strange scale activity

[Karl B. Andersen]

I opened the door of my oven last night just prior to quenching 2 - 5160 blades, and saw something I had never seen before. It was as if each blade was surrounded by an "envelope" of scale. It was a perfect blanket around the blades, yet un-attached from the blades, standing out away from the blades all on its own!
I did nothing different that I can think of on these two blades from any others.
Same temp, same soak time, same degree of finish, etc.
Now, I went ahead and quenched them, and to my surprise, they didn't harden completely.
Could that "envelope" of air between the blanket of scale and the steel insulated the steel from the full temp of the oven and not allowed the steel to come up to full autenizing temp?
Only the last inch or so of the tips got full hard. A file bit into the cutting edge back by the ricasso rather easily.
Where did all that scale come from?
Any experience with this folks?

[skipknives]

(I don't know squat about this stuff) but i would like to find my way back to here so i can read more,,,

??does some metals exple a gas near the melting point,,,then as it moved away the raident heat lowered enough for it to re-solitify into a solid????

[RJ Martin]

Scale is formed as the result of Oxygen in your furnace. The longer the blade is exposed to the oxygen, the greater the scale formation. Of course, the formation of scale removes carbon from your blade. That is why they didn't harden properly, IMO.

If you are heat treating carbon steels in an oven, you need to protect them from oxidation by using one of the commercially available anti-scale compounds like TURCO.

And, I'd recommend not inserting the blades into the oven until the oven is at Austenitizing temperature. Carbon steels don't need a long soak.

[Karl B. Andersen]

5160 will benefit a great deal with a little extra soak time.
I don't put the blades in until the oven makes it up to temp and then also stabilizes at that temp for about 1/2 hour.
I start a 15 minute soak on 5160 when the oven rebounds to full austenizing temp, BUT the blades are about 4 minutes behind, leaving about 10+ minute soak.
I'm beginning to think the door just simply wasn't latched. I admit it.
I did a spherodizing anneal on the blades, cleaned up the scale, re-did them exactly as before (making sure the door WAS latched!) and they got hard, hard!, HARD!