Mark covered this somewhat.
As you increase the hardness of steel through heat treating and somewhat as
you increase the hardness through improved metallurgy, there is a tradeoff
between strength and ductility. Hardness and strength are somewhat the same.
So if you make a knife and you heat treat it, you will make it harder and
stronger, which would primarily show up as its ability to hold an edge longer
and to pry on things without permanently bending. BUT along with the
increased hardness you lose some ductility, so it is more prone to chipping
and breaking. If you want a knife that is good for cutting, then you can
harden it a lot. If you want a knife that you will be chopping and prying
with, then you don't want it to be so hard.
This balance between strength/hardness/brittleness/ductility will vary
depending on the specific metallurgy and the exact way it is heat treated. A
steel with high strength but without being brittle is sort of the holy grail
of metallurgy and custom knifemakers have searched for this for a long time.
Some of the knife steels that we have available now are better than what we
had 20 years ago, but I don't think by a whole lot. Another characteristic is
called "toughness" and this considers strength and ductility together. Once a
piece of steel has been impacted to the point that it starts to fail, how much
it yields in a ductile mode before it fails completely is a function of its
toughness. The tests for this are very interesting and use small machined
specimens and a machine with a big heavy pendulum and a small knife edge. It
would be best to make automotive parts from steel with high toughness but
common cheap mass produced items don't typically get into it that much. We do
worry about it somewhat in building construction. I've heard that some steel
from the Titanic was recovered and tested. I think its strength was
appropriate for that era but it had low toughness, meaning when it was
impacted by the big ice cube and it started tearing, it would then tear
relatively easily.
As for axles, if one is harder than the other then it is probably stronger
than the other. So at the limit the harder axle would break while the softer
axle would yield in torsion but stay in one piece. But it would take more
torque to fail the harder axle than it would the softer axle. Axles don't
undergo a steady torque but have all kinds of varying dynamic loads on them.
I suspect they are at some intermediate point of softness/hardness/strength
that somewhat balances the ductility with the strength. An axle has to have a
certain amount of strength in order to do what it does, but it might need to
limit that so that it doesn't compromise its ductility. I have seen twisted
axles, as well as all manner of broken connecting rods, cranks, etc. I also
remember pulling axles out of our old 1957 Chevy hotrod and seeing visible
twist in the splines where the axle goes into the spider gears. A harder axle
might not have twisted, or OTOH it might have sheared completely without
twisting a little bit.
> Here's a question for you metallurgists out there. If one axle is harder
> than the others, wouldn't it be more brittle and prone to breakage? Is a
> softer metal better able to handle being banged by the engine power and
> brake force?
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