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a strange failure of an unalloyed Tantalum bolt

a strange failure of an unalloyed Tantalum bolt

a strange failure of an unalloyed Tantalum bolt

(OP)
I have a 8-33 Tantalum bolt which we manufactured according to ANSI Standard.
When tightening the bolt we got a strage and unfamiliar (at least to me) failure of the bolt's thread (I attached photos).
I thought that when designing according to the standard the shank should fail before the threads.
In addition, even more strange is the thread didn't really tear off the shank but it looke like it plastically deformed.

What can you say about it smile ?

RE: a strange failure of an unalloyed Tantalum bolt

I can say - how much engagement was there?

RE: a strange failure of an unalloyed Tantalum bolt

(OP)
5 pitches

RE: a strange failure of an unalloyed Tantalum bolt

The screw failed in torsion and wrinkled the thread in the area of failure. Presummably the threads not engaged in mating threads.

Ted

RE: a strange failure of an unalloyed Tantalum bolt

(OP)
hydtools - very interesting!
I read a little about common failures and it wasn't mentiined.

RE: a strange failure of an unalloyed Tantalum bolt

A cross section sketch of the bolting arrangement might put a bit more light on the problem, and would confirm hydtools observation / comment that certainly looks like the answer.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: a strange failure of an unalloyed Tantalum bolt

To me it looks like the bolt yielded in torsion while tightening and the yielded again during loosening to give that back and forth wrinkle. It seem like the threads seized or the bolt bottomed out in the hole.

RE: a strange failure of an unalloyed Tantalum bolt

Did you try an anti-seize treatment? What are the tolerances of the threads?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: a strange failure of an unalloyed Tantalum bolt

Pure tantalum has a 50% elongation - per one bolt source. And still has a non-zero yield strength. That's an uncommon material specification, so it's not a surprise about the uncommon failure.

I agree that the screw must have either bottomed out or snagged something in the hole as it isn't a tensile failure. I've seen those in stainless steel and the main ductile change was a large increase in thread pitch.

RE: a strange failure of an unalloyed Tantalum bolt

so I tried to find a data sheet on this material and all I found was this white paper.
depending on the alloy tensile properties and hardness value are all over the spectrum.
please advise alloy of this material. material certification would give important data.
my opinion is the tensile properties (hardness) is in adequate for this application.
the material seized while installing and failed during assembling and then on removal.
as hydratools posted. or the the threads were incorrect on the mating part or this bolt.
was the threads on both parts verified with thread gages.
https://apps.dtic.mil/sti/pdfs/AD0266415.pdf

RE: a strange failure of an unalloyed Tantalum bolt

(OP)
Hey, nice to see a lot of answers.
First of all here is a link to the manufacturer material in which you can find some data regarding Tantalum:
https://www.plansee.com/en/materials/tantalum.html
The bolt couldn't bottom out since the mating thread is a through one, so the bolt might protrude out.
I also suspected the bolt seized (cold welded) but I didn't feel it stuck while tightening it, and there was no typical sound I know when such thing happens.
About the tolerance - in the drawing it was written to stick to the standard definition.
Can you explain what you meant by a "a large increase in thread pitch"?

RE: a strange failure of an unalloyed Tantalum bolt

I agree that this appears to be torsional yielding of the bolt where the threads were not engaged with other threads. The waviness is due to compression buckling of the thread tips.

RE: a strange failure of an unalloyed Tantalum bolt

As tension is developed by tightening the bolt elongates and the thread pitch increases with it, much like a Slinky toy the pitch increases with tension. If the bolt has significant elongation ability the un-engaged threads in tension will stretch. In this case the pitch appears unaffected, just angular displacement, so the waviness wasn't a result of tension.

The material is soft, so there would be no noise to its galling.

RE: a strange failure of an unalloyed Tantalum bolt

How did you take it out might direct us to answer correctly. Did you find any failure on the inner thread? What exact materials and heat treatment were used on both sides? Note that first four threads did not fail like the others.

