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Tempering steel


#1

In the fabrication of chasing tools, the steel is first annealled,
then shaped, and finally, hardened and tempered. My
question/confusion concerns the tempering step. I do not understand
what one should do if one quenches the steel at the incorrect
temperature ie. misses that change to a light straw color. Some
publications suggest that you should repeat the entire
annealling-hardening- tempering process, while at least one other,
Marcia Lewis’ book (Chasing), suggests that one should only repeat
the hardening and tempering steps. It makes sense to repeat the
hardening step to remove the structural inconsistencies caused by
the previous tempering attempt, but reannealling does not seem
necessary. I would appreciate hearing from anyone who could offer
some help with my question.


#2

You should take the tool back to the annealled state, to remove
the initial temper you put on the piece. As for your hardening,
then annealing, I could be wrong, but hardening, and tempering are
seperate things. Hammers and blunt chasing tools should be hardened
while cutting tools, scribes, etc. should be tempered.

-T


#3
    In the fabrication of chasing tools, the steel is first
annealled, then shaped, and finally, hardened and tempered. 

Annealing, hardening and tempering each have a definite purpose.
Annealing does more than just make the steel soft enough to work
cold. It also affects the grain structure. If steel is reheated
for hardening, without going through the annealing phase, the
grain structure may enlarge, making the final product brittle, and
coarse. Some steels, although they may have become brittle
without proper annealing, may not reach sufficient hardness.

  My question/confusion concerns the tempering step. I do not
understand what one should do if one quenches the steel at the
incorrect temperature ie. misses that change to a light straw
color.

Yes, steel should go through all three steps: annealing,
hardening, then tempering. These three are part of a single
process, and not separate processes. Some steels are forgiving
enough to allow reheating to critical temperature (the point where
the steel loses it’s magnetic nature, but not beyond) and quenching
to make them hard without annealing. I’ve never been absolutely
sure which steels will let you do this, so I go through all three
steps. Someone else seems to have suggested that some tools should
be hardened, and some tempered. This is not quite right; I may
have misread their message. Unless you want to use the tool in the
annealed state, it should be hardened. This will likely make it
too hard, so this hardness should be modified, or tempered, to the
point where the tool is still properly hard for the task, but not
so hard as to be fragile, and shatter when working. Some tools are
tempered less than others. Scribes, etc., are left with a lot of
hardness. Punches and chisels are tempered more, i.e. heated
more, so they will not be as hard, and not shatter in use.
Hammers, etc., are tempered more still.

  Some publications suggest that you should repeat the entire
annealling-hardening- tempering process, while at least one
other, Marcia Lewis' book (Chasing), suggests that one should
only repeat the hardening and tempering steps. It makes sense to
repeat the hardening step to remove the structural
inconsistencies caused by the previous tempering attempt, but
reannealling does not seem necessary.

Without going all the way back to the annealed state, some of
these “inconsistencies” may remain – primarily large grain
structure. Large grain structure is not good! To make the grain
structure smaller, you should anneal.

What’s the big deal? Simply heat to critical (non-magnetic) and
cool as slowly as possible. Bury the hot tool in a pan of wood
ashes, or sand, or kaowool, or lime, or the pumice of your
annealing pan, and let cool slowly. If you have a burnout oven,
or an enameling kiln, get it hot (but not hotter than the steel),
and put the hot, non-magnetic steel into it. Turn the oven off
immediately, or even perhaps before you put the steel into it, and
let the whole thing cool.

When the steel is cooler than about 150-200 degrees, or cooled all
the way down to room temperature, you can reheat to critical, then
harden by quenching in the appropriate medium for the steel. This
is usually oil or water, depending on the steel. There are other
quenching mediums, but these are the most common. Once hardened,
polish the steel, and heat the tool an inch or so above the
working face. When the ‘colors begin to run’, pull the torch.
When they slow, give a little more heat. As long as you don’t
exceed the tempering color (temperature) you can go back and temper
(or soften) the hardness by heating until the face of the tool just
reaches the color you want. Some people heat the tool all the way
back at the striking surface, and let the colors run. This will
partially anneal the striking face, minimizing chipping.

Find a copy of Richard Wegar’s (sp?) “The Making of Tools”. Wegar
gives a very good description of tool making, describing the
hardening of many tools, complete with pictures, including chasing
tools and stone worker’s tools. Several other books on
blacksmithing also give good descriptions.

Good luck!!

Marrin Fleet
@Marrin_and_Mary_Dell
Memphis, TN


#4
Annealing, hardening and tempering each have a definite purpose. 
Annealing does more than just make the steel soft enough to work
cold.  It also affects the grain structure.   If steel is reheated
for hardening, without going through the annealing phase, the
grain structure may enlarge, making the final product brittle, and
coarse.  Some steels, although they may have become brittle
without proper annealing, may not reach sufficient hardness.    

Marrin -

Great instructions! I’ve only played around at making some tools
because I wasn’t completely sure of the exact process. Too bad its
getting to my busy time, I’ll have to save your post for the summer
when I have some time to play. Also, thanks to the person who
asked this question.

