Knife Steels-Material Science

[Bill Harsey]

Originally Posted by Peregrino

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 Yarborough 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

Looks like we better figure out simple knife shop and user definable hardness in tool steels before we go any farther and I will do that later.

There are knives harder than the Yarborough that will not hold an edge as long.

The answer to this can be found in the preceding text.
Edited to add:
Or maybe not, after review I note it was alluded to it but not explained.
Class?

[HOLLiS]

Originally Posted by Bill Harsey

Most files are designed to cut steel and have a very high carbon content for maximum hardness. This makes them hard to work (make into a knife) and they tend to be brittle in hard use.

A better answer is yes they will make a good knife for some applications but the difficulty of working the file into a usable knife blade would put it down on my list of materials to use.

I knew it was in this folder somewhere.

[Bill Harsey]

Hollis, That's close but there's more than just Iron carbides in play here.

[Peregrino]

OK - I tried the "search" button and did a little deductive reasoning. Are we thinking in terms of exotic carbides e.g. the vanadium carbides in the S30V and the CPM154? As in the supporting iron matrix having a lower hardness number to give it greater toughness/shock resistance and the sharpness/edge holding characteristics being derived primarilly from the exotic carbides? As best I can grasp it - the only way to accomplish this is with the Crucible steels or some similar sintered metal process. Peregrino

[Bill Harsey]

Originally Posted by Peregrino

OK - I tried the "search" button and did a little deductive reasoning. Are we thinking in terms of exotic carbides e.g. the vanadium carbides in the S30V and the CPM154? As in the supporting iron matrix having a lower hardness number to give it greater toughness/shock resistance and the sharpness/edge holding characteristics being derived primarilly from the exotic carbides? As best I can grasp it - the only way to accomplish this is with the Crucible steels or some similar sintered metal process. Peregrino

Peregrino, Pretty good thinking for a gunslin... Uh- I mean Good job.

Yes it is the hard carbides suspended in the matrix that give this steel and others like it the "high performance" edge holding.

A Rockwell hardness test involves the entire steel, matrix included and this test cannot show the hardness of the individual carbides.

Vanadium carbides are harder than the aluminum oxide that makes up sharpening stones. This is one of the carbides responsible for the high performance of the CPM S-30V steel.

Note, CPM S-30V is NOT a sintered metal even though a step in the manufacturing process might suggest that. It is a 100% solid steel.

[Bill Harsey]

The fastest simple time honored hardness test is to grab a steel cutting file and file on the work piece.

Most steel cutting files are in the Rockwell "C" Scale 62 range.
This means if the file cuts the test piece easy it is softer than R "C" 62.
If the file can't cut anything and just skids off the surface of the test piece, it is harder.
If one tests many pieces of tool steel this way it can become possible to "feel" the subtle differences.

Before I take my CPM S-30V blades to the Rockwell tester, I file them (carefully over the unsharpened edge) to see if I can guess the hardness.

The last time I did this almost caused panic because I thought I was going to have to do the heat treat over again for not being hard enough but the blades tested right at R "C" 58.5, right in the hard use hardness range we like.
There's a lesson in this story too.

[HOLLiS]

Originally Posted by Bill Harsey

The fastest simple time honored hardness test is to grab a steel cutting file and file on the work piece.

Most steel cutting files are in the Rockwell "C" Scale 62 range.
This means if the file cuts the test piece easy it is softer than R "C" 62.
If the file can't cut anything and just skids off the surface of the test piece, it is harder.
If one tests many pieces of tool steel this way it can become possible to "feel" the subtle differences.

Before I take my CPM S-30V blades to the Rockwell tester, I file them (carefully over the unsharpened edge) to see if I can guess the hardness.

The last time I did this almost caused panic because I thought I was going to have to do the heat treat over again for not being hard enough but the blades tested right at R "C" 58.5, right in the hard use hardness range we like.
There's a lesson in this story too.

I have use the file test before, it is easy and often available to most people.

[Bill Harsey]

Originally Posted by HOLLiS

I have use the file test before, it is easy and often available to most people.

