Heat and titanium wood stoves – ductile to brittle

[monkey]

Browsing recent discussions on woodburning stoves I’ve come across comments which suggest that titanium may become brittle as a result of being exposed to the fire. Such as for example @stridertim writes here:

How is your Ti Goat stove coping with high temperature. My early Ti metal stoves burnt like rockets so that you could almost see the flames through them (Article 2), but I could not get more than about 100h of burn time before the titanium was brittle and oxidized.

Other people however suggest that the temperature in a small woodburner may not be high enough to cause the metal to become brittle (see, for example, here)

So I’m wondering, which is the truth? Is the temperature of a wood fire high enough to cause brittleness? And if so, how long does it take for the titanium to become brittle, a mere 100h of burning or perhaps longer?

I know there are many alloys out there that would affect the answer to this question…. But say we are talking about some “standard” ti-foil that is used by Ti-goat for example?

 

 

[Franco Darioli]

Never heard of a Ti Goat or the like stove becoming brittle but I suggest you contact Josh at Ruta Locura , he knows about Ti and stoves…

http://www.rutalocura.com/Contact.html
BTW, yes Josh is the Ti Kid…

[DAN-Y/FANCEE FEEST]

Interesting……………I don’t think there are but 1 or 2 people that put half that amount of hours on a wood burning stove. I think most of the folks out there are wanna be wood fuel users.

[Anonymous]

Josh at Ruta Locura is the Ti Goat’s guy son.

As far as the question goes, i’ve had some blazing hot fires in my SeekOutside “Large” Ti box stove, and while there has been some oxidation (pretty colors of blue and purple), so far i haven’t noticed brittleness.

If this is an issue, i suspect it’s probably related to specific alloys.

[Colin Krusor]

Yes, as Justin suggested, this is completely dependent on alloy. I’ve made a lot of titanium wood stoves, and certain alloys (15-3-3-3, for example) transition to the brittle beta phase immediately upon exposure to wood fire temperatures. CP alloys (soft, low-strength “commercially pure” alloys with low levels of alloying elements) are much more resistant to embrittlement.

In my experience, 15-3-3-3 becomes brittle immediately, Grade 5 (Ti-6Al-4V) becomes brittle gradually, and CP Ti doesn’t become brittle except where it is in direct contact with the hottest coals. Most alloying elements make a Ti alloy more prone to embrittlement, so the high-strength, heavily alloyed Ti foils will be more problematic in this respect. The exceptions are O, N, and C, which actually drive up the beta transus temp somewhat, and Zr and Sn, which have little to no effect on the beta transus temp.

[Roger Caffin]

A lot of Ti sheet is delivered annealed, which means it is a bit softer and more bendable. It tempers when it gets to red heat, and that means it gets harder. The 6Al4V alloy, which easily dominates the supply chain, certainly does this. I have no experience with 15-3-3-3, so I can’t comment about it.

Mind you, I would not really describe it as embittlement, at least not for 6Al4V. It just gets very hard and cannot be bent at all. It will crack rather than bend (unless you bend it red hot, which is easy).

Cheers

 

[Pigeon]

Any comment on the Ti Caldera Cone and inferno inserts? Is excessive “testing” a bad idea?

Aside from home testing…my actual use while camping might include three boils (usually just two, for cleanup, lots of herbal tea, dry food) a night so 100 hours would come along faster than expected.

 

[DAN-Y/FANCEE FEEST]

Does your inferno get red hot? I recall the flame comes out an opening on one side of the cone, is that right?

[Pigeon]

No, it definitely doesn’t get red hot, just figured since it’s adjacent to red hot coals… I guess Colin’s science jargon confused me.

[Jon Fong / Flat Cat Gear]

I am pretty sure they (TD) use a CP Grade 2 Titanium.  Not a problem.

[monkey]

@rcaffin wrote:

Mind you, I would not really describe it as embittlement, at least not for 6Al4V. It just gets very hard and cannot be bent at all. It will crack rather than bend (unless you bend it red hot, which is easy).

This is what I had in mind I think, that it is no longer ductile/bendable….. Just thinking of packing ti-foil stoves the way we usually do, rolling them up tightly, accidental denting etc, if the risk of cracking increases with time..

