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View Full Version : cpm steels vs. regular steels



olpappy
10-31-2012, 06:46 AM
Many people have commented that cpm steels seem to lose their fresh off the stones sharpness relatively quickly but will keep a working sharp edge for a long time. This is in contrast to standard steels which by inference lose their sharpness in a more linear fashion. Knifemakers, Larrin, Butch, can anyone explain this behavior? If the cpm process gives smaller more refined particles and grain size, I would think they should have better edge stability and be able to hold a very refined edge longer than the regular steel with bigger carbides.

How do the properties of 154CM vs cpm 154, D2 vs cpm-D2 compare? I think I've heard knifemakers say the cpm versions are easier to get a nicely polished finish on the surface of the steel when making knives. This seems to make sense, if there are smaller more evenly distributed carbides the finishing properties of the steel should be more consistent. In what other ways does the cpm process affect the properties of the final product?

bluntcut
10-31-2012, 11:49 AM
3rd generation PM steels marketed as clean and fine grain. The 'clean' is true, while 'fine grain' is relatively speaks. M390 grain is ~1-2microns in size, which is quite large comparing to carbon grain of 50-200nm. PM processes prevent carbides seggregation and other lattice maladies, especially critical for high alloy steels. Without it, there won't be any usable steels with excess 1% of hard alloy carbides.

A year ago, I would readily agree with how PM edge quickly lose its crazy sharp - after head bricking s30v, sg2, etc.. Now, it's no longer the case, my s30v will stays crazy sharp even after cutting cardboard. It's about knowing steel's properties + abrasives + edge geometry.

Regular D2 grain size can be as large as 50microns but average out around 15microns. I do believe SKD11/D2 grain is around 10microns. While CPM-D2 grain is consistent/uniform about 5microns, which make it a lot cleaner and possibly avoid large grain fracture that lead to large chip. However micro-chip is still part of PM life and extra bad for super thin edge. btw - I used to hate my D2 blade and not too fond of skd11 but now I like D2 almost as much as cpm-m4.

my 2cents morning.

G-rat
10-31-2012, 12:05 PM
3rd generation PM steels marketed as clean and fine grain. The 'clean' is true, while 'fine grain' is relatively speaks. M390 grain is ~1-2microns in size, which is quite large comparing to carbon grain of 50-200nm. PM processes prevent carbides seggregation and other lattice maladies, especially critical for high alloy steels. Without it, there won't be any usable steels with excess 1% of hard alloy carbides.

A year ago, I would readily agree with how PM edge quickly lose its crazy sharp - after head bricking s30v, sg2, etc.. Now, it's no longer the case, my s30v will stays crazy sharp even after cutting cardboard. It's about knowing steel's properties + abrasives + edge geometry.

Regular D2 grain size can be as large as 50microns but average out around 15microns. I do believe SKD11/D2 grain is around 10microns. While CPM-D2 grain is consistent/uniform about 5microns, which make it a lot cleaner and possibly avoid large grain fracture that lead to large chip. However micro-chip is still part of PM life and extra bad for super thin edge. btw - I used to hate my D2 blade and not too fond of skd11 but now I like D2 almost as much as cpm-m4.

my 2cents morning.

What changed in your sharpening routine?

bluntcut
10-31-2012, 12:40 PM
What changed in your sharpening routine?

Just to clarify. Fine grain supports edge thinness. While edge sharpness & retention are different.

For high alloy steels (exceed 0.5% of V,W,Nb combined), I use diamond/cbn (occasionally SiC) to abrade the carbides & steel lattice. E.g I mostly use diamond/cbn to sharpen my EDC s30v blade - 24* back bevel, 30* micro bevel. s30v (avg ~2microns) grain size is not practical for sub 30* inclusive angle. It's fine to use non-power non-diamond/cbn abrasives in low grit (particles greater than 12 microns in size) but sub 12 microns abrasives will gradually abrade around hard carbides, thus lead to a weaker apex <= exactly reason why that insane-sharpness go away quickly.

Also knowing (well, I least I deluded/think I know :D) how burr/wire formation affect the edge, would help with the edge sculturing process. Here is my post on another forum link (http://www.bladeforums.com/forums/showthread.php/992879-Apex-Bevel-Geometry-cross-sectional?p=11390828#post11390828). When time permits, I'll post a sharpening video (vg-10 using clay-pot, SiC, ceramic knife spine, rock, ceramic, alox, waterstone, diamond) for creating a durable newsprint slicing edge.

olpappy
10-31-2012, 04:15 PM
A year ago, I would readily agree with how PM edge quickly lose its crazy sharp - after head bricking s30v, sg2, etc.. Now, it's no longer the case, my s30v will stays crazy sharp even after cutting cardboard. It's about knowing steel's properties + abrasives + edge geometry.



