HARDENING SIMPLE STEELS
All of the really low alloy steels have one feature which make them virtually foolproof
when it comes to cooking them for hardening. When one of the low alloy steels reaches the
critical temperature where it can be hardened by quenching, it turns non - magnetic. As
the steel heats, check it with a magnet. At a certain point, the magnet won't stick.
That's usually at a temperature (color) far lower than you would think. Once the magnet
won't stick to the blade, give it a moment more in the fire and then into the quench it
goes. (An extra 50 degrees over the critical temperature insures better hardening and
won't hurt the steel.)
The simple alloys can also be selectively hardened, not with a fancy temper line, but
with a softer back that will make the spine and tang less susceptible to breakage. All you
have to do is take the blade up to heat very quickly, getting the thin parts along the
edge hot before the thicker spine. You could also just dip the cutting edge into the oil,
allowing the spine to cool more slowly, not hardening it.
Should you err and get the spine too hot and inadvertently harden it,
you can use a torch to partially anneal the critical areas. I recommend that all stick
tang blades have a softer section where the tang joins the blade.
When the hot blade hits the oil, you will almost always get some fire.
Don't leave the tang half out of the oil. It is near red hot, sticking out of the oil and
acting like the wick of a candle to start a fire in the tank. NEVER use a small tank of
oil to quench a lot of blades. Sooner or later, you'll find the flash point of the oil.
That's where it starts burning all by itself, and you won't like that one little bit.
Forged blades will always have stress than needs to be
worked out before hardening. They should be annealed, preferably several times before
hardening. Some knife makers have had wonderful results by annealing their blade steel
three times before hardening. It reduces grain size significantly, making a much tougher
SERIOUS HEAT TREATING
1040 to 1050 steel Water quench from 1525
to 1550 F. Hardens to approx. RC 58. Very easy to get cracks with water quenching. Draw at
350 F. for spring temper, best for daggers, etc. Shallow hardening and can be done with
beautiful, Japanese style temper lines.
1050 to 1095 steel Brine quench from
1475 to 1500 F. hardens to RC 60 to 65. Draw immediately. Oil quench at the same
temperatures for slightly lower hardness. Shallow hardening and can be done in the
Japanese style with a decorative temper line.
4150, 5150, 6150 steel Oil quench from 1525 to 1600 F.
5160 steel Oil quench from 1525 to 1600 F.
52100 steel Oil quench from 1525 to 1600 F. Harden, cool and
let the blade settle for a day (24 hours). Re-harden twice more at the same interval.
Cutting edge toughness is fantastic. Draw at about 350 F, three times, and the spine may
be torch drawn to spring temper. Ed Fowler gets incredible cutting and flexibility with
O-1, O-2, O-6, O-7 steel Preheat slowly to 1200 F. Oil quench from
1450 to 1500 F. Draw at about 350 F. O-6 reaches RC 65.
W-1, W-2, W-3 steel Preheat slowly to 1050 F. Water (brine)
quench from 1400 to 1500 F. Draw immediately. May also be oil hardened if cross sections
are radical, or simply for less chance of cracking. Shallow hardening and will work with
Japanese temper lines.
L-6, L-7 steel Quench from 1450 to 1550 F in water or brine.
Doesn't really need to be drawn. L-7 will give slightly more hardness.
D-2, D-7 steel Preheat at 1500 F. Harden from 1850 to 1875 F.
A-2 steel Preheat at 1450. Air harden from 1700 to 1800 F. Draw
at 350 F.
S-1, S-2, S-5, S-7 steel Preheat at 1200 to 1300 F. Harden from
1650 to 1750 F for S-1, 1550 to 1650 F for S-2, 1600 to 1700 F for S-5 and 1700 to 1750 F
for S-7. Draw at 350 to 400 F.
M-2 steel Preheat at 1400. Oil or air quench from 2175 to 2250
F. Draw at 1000 F.
440-C steel Air harden from 1850 to 1950 F. Draw at 325 F.
Freezing to dry ice temperature for several hours before the draw will enhance toughness
and hardness remarkably. Don't bother with 440-A or 440-B.
154 CM, ATS 34 steel Air harden from 1975 F. Straighten before
they cool below 250. Freeze at -220 F for 6 to 8 hours. Double draw at 950 F, two hours
each cycle. (Paul Bos method) Gives RC 59 - 60 and marvelous durability.
Damascus Harden to specifications of the highest carbon content
component. Damascus may be treated a bit rougher than homogenous alloys. San-mai Damascus
sometimes has severe shrinking problems which will pull apart the center layer.
