Mr. Zowada
I have spoken with several knife
makers who say that all knives should be put in liquid nitrogen as part
of the tempering process. Others say it is a waste of time and money,
that "it is just propping up a poor heat treat" or "you can't get any
further improvement". What, if any, are the advantages to the cryogenic
treatment of knife blades?
Thanks,
Fred C.
Rapid City, SD
Dear Fred,
Thank you for your question. You may be aware that this is a rather hot
topic among knife makers lately. As always, there are many different
opinions and explanations to back them up.
Let's start with what most makers seem to agree on. Cryogenic treatment
does help stainless and high alloy blades. The reason is that there is
retained austenite in the steel after the quenching of the blade. This
retained austenite weakens and reduces the hardness of the steel.
Freezing the blade in liquid nitrogen transforms the retained austenite
in to martensite. This will greatly improve the cutting and strength
qualities of the knife.
An example of M2 tool steel can be seen in the transformation diagram
below. Note that at 0F the blade still has only 80% martensite. 20% is
retained austenite. You need to get the blade way below 0F to get the
retained austenite out.
The controversy begins when you start talking about the simple carbon
steels. Since carbon steels will convert essentially all of their
austenite in to martensite with normal quenching techniques, many
makers feel that freezing them doesn't do any additional good. The
transformation chart below shows that O1 has 99% martensite at 100F.
So, if you don't have any retained austenite, what is the point of
freezing simple steels?
Here is where we get in to my opinion. I believe that the deep freezing
of steels not only converts retained austenite, but it also acts like a
super stress relief.
In the simple steels you aren't after, and don't get, higher hardness
from converting retained austenite in to martensite. What you do get is
less residual stress after hardening and more strength in both bending
and impact.
The mental picture I have is; if you have a room full of balls that are
in random arrangement and then shrink that room by freezing it, the
balls will be forced in to a more orderly arrangement. Then, as the
room grows bigger again, the balls will hold their relative positions.
This will result in more regular and stronger bonds. This might not be
anything like how it really works. But, for right now, it seems to be a
good way to think about it.
A few years ago I did some simple testing to see if freezing had any
use at all. I had the testing done at a real metallurgy lab to keep my
biases out of it. The results are in the chart below. The samples were
Starrett O1. The sizes are non standard so please don't compare these
values to those printed elsewhere. They were either oil quenched or
martempered to the hardness stated. The frozen piece was simply thrown
in to a bucket of liquid nitrogen after the final temper. After twenty
minutes it was removed and allowed to come back to room temperature
slowly.
As you can see, the results are very encouraging. The frozen piece was
stronger than all the other pieces at all hardneses. Failure in
bending, energy to max. load and total energy are all higher than the
other samples. It is interesting to see that it is possible to make a
knife at 60HRC, with its' high tensile strength, that has the impact
strength of a non frozen knife at 53HRC. See sample C.
There are several other interesting things to note from this chart.
Notice how poorly samples A and B do on the impact test. They were
tested immediately after quenching and were not tempered. That is why
you want to temper your blades right away and be careful not to drop
them. Yet, samples A and B also clearly show the advantages of
martempering at high hardness.
For now, I am putting all my knives in liquid nitrogen after the first
temper. The blades are simply immersed in the dewar and held there for
œ hour after they reach the temperature of the nitrogen. They are then
removed and allowed to reach room temperature slowly. Two standard
tempers follow.
I understand that there are others who will disagree with me on this
one. I would love to see their test results and learn from them. The
search for better methods to produce a better product is one of the
things that helps keep knife making interesting.
For questions or comments contact:
Tim Zowada
4509 E. Bear River Rd.
Boyne Falls, MI 49713
tim@tzknives.com
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BENDNG
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|
|
IMPACT
|
|
SAMPLE
|
HARDNESS HRC
|
QENCH METHOD
|
|
FAILURE
|
|
ENERGY TO MAX LOAD
|
TOTAL ENERGY
|
|
|
|
lbs
|
lbs
|
|
ft-lb
|
ft-lb
|
A
|
64
|
M
|
|
|
|
4.1
|
4.5
|
B
|
65
|
O
|
|
|
|
1.6
|
2.4
|
C
|
60
|
MF
|
1750
|
2110
|
|
10.1
|
17.2
|
D
|
60.5
|
M
|
1790
|
1978
|
|
8.4
|
13.9
|
E
|
60
|
O
|
1690
|
1893
|
|
7.4
|
11.1
|
F
|
52.5
|
M
|
1650
|
1850
|
|
8.5
|
16.2
|
G
|
53
|
O
|
1610
|
1800
|
|
5.1
|
14.7
|
Quench method: O = Oil
M = Martempered
MF = Martempered and frozen in
liquid nitrogen