CPM MagnaCut | Edge Performance

[JRinFL]

Roger, roger DBH/BBB!

[S-3 ranch]

Ok , if the optimum hrc is pinpointed, will it actually be made to those specific places?
IMO, it seems that those numbers are always reduced by a # or 3 , every thing always seems like it’s run soft? On average is production steels run softer than optimum??

[kennethsime]

Ok , if the optimum hrc is pinpointed, will it actually be made to those specific places?
IMO, it seems that those numbers are always reduced by a # or 3 , every thing always seems like it’s run soft? On average is production steels run softer than optimum??

I'm sure that Spyderco is conducting their own tests on the steel to try to determine what an "optimum" heat treat will be.

I would venture to bet that their version of "optimum" would differ from your definition of "optimum" if you feel that their steels are usually run too soft. Most enthusiasts would prefer harder steel, because it will have high edge retention. Most makers would prefer softer, because they know all the reasons why we can't have nice things.

[James Y]

Perhaps because beyond a certain point, a higher and higher HT may create more brittleness and/or difficulty of maintenance?

Also, many do prefer something a bit easier to maintain.

Jim

[Vaugith]

I've read that on some steel compositions higher HRC also results in lower corrosion resistance. Is this true of magnacut? If so, Spyderco aiming to make this a salt steel could result in them choosing a different heat treat than optimal for edge retention or apex stability.

[JRinFL]

From Larrin's article (https://knifesteelnerds.com/2021/03/25/cpm-magnacut/):

"There is a tradeoff between hardness and corrosion resistance, explained in this article on Vanax. Steels like Vanax and LC200N have excellent corrosion resistance but are limited to about 60-61 Rc even with cryo treatments. This steel was targeted for a good combination of high hardness and corrosion resistance, though the corrosion resistance ended up better than expected as will be discussed further on."

So, I think it would be possible that Spyderco chose to run it a little softer to bring the corrosion resistance up. We won't know until we have the Native 5 MG in our hands.

[Matus]

I don’t think that was the point that Larrin was trying to make. I think he meant that steels like LC200N were developed with rust resistance in mind, while MagnaCut was trying to also be able to provide higher hardness while still being very resistant to rust. I would not expect any particular depends of rust resistance of MagnaCut on its hardness (once hardened that is)

[JRinFL]

I'm taking to mean that MC has the same tendencies as other steels in that corrosion resistances drops with increased hardness, but that the drop off with MC is less severe compared to other steels. I may very well be reading it incorrectly.

[Matus]

Or maybe just my knowledge on the matter is even worse than I made myself believe. The more I think about it, the more probable it seems.

[Ramonade]

Things just got serious !
Happily waiting to see what comes out of this (and soon, waiting with my 2 mules cutting in the meanwhile, hopefully) !

[Larrin]

Corrosion resistance vs hardness is when comparing between different steels not within a given steel. The tradeoffs for heat treating for high hardness usually don’t involve a reduction in corrosion resistance. One of the major ways to increase hardness is by austenitizing higher which also provides increased corrosion resistance.

[Matus]

I feel a little better now, thanks Larrin

[JRinFL]

I stand corrected and perhaps a bit more confused.

[Larrin]

I stand corrected and perhaps a bit more confused.

This is too much to explain effectively in a forum comment.

Corrosion resistance is controlled most by chromium “in solution.” That chromium forms an oxide layer at the surface that prevent iron oxide (rust). More chromium in solution means better corrosion resistance.

Hardness in knife steels comes from how much carbon is “in solution” in the quenched martensite. More carbon means higher hardness.

Why not just add more carbon and more chromium and get high hardness and high corrosion resistance? (Dumb metallurgists never think of the obvious). The limiting factor for hardness in high carbon stainless steels is retained austenite. Too much carbon and alloying elements in solution and during quenching the steel doesn’t fully transform to martensite and still has some austenite left over; austenite is the high temperature phase of steel prior to the quench. “Retained austenite” reduces hardness and excess retained austenite is bad. Chromium is one of the alloying elements that also stabilizes austenite, so a high corrosion resistance steel with a lot of chromium in solution is more limited in hardness.

I could say more but instead you guys need to read more Knife Steel Nerds and Knife Engineering I can’t re-write the whole book here.

[JRinFL]

I stand corrected and perhaps a bit more confused.

I could say more but instead you guys need to read more Knife Steel Nerds and Knife Engineering I can’t re-write the whole book here.