I am with Compositepro, 3DDave and Hydtools for the cause of failure until I see further information.

RE: a strange failure of an unalloyed Tantalum bolt

(OP)
saplanti, what do you mean by how did I take it out?
If I understand the question, so I unscrewed it without any excessive force.

RE: a strange failure of an unalloyed Tantalum bolt

Yes, you understood corrrectly.
You say you unscrewed without any excessive force. In that case, when you screwed it first time the first four threads engaged without any problem and fifth and sixth were damaged as given on the photos. This seem to be unusual isn't it?
In that case, if possible, I would check the threads on the bolts, there might be a manufactural mistake between pitches on the same bolt.

Or there might be something on the fift and or sixth threads during the tightening that caused this kind of failure. Is this happening on all the threaded connections? If not this might be the cause. If it is happening I would question the pitches of the threads on the same bolt and/or between threads on the oppsite connected members.

RE: a strange failure of an unalloyed Tantalum bolt

Is this the first time you have used this bolt/material in this application?
Could be worthwhile repeating the assembly / bolting while carefully observing what is taking place, ie is the bolt free in the tapped hole, any signs of binding, what is the surface finish on the female thread -rough /smooth any sign of galling, try lightly torquing the bolt, remove and check thread surfaces for damage, was the failed bolt over torqued?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: a strange failure of an unalloyed Tantalum bolt

elinah34

can you please explain the the machining process? were these single pointed on a lathe. please specify the thread type and size.
on the link you provided it states this material does not machine very well, because it is dead soft it will have a poor finish if machined.
were there burrs present, were these deburred properly, if there can not be a material change then rolling these threads may be a better
application instead of single pointing.
one other question why was this material chosen?
Edit: also can you provide a screen shot of the thread requirements, I am also puzzled by the question you asked about the enlarged thread pitch.
and I really recommend to purchase thread gages, what this will provide that the thread bolt will assemble to the mating part. it incorporates, all errors in the thread well as
to verify the correct geometry, and pitch diameter. and plug gages for the mating part. since this material is soft it will be prone to galling and seize.
a coating of either dry film lube or anti seize as mention earlier.

RE: a strange failure of an unalloyed Tantalum bolt

(OP)
Hey mfgenggear
I can't give details why using this specific material, though I am sure it's interesting. I can just say it's a requirement/constraint.
You are definitely right - this material isn't ideal for machining, and the right way of producing bolts made of tantalum is rolling.
But when contacting manufacturers about rolling tantalum bolts we got an enormous minimum ordering amount since this product is a custom one for us, and as for now we can't afford it.
So, we machined it by turning (lathe). The quality wasn't perfect, and indeed we could recognize the surface wasn't smooth but quite rough. I am attaching the drawing that was used to manufacture this bolt.
About thread gauge - I was sure it's the manufacturer role and duty to check with this instrument:/

RE: a strange failure of an unalloyed Tantalum bolt

elinah34

so I don't have a copy of the ASME b18.3, so I pulled out my old 18Th edition machinery handbook and took the dust off it. :)
there is nothing special about this thread. it appears to be a standard thread. I observed the the manufacture is required to verify with thread gages.
Purchase your own they are relatively inexpensive and reverify the threads. pull a sample and reinspect them.
as a manufacture will only do a sample inspection procedure unless otherwise stated on the contract. if it is a small lot it
should have been inspected 100%. simple to inspect the major diameter with blade mic's. on a pinch a measurement over 3 wires to inspect size.
but this does not inspect the other geometry as thread gages. correct P.D size, lead, included angle of thread and so on.

so first very the thread geometry is to the standard,
second look for burrs, interference will exacerbate seizing problem
third oil the parts well to prevent corrosion,
I personally feel the pitch had errors, thus causing the last threads to fail.
as pointed out buy others,
if a small volume, a buffing operation of the threads very lightly with red wheel(scotch bright)
will help immensely. and a cleaning operation to remove debris.
edit: one other suggestion, have the threads ground, it will produce a smoother better surface finish, and much more precise on geometry and
size, generally a thread will require 7 passes, this part may require more on the last cuts to improve finish.