 

And is this true? (from the post I linked in the OP)

“the effect of heating and cooling titanium is generally to make it more ductile, so it shouldn’t become more brittle, as long as you let it cool slowly rather than chucking a pan of water on it”

Can “hardened/brittle” titanium be brought back to its ductile state?….

 

[James Marco]

“Can “hardened/brittle” titanium be brought back to its ductile state?”

For metals, the answer to that is yes. Usually this is the opposite of tempering. But with light metals, aluminum/ti specifically, you might find that it will be a matter of a few degrees of heat. I believe that the aluminum alloys will eventually burn off sheets leaving it fairly brittle anyways.So, for practical purposes, it is probably not possible.

 

[Colin Krusor]

I think in the last couple of posts here there might be a misunderstanding. When you heat aluminum it softens and stays soft because it loses its temper. The heat causes the aluminum to “anneal”.

Titanium is completely different. Depending on the alloy, heating titanium can cause it to undergo a transition to the “beta” phase. This means that the atoms in the titanium have permanently rearranged into a harder, more brittle configuration. The titanium will be soft and bendable while it is hot, of course, but after cooling (no matter how you cool it), the titanium will be harder than before and more prone to cracking.

The difference between titanium alloys is that some transition to the brittle beta phase at relatively low temperatures, others require very high temperatures, and some won’t rearrange into the beta phase at all. For example, 15-3-3-3 alloy titanium foil seems like a good material for wood stoves. It is fairly cheap for a titanium alloy and available in lots of different foil thicknesses, and it is tremendously strong (stronger than most other alloys). But, it turns out, it is useless for wood stoves because, after the first fire, rolling the foil up causes cracks to appear at the edges. The softer (and weaker) CP titanium alloys don’t have that problem. The beta transition temperature for the CP alloys is much higher than for 15-3-3-3, so a wood fire is not hot enough to cause embrittlement of the CP alloy. I’ve only seen CP alloy embrittlement when it was used for the pan beneath the coal bed, where temperatures are very high.

[Roger Caffin]

Yes, as Colin has pointed out, some titanium alloys have alpha and beta phases which differ significantly in their properties. I am sure it is possible to bring the harder beta phase back to the softer alpha phase (because my 6Al4V sheet is delivered annealed), but I have never attempted that. I think some fairly serious temperature control gear would be needed.

Tricky stuff.

Cheers

 

[James Marco]

Yes. Some metals require slow cooling to maintain an annealed state. Some simply require fast quenching. Piano wire is one that requires low heat to be gradually reduced to become annealed. Over heating it will cause it to over-temper forming crystals in the metal, very weak, almost crumbly. Copper, aluminum requires heat to soften after work hardening. I agree, ti can be tricky.

[monkey]

Many thanks all for replies and especially Colin for the science explanation! Its all much clearer now.

Hmmm wondering how does stainless steel compare to titanium in this department….. can it also become brittle or other issues? (thinking of Kifaru wood stoves – heavier, but perhaps more reliable long term than titanium….?)

 

[Roger Caffin]

By and large, SS alloy does not embrittle the way Ti alloy does. But SS is about twice as heavy (or dense). Talk about a trade-off!

Cheers

 

[Roger Caffin]

To monkey.

Hi monkey and fellow stovies, I have read your discussion “Heat and titanium wood stoves – ductile to brittle” with interest but have been locked out from commenting, (sorry for the delay) so I have relayed this through Roger Thanks again Roger. It is great to have some discussion about experiences that appear to be at odds with those of many others. I think this is the interface where we can learn the most.

My experience of metal destruction in stoves as detailed above was at first with stoves made of pure titanium foil (~0.12mm thick) in the prehardened form. Subsequently, I repeated my tests with similar stoves made from grade 304 stainless steel (~0.10mm thick) again in the hardended form. The SS gave a small improvement in the burn hours before self destruction. I describe my struggle with this throughout my four part article “Backpacking Wood Stove for Alpine/Snow Camping” .

I have for some time suspected the my stoves were very different to predecesors:

They got hotter (~550C plus on the outside, bright red see photos in articles) and who knows how hot on the inside [possibly 1000C by the white colour seen when you peep through the primary air port when the valve is removed]. It is a bit like being unable to know if the fridge light is off when you shut the door.