It will tell you a lot, like can I drill or machine this steel in this condition?

Did the heat treating make any changes, hopefully for the better?

[gunnerjohn]

I believe the answer to if you can drill or machine the steel in the hardened steel is maybe. If you have rigid enough of a set-up and properly designed tooling with TiAlN coating, you will be able to cut the hardened steel. You may not be able to work it very quickly and you may burn up some expensive tooling, but yes you can cut it. With some of the carbide drill bits you would be able to get into the surface some before the bit melts.

Note:
TiAlN coating is Titanium Aluminum Nitrite ceramic coatings applied the the surface of the carbide tool. This allows many steels to be cut like it was aluminum. The coatings actually "like" it when they get hot and have constant friction. It was developed for Boeing to ease the machining on all of the titanium and tool steels they use.

[Bill Harsey]

John, Great technical data but I wasn't making it that complicated.

I wanted to use the file to figure out if the steel in question was simply hard or soft.

[Bill Harsey]

Guys and Gals,
We've laid down enough stuff so far to make common folk hate reading about steel.
Good thing we don't have any common folk around here.

You've noticed in this thread that a concern when heat treating hardenable steels is cracking if we cool it too fast from the initial hardening temp. Welding heat can cause this too.

Here is a tip to lock down and print out for any of you guys and gals are out in the field that might have to make an emergency weld on unkown steels.

Pre-heat your weld zone (the steel) to 300 degrees F. Use your best judgment. Make the weld and if possible, post heat for a couple minutes with the same torch or heat source. DO NOT COOL WITH WATER!

This will greatly increase the chances of the weld not cracking on castings, shafts, springs, etc. Use 7018 Low Hydrogen rod on DC Reverse polarity of possible. If your welding on cases or around things that have bearings in them, place the ground clamp within 6 inches of the weld, either coming or going as the magnet will deflect the arc.

This stuff is so good I should charge money for it.

Edited to add:
AmbushMaster, I have no technical data on using various and interesting alternative heat sources for a controlled pre-heating of steel that may be readily available to the Special Forces Soldier.

[Ambush Master]

Originally Posted by Bill Harsey

Guys and Gals,

This will greatly increase the chances of the weld not cracking on castings, shafts, springs, etc. Use 7018 Low Hydrogen rod on DC Reverse polarity of possible. If your welding on cases or around things that have bearings in them, place the ground clamp within 6 inches of the weld, either coming or going as the magnet will deflect the arc.

This stuff is so good I should charge money for it.

What do you think about "Square Wave" Machines?!?!

[Bill Harsey]

Originally Posted by Ambush Master

What do you think about "Square Wave" Machines?!?!

I've never had the privilege of welding with that type of welder. All my welding has been with "tough guy" welding machines that do not have the exotic manipulation of the electrical current that you big city kids like.

[LibraryLady]

Mr. Harsey,

You mentioned in another conversation, knife makers like the waterjet vs the laser as there's no heat residual when working with the high end "tough stuff". So if you cut the knife blank, (and I'm assuming you do this first) then go through the Martensitic Transformation process, won't that negate any of the heat issues from the laser? Or does repeated applications of heat affect the carbon atom structure development?


LL

[Peregrino]

Originally Posted by LibraryLady

Mr. Harsey,

You mentioned in another conversation, knife makers like the waterjet vs the laser as there's no heat residual when working with the high end "tough stuff". So if you cut the knife blank, (and I'm assuming you do this first) then go through the Martensitic Transformation process, won't that negate any of the heat issues from the laser? Or does repeated applications of heat affect the carbon atom structure development?


LL

Looks like we're going to have to watch this one. She lurks quietly for "a while", does her homework before posting, follows the rules, and all of a sudden - out of left field, detonates a claymore. Got to watch them librarian types - the ones I know suffer from insatiable curiosity and a true "garbage collector" mentality (always learning little bits and pieces "cause you never know when it might be useful"). Nice question though. I wouldn't mind knowing the answer to that one myself. JFTFOI - Peregrino