: | |||
| Grain size: Bars under 9 in2 in cross-sectional area – ASTM 5 or finer (up to 20% may be ASTM 3-5) Bars 9 in2 to 78 in2 cross-sectional area – ASTM 4 or finer (up to 20% may be ASTM 2-4) | ASTM 2 or finer No topological duplex grain size allowed |
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| Deleterious phases: Free from laves phases | Deleterious phases (at 100x and 500x magnification) (free from acicular phases except for individual and isolated grains, free from continuous networks of secondary phases along grain boundaries, and no laves phases). | ||
| Tensile Properties: | |||
| Not formally required other than capability results for conformance to AMS5663 – please see the AMS5663 paper in this series for details of all material test properties. | See 718 API 6A CRA paper | ||
| Impact Toughness: | |||
| Not formally required | See 718 API 6A CRA paper | ||
| Hardness Test Requirements: | |||
| <277 HB <21 HRC | 32 - 40 HRC | ||
| Non-Destructive Examination: | |||
| Not formally required | 100% ultrasonic tests shall be carried out in accordance with API 6A PSL 3 or PSL 4 (with Operator qualified to minimum SNT level 2.) | ||
The next paper in the series will consider AMS5663 properties.
Please contact Broder Metals Group Limited, either through the webform section of the website or by any of the other contact methods list on the website if you have any questions.