Water hardening is a mis-used term. In virtually all cases where a steel is referred to
as water hardening, they're actually talking about quenching in brine, heated to a
temperature of 170 degrees, (F) or above. Brine is made by dissolving non-iodized salt in
water until a egg will float in it. Jim Hrisoulas uses bluing salts to make his brine. The
whole idea is to raise the boiling temperature of the liquid and make it transfer heat
better. Brine will eat right through an ordinary steel barrel in a very short time. Jim
Ferguson just mixes in borax and detergent to reduce the surface tension.
Drawing is normally done for 30 minutes when one has the equipment to properly maintain
the temperature. Flash drawing, that is, heating to show a certain oxide color on the
metal surface and then letting it cool, is fine for softening a spine, but not best for
the cutting edge.
Any steel which has a low draw temperature may be drawn to a softer temper along the
spine to give it better shock resistance. If this is done, the blade will almost always
curve a bit towards the softer part of the blade. This works better on shallow hardening
steel than on others but can be done on 5160, 52100, A-2, 440-C and others.
Many of the low alloy steels, like 0-1 and 5160, will show a temper line
if the blade is selectively hardened, but the line is rather plain, merely a division
between hard and soft. A proper, decorative line, needs either the 10 series, or W series
of steel. Oil quenching will produce a temper line in those steels, but water quenching is
necessary if one desires the more intricate details of the Japanese style line.
OVEN DRAWING COLORS
If you want to do your own heat treating on some of the easier steel types, you will have
to first harden the steel all the way, then draw its' hardness back a bit to keep it from
being too brittle to use as a knife. You can do this in an ordinary kitchen oven, but most
ovens have a crude thermostat that is not much better that taking an educated guess at the
temperature. The scientific term is SWAG, scientific wild assed guess.
A note here, to keep peace in the household. Any blade that goes into the kitchen oven
for drawing must be wiped clean of quenching oil and then soaked in the sink with plenty
of dish washing detergent for awhile, getting the last traces of oil off. Why? Putting an
oily blade in the oven and heating it to four hundred degrees creates an absolutely
terrible smell that lingers for days. Your wife won't let you forget it for a lot longer
Oxide colors will form on clean steel as it is heated. You've seen them many times, as
the steel got hot where you're grinding.
Contrary to some belief, these do not indicate that the steel has had its' carbon
burned out. All it means is that the steel got hot, and if hardened, lost some of the
hardness. (Burning carbon out of a blade takes bright yellow heat applied for more than
just a minute.)
The colors are a very precise indicator of temperature with simple carbon steel.
400 degrees, pale straw
425 degrees, straw
490 degrees, golden yellow
500 degrees, brown
525 degrees, brown purple
570 degrees, purple
600 degrees, bright blue
650 degrees, pale blue
There is one little catch when using the color with any steel more
complex than 0-1. They don't all react to a given temperature with the same color. D-2,
for instance, looks a lot different from 0-1 at 450 degrees. It's best to put a scrap
piece of 0-1 or 1095 in with the complex stuff when running the draw cycle, and use it as
a precise indicator. [Return to Top][How
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RED HEAT TEMPERATURES
Faintest dull red - 900F
Dull red - 1200F
Cherry red - 1400F
Orange red - 1500F
Bright salmon orange - 1600F
Brilliant orange - 1640F
Temperatures should be judged in a dimly lit area. If you are working
outside, colors will appear at least one or possible two steps too low, and you will
severely overheat the steel. I have become intimately familiar with this problem. One
fellow has a open barrel set horizontal beside the forge, providing a shaded spot to stick
the blade into for judging steel temperature.
If you want the spine of a carbon steel blade drawn softer than the cutting edge, the
process isn't nearly as difficult as you might think. All you need is an inexpensive
wallpaper tray, filled with water, and a torch. A propane torch will work, but the
oxyacetylene type works a lot better.
You hold the blade above the tray, edge down, and begin heating the spine until you
start to see some color. You always start at the heaviest part of the blade. The first
color will be a pale yellow.
At this point, try to keep at least half an inch of the cutting edge in the water. This
will keep the heat from running down to the parts that have to stay hard. Work the heat
only on the very spine, and work slowly towards the tip. Stop well back from the tip. It
will heat easily and you want it to stay hard.
If you are using a acetylene torch. the flame must be aimed directly down at the spine
of the blade. If you heat from the side, the blade will warp. This is not a problem with
the slower heating L P torch. After you melt a tray or two, you'll also want to use a
When the length of the spine has a rich blue color, it will be drawn to a high 40 or
low 50 Rockwell, which is right where you want it. On stick tang blades, you should cook
the junction of tang and blade to at least a good rich brown to prevent any brittle
fracture at that point. The end half of a tang doesn't need to be hardened at all.
Some of the more complex alloys with low drawing temperatures, like
440-C and A-2 may also be treated to soften the back with this technique.
Copyright Bob Engnath
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