CPM 3V hardness questions for LONG blades.

The billet was 3/16" thick. Could you expand a bit on the slenderness ratio? I'm not sure I know exactly what you mean, and I'm wary of things that sound somewhat self-explanatory.

I've definitely got it 95% by now, and I've been working on hand-sanding it. I'm working on 400 grit now. I probably won't take it higher than 600 grit before HT, and maybe not even that far. Thanks for the advice; I'll definitely keep on working until I get it close to final finish before HT, so there are fewer tears later.
 
Personally, I wouldn't beyond 400 grit, and actually stop mine at 120 pre-HT. The high temper makes an ugly surface so I doubt you gain too much by being fussy before that happens. Truthfully, I don't know because I've always done it the way I've always done it. Maybe I'm just paranoid, but I do worry about warpage during HT, and that might be because my blades start thin and tend to have more metal ground away than most and tapered tangs can be vulnerable, so I never grind beyond 70-80%. After HT I usually start again at 60 or 80 grit to smooth everything and iron out the small warpage twists and bends that can and do happen to mine with some frequency. I'm probably breaking every rule there is on how you're supposed to grind a blade, but I get there eventually. While not as bad as some, 3V is certainly not an easy steel to work. I'm doing a batch of CPM-154 blades right now, having finished a batch of 3V last month. It's like a walk in the park, and my wife says I'm a nicer person when not working with 3V. :)
 
Jerry, do you do your own HT?

Here's a progress pic, since I've been trying to figure out balance and weight. Maple burl scales from Burl source, brass fittings.

 
Cashmore said:
Don't hesitate to do it Crismonfalcon. Dan's a great guy. Last year he spent almost an hour on the phone with me explaining what he knew about the differences between 3v and m4 just because I was interested, and even though I wasn't purchasing a knife from him. He's a wealth of information and just super friendly to boot.
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Would you please elaborate me the difference between 3V and M4? From what I gather here M4 has far superior edge holding while give up little toughness.
 
From the manufacturer:

cpm3vcompgraph.gif
 
M4 has slightly higher edge holding, but 3V is nearly 3 times as tough.

cpm3vcompgraph.gif


Whoops, Dan beat me to it.

For a sword, I'm looking for toughness, but I don't want to sacrifice edge holding too much. S7 is tougher, but it's not even in the same realm for edge retention. CPM 9V might be interesting, but I don't know of anyone who makes 9V blades, and would have no idea on where to source a precision ground billet to try out (or who would be able to heat treat it).
 
Jerry - I switched from *** over to Peter's - haven't had a warped blade since and each blade comes "dimpled" (hardness tested). I've even sent "curved" blades (around 1/16" of deflection) and had them come back straight.

crimsonfalcon07 - slenderness ratio is determinant factor in a steel's tensile strength under a lateral load. (for example, sticking a knife in a vise and bending it 45 degrees). On the surface, this makes perfect sense. A 1" wide bar of 1/8" thick steel will bend more readily than a 1" wide bar of 1/4" steel. Go thin enough (1/32") and you get a fillet-knife-type action (can bend 30 degrees but it comes back on its own). Whereas going thicker (say with a 1/4" thick bar, 1" wide) means that the slenderness ratio and increased and by the time you apply enough force to bend the steel 30 degrees, it "retains shape in memory". Every material has an applicable minimum slenderness ratio. I learned a thing or two about steel during my stint as an architect working with structural engineers. Fun stuff.

Basically, in nut shell, not all steels are equal (yes, i know = duh). They each have a different minimum slenderness ratio. And it changes after heat-treat. 3V is one of those steels that is different than others. It has to do with different ingredients/proportions. If you look into why certain elements are added and how they affect the overall mixture, you'll see that 3V was built to resist deflection. That's part of what makes it so tough.

Dan
 
Thanks for the explanation. That actually makes a lot of sense. I suspect that's true of a lot of materials, now that you've explained it that way. Take wood, for instance. Would the same principle apply to, for instance, bow making? And perhaps that's part of why the tillering process removes more wood from the tips, so they can retain their springiness? Do you have any data on 3V slenderness ratio, by any chance?

As far as the metallurgy, my incredibly basic understanding leads me to gather that you get some rust resistance from chromium, along with some wear resistance, and hardenability, although I'm guessing the molybdenum has more to do with why you can get such high hardness without losing toughness? And the vanadium I'm already a bit familiar with, since I think it makes the grain structure finer. That's about as far as my understanding goes, however, so I'm not sure how the steel is built to resist deflection. Is that what we mean by toughness? I understand toughness to mean something along the lines of the ability to rapidly distribute stress/strain from a sharply applied force.

