Friction Forged Blades : CATRA tests

[cds4byu]

Could you ff an entire blade, or FSW a d2 edge to a high chromium spine? Just thinking that avoiding rust on the whole knife might make it useful in some applications

It it possible that we could ff an entire blade, but probably not cost effective.

It is also possible to FSW/FF a high-performance edge to a stainless back. We have done this in the laboratory, but can't yet do it in a production-ready manner. It's on the table for future research.

My personal opinion is that we can find a stainless alloy that will be provide an FF edge of quality similar to or better than FFD2. That's also on the future table.

All of this future work will take place once the generation 1 knives are on the market.

Carl

 

[Cliff Stamp]

My personal opinion is that we can find a stainless alloy that will be provide an FF edge of quality similar to or better than FFD2. That's also on the future table.

It isn't opinion that ingot stainless exists like this now (superior edge stability).

-Cliff

 

[kneedeep]

It isn't opinion that ingot stainless exists like this now (superior edge stability).

-Cliff

Just my opinion, but I think Carl was referring to finding a stainless that will benefit from being processed by the friction forging technique and result in an improvement in the ff'd material along the lines of the improvement they have observed in FFD2 over normally processed D2.

 

[tnelson]

It isn't opinion that ingot stainless exists like this now (superior edge stability).

-Cliff

Cliff,

I think you meant to say "...wrought stainless....", not "ingot". Ingot is a casting [1].

1. Verhoeven, J.D., Fundamentals of Physical Metallurgy, John Wiley & Sons (ISBN 0-471-90616-6) pp. 285.

TN

 

[Cliff Stamp]

... result in an improvement in the ff'd material along the lines of the improvement they have observed in FFD2 over normally processed D2.

Yes, there are materials out now which are already pretty much at the limit of refinement. You don't see CPM 1050 steel for example for a few reasons. The main benefit of friction forging would be to break up the really coarse carbide structure and produce a very fine aus-grain. The Sandvik steels are already like this naturally because of the very low carbide volume. For a similar reason you don't see P/M versions of very low alloy steels, it mainly is used when segregation is very high.

-Cliff

 

[beyondmyken]

Enough of this stuff. I volunteer to take a FF blade with me on my moose hunt this fall and give it a real world test.
In all seriousness, have you thought about FF broadheads for bowhunting?

 

[kneedeep]

Yes, there are materials out now which are already pretty much at the limit of refinement. You don't see CPM 1050 steel for example for a few reasons. The main benefit of friction forging would be to break up the really coarse carbide structure and produce a very fine aus-grain. The Sandvik steels are already like this naturally because of the very low carbide volume. For a similar reason you don't see P/M versions of very low alloy steels, it mainly is used when segregation is very high.

-Cliff

So an ideal candidate for the FF process would be a very stainless steel, with poor edge potential due to a coarse grain structure that can't be (or is impractical to be) refined through traditional forging and HT methods?

 

[Cliff Stamp]

Yes exactly, similar to P/M.

-Cliff

 

[cds4byu]

Yes exactly, similar to P/M.

-Cliff

Similar to P/M, but even better.

1) Grain size is smaller (sub micron)
2) No oxide between the grains, which means higher ductility.

The small grain size increases martensite transformation kinetics and prevents retained austenite. It also increases fracture toughness.

Carl

 

[Cliff Stamp]

On a note of consideration, there are a number of very well respected knife makers who will speak positively about retained austenite, specifically in D2. Not a viewpoint that I share however. The aus-grain in the P/M steels is of course very dependent on how it is heat treated, specificially the rates of heating and cooling.

-Cliff

 

[gator68]

From the other thread...

The term “soft back” doesn’t necessarily mean the back is dead soft. When a bladesmith or stock removal maker does a soft back draw/temper on the back of a blade it is taken to spring temper, usually in the mid 40’s.

Have no fear about the FFD2 blades, the “soft” backs are spring tempered.

...
Wayne G

Two points. It is easy to find examples of bladesmiths today making knives using a clay quench to get a hamon. In this case, the back is unhardened. This is not a spring temper. In any case, is a spring tempered spine necessary for a hunting knife?

The DiamondBlade press release mentions the Japanese clay quench method specifically. For the FFD2 blades, is the back hardened as well as the edge?

 

[cds4byu]

From the other thread...


The DiamondBlade press release mentions the Japanese clay quench method specifically. For the FFD2 blades, is the back hardened as well as the edge?

