how many times can a hardened blade be re-annealed?

[overmodulated]

Hi Folks: a hopefully simple heat treatment question:

Once hardened and essentially unfileable, could a blade be annealed again for any major reshaping (by file)?

I read somewhere on this forum that it is recommended to happen no more than once. Am wondering how subjective that is, and what the drawbacks are of annealing more than once.

Thanks for any feeback!

[fisher_norris]

it depends on the steel and i have done this several times on 5160 or 1095 i heat treated them 2 times as i tempered too soft in my forge. but i don't know about other steels. the admin's would know or some other better smith.

[Rich Hale]

Actually if you really wish an answer you can find it in yoiur shop. Using a piece of the steel you make blades from do some tests. Harden and temper. Then test the blade for flexibility and edge holding. Then anneal and repeat. When you create an edge for the first test you will have to repeat that for subsequent testing as you will lose some stock. And keep in mind that whatever you find out about one steel may not work with different steels or maybe even a different batch of the same steel. Keep records of you work for reference.

[ThomasPowers]

Some steels even profit from multiple *hardenings*. (special cases of course).

[Quenchcrack]

Every time you anneal a blade, the carbides tend to get bigger. This means the carbon is all tied up in the carbides and is not available to actually harden the steel when you quench it. You can theoretically anneal it as often as needed but normalize it a couple of times before you re-harden it to reduce the size of the carbides. This puts the carbon back into the iron and will harden more effectively. See BP0078 for details.