RE: a strange failure of an unalloyed Tantalum bolt

(OP)
Thank you very much for your detailed answer

RE: a strange failure of an unalloyed Tantalum bolt

The drawing notes 40.1 say the threads must be checked with go and n0-go gages.

What are you using for cutting oil when turning the threads?
Some exotic inserts supposedly need to run hot to perform at their best.
Are the threads created with Interpolated CNC, more of a manual thread turning operation?
Changes in tool relief and rake can make big differences in surface finish.
Is the tool the full thread profile?
With such a small diameter getting the cutting edge "on center" and providing sufficient rake and clearance can be a problem.
I'd paint the sides of the insert and look for "dragging" which can make a real messy surface finish.
So can "gummy materials, for which the best defense has to be found by trial an error. Tool sharpness, top rake, relief etc and .

Here is a link that talks about the various kinds of tool infeed with advantages and disadvantages.
https://www.mmsonline.com/articles/threading-on-a-...

Here is link to a guide for that includes screw thread surface finish/roughness.
https://www.engineersedge.com/manufacturing/maximu...

RE: a strange failure of an unalloyed Tantalum bolt

I will do a machinability harden when possible and it is with in the permissible thermal treatment such as AMS2759/2.
being this material is gummy there are limitations on machining properties , where as the geometry of the tool, feeds and speeds,
as well as the coolant used, most high volume shops in America, do not use oil any more unless it is grand fathered in.
most use a soluble based coolant. it gets. tricky to dispose of chips, and lubricating oils or coolant.
EPA issues. no more dumping coolant down the drain. and has to be disposed of properly.

in softer materials it is important on the rake, but for threading there is a limit on available inserts, and I may be out of touch there may be more these days, unless
one fabricates or modifies the carbide insert with a diamond wheel. on a tool and cutter grinder. then obtaining the correct geometry becomes important.
a machinability harden steel get , better finish and the heat is removed with the chip, as with soft gummy material it tears, and has a poor finish.
and it is stringy, and it is difficult to break the chip, like harden steel.
also depth of cut as well as higher RPM is an important factor, harder, tougher material lower rpm and deeper depth of cut. except for the final pass.
softer the material higher rpm and shallower depth of cut to obtain better finish, on gummy material there has to be enough depth of cut
that the insert can cut correctly and not rub or tear. it's actually a science to it.
it is what is.

RE: a strange failure of an unalloyed Tantalum bolt

The release date on that drawing 1601 is a bit suspect, unless they are using some calendar I am not aware of.
B.E.

You are judged not by what you know, but by what you can do.

RE: a strange failure of an unalloyed Tantalum bolt

(OP)
I have to admit that I wasn't really involved in the manufacturing process of this part. It was produced by a manufacturer who claims to be familiar with working with this exotic material, and as a designer who isn't really familiar with all the tips you mentioned regarding manufacturing, I let him do his work.

RE: a strange failure of an unalloyed Tantalum bolt

Yes it does not appear fun material to work with.it was probably given to a young machinist and was told to handle it.
I don't believe I would try to machine
This material.cutting threads is probably easier said than done.probably the thread rolling guys probably quote high because they did not really want it.
An other suggestions would be to thread roll on the lathe.

RE: a strange failure of an unalloyed Tantalum bolt

elinah34,

Your initial post describes the thread as 8‑33. Is this a typo?

--
JHG

RE: a strange failure of an unalloyed Tantalum bolt

(OP)
drawoh, you are right.
It was a mistake.
The thread is 8-32

RE: a strange failure of an unalloyed Tantalum bolt

elinah34,

That is good, although it eliminates a good explanation. smile

--
JHG

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