[TheForge] Ed- heat treating S-7

Tue Sep 2 20:10:01 2003

Yo Guys!
Introducing the Rt. Rev. Hochewa of the Church of Orthodox Metallurgy.
Transformation grain size is a funny thing.  It gets really funny when you 
try to control it in anything short of a pyrometer controlled heat treating 
furnace.  
Grain size, whether it is the result of transformation or recrystallization 
from the strained state, is a temperature controlled reaction.  As you begin to 
get new structure or recrystallized grains, there is competition between the 
energy of the volume of the grain and the energy of the grain surface.  When 
they are in equilibrium, you have the terminal grain size.  For a material at a 
temperature with a certain thermo-mechanical history and chemistry, this size 
is relatively fixed.  After the reaction is complete, time is no longer a 
factor.  Holding it forever at the temperature will have small effect on the 
grain size.  If you increase the temperature of the work, you will get grain 
growth but at a much slower rate.  Increasing deformation, hot or cold, will create 
more sites for nucleation of new grains.
As to the forge versus grind to shape question.
The best condition that any material will be in is when you pick it up at 
your local neighborhood steel supply center.  The first time you heat it in a 
forge, you screw it up.  The first time you hit it with a hammer you screw it up 
more.  Without knowledge of working ranges, temperature control, reductions 
between reheats, reduction rates, etc., we turn metallurgical jewels into 
costume jewelry.  This is not to say that what comes from our work is not completely 
suitable for our purposes.  It is as sure as hell not in an optimum condition!
At my day job, the majority of the material I work with is better than 90% 
Platinum and it goes into biomedical devices for implantation.  There are many 
times that I just have to walk away from watching an operator allegedly follow 
the process sheet.  We do things that are not exactly right all the time but 
we get away with it because metals tend to be forgiving.  Some are more 
forgiving than others.  Plain carbon steels are pretty hard to hurt.  The more alloy 
that you put into a steel, the smaller the window is that you have to work in 
without doing some damage.
All tool steel suppliers have data sheets that go into some detail as to how 
to work and heat treat their materials.  They are usually free or at least 
downloadable.  Read them. Try to understand what they are trying to tell you.  
Following their recommendations may be a little tough unless you have the 
facilities.  Generous applications of black magic and foo-foo dust do not replace 
knowledge or proper procedures.

Regards,

Hochewa

In a message dated 9/2/2003 2:34:19 PM Eastern Standard Time, 
[email protected] writes:
The issue is not normalizing or annealing the steel per say.  (Let me say
also, that I did not understand this for a long time myself.)  The issue is
that what give us smaller grain size is the action of going through the
transformation point.  (in the cooling direction)  As the steel is
transformed from the FCC form to the BCC form we get the formation of
smaller grain size.  This is why Jim Batson goes through this step 3 times
before heat treating his knife.  The only other thing we can change (the
chemistry of the steel is already set) to get small grain size is forging
itself.  This physical deformation also reduces grain size.  Only problem is
that every time we heat into the forging temperature range we are helping
the formation of larger grain size.  This is one reason that fewer heats are
better as is the fact that fewer heats means more deformation per heat which
is good.
This grain size issue is why some, George Dickson for example, suggest as
much machining / grinding rather than forging for making tools.  Unlike
George, I would never quench S-7 in anything faster than oil from a cooling
stand point.  I must say that George has made 100's of tools quenching in
brine as have the many many students that have taken his classes.  The S-7
we buy from the suppliers comes with a very fine grain size -- because they
have been very careful to obtain this fine grain size to aid in the end use
of their product.  The same can not be said for many of the non-tool steels
we might work with.

Dave Smucker

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