Fine Grain and Edge Holding

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me2

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Oct 11, 2003
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I've asked this on other forums. I'm just expanding my sample size.

Can anyone explain the mechanism that is supposed to allow finer grained steel to cut longer? I've seen several makers tout their blades as having longer edge holding due to extremely fine grain structure, but for the life of me can't figure out a mechanism for it, at least not one attributable to fine grain alone, or in a direct path.
 
Finer grains give better edge stability, resistance to lateral forces. Imagine two batches of concrete, cured to the same psi hardness. The difference is that one is made with pea gravel and the other with 2" river rock as the large aggregate. Now grind a knife edge on each concrete sample. The one with the pea gravel is stable closer to the edge because the smaller gravel (analogous to carbides) is better supported by the mortar matrix (analogous to the iron matrix of cutlery steel) than are the river rocks.
 
Those both sound like they refer to the carbides, not the steel grains themselves. I'm talking about the matrix itself. I've used a similar example before, but it was clay and marbles vs. clay and glass beads and how thin can you roll them. Fine carbides are required too, and I really like the Sandvic knife/razor blade steels. Easiest to get razor sharp that I've seen, as expected, since some are actual razor steels.
 
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in sharpening, you actually erode each individual exposed crystals (those right at the edge.) intuitively, you will know that larger grains will erode producing a rougher outline and texture compared with a fine grained or uniform substance. the edge of the sharpest surgical diamond scalpel looks pretty much the same at 500X magnification, whereas that of the best surgical stainless steel looks something like the andes mountain range.
 
IIRC, according to the metallurgists, finer grain steel (steel, not carbides) can be hardened to a higher hardness and still maintain a given level of toughness. It's the higher hardness which can give a finer grained steel better edge retention, rather than the grain fineness in and of itself.
 
Never heard of grain size contributing to edge life. It improves toughness, and going too far in refining grain size through thermal cycling reduces hardenability. Though I also don't know why grain size is continuously mentioned as something PM does, it reduces carbide size, grain size is determined in heat treat.
 
Wouldn't the bonding of the particles themselves come into play? I do understand that a coarser grain would grind off into an edge that looked like a mountain range at 1000X and finer grain would look obviously smoother. I also have seen enough educated evidence on this forum that a smooth fine edge cuts better than a toothy edge. But does it stay sharper for longer? What are the factors other than the grain size itself?

Bonding is a result of the chemical composition of the blade and its heat treating, I assume. And wouldn't bonding be a factor of hardness and toughness? A badly bonded fine gran steel v/s a well bonded steel in coarser grain?

Another question I have is on the relation between hardness on a rockwell scale and edge retention. Is a harder blade necessarily a better edge holder?
 
knarfeng said:
IIRC, according to the metallurgists, finer grain steel (steel, not carbides) can be hardened to a higher hardness and still maintain a given level of toughness. It's the higher hardness which can give a finer grained steel better edge retention, rather than the grain fineness in and of itself.
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Thats the best explanation I have seen. When sharpening, I dont think the grains are removed as the edge is sharpened. There is the infamous carbide tearout, and coarse enough abrasive can certainly take off entire grains, but the grains can be cut without removing them.

Some makers who promote extremely fine grain structure point to it as a reason for increased edge holding, which may well be true. I've just never heard of the mechanism that causes it.
 
me2 said:
Some makers who promote extremely fine grain structure point to it as a reason for increased edge holding, which may well be true. I've just never heard of the mechanism that causes it.
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I couldn't see how that would work. In my mind, any nonuniform aspect of the steel would be indicate a point of increased stress around it. Just like how in a knife with a rat tail tang would be weaker than a full tang knife with a blade and tang that is equally thick to the tang in the rat-tail tang
 
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