Inconel 718 Impact Strength

[Jughandle]

We are experiencing occasional surface denting from rough handling impacts (at ambient temperature) on a thin wall pressure vessel made of Inconel 718. Mechanical properties at ambient temperature for this part have been spec’ed as follows. Test bars are prepared in the longitudinal direction to forging.

Yield = 165ksi, min (@ 0.2% offset)
El = 15%, min.
(Strain rate .005/minute.)
CVN Impact Strgth = 3 ft-lbs min

Dealing mainly in steel alloys, I was astounded at the low CVN specified for this material. Test bars are breaking at around 7 ft-lbs, so we don’t have a problem meeting the spec, however can anyone offer an academic explanation at the microstructural level as to why this alloy has such a low impact strength in view of the pretty decent El? Yes, strain rates are vastly different; maybe dislocations don’t ‘like’ moving quickly around the precips, etc. Could that be why? This may not solve our rough handling problem but I was just wondering about this apparent contraindication between CVN and El. (No, I’m not a student, wish I was still.) Thanks.
PS - There is no internal pressure at the time of denting.

 

[EdStainless]

What is your UTS? 185ksi?
Which heat treatment was used? This is very important since you can heat treat this alloy either to maximize strength or to maximize the stress-rupture.
Being an age hardened alloy the structure is loaded with very hard secondary phases. The alloys used to make turbine blades have the same issue.
Your values seem low, but I am not sure which condition we are talking about. I have seen them range from 20 lb-ft to 50 lb-ft.

http://www.specialmetals.com/assets/documents/alloys/inconel/inconel-alloy-718.pdf

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[metengr]

For what it is worth, if it were me I would evaluate the fracture surface morphology of one or more broken CVN specimens. The CVN impact data is lower than I have seen for Inconel 718 used in pressure vessel construction.

 

[MagBen]

From the tension properties given, this is likely a maximized strength version with a minimized stress-rupture (a low solution temperature possibly), the impact strength should also be sacrificed. However, 7ft.lbs is way lower than what it is supposed to be. I would first check the testing specimen and/or the testing itself. you won't find an academic logic.

 

[mrfailure]

Just wanted to confirm that your Charpy specimens are standard sized 10x10x55 mm with 2 mm notch?