Historical armour precipitating (sweating) carbon thread?

Castings · Post by **Castings** » Thu Apr 06, 2006 9:28 pm

Greetings all-

I thought I read a thread on this board some years ago about a historical piece of armour that was actually prepicipitating (sweating) carbon. I've searched the board and googled the subject but I can't find it now. Does anyone else on the board remember what I'm talking about? If so, can anyone point me to the original thread and/or an article on the subject (or even the museum and name of the piece)?

Thanks in advance,
Josh Stender

Alcyoneus · Post by **Alcyoneus** » Thu Apr 06, 2006 11:18 pm

There are some steels that are precipitation hardening (PH), but it doesn't sweat out like nitroglycerin from old dynamite. ;-)

matthijs · Post by **matthijs** » Fri Apr 07, 2006 7:02 am

Sweating as in carbon migrating from in the surface to on it? Never heard of that. Sound a bit like reverse case hardening. Has anyone here experimented with that? Might work quite well, making the piece first in mild steel and then case hardening it.

Thomas Powers · Post by **Thomas Powers** » Fri Apr 07, 2006 12:26 pm

Speed of carbon migration at room temps precludes this from happening.

Thomas

Castings · Post by **Castings** » Fri Apr 07, 2006 6:55 pm

Perhaps I did not read the article here... FWIW this came up because I'm in a materials science course right now. According to my professor (who holds a doctorate in materials science), given a long enough timeline (on the order of a thousand years or more), carbon will actually precipitate out of steel. Anyway, thanks for the responses.

Josh Stender

Andrew Young · Post by **Andrew Young** » Sat Apr 08, 2006 2:44 am

Id be willing to bet that in the next 200-500 years, ie, before most plate reaches 1000 years old, we will have figure out how to reinsert carbon via some tool found at Home Depot

Minotaur · Post by **Minotaur** » Sat Apr 08, 2006 2:50 am

Did he say why this would happen?

RalphS · Post by **RalphS** » Sat Apr 08, 2006 11:22 am

Isn't that only the case for hypereutectoid steels, and then only theoretically so? So far, all the numbers I've seen for carbon analysis of parts of historical armour were hypo-eutectoid (low to medium carbon)

Andrew Young · Post by **Andrew Young** » Sat Apr 08, 2006 11:12 pm

Isnt the argument for hair line cracking carbon sweating or vice versa. Yet seems to me that we'd see a nearly homogenous mesh of lines rather than a series of "martian canals"---that my own armor left...ahem, neglected during busy periods has done this has me wondering if we're not seeing water etching. I dunno, not my area of expertise.

Castings · Post by **Castings** » Sun Apr 09, 2006 1:04 pm

RalphS-

Well, without you having a least a little introduction to metallography (which is about all that I have had), its hard for me to put it into terms that will make sense. Keep in mind that I am just learning this same material so my explanation will be imperfect, but here goes: If you look at a sample of steel under a microscope you will (generally speaking) see a large number of small dots which is actually the carbon "phase" of the material. Apparently, this phase is not 100% stable, so, on a long enough timeline, these dots (actually spheres, but your looking at a cross section of them) will tend to migrate together and thus eventually begin to precipitate out. That is, at least, my rudimentary understanding of the subject. I'm sure there are at least 1-2 other people on this board or on SFI that can explain it better.

RalphS-

I honestly don't know, we were talking about both hypo and hyper eutectoid steels that day, and I don't remember what she said. I'll ask her next week and get back to you.

Josh Stender

RalphS · Post by **RalphS** » Sun Apr 09, 2006 5:28 pm

Castings wrote:Well, without you having a least a little introduction to metallography (which is about all that I have had), its hard for me to put it into terms that will make sense.

Hit me with all you've got!

Metallurgy is a bit of a hobby of mine, and I have a degree in physics (though a bit rusty in this field), so I should be able to hang on.

After a bit of googling I came up with this interesting link:
http://www.msm.cam.ac.uk/phase-trans/20 ... nsite.html
Seems that the stored energy for non-graphite containing structures is positive, so you do have a point about the possibility of C-precipitation.
But on the other hand, the energy of formation for cementite is negative, so that should balance the effect somewhat. I don't have the right numbers and have never done the math to see which effect dominates for equilibrium determination.

But in any case, carbon diffusion rate seems to be the crucial factor here.

Konstantin the Red · Post by **Konstantin the Red** » Mon Apr 10, 2006 1:29 am

Durasteel Corporation wrote:. . .seems to me that we'd see a nearly homogenous mesh of lines rather than a series of "martian canals"---that my own armor left...ahem, neglected during busy periods . . .

Mars is the Planet of Rust, Dura! :wink: