I think it's to help keep oxygen away from areas you don't want it (inside). That way you don't get internal pitting/weakness. But, I could also be wrong. I have asked so many questions that I'm now getting the answers all confused. Haha. Although, Matt, you might be right...maybe it ensures more even heating, which allows everything to "mix" properly.
It keeps the carbon from burning off. Oxygen steels carbon.
"The Buddha resides as comfortably in the circuits of a digital computer or the gears of a cycle transmission as he does at the top of a mountain."
The foil keeps the steel from oxidizing at high temperature. The oxygen can combine with the carbon at the surface of the steel to form CO and CO2 and cause some decarburization. Oxides also form which are ugly and very hard.
It protects the steel while at the hardening temperature.
Thanks Mark, back at it today. I had to rescue my daughter's car that broke down 100 miles from home.
This may be my favorite thread on the interweb.
More handle making.
I'm not sure why the photos come out in order sometimes and sometimes in reverse.