Machining Inconel 718 Per AMS 5962 (Cold Reduced) 6

[FastenerDude]

I have a customer who wants me to make a double end stud out of Inconel 718 per AMS 5962 (Capable of 220 KSI Tensile). The stud size is a 1-1/4" diameter (with an unthreaded body diameter of 1.111"). The only material I can get is 1-3/4" round. I raised the concern to my customer that machining off that much material may remove the cold work that was applied to the bar. My customer is saying that isn't true. Am I correct in my thinking that turning off that much material will remove most of the cold work?


Greg
System 22, Inc.

 

[CoryPad]

It depends on the area reduction, bar diameter, tool contact length, and friction coefficient. The loss may be low or high, so either of you could be correct.

 

[EdStainless]

As we say, it all depends.
Regardless of how it was cold worked I would expect more work at the surface than at the core.
But the tensiles should have either been full section (I doubt it) or machined from mid-radius.
In either case you are not removing so much material that I would be concerned.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

Why guess? Machine a prototype and test it.

 

[FastenerDude]

Thank you all for your responses!!
@metengr: We did consider a prototype but my customer doesn't have the time to wait for that. They are going to approve the use of larger diameter bar so we should be good-to-go.

Greg
System 22, Inc.

 

[metengr]

Your customer is penny wise and dollar foolish.

 

[FastenerDude]

@metengr Welcome to the fastener world. It'll drive you nuts.

Greg
System 22, Inc.

 

[MagBen]

Your custoemr is probably right!

Inconel 718 was designed a precipitation hardable alloy, it will be precipitation heat treated after cold worked for final applications (although it can be working hardened by cold reduction to 220ksi, it needs >60% reduction to get there, in the meanwhile, the elongation will be way below 8% as AMS specifies.)

I donot see how the material removal from OD affects the capability to meet 220kSi, for which, AMS 5962 indicates the specimen shall be from the enter of product. So even the cold reduction from the surface is higher than from the center, the surface removal would not affect the capability for the center to meet 220ksi.

 

[btrueblood]

"Welcome to the fastener world. It'll drive you nuts. "

Star for that. Does it ever get so bad you consider bolting? It sounds like your job is pretty screwy. Though at times I'll bet it's riveting.

 

[CoryPad]

Not only riveting, but fastenating (fascinating).

 

[TVP]

Boo to all of the above puns...

 

[EdStainless]

I guess that the pun-ishment was too much for him.

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P.E. Metallurgy, Plymouth Tube

 

[TVP]

Ok, I had to give a star to Ed for that one!

 

[tbuelna]

What process do you intend to use to produce the threads? If you're going to bother making a stud using 718 Inconel at 220ksi, wouldn't you want to also roll form the screw threads? This would give the threads much better fatigue properties than cutting or grinding them. Of course, the screw threads would need to be roll formed with the material at a much lower hardness (around Rc38 max). And then the material would need further heat treatment to increase its mechanical properties.

 

[EdStainless]

tb, you hit the issue.
This part should be roll threaded, but they can't use cold worked bar then.
In order to get the strength of cold work + age they must grind threads.
And then live with the lower fatigue performance of the threads.

Perhaps they could take cold drawn bar, machine and roll thread it, then age harden it.
But this would take planing, not just grabbing whatever stock they could find.

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P.E. Metallurgy, Plymouth Tube

 

[tbuelna]

Ed,

That's the point I was trying to make. Once you add in the knockdown factor of a cut/ground thread vs a rolled thread, you should be able to use the 718 Inconel at a lower TS and still get an acceptable fatigue life. The difference in calculated fatigue life between a rolled and a cut/ground thread can be quite significant.

Also, aircraft tension bolts are routinely manufactured from AMS 5662 718 Inconel up to a 1.5" thread size and HT'd to 220ksi min uts. The threads of these bolts are roll formed IAW MIL-S-8879, and the bolt is made from a forged blank. So it is possible to do. Maybe FastenerDude could have a chat with his customer about this situation.

 

[FastenerDude]

@tbuelna & @EdStainless: This has all been discussed with the customer. Threads will be rolled before aging. Customer had asked for thread rolling after aging but that is borderline impossible. There isn't a thread roller in the United States that would roll or could roll the threads after aging.

Greg
System 22, Inc.

 

[MagBen]

I doubt anyone in the world could roll threads after aging, the hardened material is too strong.

The cold work created by rolling can be beneficial to increase the strenghth at aging. Normally we will need about 20% cold reduction to reach 220ksi at aging. Without any reduction, you will only get min 180ksi TS, min 38HRC hardness.

 

[WKTaylor]

Guys... the AMS5962A spec is very clear about material properties, usage, etc...

Suggest You review following Paragraphs VERY closely with a metallurgist [or M&P engineer] in-hand, in-regards to Your 'stud' fab.

NOTE.
Unfortunately SAE updated it in PDF format w/out regard to OCR.. hence I can't copy/paste exact details without a lot of re-typing, Grrrrrrrr.

1.2 Application

3.3 Condition

3.4 Heat Treatment [and cold working]

3.5.2 Response to Heat Treatment

3.5.2.1 Tensile Properties

3.5.2.2 Hardness

Machining this stud per guidance by customer should NOT be a problem.

Roll-forming threads with appropriate tool steel alloys with appropriate coatings [TiN] will NOT be easy. HOWEVER I believe the forces could be reduced with appropriate 'elevated-warm temperatures' [<900-F] and suitable hot-work lubricants.

What really bugs me is I cannot find authoratative mechanical allowables ANYWHERE.

Regards, Wil Taylor

Trust Me! I'm an engineer!

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

Unfortunately, in science what You 'believe' is irrelevant û "Orion"

 

[CoryPad]

Wil,

How about final part properties in AS5270 Bolts, Screws and Studs, Nickel Alloy, UNS N07718, Work-Strengthened, Tensile Strength 220 ksi, Procurement Specification?

Cory