Titanium alloy (Ti-6Al-4V) Thermomechanical/chemical properties

[corvuscorax]

I'm designing some chains for a fire-juggler who attaches Kevlar wicks to the end and spins them as a sort of martial-art... The wicks and hardware immediately attached to it can experience temperature excursions of up to 400-500 Celsius (800-1000 Fahrenheit) for periods of 3-4 minutes.

My previous design used 304 stainless wire, but were too heavy for the customer.

I have been researching common Ti-6Al-4V (Grade 5) titanium alloy wire as a material, but there seems to be documentation suggesting that Grade 5 Ti has transient temperature limits around 400C (750F) due to tensile and oxidation factors.

Mild steel chain constructed of the same gauge wire has been demonstrated to easily deal with the heat and forces typically present in a performance. I am concerned however that the Titanium equivalent, while initially superior, will oxidize and become brittle over time and possibly fail during a performance, which could endanger people around the performer.

Are these concerns valid? If so, what materials could I consider as an alternative keeping light weight in mind?

Many thanks...

 

[EdStainless]

What diameter are you looking at using? Could you use small diameter SS tubing to form the links?
We make some aircraft hydraulic tubing that is very high strength and still very tough.
Ti strength drops off quickly when you reach critical temperature.

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Plymouth Tube

 

[corvuscorax]

I'm hoping for between 1.2 and 1.6mm wire formed into circular 6.5-8mm I.D. rings...
Previous designs used 8.7mm ID X 1.6mm wire rings made from 304 spring-tempered steel and were simply too heavy, but were also structurally overkill.

Since you mentioned it, I looked up hypodermic tubing suppliers and found some T304 stainless tubing at .062" which is roughly 1.6mm - .006 to .028" wall thickness:

http://eagletube.thomasnet.com/viewitems/all-categories/standard-fractional-tubing?&forward=1

Welding the rings closed may represent somewhat of a problem for me... and I'm not sure if it would be able to withstand the forces subjected.

 

[EdStainless]

You use a small piece of slightly smaller tubing as an ID sleeve and then you do a full penetration butt weld.
Yes welding 0.010" wall takes a touch, but the stuff is plenty strong.

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Plymouth Tube

 

[Albigger]

This isn't a material suggestion, but is there any chance you would eliminate enough weight just going down to 1.2mm diameter but using the same material? You say the 1.6mm was structurally overkill, so if the 1.2mm will work you will save about 44% weight (based on diameter alone).

Also - if only the "ends" are exposed to the heat perhaps you could use steel in the hot zone and titanium for the cooler part of the chain? (Although, judging by your description there may be a chance the whole chain is exposed to heat)

 

[corvuscorax]

A valid suggestion... The original was 2.75 OZ, and I can theoretically bring it down to around 1.54 OZ with 1.2mm wire. The same chain produced from 1.2 mm titanium wire would be .89 OZ though, which is tough to pass up.

As mentioned, it is also completely conceivable that the ring materials could be "hybridized" so that the hotter parts were made of stainless and the rest titanium... maybe bringing the overall weight of the chain to around 1.22 OZ if it was 50/50 by length.

But since this is a metallurgy forum: can anyone definitively say if titanium will suffer tensile/oxidation performance degradation much below that of 304 stainless at 400-500C?

 

[moltenmetal]

That's a small cross-section, so it wouldn't take much to crack it through. You're talking about relatively brief exposures rather than continuous duty, so that goes in your favor. I doubt anyone is going to definitively tell you that the Ti gr 5 is going to be OK, especially with the potential for injury if it isn't.

The Ti gr 5 will certainly be OK for some period. It doesn't turn into candy on first exposure to air and nitrogen at 4-500 C. What about limiting the service life on these, throwing them away after a time?