Tim,
I was very glad to see you now have a
column in KI.
Question for your next article:
I have heated knife up really hot to
forge to shape... grains are big and
fat sluggish and slow
What various techniques are there to
make-um small sharp eyed and fast. Pros and cons.... your
preferrences... why? Hope things are going well for you in the land of
ice and snow
Wil Smith
(Austin, TX)
Wil,
Thank you for your question. I'm glad to see that someone is reading my
column. A small grain structure us usually desirable in a blade to
increase its strength. If the grain size of the blade is too large the
edge will tend to crumble as it is used. Conversely, if the grain size
is too small you can lose wear resistance. The edges may also tend to
wear to a smooth polished one rather than a fine micro tooth. Finding a
happy medium is important.
A lot of different things go on inside a blade that has been forged.
These will all relate to the variables of: temperature, atmosphere and
how you forged it. The internal structure of a blade forged at 2000F
and one forged at 1600F will be quite different. Areas of the blade
that receive more forging will have a different structure than little
forged areas. Fortunately, it is possible to straighten out almost
every forging problem by proper thermal processing.
First, after forging, the blade should be normalized. I am assuming you
are working with a water or oil hardening steel. Normalizing is not
usually recommended for air hardening steels. Normalizing will break up
non uniform structures, relieve residual stresses and produce greater
uniformity in grain size. A great deal of distortion or warping during
heat treating can be traced back to skipping, or poorly performing,
this step.
To normalize, slowly and evenly heat the blade to above its
transformation range, then cool in still air. The temperatures for
normalizing simple steels usually falls in the 1550F - 1650F range.
After normalizing, the blade should be annealed. This will soften the
steel, allowing it to be drilled and cut. Typically the blade is held
at a temperature at or near its transformation range for an hour, and
then cooled at a rate of 30 to 50 degrees per hour. The actual
temperatures and cooling rate depend on which alloy you are using.
After grinding and finish shaping the blade is ready for hardening.
The heating for hardening, or austenizing, is the most critical part of
keeping the grain size small. Overheating a piece of simple steel by as
little as 100F can turn an otherwise good blade in to junk. The
pictures here tell the story. Even though these pictures are of M2, the
same thing happens with steels like 1095 or O1. If you have a kiln or
controlled furnace available, use it. It takes a great deal of practice
to get the temperatures right with a forge or torch. I'm not saying
that you can't get good results with a forge or torch. It is just
easier and more consistent to take the human factor out of the equation.
Here is a simple exercise to demonstrate what the critical temperature
should look like and the importance of getting it right.
1. Take an old file and grind grooves across it. Use
a large and good quality file such as Nicholsen. The grooves should be
about one inch apart and go half way through the file.
2. In a dark shop heat one end to yellow while
leaving the other end cold. What you are doing is setting up a
temperature gradient, just like on the wall charts. You will notice
that, starting from the cold end, the bar will get brighter. Then,
there will be a definite "shadow" that goes across the bar. This is not
a cold spot. It is the area where the steel is transforming into
Austenite. Just to the hot side of that shadow is the correct critical
temperature.
3. After doing step 2 a couple of times, just to get
used to it, quench the file in water. Note which groove the shadow is
at.
4. Put the file in a vice at the first, hottest,
groove and break it off. Use safety glasses and gloves. A gentle tap
with a hammer will be all it takes. The grain will be very coarse and
weak.
5. Continue down the bar noting how the grain gets
finer and stronger as you go. Pay special attention to the groove where
the shadow was at and the one just above it. This will illustrate a
temperature difference of about 100F. Also, try to remember how subtle
the color difference was between those last two pieces. You will see
how difficult it is to get it "just right" in a forge or with a torch.
Holding the blade at the critical temperature for too long will also
promote grain growth. For simple steels the standard rule is five
minutes per inch of thickness. So for a 3/16" blade you would want to
soak it at temperature for about a minute.
There are special techniques to further reduce the grain size. The most
common is the use of multiple quenches. This involves hardening the
blade, then repeating the austenizing and quenching process one or
several times. Done properly, the subsequent austenizing must take
place in less than 45 seconds, with no soak at temperature and have a
temperature control capability of less than ten degrees. Usually, this
needs to be done is salt bath furnaces with a contact thermocouple on
the blade. Slower heating or overshooting the critical temperature will
give little or no grain size reduction.
Remember, with the grain too small you may be giving up both sharpness
and edge holding for increased strength. Overly large grain size is
always bad. Techniques such as martempering and cryogenic treatment can
give greater strength increases than an ultra fine grain structure.
Usually, if you hit the critical temperature just right, the grain will
be the right size for both edge holding and strength.
For more information see:
Metals Handbook, Vol. II 8th ed., ASM
Tool Steel Simplified, ISBN: 0137558056, pp. 331-335
The Rapid Treatment of Steel, R.A. Grange
Metallurgical Transactions, Volume 2 - January 1971
Corrections:
The ISBN numbers have changed for two of my reference books. They are:
Tool Steel Simplified - ISBN: 0137558056
Metallurgy Fundamentals - ISBN: 0870069225
Thank you Matt for bringing this to my attention.