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I'm not a metallurgist, but I was a machinist for the last half of my working life. I can tell you when making small springs, 350 degrees was the 'tempering' range for drawing down the hardened spring from it's initial point. This was 1095 in the oven for one hour. A lot of it all depends on what steel it is and what hardness it was to start. I'm not sure where the table is on Sandvik products, but I'll bet that 350 degrees for a whole 90 minutes is going to draw down that blade headiness. To put a finished knife blade in an oven at that temp is insane. Why not just toss it in the trash and be done with it?
I sure wouldn't do it with any knife of mine, even a low cost Opinel.
I agree with Carl.
Opinel uses 1075 for Carbonne blades. An hour at 350F is enough to affect the hardness.
Here's a chart that shows the effect of 30 minutes of tempering at various temperatures. (An hour would have a stronger effect than 30 min):
http://tienda.suministrosparacuchillos.com/en/carbon-steels-for-knives/585-steel-1075.html
Opinel used 12C27Mod for their stainless. An hour at 350F is also enough to have an effect on the hardness.
Here is the Sandvik tempering chart for 12C27Mod.
As with the 1075, Sandvik's standard tempering time is 30 min, so an hour will have quite an effect:
http://smt.sandvik.com/en/materials-center/material-datasheets/strip-steel/sandvik-12c27m/