Sparks...
E-T--IMX, sparks do not necessarily mean any steel is being ruined, sharpening a knife on a belt sander. It mostly depends on the alloy of steel used in the individual knife.
Most tempered-steel knives don't spark much, if at all, when being sharpened on my 1x30. But, I recently came upon a knife blank (unfortunately, with no provenance, so I have no idea of the alloy or tempering, except that it does seem to be tempered) which sparks like crazy when applied to the belt sander. I mean a fountain of sparks--startled me at first.
I guess I'll just make up a handle for that blank and see how it performs in use. Anyhow, sparking varies considerably when the whirling grit is applied to the steel.
As to heat--You mustn't allow a knife being sharpened by any mechanical means, to heat up. I do mine bare-handed, with the idea that if my pink patty-paws can stand the heat the steel certainly can, and when the knife becomes warm I dip it in water to cool, and wipe it off before proceeding. The point of the knife, where the heat can get concentrated, requires the most care to keep it cool. Using this technique, my knives' temper doesn't seem to be affected.
As I say, this is my experience. Your results may vary.