Overaging IN718

[Augustus1]

Good Afternoon,

I am after some information about IN718. I have reviewed a number of documents and the maximum HRC is around 40-42, I am wondering why you couldn't increase that to 46HRC value.

I am looking at a HT cycle of solution treat, 1010C 2hrs, then a double age 750C 8hrs furnace cool to 650C for 8hours. I assume once you go about 42HRC you run the risk of the material being too brittle, but why is this? what's the mechanisms behind this change?

kind regards
Augustus

 

[EdStainless]

There are three published aging treatments for 718.
One maximizes the stress rupture and rupture ductility; it uses the lower temp 930-1000C +quench and then age 720C x8hr FC to 620C x8hr.
Because of the hardening mechanism the max strength is limited by the amount of precipitate that you form.
And the composition limits that.
The higher temp treatment uses 1038-1065C +quench and then age 760C x10hr FC to 650C x20hr.
This gives the highest toughness, especially in large sections.
There is another HT that uses 1000-1040C and it is designed to keep the hardness below RC 40 to maximize SCC resistance.
This is the NACE oilfield HT.

As you push the strength higher the first thing is that you lose elongation, along with that you the material becomes more and more notch sensitive (notched tensile and stress rupture drop). This also means that your critical defect size drops. And your SCC sensitivity goes up.
What is your service temp?
There are plenty of other Ni based alloys with different balances of properties.

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P.E. Metallurgy, consulting work welcomed

 

[Augustus1]

thank you very much for this, Is there a way to calculate the maximum hardness you can achieve by the composition?

Thanks again for this, I will look into what you have said. It looks like we use the NACE oilfilled HT then

kind regards
Augustus

 

[WKTaylor]

A1...

Nickel alloy 718 [trade-name Inconel 718] can ONLY safely attain ~192-to-194-KSI tensile by heat treat [HT] processes [for instance-> AMS2774], from raw solution HT [SHT] precipitation HT-ready [PHT] stock material... as noted in MMPDS-* documentation.

However, FASTENER manufacturers... and some forgers... are able to safely attain 220-KSI tensile, with good toughness, when processing raw-stock into certain parts. HOW...???... by strain-hardening within context of the material processing and HT. This 'strain processing addition' is carefully guarded proprietary info and I'm uncertain exactly WHEN/HOW it is accomplished [although I have a 'hazy' SWAG].

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
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[EdStainless]

For all compositions that I have ever seen the max hardness of HRC 40-42 is it. Period.
You can hop the alloy up a bit, but that is a different animal.
You can do cold work + direct age on 718, but you have to be very careful.
The cold work needs to be uniform through the cross section. This usually leads to working with fairly small diameters.
The amount of cold work needs to be carefully controlled.
You have to use the low temperature aging treatment.
There are huge risks in this. And your HT control has to be very good (+/-5C).
It is easy to blow the grain size way up and lose toughness.
You can also end up with a material that has significant SCC issues.
I have worked with 718 wire at >225ksi and it is not fun.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed