On 11 Dec, 21:11, "George" wrote:

Hmmm! my chisels havn't lost their tempred factor in the past three years of
sharpening them on a grinder wheel

How do you know? Have you measured their hardness?

but then again if you remember your
school metalwork classes you always used a cooling process inbetween heated
metal.

Which will cause cracking (too small to be visible) in high-carbon
steels, such as chisels. This hasn't been a recommended practice for
decades. When it was, back then "powered grinders" meant bench wheels
rather than high speed angle grinders, and even then it was dubious
(especially for woodworking tools). Bahco might survive (because of
their alloy) but cheap Chinese from Lidl won't, even though these will
take a better edge (those £4 sets are actually pretty good).

Wood chisels (and plane irons) are _really_ sensitive to overheating
during grinding, because of the narrow angles at the edge (minimal
conduction cooling) and the high carbon content. Techniques that work
fine on a Swiss Army Knife in 440 stainless will ruin even a simple
chisel in short order.

Of course it's possible to use a powered grinder to sharpen chisels. A
slow water-cooled grinder (and I've seen these for £20) will do it
fine. If you change the alloy away from a simple carbon steel then you
can also make it resilient against heating and do it on a dry wheel.
That's why so many modern plane irons trumpet that they're "tungsten
vandium steel". This is a poor alloy for a plane iron and never takes
a good edge, but it makes manufacturing quicker and cheaper on simple
machines.