Fatigue properties of titanium alloys marked with a pencil 1

[frank72]

we are currently testing parts made from Ti6Al7Nb - which should behave similar to Ti6Al4V for fatigue life properties. Actually we got an early failure that was unexpected and now we are looking for explanations.

One thing that bothers me is the fact that somebody made pencil marks on the surface of the test specimen. I have something in mind that you should not pencil mark titanium parts but I could not find a reason.
Q1: is it possible that the pencil mark has an influence on fatigue life of titanium parts?
Q2: if yes what is the reason?
Thank's for any Feedback.
Frank

 

[TVP]

When you say pencil do you mean a conventional writing instrument that has a soft graphite based tip? If so, this is not the cause of the early fatigue failure. Have a proper metallurgical analysis performed so that the root cause can be determined.

 

[frank72]

... the interpretation of the writing instrument with the soft graphite tip is correct. Nevertheless I have heard that no such markings shall be on titanium surfaces...

 

[EdStainless]

Was this room temp testing? If so then writing on the surface should have no impact.

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Plymouth Tube

 

[frank72]

Test was performed at slightly elevated temparture 37°C in low concentraion salt water solution (.9%).

 

[EdStainless]

What is the galvanic potential difference between Ti and graphite? could there be some hydrogen charging at these locations?

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Plymouth Tube

 

[TVP]

The galvanic potential is not insignificant in seawater: graphite is + 0.25 V and Ti is - 0.10 V. Combined with a somewhat increased temperature and potential environmental effects (any agitation of the water, crevices, etc.?) this may create enough hydrogen to embrittle the Ti. Have you tried testing without graphite pencil marks?

 

[metengr]

Going back to what was proposed by TVP, have a proper metallurgical failure analysis performed, end of story.

 

[frank72]

sorry metengr your post is not helpfull. I know about the possiblities of a proper failure analysis. it is not the intention of this thread. the intention is to figure out whether markings on the surface might influence fatigue live.
@TVP yes we have quiet some testing without markings and without early failures - this was the reason for the question of a possible influence. We will follow up and do further rotating beam fatigue testing with markings... Thanks!

 

[CoryPad]

frank72,

[blue]metengr[/blue] has been identified as the most helpful member of this forum (see the MVP's list on the right side of the page). You would be wise to heed his advice.

TVP has provided something that is a possibility, but it seems so unlikely. Pencil leads are ~ 70 % graphite with ~ 30 % clay and wax. The amount of graphite on the titanium surface couldn't be more than micrograms. That amount shouldn't be able to generate much hydrogen. Titanium can be embrittled by hydrogen, but it isn't as susceptible as high-strength steels.

I think you need to look elsewhere.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[NickE]

Frank72- A good failure analysis would definately tell you how the specimen failed. Since you are in an aggressive environment (for steels at least) I would third the reccommendation for a real failure analysis, not conjecture.


Nick
I love materials science!