What happens on the edge of mild steel when it's flame cut?

I've heard that gas or plasma cutting mild steel creates a hard edge. Is that right? If so, what is the mechanism? I.e., what causes the
hardness if there's not enough carbon in the steel to start?
Still struggling up the learning curve, Bob
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Bob Engelhardt wrote:

This is certainly true for oxy-acetylene cutting, where the carbon comes from the flame. I'm not sure it's true for the electric "flame" of a plasma cutter.
However, torch cut edges can be machined using carbide tooling.
Grant
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We flame cut (Mapp/Oxy) and Plasma (compressed air) a great many of the pieces we wind up installing on the boats, and yes, the cut edges do get harder. I believe Grant is right about the carbon in the flame adding to the hardness, but I also think the heating of the base metal, even though the Heat-Affected Zone (HAZ) is mostly removed, causes some migration of carbon to the cut edge.
I seem to remember some photomicrographs in an AWS journal showing how the carbon in the steel "moved," and concentrated along the edge. Also, the crystalline structure changes...(have to hit my books--austenite, martensite, something-ite...)
Easy to clean up- just touch it up with a grinder.
TL
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Bob Engelhardt wrote:

Do you mean the surface next to the kerf? I have to admit that I've not systematically studied the subject, but experience would seem to suggest that, given a clean cut, any hardening is at least as much due to temperature changes as it is to a change in carbon content. The time and temperature required for the steel to change composition, due to available carbon, to any significant depth is much greater than that offered as the cut travels, and the cut is taking place in a very oxygen-rich environment (in the case of the torch at least).
A poor cut, with slag hanging in the kerf, can certainly result in spots on the surface with quite high carbon content. If you've ever used a chipping hammer or grinder to clean slag from a poorly cut surface you'll have noticed that there are sharp spots that got surprisingly hard, but again, I don't believe there's much depth to the hardness. If you were going to weld on this cut edge, grinding it back until the sparks reveal that the area of raised carbon has been removed wouldn't be an all bad idea.
As to the source of the carbon in these hard spots, I'm not sure of the source of the carbon but I suspect it's more from the steel that was inadequately cut. Experience with the plasma cutter may confirm this, as a poor cut with that also gives the shape edges that can be impressively hard, and there's not all that much carbon in compressed air- a little bit of atmospheric CO2 and a bit of CO from the compressor itself, but compared to the amount in the steel already, not much. I suspect what happens is similar to the effect that everyone's familiar with: the rusty old knife that you find stuck somewhere turns out to be the best knife you ever had, 'cause the rusting process effectively raised the carbon content of the steel.
Humorous anecdote to finish with, concerning losing knives and suck.. my grandfather told me about the time, years before, when he had lost his knife. Really liked that knife, just didn't find it so he got another. Winter comes along and he goes hunting, sits down someplace in the woods to have a little nip and cut a plug of tobacco, goes to stick his knife in the fencepost he's sitting next to (instead of getting up right then to put it back in his pocket) and right where he looks to stick it is the knife he'd lost the winter before. Heh.
John
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The edge ends up a blue line and is a complex zone. I have plasma punched holes and then had to use a carbide drill to true up the hole.
Now consider Stainless - oxides of chrome are nasty! They are hard, sharp and jagged.
Remember the metal goes to 3K degrees with lots of oxygen pouring on the cut (to flush out the melted metal and cool the cutter.)
Pre-hardened metal tends to revert to soft on the edge. But when welding, that part is re-hardened if quickly cooled.
Martin
Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net TSRA, Life; NRA LOH & Patron Member, Golden Eagle, Patriot's Medal. NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member. http://lufkinced.com /
Bob Engelhardt wrote:

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With Flame cutting you will get a thin skin of hardened metal. It is hardened by the process of being heated to near melting temperature and then cooled rapidly by the remaining mass of the steel. While mild steel doesn't have much carbon in it, the process is so fast that it will still cause martensite (hard steel) to form.
Plasma cutting is a bit different. With plasma the heat of the cut is so intense (around 24,000 degF) that it breaks down the steel compound and causes the iron and carbon to combine on a molecular level into Iron Carbide. This Iron Carbide skin is incredibly hard, but is also very thin. You can usually remove it with a hand grinder. If you don't remove that skin it will rip the teeth off of a saw blade, milling cutter or drill bit.
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Thanks to all those who replied. Good stuff.
Bob
Thanks
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