Grains, Carbides, and You

CPM vs ingot, cryo treated with the same HT protocol at the same facility. Two of the sixteen blades we did the CATRA test on.

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Thanks for those micrographs, Hardheart. It's obvious that the powder steel has much finer, better distributed carbides, but I can't tell about the grain structure. Do you have a sense that the powder steel is finer grained? It looks like it a bit, but the grain structure on the powder steel is more difficult to discern than on the ingot steel, and I can't tell for sure.
 
Yep, spreading out the carbides makes the rest hard to see. I can see how the cpm versiin might be a little finer, but its not obvious. One would have to do the count and math to be sure. Just eyeballing it, both seem to be in the 10 to 25 um range, with a few smaller and a couple bigger sprinkled in for fun. HH can you reveal what steel that is?
 
We have edge-on SEM, cross section images in the resin for angle measurement (also done with a goniometer when the test was run), and metallographic images for all of them.
 
You have a great deal of time and resources tied up in that work Hardheart. If you get it ready for download please let us know. I'd purchase one for sure.

Joe
 
on the spec-metals chart SB wear and PD#1 are the same steel. Probably Cruwear for SB wear and PD #1 is, CTS version in powder form. Both steels are at rc 62 on the chart. I guess that's a decent idea of how powder steel can improve things a bit. Toughness is easy to understand. Why it gives greater wear resistance is still beyond me.

I'm one that actually likes D2 over CPM D2, and ingot Cruwear over powder cruwear. It seems to have better bite on some media I cut, such as rope. I sharpen both to around 200 grit, or if I'm using DMT's the extra coarse, whatever grit that is.
 
I don't actually think the wear resistance is that much better. It is just more even. In fact, one of the benefits of pm steel is it's easier to grind (rough). Wear resistance is tricky. There are some issues with matching the wear environment to the steel microstructure. Wear resistance is dominated by carbide volume and type. Both of these stay the same with PM vs. ingot versions of the same steel. Distribution of carbides and hardness are influential, but not as critical.
 
me2 said:
I don't actually think the wear resistance is that much better. It is just more even. In fact, one of the benefits of pm steel is it's easier to grind (rough). Wear resistance is tricky. There are some issues with matching the wear environment to the steel microstructure. Wear resistance is dominated by carbide volume and type. Both of these stay the same with PM vs. ingot versions of the same steel. Distribution of carbides and hardness are influential, but not as critical.
Click to expand...

Distribution and SIZE are the major factors after volume, type influences size and hardness, but hardness is only critical depending on the abrasiveness of the medium being cut or the narrowness of the apex being achieved. Harder carbides can achieve a thinner apex-diameter and also resist cutting from the abrasive particles. Larger carbides cover/protect a greater area from abrasion, but a random distribution of such carbides can leave vast areas of soft matrix unprotected. As those areas wear down, there is less 'binder' to hold the carbides in place so they tear-out more easily, leaving a large gap of unprotected matrix behind which then quickly wears down until the next set of protective carbides is reached to slow the abrasion. With smaller, well-distributed carbides, you have a lot more protective plates covering the surface or apex (the meeting of 2 surfaces) and able to provide protection to the surrounding matrix. They hold the abrasive particles away from the matrix-surface, and while each carbide is smaller there is also less open space between them that needs protecting, so together they effectively provide more protection of the surface than a single large carbide would provide.

A poor analogy is body-armor: one central plate that covers a single large section of the body vs. many small plates than can be distributed to provide more protection to the entire body. Your own skin is another example - MANY tiny armored cells (corneocytes) vs one large cell.

However, as me2 mentioned, wear-environment is critical to consider. If the abrasive particles in the medium being cut are quite large (much larger than the small distributed carbides), then the carbide "plates" provide effective protection and the abrasive cannot cut into the surface and plough away matrix of gouge out carbides as easily. However, if the abrasive particles are relatively small, they might cut between the carbides and so gouge away the matrix and leave the carbides with llittle 'binder' to hold them in place, resulting in faster wear (though still MUCH slower than if there were no carbides at all). In contrast, large carbides generally require that much more "binder" be removed around them before they lose adherence, and small abrasive particles cannot reach deep enough around the carbide to increase wear, they must first wear-down the carbide itself, so resistance is increased. But if the carbides are quite large, they can gouge deep into the matrix around the large carbide and also impact it with sufficient mass to dislodge it, resulting in much quicker tear-out.

As a curious result of this, one can use the different behaviors to maximize performance not just in use but in manufacture. Large-particle low-grit abrasives are actually more effective at grinding down large-carbide materials vs. small-carbide of the same volume, easing rough shaping of the tool. However, one must then progress to finer abrasives more slowly in order carefully work down the surface/apex in uniform fashion that doesn't leave large craters from carbides tearing out during the low-grit grinding. With small-carbide materials, the behavior toward changes in grit-size is more uniform & predictable. Large particles can chip/flake off large sections of material if the carbide volume is too high, but these are usually less deep so it is easier to polish the surface as one proceeds to finer grits - again, the wear-resistance advantage of small well-distributed carbides.
 
good post:thumbup: Furthermore/augment on the bold part. Oh, not addressing matrix properties nor carbide types/friability/etc...

When the abrasives (contained in cutting material) flow is around normal/perpendicular (such when push cut) to the edge, then matrix space/gap between carbides subject to direct wear by material abrasives. Which under cut the matrix binding to carbide, led to tear out. However in normal cutting motion with back/forth edge movement, the wear/abrasion vector (AV) is similar to trajectory vector - Tan angle (normal force(NF) / edge distance travel (EDT)). The gap between carbides limit the EDT, in turn reduce the NF depth. If NF depth is 2/3 or less than carbide height, over time (repeat abrasion), there still will be sufficient matrix to hold onto carbide.

