Knife Steels-Material Science

[The Reaper]

Not sure that we really want to get into the details here, even if we knew.

Some jerk in China is probably already trying to figuire out how to knock off a copy of CPM S-30V and make it for half the price.

Just my observation.

TR

[Alchemist]

Mr. Harsey,

Fascinating thread! I don't know much about materials science, but there's enough chemistry here for the topic to feel just a little familiar, and I've enjoyed learning more.

Here are some of the alloys added to tool steels I know about, anyone notice anything a little unusual in the list?

Speaking just as a fool chemist, I wasn't surprised by nitrogen, because I'd read that metal nitrides can be extraordinarily hard. If I were more timely I'd have offered phosphorus. (Since it's considerably larger than nitrogen, if you could take a metal nitride structure and substitute P for N, you'd disrupt the crystal lattice quite a bit.) From a quick Google search, most hits had to do with removing phosphorus that was present as an impurity, to avoid excessive brittleness. Are there instances where you'd want to add it on purpose? Or is it an element that tends to be present anyway, and you have to go to some effort to remove it? Sorry if this naive or off-topic.

[Bill Harsey]

Originally Posted by Alchemist

Mr. Harsey,

Fascinating thread! I don't know much about materials science, but there's enough chemistry here for the topic to feel just a little familiar, and I've enjoyed learning more.



Speaking just as a fool chemist, I wasn't surprised by nitrogen, because I'd read that metal nitrides can be extraordinarily hard. If I were more timely I'd have offered phosphorus. (Since it's considerably larger than nitrogen, if you could take a metal nitride structure and substitute P for N, you'd disrupt the crystal lattice quite a bit.) From a quick Google search, most hits had to do with removing phosphorus that was present as an impurity, to avoid excessive brittleness. Are there instances where you'd want to add it on purpose? Or is it an element that tends to be present anyway, and you have to go to some effort to remove it? Sorry if this naive or off-topic.

Alchemist, It's an honor to have you here, thank you for reading.
I have someone hanging out in the shadows that may be more able than I to answer your question but yes I do use steels that try to minimize the phosphorus.

You have some (deep) background in organometallic chemistry, that sounds close enough to knifemaking for me, Sir.

[Bill Harsey]

Here's a red-neck engineering attempt at tying up some of the loose ends about how tool steels harden.

We've already used the term Austenite or Austenitizing here without much of an explanation. If we get some understanding of this word, it will help us know what happens when heat treating tool steels for use as edged tools.

Austenite is the word used to describe the solid solution of Iron and carbon, when steel is heated above it's critical temperature (something like 1450 Degrees F for simple water or oil hardening tool steels, much higher like 1950-2150 F for air hardening steels).

When the steel is cooled quick enough this turns the carbon and other alloys which can form carbides into hard Carbides and that's called "Martensitic Transformation". All during this the steel remains the exact same chemical composition but it's a different shaped atomic structure because of where the carbon and Iron atoms freeze into place depending upon rate of cooling. Cementite (Iron Carbide) is produced in here somewhere too.


If the steel is cooled slowly, this hard carbide formation doesn't happen.

We control the rate of cooling from the austenitizing temperature to get a particular range of physical characteristics, like hardness, toughness and edge holding in the tool steel being made into knives.

Usually this "rate of cooling" is exactly as fast as we can get away with while not cracking the work piece. The thicker the work or more complex the shape, the more careful we have to be about cooling too fast.

The alloys and percentages of these alloys will determine the physical characteristics of the steel and how it is heat treated, that is quenched in water, oil or air OR a combination of oil/air interrupted quench etc.

[Bill Harsey]

For The Reaper and other scientifically distinguished company around here like Mr. Dick Barber and Alchemist:
Yes I left out the other good stuff in steels like pearlite, bainite and other phase changes in the steel structure and time/heat graph because quite simply, I can't explain them.

There's a whole lot of rockin' and rollin' going on at the atomic level when we heat treat this stuff.

[Ambush Master]

Cool............Very Cool...........What about Cryogenic Tempering?!?!

