A question of principle? Warning sharpening!!!

[Lasse Hilbrandt]

Im having a discussion on another forum with a guy who claims the only parameters in sharpening is the angle and the smoothness of the edge. So far I agree. But he also claims that all steels are able to achieve the same smoothness of the edge. Its only a question of difference in how difficult it is.

Im sure Im right, but how do I dokument it?

[Patrick Chase]

Im having a discussion on another forum with a guy who claims the only parameters in sharpening is the angle and the smoothness of the edge. So far I agree. But he also claims that all steels are able to achieve the same smoothness of the edge. Its only a question of difference in how difficult it is.

Im sure Im right, but how do I dokument it?

One word: Microstructure.

Steel isn't homogeneous, but instead has a grain structure. That's particularly true for hyper-eutectoid tool steels, i.e. ones with Carbon content higher than ~0.8%. In those steels the excess Carbon forms carbide particles, which are dispersed throughput the metal matrix. The size of those particles depends on the composition of the steel and its processing. When you sharpen, and particularly at low edge angles, those particles tend to come loose along the edge leading to a rough edge.

Steels with fine structure and small carbides (if any) are typically regarded as easy to sharpen and as taking fine edges. Steels with coarse structure and particularly ones with large carbides are difficult to sharpen (the carbides make them very abrasion-resistant, which is one of the reasons why high-alloy, high-carbide steels exist to begin with) and take rougher edges.

The sharpening medium matters, too. If the medium is softer than the carbides then there is no way to avoid chipping of the sort I described above. In that case sharpening merely erodes the metal around the carbides until they fall out, leaving chips in the edge. That's why JNats and Arks (SiO2) get poor results on relatively coarse-grained, high-carbide steels like A2 or D2.

Conversely if the medium is significantly harder than the carbides then there is some chance that it will abrade and sharpen the carbides in situ instead of knocking them out. This is mostly applicable at higher edge angles, because at lower angles the carbides are poorly retained in the matrix, and the forces applied by sharpening will tend to knock them out regardless of how hard or soft the abrasive is. Even if you did manage to get a clean, low-angle edge the carbides would chip out the first time you tried to use it. That's why I exclusively use diamond media and fairly high angles (>=35 deg) on non-PM high-alloy steels, but even so I can't match the edge quality I get from O1 (or a PM steel like PM-V11).

Ron Hock has posted some grain shots comparing O1 and A2 excerpted from his book. There are links to more in his Internet references post for his sharpening book.

[Todd Stock]

...and all women are exactly the same because they have a pair of X chromosomes. Sheesh...

[Malcolm Schweizer]

Ron Hock says it best:
http://hocktools.com/tech-info/toolsteel.html

[Frederick Skelly]

Ron Hock says it best:
http://hocktools.com/tech-info/toolsteel.html

That was good readin'.

[Patrick Chase]

Ron Hock says it best:
http://hocktools.com/tech-info/toolsteel.html

That was good readin'.

If you want to go deeper then the posts from Hock's blog that I linked in my previous post are excerpted from his sharpening book. They have most of the content from the chapter he references at the bottom of the page Malcom linked.

[Stanley Covington]

One word: Microstructure.

Steel isn't homogeneous, but instead has a grain structure. That's particularly true for hyper-eutectoid tool steels, i.e. ones with Carbon content higher than ~0.8%. In those steels the excess Carbon forms carbide particles, which are dispersed throughput the metal matrix. The size of those particles depends on the composition of the steel and its processing. When you sharpen, and particularly at low edge angles, those particles tend to come loose along the edge leading to a rough edge.

Steels with fine structure and small carbides (if any) are typically regarded as easy to sharpen and as taking fine edges. Steels with coarse structure and particularly ones with large carbides are difficult to sharpen (the carbides make them very abrasion-resistant, which is one of the reasons why high-alloy, high-carbide steels exist to begin with) and take rougher edges.

The sharpening medium matters, too. If the medium is softer than the carbides then there is no way to avoid chipping of the sort I described above. In that case sharpening merely erodes the metal around the carbides until they fall out, leaving chips in the edge. That's why JNats and Arks (SiO2) get poor results on relatively coarse-grained, high-carbide steels like A2 or D2.

Conversely if the medium is significantly harder than the carbides then there is some chance that it will abrade and sharpen the carbides in situ instead of knocking them out. This is mostly applicable at higher edge angles, because at lower angles the carbides are poorly retained in the matrix, and the forces applied by sharpening will tend to knock them out regardless of how hard or soft the abrasive is. Even if you did manage to get a clean, low-angle edge the carbides would chip out the first time you tried to use it. That's why I exclusively use diamond media and fairly high angles (>=35 deg) on non-PM high-alloy steels, but even so I can't match the edge quality I get from O1 (or a PM steel like PM-V11).

Ron Hock has posted some grain shots comparing O1 and A2 excerpted from his book. There are links to more in his Internet references post for his sharpening book.

Well said, Patrick.

[michael langman]

I enjoy your explanations when I understand them Stan.., through no fault of your explanations. Nice answer.