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
However, torch cut edges can be machined using carbide tooling.
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,
Easy to clean up- just touch it up with a grinder.
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.
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
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 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.
Bob Engelhardt wrote:
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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