Nancy
Bacliff, Texas Gulf Coast USA


#5

Ron:

If you missed the straw color by not going far enough, then just
continue the tempering until you get to the correct color. If you
already went past that color (to a purple or blue) then you must
re-harden the steel. Annealing brings the steel to its softest
state - you already did that before you formed the tip. If you
haven’t built up any stresses in the steel, just harden then
temper. If internal stresses are a concern, you can normalize the
steel. Heat the steel to a dull red color then wave it in still air
until the color is gone. Repeat that three times and it will
reduce the grain size in the steel and relieve the stresses. Next,
harden and temper as usual. You really only need to heat from the
tip of the tool back a couple of inches.

Hardening and tempering always go together. Hardening brings the
steel to its maximum hardness but it will be very brittle. If you
drop a fully-hardened piece of steel on the floor, it will shatter
like glass. Tempering reduces the hardness to the level you need
and reduces the brittleness. You can’t temper steel that hasn’t
been hardened first. Sorry for being long-winded.

Terry Johnson
knifemaker
Arkansas, USA


#6
Annealing brings the steel to its softest
state - you already did that before you formed the tip.  If you
haven't built up any stresses in the steel, just harden then
temper.  If internal stresses are a concern, you can normalize the
steel. Heat the steel to a dull red color then wave it in still air
until the color is gone.  Repeat that three times and it will
reduce the grain size in the steel and relieve the stresses. 

This apparently depends on the steel. With older alloys, this was
a practical approach, according to my sources, but not a perfect
solution. With some of the new alloys, it is safest to anneal all
the way back to full dead soft, or so I am told.

 Next, harden and temper as usual.  You really only need to heat
from the tip of the tool back a couple of inches.

Just don’t leave the striking face of the tool full hard! If the
entire tool has been hardened, temper the striking face for
safety.

Marrin Fleet
@Marrin_and_Mary_Dell
Memphis, TN


#7

I think we’re making this too complicated for someone who just
wants to use a torch to make a simple tool for a project. For
safety, if you harden the steel, you must temper it. After you
have shaped the tool hold the end that the hammer will hit in tongs
or a pair of pliers. Aim the torch at the other end, play it back
and forth from the tip back a couple of inches until the tip is a
bright red or becomes non-magnetic (don’t heat the magnet with the
torch). Quench the steel immediately (ideally you should know
what kind of steel you’re using to choose what quenching medium to
use. If you don’t know, a light oil will be the safest
bet-automatic transmission fluid or even cooking oil will work.)
Submerge the steel completely and stir it approximately 30 seconds.
Test for hardness-a new file should slide off the steel without
biting. Rub the steel with some sandpaper to get it bright, then
temper. To make this easier, rest the tool on a larger piece of
steel and heat the bigger piece or rest the tool on an electric
burner on a stove. Direct the heat an inch or so from the tip.
This will allow the change of colors to progress slow enough that
you can catch the color you want. You can quench the steel when
the color is right (water is ok for this). If you don’t, the color
will continue to change after you remove the heat.

M. Fleet is correct about alloy steels. Anything other than plain
carbon steels need specialized equipment to heat treat. Most are
air-hardening steels. They should be pre-heated then hardened at
the temperature recommended by the mill in a heat treating oven,
quenched in still air or a dry air blast, snap tempered to relieve
stress, sub-zero quenched, then triple tempered. While I follow
these procedures to the letter for a knife blade that someone’s
life may depend on, it is overkill for the application we’re
speaking of here. It’s possible to have a detailed metallurgical
discussion on the proper heat treatment of steels ( many so-called
experts don’t even agree on everything). Technically, the
tempering colors everyone cite are only accurate for plain carbon
steels. Other steels should be tempered at the temperature
recommended by the manufacturer ( which is only accurate if it was
hardened at the specified temperature, etc.)

For a light-duty tool, I don’t believe it’s critical whether the
steel was fully annealled or not. Don’t let yourself get
intimidated by an academic discussion of metallurgy, have fun and
make the tool. As long as the tool does what you want, that’s all
that matters. One more thing, always wear your eye protection.

Terry Johnson
knifemaker
Arkansas, USA


#8
    or  rest the tool on an electric burner on a stove.  

Terry

I’m not sure I understand. Are you using a torch on the stove
burner (??), or letting the tool heat up by turning the burner on
and leaving it there?

Nancy
Bacliff, Texas Gulf Coast USA


#9

Nancy:

Yes, for tempering you can turn the electric stove burner on
(med-high) and rest the tool to be tempered on it. With a pair of
pliers move the tool back and forth on the burner to direct the
heat where you want it. This lets it heat up slow enough so you
can see the colors change, when it’s right, quench in water.

Terry


#10

An old Austrian goldsmtih I knew and respected swore by a quench
in stale beer. He said it was better than water or any oil.L.
Michael Flynn

L. Michael Flynn, Platinumsmith
3727 Palm Drive
Punta Gorda,Fl
33950-2900
941-575-1913