Only Case Hardening is surface hardening. True hardening of the steel involves
raising of the temperature of the steel to a point where the grain size is
modified to be smaller and is more orderly thus increasing the
intercrystalline bonds reducing the potential for dislocations. The trick with
hardening is to get the crytalline structure consistent inside to outside and
then 'freeze' it by quenching at that state.
Subsequent to quenching, most steels are then annealed to increase the
toughness (resistance to deformation).
Harder does not equal tougher. Toughness is an independent variable based on
the amount of abuse (non-engineering term here) that it will take before it
starts to show signs of yielding.
In gross generalities, a stronger steel is more brittle, however this is a
relative term. My point being that as a steel becomes stronger, the percentage
difference between the yield point (where it starts to deform plastically) and
the failure point (where it catastrophically fails) is reduced. Thus as
stronger steel in the same application will yield later (at a higher load),
but it won't put up with the same percentage increase in load beyond that as
long as a weaker steel will before it fails.
Hardening of steel is more of an art than a science unless you have a
limitless database of minor variations in each specific steel you are working
with.
So to answer the question, at a given load assuming no annealing has taken
place, a harder steel will fracture more suddenly than a weaker steel, not
necessarily at a different load though. Some tougher steels are harder, some
harder steels are more brittle, some softer steels are tougher, it all depends
on what the alloy and the heat treatment is.
Sorry for the dissertation, I'm just off of my load frame testing Aluminum at
4 K (-269*C or-453*F).
Mark Haynes
It only goes one way-Pay it Forward
HAN6L12977
HAN5L8016
Date: Tue, 16 Aug 2011 12:14:00 -0400
From: Allen Hefner <lbc77mg at gmail.com>
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?
Just askin'.
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