Grain size v sharpness

[Goldfinger234]

Morning everyone,

Is there a correlation between grain size of tool steel and the maximum grit to which you should sharpen it?

For arguments sake, if a well treated O1 tool steel had a grain of 3ųm, is there any benefit to sharpening beyond a 3 micron abrasive? I’m thinking in terms of woodworking tools, chisels etc., that will need to withstand the normal impact forces placed on them by hand, not something like a razor, that will generally be sharpened at a shallower angle and only has to cut hair. Chisels are generally 25 degree primary and 30 degree secondary bevel.

I’m not a metallurgist and it’s one of those subjects shrouded in myth, mainly because most woodworkers aren’t metallurgists!

To the layman it just seems that if the grain of the steel is a given dimension, how can it support a finer edge and stay robust enough?

Thanks in advance for any advice.

 

[Maui]

In general, no. There shouldn't be any correlation between the level of sharpness that can be achieved on the cutting edge of a tool and the prior austenitic grain size of the microstructure. The level of sharpness you require for a given application however depends on what you are doing. As you mentioned, a wood turning chisel requires a different edge profile than a razor blade. What sometimes happens during use is that the condition of the cutting edge of a chisel degrades due to microspalling. This occurs when constituents of the microstructure such as individual grains or individual carbide particles break off of the tool at the cutting edge. It effectively dulls the tool by degrading the condition of the edge. Regardless of the grain size, you can sharpen your cutting edge to whatever level of sharpness your preparation technique (and abrasives) will allow.