Juergen Hannappel wrote...
Bay Area Dave writes:

wouldn't they be discolored? I use to harden chisels for automotive

Not necessarily, softening starts well before coloring (depending on
the steel type, of course).

They might not be discolored because oxidation colors are easily removed
by abrasion -- grinding, polishing, honing or other normal tool finishing
practices. They may also have been tempered in a controlled environment
(inert atmosphere or vacuum) that prevents the oxidation that causes the
coloring.

However, if heated in air, the tool steel used in Marples chisels will
certainly discolor before it softens beyond its intended use hardness. In
fact, it discolors if heated to its normal tempering temperature in an
oxygen-bearing atmosphere.

Jim