The whole blade won't take an edge

I just sharpened a couple of Shun's for a chef. I don't know the history of the knives but they were pretty worn. On both of the knives, after more than fifty strokes to the front third of the blade, I barely raised a burr. While the rear two thirds raised a burr after about twenty. I know the heat treatment can get ruined, but for just a portion of the blade? What else could prevent that portion of the blade from taking an edge?

That's weird. What kind of shuns are they? vg10 or sg2 steel? The sg2 is not an easy steel to sharpen, especially over curved sections and doesnt raise a huge bur.

As for damaging the heat treat , i don't know what would be needed to do that...maybe using knife like a turner on the grill?

Just to make sure: is the blade straight, thickness equal along the edge, any twist, overgrind??

What sort of bevel angle did it have? Has it possibly been steeled improperly to death and needs a good thinning?

k.

You're probably not hitting the edge yet on the front third. Have you used the "magic marker trick" to see where on the bevel you are sharpening?

Either your angle isn't consistent in your hands, or on the knife. Which means either wobbly/inconsistent technique for you, or the knife has over/undergrind(s).

I'd say the profile is screwed up and you just need to go lower in grit. Maybe the person who sharpened it before did a really half-assed job and it's just blunter in one area than another?

fatigued steel may have something to do with it

The one blade was bent and I straightened it. I'm not worried about my technique. I think maybe a combination of poor previous sharpenings, the bend and some steel fatigue makes sense to me. Thanks.

Pensacola Tiger said:

You're probably not hitting the edge...used the "magic marker trick" to see where on the bevel you are sharpening?

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+1. Regardless of what the problem is, this will fix it. I'm pretty sure the condition of the HT, etc. has nothing to do with it. Just apply the marker along the entire edge and use a coarse stone.

bjg said:

I just sharpened a couple of Shun's for a chef. I don't know the history of the knives but they were pretty worn. On both of the knives, after more than fifty strokes to the front third of the blade, I barely raised a burr. While the rear two thirds raised a burr after about twenty. I know the heat treatment can get ruined, but for just a portion of the blade? What else could prevent that portion of the blade from taking an edge?

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IMO that's where your solution lies. Raise a burr along the entire edge that you're 100% sure about and you'll be sharp.

yeah... thats why i guessed fatigued steel... it would make burr development a little more difficult in that area.

What is "fatigued steel" in a knife, and what causes it?

I'm not sure that fatigued is the proper term.

probably not, but heres the idea... all day long, you are banging your edge against something relatively hard (cutting board and food). You can try to strop to bring back an edge, but after a while what happens is that the steel at the edge is no longer capable of holding an edge. Its not too dissimilar from bending a paperclip back and forth a bunch. Eventually, it wont hold up anymore. At that point (though ideally before), you need to grind away enough steel to remove that fatigued steel and expose fresh steel that is capable of taking and holding an edge.

I'm sure Larrin knows much more about this than i do, including the proper terminology, but thats the gist of it.

What you're describing is fatigue, I'm just not convinced that fatigue occurs in normal knife use. Fatigue leads to fracture not "doesn't take an edge." I would think the brittle steel would grind away easily.

Make sure you are sharpening from the top of the bevel all the way down till you are hitting the edge and all along the whole blade.

I have had this problem before with a shun one of my teachers had. The knife had been sharpened using a pull through carbide or some other ridiculous abrasive and it had developed an S shaped edge. I think an easy way to do it is doing full strokes on the whole knife like C Dawg's sharpening method that way you keep a more consistent shaped edge, while removing the S.

Larrin said:

What you're describing is fatigue, I'm just not convinced that fatigue occurs in normal knife use. Fatigue leads to fracture not "doesn't take an edge." I would think the brittle steel would grind away easily.

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I wouldnt say this happens in normal knife use... i see it in knives that havent been sharpened properly for long periods of time (i.e. sharpening behind the edge but never hitting the edge) and knives that havent been sharpened for long periods of time. I think its worse when the users are particularly abusive with the blades. The fatigued metal does grind away pretty easily, but you have to make sure to get it all.