Later
Martin

[The Reaper]

Originally Posted by Bill Harsey

For The Reaper and other scientifically distinguished company around here like Mr. Dick Barber and Alchemist:
Yes I left out the other good stuff in steels like pearlite, bainite and other phase changes the in the carbon and iron time/heat graph because quite simply, I can't explain them.

There's a whole lot of rockin' and rollin' going on at the atomic level when we heat treat this stuff.

Sir, I am afraid that you have me confused with someone else. I am just an old, broken-down, former action guy.

TR

[Bill Harsey]

Originally Posted by Ambush Master

Cool............Very Cool...........What about Cryogenic Tempering?!?!

Later
Martin

Freeze treatment (from -100F to -320F) after the initial hardening steps described above continues the transformation of the carbides for better and stronger steel. Said another way:

The deep freeze increases the driving force inside the steel to force the completion of the austenite to martensite transformation.

It is critical that this step is followed by a heat tempering cycle, well under the transformation point, to remove just a bit of hardness from the steel to keep it from being too brittle for use.

I use liquid nitrogen for the cryo step here, that's -320 F.
Personally I won't let steel set overnight from the deep freeze cycle but always get it right back up to room temp and into the first temper cycle to make sure it doesn't sit there and break itself because of the tremendous stresses being generated inside.

[Bill Harsey]

Originally Posted by The Reaper

Sir, I am afraid that you have me confused with someone else. I am just an old, broken-down, former action guy.

TR

Sorry Reaper, not buying this one.

[Bill Harsey]

I don't think we talked about a question asked earlier:

Can a knife steel that isn't as hard as another knife steel have better edge holding?

[The Reaper]

Originally Posted by Bill Harsey

Can a knife steel that isn't as hard as another knife steel have better edge holding?

Yes.

TR

[HOLLiS]

Originally Posted by Bill Harsey

I don't think we talked about a question asked earlier:

Can a knife steel that isn't as hard as another knife steel have better edge holding?

Yes,

[Peregrino]

Originally Posted by The Reaper

Yes.

TR

OK - I'm sacrificing myself for the greater good. I don't normally throw myself on live grenades but this one I've got to see. What's confusing me is the definition of hardness. Rockwell #s aside, I thought hardness (as a factor in edge holding ability) was a function of % of carbon in the steel, the heat treat, and the resulting grain size and density of the carbides. Softer steel = fewer carbide crystals in the iron matrix. (Assuming a quality heat treat with controlled grain size/growth that doesn't leave the steel brittle and prone to cracking.) Since the exposed carbides (ideally troosite after martensite is converted during tempering) along the edge (earlier lecture on S30V) do the cutting, and resist wear better than the iron matrix, how is a softer steel going to hold a better edge? I've been able to get softer steels sharpened with less work and (without an electron microscope to make the comparison) they seemed as sharp as the harder steels, but heavy use - dressing game, cutting meat, hides, leather, etc. - required frequent touchups to maintain the edge. Every time I've used a cutting tool (machete, axe, sickle, sythe, etc.) it's been a softer steel and it always lost its edge with use. I didn't pay much attention to the particular steels used because they were cheap work implements often locally manufactured from scrounged materials with more art than science. On the other hand my Yarboro knife still has the factory edge and it will still shave anything that'll hold still long enough. Am I reading too much into this? Inquiring minds want to know - Peregrino

[Maytime]

...hardness is the resistance to penetration of the surface of a material by a hard object.

IIRC, hardness is tested with a small diamond punch and a machine that applies a given force, so afterwards you measure the depth of the protrusion (with a computer?). From there hardness is referenced in a big book of numbers or I suppose the computer can just tell you. Mr. Harsey can elaborate or correct me; he knows volumes more than I.

[Bill Harsey]

Originally Posted by Maytime

IIRC, hardness is tested with a small diamond punch and a machine that applies a given force, so afterwards you measure the depth of the protrusion (with a computer?). From there hardness is referenced in a big book of numbers or I suppose the computer can just tell you. Mr. Harsey can elaborate or correct me; he knows volumes more than I.

Maytime, You have already answered this correctly and I have already explained it further in the preceding text. We don't use computers in our Rockwell testers but just an old fashioned caveman analog dial.

Hardness is not the only characteristic which explains edge holding of which abrasion resistance is also a component.