I appreciate your unexpected reply. I know where to find the knowledge you have provided, but I’m out of my depth and just hoping to barely keep up with the baggage train.

[S-3 ranch]

I stand corrected and perhaps a bit more confused.

I could say more but instead you guys need to read more Knife Steel Nerds and Knife Engineering I can’t re-write the whole book here.

I appreciate your unexpected reply. I know where to find the knowledge you have provided, but I’m out of my depth and just hoping to barely keep up with the baggage train.

Yep ,kinda over my head , I need the Clift notes version, like you can probably have a working edge after cleaning a elk or hog or if a cow goes poo on it and I don’t find it till next week
You can just wash it off and don’t need any fitze to get some rust off , before You cut my sandwich in 1/2

[Bemo]

S-3, you're a treasure.

[Fireman]

BBB aka “knife mythbuster” Some tests I have seen give higher cutting results after a few sharpenings, supposedly to get rid of “Burnt” steel from the grind process. Will you be willing to retest after a few sharpenings to test this theory? I am doing some testing with my mules but have not sharpened mine enough yet and your tests are more credible than mine will be.

[Beanie-Bean]

Outstanding post, Shawn! Thank you for the work you did to show us the MagnaCut performance. Your presentation wsa very informative.

[Deadboxhero]

Here are the test results for the pm2s that I tested with this rope cutting method.



The objective of this testing was to observe the loss of "front end sharpness" or "initial sharpness" and the transition into the working edge between these steels on the same model as you can buy them off the shelf all with a 14°degree per side, 400 grit cBN edge finish that has been carefully deburred without stropping and confirmed with microscope prior to testing.

All of these were hardness tested using my calibrated hardness tester using the ASTM e18-20 standard for best practices.

A disclaimer about the testing.

This test is a stand alone set of data and should not be compared to other tests. I did multiple test runs and ran an average of BESS scores, edge dulling is not consistent in real world and there is less consistency as the edge wears meaning more BESS tests are needed to average from as the edge gets duller.

These results will be different from the mule team because I spent more time retesting and running more averages on the BESS testing especially at the end of the test.

With the MagnaCut mules I only needed enough resolution to see a clear difference between hardness between those two in that data set.

We can retest those in the future if I have more interest and time to do so.



Also the mule knife is not pm2, the profile of the mule team edge doesn't have as much curvature as the PM2, so the dulling may be different which is something that can be explored further and ruled out in the future.

There were some effects of edge fatigue which made it so I had to retest especially on some of the knives more than others but that's for another discussion.




This test should be looked at as how these specific knives/batches line up.

There will be variation between batches in the same steel and even within the batch when it comes to hardness.

I think folks need to really respect the volume these are made at, which will always be a trade-off for precision, and exactness but at the same time custom work is not as scalable as production work and you have to appreciate the value you can get these knives at which are also more available than custom knives.

So none of these should be thought of as fixed points to say these steels will always line up like this.

Some of these knives will be harder or softer within the tolerance.

So we should look at these values as to how these specific knives line up with each other.


Here are the hardness results.







The MagnaCut Pm2 I have here is not a full production model yet, this was part of a small experimental batch and will not reflect the actual production MagnaCut that will be available in the future. So, hopefully those reading here have attention to detail and can put this into context if not I won't share such detailed testing in the future if it's too much for the lay person to handle.

We have already seen that the Mule team has a higher hardness range and there is potential to see even higher hardness for increased edge retention in the future which should take more advantage of synergizing MagnaCut's attributes.

The CPM Cruwear Pm2 Micarta is significantly harder, even harder than what has been run on past CPM Cruwear knives. I would look for this model specifically to get this hardness range rather than some that were made in the past.


The M390 from DLT red g10 was also surprisingly hard, I've seen past batches of M390 Pm2 knives run at 61-62rc more. I am not sure if this is an outlier in the batch. I didn't heat treat it and I don't have the time and temperatures used so I can't comment further.

The Elmax and the s45vn Run slightly harder than the s30v we've seen in the past which is pretty exciting. Yet, it would also be interesting to see s30v run harder as there have been slight modifications to its chemistry since its first inception that haven't been advertised such as the nitrogen addition.

It's nice to see the 52100 with a respectable hardness, I saw another 52100 Pm2 knife test at 62.8rc well the carbide volume is low and the iron carbides are soft higher hardness is better for taking advantage of the increased edge stability 52100 can offer.