Carbide tear out mostly cause by shear/lateral force (SF) relative to carbide protruding top/head. When SF strained (resultant force) beyond matrix-carbide bond to exposing carbide structure, tear out taken place. Max SF limits by unprotected/protruded carbide facets. OK, what all these means?

Small and uniform distributed carbides (in compare to sparse of very large carbides)

1. have more matrix binding surfaces. e.g. per 1 earth size volume. 1 earth size surface area is much less than surface sum of many moons.
* For spheroid carbide, impact torque could be as much as power(1/3) due to surface curvature.

2. gap between carbide is smaller, thus providing matrix protection for wider range of AV.

3. less carbide tear out. reasons (lower NF depth, less SF, more binder).

Too hard to compose a more coherent post w/o a ghost/tech writer:p

chiral.grolim said:
Distribution and SIZE are the major factors after volume, type influences size and hardness, but hardness is only critical depending on the abrasiveness of the medium being cut or the narrowness of the apex being achieved. Harder carbides can achieve a thinner apex-diameter and also resist cutting from the abrasive particles. Larger carbides cover/protect a greater area from abrasion, but a random distribution of such carbides can leave vast areas of soft matrix unprotected. As those areas wear down, there is less 'binder' to hold the carbides in place so they tear-out more easily, leaving a large gap of unprotected matrix behind which then quickly wears down until the next set of protective carbides is reached to slow the abrasion. With smaller, well-distributed carbides, you have a lot more protective plates covering the surface or apex (the meeting of 2 surfaces) and able to provide protection to the surrounding matrix. They hold the abrasive particles away from the matrix-surface, and while each carbide is smaller there is also less open space between them that needs protecting, so together they effectively provide more protection of the surface than a single large carbide would provide.

A poor analogy is body-armor: one central plate that covers a single large section of the body vs. many small plates than can be distributed to provide more protection to the entire body. Your own skin is another example - MANY tiny armored cells (corneocytes) vs one large cell.

However, as me2 mentioned, wear-environment is critical to consider. If the abrasive particles in the medium being cut are quite large (much larger than the small distributed carbides), then the carbide "plates" provide effective protection and the abrasive cannot cut into the surface and plough away matrix of gouge out carbides as easily. However, if the abrasive particles are relatively small, they might cut between the carbides and so gouge away the matrix and leave the carbides with llittle 'binder' to hold them in place, resulting in faster wear (though still MUCH slower than if there were no carbides at all). In contrast, large carbides generally require that much more "binder" be removed around them before they lose adherence, and small abrasive particles cannot reach deep enough around the carbide to increase wear, they must first wear-down the carbide itself, so resistance is increased. But if the carbides are quite large, they can gouge deep into the matrix around the large carbide and also impact it with sufficient mass to dislodge it, resulting in much quicker tear-out.

As a curious result of this, one can use the different behaviors to maximize performance not just in use but in manufacture. Large-particle low-grit abrasives are actually more effective at grinding down large-carbide materials vs. small-carbide of the same volume, easing rough shaping of the tool. However, one must then progress to finer abrasives more slowly in order carefully work down the surface/apex in uniform fashion that doesn't leave large craters from carbides tearing out during the low-grit grinding. With small-carbide materials, the behavior toward changes in grit-size is more uniform & predictable. Large particles can chip/flake off large sections of material if the carbide volume is too high, but these are usually less deep so it is easier to polish the surface as one proceeds to finer grits - again, the wear-resistance advantage of small well-distributed carbides.
Click to expand...
 
I don't believe in abrasion wear in modern knives.

I think that most wear is micro and macro deformation, from various sources.

Our hand cutting is not industry cutting. We use different forces, angles, and cut different materials.
Materials are not uniform. We bend, we chip, we roll the edge. We do not use any standard "wear".

Therefore knives with highest hardness and good edge stability works best.
 
idaho said:
I don't believe in abrasion wear in modern knives.

I think that most wear is micro and macro deformation, from various sources.

Our hand cutting is not industry cutting. We use different forces, angles, and cut different materials.
Materials are not uniform. We bend, we chip, we roll the edge. We do not use any standard "wear".

Therefore knives with highest hardness and good edge stability works best.
Click to expand...

Wear isn't the easter bunny.

Abrasive "wear" is microscopic deformation & fracture, even on low-carbide steels. In their case, the steel is being cut and cold-worked to brittleness. Carbides resist deformation due to their high hardness. The higher volume of small carbides, the harder the steel, harder still depending on the type of carbide, but harder = less strain for a given stress = brittle fracture when stress exceeds yield point.

Ceramic or carbide knives? Highest hardness (so least deformation), highest wear-resistance (discussed above), able to achieve the lowest apex diameter (sharpness and edge stability that steel knives are incapable of achieving), but .... low toughness :(

We need toughness in general utility knives to resist chipping, how much depends on application. We need strength/hardness in utility knives to resist deformation (rolling, denting). And we need wear-resistance in utility knives because wear is microscopic rolling & chipping. I experience wear cutting rope and cardboard and wood (planing, scraping), shaving, and animal hide. I experience chipping only when I impact things with the blade, so that is important for my axes & choppers, saws (including chainsaw), and mower blades (including a scythe).
 
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