They have a small heat exchanger surface that is designed to emit strong heat radiation to tent ocupants and provid a little cook top.

They have a generous oxygen supply for clean complete burning which is beyond the stoichiometric requirment of the available fuel gases

And lastly they have an essential internal labyrinth/after-burner/heat exchanger/cook top/spark arrestor components that are poorly connected (conductively) via spot welds to the outside world and have little capacity to quickly dissipate the heat that comes from bombarding them with a fireball of flame and oxygen for hour upon hour.

These differences and resulting problem were not unexpected when the design is much like the the rocket mass heaters without the mass and without the protection of baldosa or fire brick and having a little bento box sized heat exchanger for radiating intense heat, rather than a big oil drum to heat room air.

My contention is that my stove metal/s is destroyed by the combination of higher temperature, lack of carbon deposition and metal oxidation in an oxygen rich environment.

In other very small and powerful oxygenated stoves that I make with metal components there appears to be a clear correlation between rapid metal destruction (at and down stream of the burn) and the absence of carbon deposition. This could mean that the temperature that is low enough to condense carbon it is not hot enough to destroy the metal. Alternatively it could mean that the carbon sitting on the surface of the metal it is protecting the metal surface below it from oxidation.

Either way I think it means that there is likely to be a metal oxidation problem for clean burning high temperature stoves if the metal is not protected by a suitable refractory coating.

A simple question to you all is: do you get carbon deposites on the inside of your metal stoves that are not degrading after many hours of use?

From my experience most wood burning fires and stoves (even the stickman stove, an inverted gasifier stove with a fan) have a little yellow colour in the flame and pots get blackend quickly.  This indicates to me that there is less than full combustion of the carbon and the temperature may not be as high as in my stoves. The solution for me could be to just go back to dirty inefficient smoky stove, but that is not for me. I also reject runing the stove at lower temperatures because I want the maximum radiant heat from it when I am camping in the cold.

Supporting my oxidation theory of metal destruction, I have made a repeated interesting observation: If I accidentally leave a tiny pin hole in my stove body (resulting from a faulty spot weld), that little hole will just keep growing in size with each hour of strong burning. To me this is counter intutitive as I thought the air that would be sucked in through such a hole would be inconsequential or even ‘cooling’. However this is not so. I think the extra air/oxygen entering the hole contributes to rapid oxidation around the hole. A simple finger tip wipe of my magic refractory render (described later) will fill the hole and that is the end of the problem that would have otherwise kept growing.

My next action was to coat the metals with 1050C exhaust paint or even glass enamel and even then they broke down rapidly after ~10h of fierce burning and I sense in these cases it was not the metal failing, but the ravages of the high temperature combined with the excess oxygen was first destroying the coating. The metal could be had for desert.

The one ray of hope was my home made sodium silicate refractory render. It relishes a good burn out (as that is how it has to be treated to finally cure to its most heat resistant form…..-born-in-fire lives-in-fire) and with repeated heat exposure it changes into a ceramic and gets harder and harder with each firing. This greatly extended the burn life of the metal (~3 fold) and as discussed in comments to Part 4 it is likely to lift the emissivity of the stove surface to an impressive 0.95. If this refractory render is coupled with a thicker metal stove body (which is discussed in Part 2 considering the weight offsets of not needing a protective container for backpacking) and removable/replacable labyrinth components (that can not reject enough heat and will be destroyed more quickly) that there may be a sustainable design for a hot little stove with a long service life.

I also tinker with zirconia fibre felt that is an ultra light high temperature insulator and I can bond this to metal surfaces with a variant of my sodium silicate render. It survives the worst temperatures but is very delicate strenth wise (like meringue) and will protect the metal when applied on hot spots on internal surfaces where there is capacity for heat rejection on the outside surface, but would simply slow the temperature rise for internal components in the labarynth and they will eventually end up at the same destructive terminal temperature.

I also should say that when my stove metal dies it usualy warps or buckles, becomes very weak and cracks and closer magnified examination of the residue break indicates that it is no longer a metal and looks more like a metal oxide.

Hope this makes sense and happy to answer any questions.

[Author]

Posts