The FFD2 blades start at HRc 40-45. The FF process is applied to the edge to raise the edge hardness to HRc 67-69, The back remains at 45.

Carl

 

[gator68]

The FFD2 blades start at HRc 40-45. The FF process is applied to the edge to raise the edge hardness to HRc 67-69, The back remains at 45.

Carl

I'm sorry, I don't know the hardness of annealed D2. Is the back hardened (ie, is it martensite or pearlite?) Could you start with a hardened blade (HRC59-60) and then FF the edge?

 

[PatriotDan]

Hardness of annealed d2 is 20 hrc according to Crucible. Most steels seem to be around there 15 - 25 hrc. So ffd2 blade spine is definitely hardened.

 

[Kohai999]

Two points. It is easy to find examples of bladesmiths today making knives using a clay quench to get a hamon. In this case, the back is unhardened. This is not a spring temper. In any case, is a spring tempered spine necessary for a hunting knife?

I already indicated that the DiamondBlade literature stated mid 40's on the back RC. A quick perusal of ferrous spring material, hardened, showed some in the mid '40's. Certainly not dead soft.

Best Regards,

Steven Garsson

 

[cds4byu]

I'm sorry, I don't know the hardness of annealed D2. Is the back hardened (ie, is it martensite or pearlite?) Could you start with a hardened blade (HRC59-60) and then FF the edge?

I imagine so, but we deliberately stayed away from that for increased toughness at the back edge of the blade, to avoid fracture during chopping.

Carl

 

[kneedeep]

I imagine so, but we deliberately stayed away from that for increased toughness at the back edge of the blade, to avoid fracture during chopping.

Carl

So FF'ing an already hardened blade hasn't already been attempted or tested?
When you say "to avoid fracture during chopping" are you generalizing or referring specifically to an FF'd blade?

If what has been posted (here and in other threads) about properly HT'd and tempered high quality steels, with high HRc's being stronger and not necessarily brittle is accurate, then it seems like there would be significant benefits to FF'ing an already HT'd and tempered blade blank. Clearly I'm no metallurgist, so hopefully this comment made sense.

 

[cds4byu]

So FF'ing an already hardened blade hasn't already been attempted or tested?

I'm not sure whether it's been tested with D2. It was tested with either S30V or S90V (I can't remember which), and the steel fractured during the FF process.

When you say "to avoid fracture during chopping" are you generalizing or referring specifically to an FF'd blade?

I'm generalizing.

If what has been posted (here and in other threads) about properly HT'd and tempered high quality steels, with high HRc's being stronger and not necessarily brittle is accurate, then it seems like there would be significant benefits to FF'ing an already HT'd and tempered blade blank. Clearly I'm no metallurgist, so hopefully this comment made sense.

It all depends on your definition of "brittle". All steels, regardless of the alloy, will be stronger as they are harder. Stronger means that it will take more force to permanently deform them. At the same time, all steels will be less tough as they are harder. Less tough means that cracks will run through them more easily. In particular, when steels are too low in toughness, a small defect can cause failure at a load below the yield strength due to crack propagation.

The plot below shows hardness versus toughness for L6 steel. Toughness is measured by Charpy Impact Energy, which is the amount of energy necessary to fracture a 10 mm square specimen when hit by a pendulum.

I'm not proposing that L6 is a great knife steel, but it's a steel I have this data for. If I were making a knife out of L6, I certainly wouldn't want to go harder than HRC62, because the toughness gets too low. And if I were making a machete out of L6, then I might even keep the hardness down to 57, because the toughness is much higher.

So, in my opinion, you want to have the edge as hard as possible to allow you to make a very sharp geometry. But if the edge is too hard, it will have a tendency to chip. At the same time, once you get enough strength at the spine of the blade to avoid bending, there's no benefit to increase the hardness beyond that point, because you're increasing the probability of fracture under impact load.

I hope this helps explain our justification for leaving the spine at 45, and putting the edge at 67 or higher.

Carl

 

[cds4byu]

I'm sorry, I don't know the hardness of annealed D2. Is the back hardened (ie, is it martensite or pearlite?)

The back is tempered martensite.

Carl

 

[Cliff Stamp]

In regards to impact in chopping, the edge takes the most impact so the toughness of the edge is most critical in large chopping blades. The spine hardness is only significant toughness wise if you want to beat on it with a hammer/rock, or want extra ability to bend.

-Cliff