The other metallurgical question that arises for me is why not just add more elements, like manganese, and tungsten, for even more toughness. I'm guessing it has something to do with the limited amount of carbon available for carbide formation, and how those carbides get distributed structurally throughout the steel? What pieces am I missing here? Thanks for the discussion so far; I've been finding this very educational.
 
crimsonfalcon07 said:
The other metallurgical question that arises for me is why not just add more elements, like manganese, and tungsten, for even more toughness.
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It doesn't really work that way. Think of a tungsten carbide bit or end mill - highly wear-resistant and very hard, but way too brittle for a knife blade.

It's important to remember that a major part of what makes CPM-3V so tough is the fact that it doesn't have a whole lot of alloying elements in it. It's roughly 88% plain old iron - which of course is very tough. The moderate amount of carbon allows it to get nice and hard without becoming brittle, and there's just enough to bring the chrome, moly and vanadium carbides into play for wear-resistance.

crimsonfalcon07 said:
I'm guessing it has something to do with the limited amount of carbon available for carbide formation, and how those carbides get distributed structurally throughout the steel?
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Sort of, yeah. The fine grain (vanadium helps that) and even distribution of small carbides (the CPM process helps that a lot) do contribute to higher toughness, but it's more a case of preventing brittleness than "making it tougher". Simply adding more and more carbides would tend to make the steel even more wear-resistant, but also more brittle (such as D2 for instance, or CPM-M4). It's a question of balance... and 3V is very well-balanced.
 
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I think I understand most of that. Would I be correct then in assuming that more carbides contribute to brittleness then, and you want the minimum number of carbides necessary to accomplish whatever goal you're shooting for, be it hardness or wear resistance?
 
crimsonfalcon07 said:
I think I understand most of that. Would I be correct then in assuming that more carbides contribute to brittleness then, and you want the minimum number of carbides necessary to accomplish whatever goal you're shooting for, be it hardness or wear resistance?
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We may be oversimplifying it a tad, but yeah... for knife blades like we make, yes, I think that's true. I will of course defer to makers like Mr. Keffeler, Mr. Koster and Mr. Hossom, who all have a lot more experience than I do.

That's what I mean about CPM-3V being well-balanced... it has both high toughness and good wear-resistance. Its moderate corrosion-resistance is kind of a bonus, since most of the chrome should be tied up in carbides. I have an article saved somewhere that explains how that works... lemme see if I can find it.
 
This is quoted from a Q&A thingamabob from BLADE magazine a few years back... I'm pretty sure it was Wayne Goddard's column. The original question had to do with why "stainless" steels can still rust in their annealed state, but it also address the fact that in steels with less chrome than is considered needed to make a steel, stainless (D2 in this example), there is still some corrosion-resistance benefit from the "extra" chrome that's not tied up by forming carbides.

Ed Severson of Crucible Steel said:
Following is what he wrote in answer to the questions:
"When you anneal heat-treatable grades of stainless steel, the end result is a ferrite matrix. Ferrite can only dissolve about
0.02 carbon. This means that any grade with more than this amount of carbon will consist of ferrite and some type of carbide.

Take 440C for example, which is 1 percent carbon and 17 percent chromium. Annealed, 440C will have essentially 1 percent
carbon looking for something to combine with, and the chromium is the first thing the (carbon) will go after. By mass, one part
carbon will tie up 10 parts chromium in a carbide. Therefore, the annealed matrix of 440C stainless will have only about 7
percent chromium and that is well below that of the 11.5 percent needed to be truly stainless."

Ed continued: "We get many customers questioning why the 420 and 440 stainless bars they get (from Crucible) have some rust
on them. Annealed, the bars are not stainless. When you heat treat 440C or any 400 series grade, some of the carbide is
dissolved during the austenitizing process, putting enough chromium into the final martensitic matrix to make it stainless."

He answered about D-2: "When you do the balance, D-2 has a little more chromium in the matrix than all the alloy grades and
most of the tool steels. It is the reason D-2 is pretty good for corrosion resistance."
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Hope this helps.
 
I've also used Peters. Might be I'm tapering my tangs or grinding my blades thinner than your are Dan. Doing something wrong...
 
Makes sense - I don't do tapered tangs until after heat-treat (and I rarely do them at all). I also don't do many hollow-ground blades..and the one design I do hollow-grind, is from 0.220" thick stock.
 
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