Are you saying that before, using bevels less than 15 degrees and abrasives smaller than 12 microns used to give an edge that loses initial sharpness quickly,

whereas now with 15 degree bevels and abrasives not less than 12 microns the edges are holding up better?

bluntcut
10-31-2012, 05:35 PM
Are you saying that before, using bevels less than 15 degrees and abrasives smaller than 12 microns used to give an edge that loses initial sharpness quickly,

whereas now with 15 degree bevels and abrasives not less than 12 microns the edges are holding up better?

I mentioned angle because S30V doesn't support a usable cutting bevel below 15*. Soft abrasives can abrade hard carbides if the abrasives big enough - ~12 micron for hand-power - or abrasives move fast enough (belt/grinder). When freehand sharpening high hard alloy steels at 1.5K grit or higher (below 12microns in size), it's more effective to use abrasives that harder (mohs) than alloys/carbides.

Gator
10-31-2012, 05:38 PM
To clarify a bit more, CPM is a different process than PM. There's also Micro Melt tech from Carpenter, which is also similar. More details here - Modern Steel Making Technologies (http://zknives.com/knives/steels/steeltec.shtml).

From my experience, mainly comparing ATS-34/154CM with CPM154, sharpening is no different, but at high level finish edges, polish, aggressiveness, etc are better...

knyfeknerd
10-31-2012, 05:46 PM
Man you guys are way too smart. You've probably forgotten more about steels than I will ever know!
I appreciate the knowledge for sure.

Larrin
10-31-2012, 08:54 PM
154CM and CPM-154 should both lose their initial sharpness quickly. We're talking about edge stability which is controlled by carbide volume and size. Carbides are the hard particles that contribute to wear resistance but make sharpening more difficult and reduces toughness. Steels with a large volume of carbides lose their initial edge quickly because the edges are only stable when at a certain angle/thickness. After stability is reached the edges last a long time because of the high wear resistance from all of those carbides. Most/all PM steels have a large volume of carbides which is why they're produced with the PM process, to reduce the overall carbide size. This improves edge stability somewhat but the primary factor seems to be carbide volume. This means that PM steels should actually be better, not worse, at maintaining the initial edge. Carbon steels and low carbide volume stainless steels like AEB-L are best at maintaining the initial edge. The long term edge retention, however, is lower.

I would disagree with Gator that CPM or Micro-Melt is different than PM, they're all just trade names. The processes will differ some but it's all powder metallurgy.

Gator
10-31-2012, 09:24 PM
Well, I did say there were all similar :) Companies(Crucible, BU, Carpenter) are quite adamant about them being different in terms of efficiency, results, methods used. But yeah, in a big pic, they're all powder, particle etc...

olpappy
11-01-2012, 07:27 PM
154CM and CPM-154 should both lose their initial sharpness quickly.

Steels with a large volume of carbides lose their initial edge quickly because the edges are only stable when at a certain angle/thickness. After stability is reached the edges last a long time because of the high wear resistance from all of those carbides.

This means that PM steels should actually be better, not worse, at maintaining the initial edge. (than the non-PM version)

Carbon steels and low carbide volume stainless steels like AEB-L are best at maintaining the initial edge. The long term edge retention, however, is lower.




So if I'm understanding correctly, high carbide volume steels might be excellent for less acute bevel angles and thicker toothy edges, but might not do as well on super thin polished edges or very low bevel angles?

Larrin
11-01-2012, 08:20 PM
So if I'm understanding correctly, high carbide volume steels might be excellent for less acute bevel angles and thicker toothy edges, but might not do as well on super thin polished edges or very low bevel angles?
That is correct.

olpappy
11-02-2012, 09:01 PM
Thanks a ton Larrin, this kind of information will be very useful in choosing the right combination of knife/steel/edge geometry/sharpening method for different uses.

Knifefan
11-03-2012, 03:09 AM
Microbevels work wonders on PM steel. I put a primary bevel of about 16 degrees (included angle) on my PM steel knives plus a microbevel at 30 degrees with a few strokes on a #3000 or #10000 grit. This way I get great initial sharpness which the steel can hold reasonably well through weeks of cutting in a home kitchen.

Benuser
11-04-2012, 06:47 PM
Microbevels work wonders on PM steel. I put a primary bevel of about 16 degrees (included angle) on my PM steel knives plus a microbevel at 30 degrees with a few strokes on a #3000 or #10000 grit. This way I get great initial sharpness which the steel can hold reasonably well through weeks of cutting in a home kitchen.
That's the way I sharpen a Nogent from the 1920's.