You mean the blade had swirls on it? Kind of looks like a whole mess of shadows or banding in the steel?
This type of pattern is caused by segregation of the various types of crystalline structures and cabides in the steel. Cleared that right up, didn't I?
For a full understanding you'd really have to read "Principles of Heat Treatment" by Grossman and Bain, or "Metallurgy" by Johnson and Weeks. I haven't, but it's been explained to me a couple of times, so here it goes
Basically, if it's a forged blade the banding is most likely caused by forging at too low a heat and not normalizing properly before the heat treat. If the blade isn't forged, the explaination is still the low heat...
There are 3 temperatures to be concerned about with a rising heat. The first, designated Ac1, is "the point at which the shift from alpha iron to gamma iron first begins." Uh, simply means the ability of carbon to move around in the steel increases, but there still isn't enough heat to break the heavier carbide bonds.
**If you hang around this temperature too long while bringing the steel up to non-magnetic (which is the 2nd temperature to be concerned about: Ac2) you get the segregation and banding.**
A relatively quick, even heat to bring the blade up to non-magnetic will avoid this.
The third temperature to be concerned about is designated Ac3 (or Acm depending on the carbon content of the steel) and is the point at which all the carbon is dissolved and in solution and is the temperature you bring the blade up to to normalize the steel (a little bit hotter than the non-magnetic state)
To make the swirls and banding go away just normalize the blade and do the hardening and tempering again.
Clear as mud, huh?
Topic: hardening a blade (Read 421 times)
