How does grain size affect edge holding?

M

me2

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Oct 11, 2003
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I've heard some makers say the very fine grain structure they get from forging/heat treating is the reason for their blades edge holding ability. Has anyone done a comparison with grain size being the only variable to see if this is true? If so, can you describe the mechanism that lets finer grain size contribute to edge holding? I'm speaking here about grain size only, not carbide size. They are different things and the effect of carbides on wear resistance and edge holding is more widely known in general terms.
 
As i understood, edge losing is in fact loss of grains on the edge, as the "weekest link" of the blade is grain borders and not grain itself. So the bigger are the grains, the biggest is the "chip" left. But grain refinement is much more a matter of heat treatment and alloys in the steel than processing method (forging). This is what i came to believe, but i'm nothing of a fully qualified metallurgist.
 
Small grain = stronger steel. If the grain is too large, the fine cutting edge will just crumble away.
Grain size is very important in blade steel..
 
I'm sure its very important, but is there anything out there that shows that being finer than average contributes directly to edge holding with all other factors equal. The general question I'm trying to answer is will 2 blades from the same bar of 1086, one with a grain size of 9 or 10 and the other with a grain size of 14 or 15 hold an edge the same or will one be ahead of the other. I have no way to control grain size well enough to test this myself, so I figured someone would have tried it here.
 
I've never really tested it but have teasted a lot of blades. I do know the 1086 blade with the finer grain will hold and edge longer. Just common knowledge.
 
Yes, in every book of metalurgy you can read that finer the grain, better the mechanical properties. As it's also very known that the most martensite in the steel increase mechanical properties. We don't really know why it's so, it's mostly based on experiment, not on theory. Edge holding ability as such have never been an issue serious enough to be really studied, but it's indirectly studied for dies, i guess, which have sometimes relatively acute angles that have to stay sharp. If "general" metalurgy tell us that fine grain improves for exemple wear resistance, resilience and toughness, we can believe it's good for edge holding of knives also.
 
Yeah... If I understand this right, seeking a steel that will have a fine grain as well as various carbides when properly HT'ed is exactly why certain alloys were developed in the first place. For instance, I've read that vanadium is added to some steels because it makes very hard carbides, and also helps keep the overall grain fine. I've seen a few broken dies/tools in my past career as a pressman, and they all showed a definite large grain at the break. I can only guess, but I think that indicates improper heat-treatment.
 
You should get in tough with razorsharp44, he is an employee of Sandvik. They also have a decent website on heat treat & cutlery steels.
 
I havent read anywhere that fine grain increases wear resistance. It's well documented to increase strength and toughness. So if I'm using 5160 or 1080 or whatever, then going through all the hoops to get the ultrafine grain structures, like was done in the Verhoeven book, will yield a longer lasting edge vs a blade of the same steel not done that way?
 
Wear resistance is primarily due to carbides .
Part of the grain size effect may be spreading out the bad elements that collect in the grain boundaries. I'll see if I can find some sources for you.
 
I heard somewhere that the forging process actually increased the grain size.:confused: It was a matter of the heat treatment that came after that which refined the grain structure. But I don't know if that is true or not.
 
High heat during the forging process increases grain size, not the forging.
Reducing heat toward the end of the process refinds grain, as does normalizing after forging.
 
Failure, which is what causes your edge to go away, can be broken into two types, fracture and yielding. Yielding is essentially permanent deformation as a result of a load. On the atomic level, yielding occurs along planes of closest packing in the crystalline lattice. Different grains frequently have different lattice orientations, so it's unlikely that too many of these slip planes will line up between different grain boundaries to allow for easy deformation. Essentially, much of the force required for permanent deformation is used to re-orient grains. Since smaller grains equals more grains per given area, that also means more force has to be applied to make many different grains line up in a fashion conducive to deformation.

So basically, although counter-intuitive, grain boundaries make polycrystalline materials much stronger than just a single crystalline lattice, which leads to greater edge retention. As for wear resistance, I don't know how or if finer grains affect it. I'm also not entirely sure how grain size affects fracture mechanics although I would imagine it's roughly along the same lines as yielding, but I'll look more into it later.

Most of the information in this post was paraphrased from The Science and Technology of Civil Engineering Materials by Young, et al.

This stuff is understood on a theoretical level, to a degree.
 
Yes, this could all get a little complicated. Edge rolling, the deformation you describe Destraal, seems the most common reason for dulling. As thin as the edges get, even 64-65 HRc steels will see edge rolling, since there is such a small amount of steel that the yield point is passed even with low force. This, I think, is the reason Goddard found that hardness and not wear resistance has such a strong effect on edge holding. I was also thinking about indirect effects of a fine grain size, such as in a chopping knife. A finer grain size gives better toughness, which leads to longer edge retention for some chopping applications where dulling may be mostly caused by microchipping.

Just to clear things up, I'm assuming that one blade has a "normal" grain size, somewhere around 8 to 10 on the ASTM scale, while the other has what would be classified as "ultrafine" size, in the 13-15 range of the ASMT scale. Some say this fine size makes the knife, others say past what is the norm, its just wasting time and effort for negligible returns.
 
I can see fine grain size improve toughness, but edge holding? If anything, smaller grains can decrease strength (hardness), as smaller grained steel requires a faster quench to get full hardness, so I see it as a disadvantage. Grains are just crystals oriented in a specific direction, and you can cut through them. So I'm not sure finer-grained steel gets sharper, either.
 
Karl B. Andersen said:
Why do you see requiring a "faster" quench as a disadvantage?
Click to expand...

I did multiple oil quenches for 1084 to get the fracture grain size down to 15. Then I found out the blade was only 50 rc. Apparently I had to use a fast water quench, which worked great but introduces the risk of cracking. It's a bit of a disadvantage.
 
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