Alloy 718 – What is it?

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.  The second article contrasted AMS5662 with API 6A CRA 718 properties.  The third addressed AMS5663 properties and contrasted with both AMS5662 and API 6A CRA.  

This final and fourth article will discuss the general engineering ASTM B637 standard requirements, and contrast with its closest standard, AMS5663.   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 ASTM B637 stocks range from 22 mm – 177 mm in diameter.

Alloy 718 to ASTM B637

For rod, bars, forgings, and forging stock. Typically used for parts in turbine engines as well as liquid rocket components involving cryogenic temperatures.   

This fourth and last paper in the series will compare Alloy 718 to ASTM B637 together with AMS5663 as both are supplied in the solution annealed and aged condition.  In addition, we will look at the chemistry of  API 6A CRA with ASTM B637, as the two are often offered together as meeting “chemistry only” – although as it will be seen, it should not be taken for granted that they will be the same in practice.

Please contact Broder Metals Group Limited, either through the webform section of the website or by any of the other contact methods listed on the website if you have any questions.

Standard properties:

Alloy 718 to ASTM B637			Alloy 718 to AMS 5662/5663	Alloy 718 to API 6A CRA
Chemistry:
Element	Min	Max
C	–	0.08	Same	0 – 0.045
Mn	–	0.35	Same	Same
Si	–	0.35	Same	Same
P	–	0.015	Same	0 – 0.010
S	–	0.015	Same	0 – 0.010
Cr	17.00	21.00	Same	Same
Ni	50.00	55.00	Same	Same
Mo	2.80	3.30	Same	Same
Nb+Ta	4.75	5.50	Nb only – 4.75 – 5.50	4.75 – 5.20
Ti	0.65	1.15	Same	Same
Al	0.20	0.80	Same	0.40 – 0.60
Co	–	1.00	Same	Same
B	–	0.006	Same	Same
Cu	–	0.30	Same	0 – 0.23
Fe	Bal		Same	Same
Pb	Not Required		0 – 0.0035	0 – 0.0010
Bi	Not Required		0 – 0.00003	0 – 0.00005
Se	Not Required		Not Required	0 – 0.0005
Ca	Not Required		Not Required	0 – 0.0030
Mg	Not Required		Not Required	0 – 0.0060

 

Alloy 718 to ASTM B637		Alloy 718 to AMS 5663
Melt Practice:
Not Required	Multiple melted using consumable electrode practice in the remelt cycle or shall be induction melted under vacuum.

 

Heat Treatment:
Not Required	Equipment and controls used are to be qualified as per API 6A or to a similar standard such as AMS 2750.
Not Required	Temperatures are only monitored by either a contact surface thermocouple or heat sink.  Heat treatment monitoring charts are provided.
Solution Anneal – 924o C to 1010o C hold for 0.5 hour minimum, and cool in air or faster (liquid).	Solution Anneal – 941o C to 1010o C for time commensurate with cross-sectional thickness and cool in air or faster (liquid).
Precipitation Hardening Treatment: 704o C to 732o C, hold at temp for 8 hours, Furnace cool to 607o C to 635o C, hold at temp until total precipitation heat treatment time has reached 18 hours, and air cool	AMS5662 Age Hardening only required for capability tests.  AMS5663 – Precipitation Heat Treatment: 718o C to 760o C, hold at temp (+/-8o C) for 8 hours, cool at 48o C to 64o C per hour to 621o C to 649o C, hold at temp (+/-8o C) for 8 hours and air cool. Alternatively, instead of the cooling at 48o C to 64o C per hour, the material may be furnace cooled providing the time at 621o C to 649o C is for a total precipitation heat treatment of time of 18 hours.

 

Material Testing Requirements:
Microstructural Analysis
Not Required	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)
Not Required	Deleterious phases: Free from laves phases

 

Tensile Tests
	Alloy 718 to ASTM B637	Alloy 718 to AMS 5663
Temperature	Room	Room	Room	Room	At 649o C
Size	Not required	≤10” ≤254 mm	≤5” ≤127 mm	5”-10” 127-254 mm	≤10” ≤254 mm
Orientation	Not Required unless for large disk forgings or rings.	Longitudinal	Transverse*	Transverse	100 ksi 689 MPa
Tensile ksi	>185	>185	>180	>180	>23 hours
0.2% Yield Strength ksi	>150	>150	>150	>145	>4%
Elongation in 4D %	>12	>12	>6	>6
Reduction of Area %	>15	>15	>8	>8

 

Stress Rupture:
	Alloy 718 to ASTM B637	Alloy 718 to AMS 5663
Size	≤10” ≤254 mm	Not Required
Temperature	At 649o C
Load	100 ksi 689 MPa
Time to Rupture	>23 hours
Elongation in 4D	>4%

 

Impact Toughness:
Alloy 718 to ASTM B637	Alloy 718 to AMS 5663
Not formally required	See 718 API 6A CRA paper

 

Hardness Test Requirements:
Alloy 718 to ASTM B637	Alloy 718 to AMS 5663
<331 HB <36 HRC	32 – 40 HRC

 

Please contact Broder Metals Group Limited, either through the webform section of the website or by any of the other contact methods listed on the website if you have any questions.