In Dr Larrin Thomas's excellent book "Knife Engineering" on pg 81 there is a great graph of the CATRA testing showing cards cut over cutting cycles, the three steels here are drastically different from each other you'll often see claims of steels like AEB-L holding higher front end sharpness.

The graph here shows that they seem to dull at the same rate until we get into the working Edge which then shows more separation as the carbide volume and type in the matrix start to take over more.

It was my hope to explore this area more.



My testing will focus on this red circle area which will have less separation than if we tested further into the working edge. At 127 cuts in 3/4"manila rope we start to transition into the working edge.




A working edge is an edge that will no longer shave or cut paper as cleanly or consistently. We notice this transition around 300g-400g BESS
The hair shaving edge starts to drop off at 200g with the edge no longer able to scrap shave thin arm hair at 300g BESS. Yet when closer to 250g the ability to scrape shave remains.

This is specific to a 14°dps 400grit edge. so the values are not universal, if we change angles and finish than when we stop shaving hair etc will change. That is something to explore at a later time.


Here we can see the increase of grams needed to cut the BESS testing media over the number of cuts to a total of 127 single slice cuts in 3/4" manilla rope at 12% moisture. BESS testing was done at logarithmic intervals to watch the increase in weight needed to cut the media grow with more cutting volume.

The increase in weight need to cut the BESS media can be thought of as loss of sharpness.







Intial Sharpness decreased rapidly in the first 25 cuts with more separation in sharpness from here.

The strongest factors for retaining initial sharpness or front end sharpness is hardness and carbides. If everything is sharpened the same.

These results should also be thought of as specific to these knives not all the M390, MagnaCut etc and also specific to a 14°dps with a 400 grit metallic bonded cBN finish.

If somebody is experiencing a difference from this result I'm concluding it is because of how they are sharpening not the steel name.
There are significant factors that people need to rule out for there experience, use anecdotal accounts and testing are.


1. Edge angle- which is almost impossible to do
without using a laser goniometer a difference in 1° dps makes a huge difference in edge retention/durability if you want to compare things.

2. Abrasive type- I strongly feel its important to cut all phases inside the steel matrix.

3. Deburring and fatigue- the most difficult part is making sure that the edges are properly deburred and not fatigued.


It was interesting to see that there wasn't much separation between MagnaCut, S45VN and Elmax in Intial Sharpness if the hardness was similar.

If we increased the total cuts in rope past 127 we will see more separation however the focus of this testing is on loss of initial sharpness not pure endurance.

The S45VN being slightly harder on this exact knife compared to Elmax and MagnaCut im sure played a role in it being slightly better, MagnaCut has the lowest carbide volume next to 52100 which improves its toughness but not its resistance to wear on cutting rope.

Hardness is important for resistance to deformation so while MagnaCut does have an advantage with its finer carbides for increased edge stability at 61.4rc on this experimental knife it wasn't hard enough for it to shine.

MagnaCut should be run harder than 61.4rc to see higher performance in edge retention and resistance to edge deformation if used on a small pocket knife made to cut like the Pm2 at 14dps with a 400grit finish to cut rope which will translate to higher edge retention with anecdotal accounts in real world use.

It's very interesting looking at the 52100 the Cruwear and the m390.

While the 52100 and Cruwear were significantly harder the m390 still maintained its Intial Sharpness better thanks to its higher carbide volume.

52100 has the softest carbides at the smallest volume which is good for edge stability but not for edge retention. While some have claimed the front end sharpness was higher for them that seems to be due to sharpening technique, skill and stones used rather than the steel itself, 52100 is more responsive to mediocre sharpening.

It should be noted that the 52100 took more force to separate rope near the end of its 127 cuts will have to quantify this using a load cell at a future time.

Conversely the M390 took the least amount of force to cut rope near the end of its 127 cuts.

I was very surprised to see how well the Cruwear did but it was the hardest out of all these knives and that something more specific to this batch rather than all Cruwear on previous models and batches.

Hardness is an important component for front end sharpness/Intial sharpness. Along with carbides, their type and volume.

What increased hardness does is if done properly is it translates into resistance to deformation with high performance cutting geometry.

Resilience often doesn't get as much love as toughness it seems.

So seeking knives that have the highest toughest levels like S7 and CPM 1V, 3V will not be great for edge retention unless the primary mode of edge loss is due to sudden shock/impact, which is something that can be explored at a later time.

We can explore the edge durability between these knives at another time.

When production Spyderco MagnaCut comes in a Pm2 we will see it harder which will be interesting to test when the time comes.

Please forgive me for any typos here.