1095 warping during second temper?

[steveo83]

Has anyone experienced late warping with 1095 - during the 2nd temper??

I heat treated a 1095 blade today. 1/8" thick, clay coated.

On the first normalize (1600) it warped into a very pronounced crescent shape. It had fallen over somehow in the oven fixture so this may have caused it, or it could have been uneven grinding etc.

I straightened it out, then normalized twice more. Once at 1600 and again at 1500. No warpage at all. Straight and true.

I coated the blade with clay, soaked for 6 mins then quenched @ 1455 into 120F canola oil. Everything seemed to go well, the clay held on right through the quench and only popped off when the blade was almost down to bare-hand temperature. Still nice and straight.

I checked the knife over, everything seemed fine, put it in the oven @ 400 for 90 minutes.

Took it out, checked it over, all was well so I left it to cool and then put it back in for another 60 mins @ 400.

When it took it out, it was noticeably warped about 2.5" from the tip. Noticeable right away as in the pic (more noticeable in real life).

I didn't look at the blade again before I put it back in for the 2nd temper.. so either this happened while it was cooling after the 1st temper... or it happened while it was in the oven during the second temper.

What could cause this delayed warping? It definitely wasn't like this after the quench, and it was straight as an arrow immediately after the first temper. It has me stumped

 

[mete]

Give some details of the hardening and tempering furnaces and how the knives are set in them.

 

[steveo83]

Give some details of the hardening and tempering furnaces and how the knives are set in them.

Sure - its an Evenheat oven for the hardening. I have small notches in 2 pieces of angle iron (welded together) which allows the blade to stand spine up in the oven. Only the spine contacts the fixture.

For initial temper, I use a kitchen oven (i know..) but the kitchen oven has pizza stones top and bottom, and a fire brick in the middle to even out the temp fluctuations.
2 separate oven thermometers report very stable heats. During temper, the knife lays on the middle oven rack. Could this be the problem? laying one-sided on the rack?