Why is M390 (and its 204p/20CV counterparts) such a pleasure to sharpen?

[OhioApexing]

I really thought that they would be a pain to sharpen. Instead, I’m finding the opposite. M390 has little-to-no burr issues. When they do form, they’re small and come off with ease. The edges don’t chip at readily, I could go on. I see this sentiment reflected here and elsewhere. M4 was also surprisingly fun.

So, from a metallurgic/experienced perspective, why are some of these harder steels seemingly easier to sharpen, in spite of increased hardness?

 

[Obsessed with Edges]

Increased hardness is usually the reason burrs are minimized. When the edge is thinned adequately enough to form a burr, the hardness means the burrs will be more brittle and will break away more readily, and sometimes so early you may never notice them. More ductile ('soft') steels are the ones with burrs that will bend back & forth all day if you let them, and therefore will be more of a pain to remove. There are some exceptions, steels hardened up to even ~ 60 HRC or so, that will still show some very tenacious, tough burrs. But the 'upside' in those is that even the burrs are quite durable, and you can actually get some use out of them by aligning them straight.

So long as the stones/abrasives you're using are suitable to the hardness and carbide content of the steel, sharpening should almost always be easier. This means using something like diamond for steels with a lot of vanadium carbide content, or maybe SiC at least, at coarser finishes. But if trying to grind the same steels on something like aluminum oxide or natural stones, it won't be so fun anymore.

 

[sodak]

Then once you try a high Vanadium knife with a high hardness AND the knife has been optimized for performance, i.e., thinned out behind the bevel, sharpening gets even easier. And performance goes through the roof...

 

[Obsessed with Edges]

Then once you try a high Vanadium knife with a high hardness AND the knife has been optimized for performance, i.e., thinned out behind the bevel, sharpening gets even easier. And performance goes through the roof...

This is what I was surprised to find with a Spyderco Manix2 in S90V, a few years ago. For a long time after I'd bought it, I was intimidated by the vanadium content in the steel and avoided making any attempts at major sharpening or thinning of the edge on that knife. I'd already struggled more than once with other knives in S30V, that steel having less than half the vanadium content of S90V. Most of the difficulties with those were due to bad geometry and overly-thick edges, I later figured out. To Spyderco's credit, the geometry of the Manix2 was already very good and the factory edge was well-finished, so I never felt like I NEEDED to make any major changes with it. When I finally committed to thinning the edge a bit more with my DMT hone (Coarse), it was a breeze of a job and I found myself wondering what I'd been so scared of in the first place. Good steel, good geometry and the right tools to keep it in shape make it all much easier.