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Stones Can't Sharpen HAP40?

Wed Mar 19, 2014 8:16 pm

We started to have a discussion about sharpening HAP40 in the Recommendation Q&A section. I didn't want to high jack that thread too much so continuing the discussion here(quotes are from previous thread)

Tall Dark and Swarfy wrote:Shapton Pros work just fine. If you're going to to crazy with high grits you will want to use diamond above ~5K so as to be able to shape the Vanadium and Cobalt carbides and not just abrade away the matrix.
mark wrote:I don't have any Shaptons above the 5k, but I'm curious... Why would the Shapton pro or glass 8k be ineffective?
Tall Dark and Swarfy wrote:This should stir lively debate. The Shapton abrasive is less hard than the V and Co carbides. Thinking of the steel as chocolate chip cookie dough, the Shaptons will eat the dough but leave the chips which will eventually fall out. Using diamond when you're in the high grit range actually helps shape the carbides. At least that's the theory.

So you are saying the Shapton stones will work fine below 5k, but above 5k their magic stops working? Wish they would put more magic in their higher grit stones ;)

I understand the chocolate chip cookie dough theory. I was more or less looking for clarification on why exactly 5k was the predetermined stopping point for stones. I'm going to layout my understanding of the cookie dough theory(for those reading along) and my hunch as to why 5k is the stopping point.

Hard carbides like tungsten, vanadium and colbalt (chocolate chips present in HAP40) are unable to be cut by the abrasive in the stone as they are too hard. The largest of these carbides are between 2-3 micron in size. Stones that have larger abrasive particles than vanadium, for instance, are able to sharpen the steel as the stone abrasive can reach the softer material(cookie dough) between the vanadium carbide which holds it into the steel matrix. By removing softer material(carbon, chromium and Molybdenum in HAP40) holding the carbide it will effectively fall out.

Stones such as the Shapton 10k, with 1.5 micron abrasive, do nothing to refine the edge as they can not cut vanadium carbides. Thus they can not refine the edge past the 2-3 micron size of the vanadium carbides. In relation to the cookie dough, you can remove as much dough as you want, but there will still be the large chips exposed at the edge.

Feel free to correct any mistakes. Am I correct in my hunch that the carbides unable to be cut are 2-3 micron in size? Always the same size regardless of steel makeup and heat treat?

Re: Stones Can't Sharpen HAP40?

Wed Mar 19, 2014 9:00 pm

Carbide size is not a set value , as powdered metallurgy advances they get smaller and smaller. But other than that yes you are correct. This is why 1st gen powdered steels where such a pain to sharpen

A) because nobody had ever dealt with these carbides in sharpening knives before
B) because they were quite large

I have it on good authority that some of the 3rd gen stuff the Vanadium Carbides are getting into the 1u range. But I haven't seen any scientific studies on the matter. Im not familiar with HAP40 at all so I cannot comment on the carbide sizes there. However I can say that on any high carbide steel , Kens Diamond Films are the single best solution. They are cost effective , and fast , outperforming the shaptons , even in the lower grits , by a wide margin.

Another option is to "dope" your shaptons with either CBN or Poly. Giving them a teeeeny spritz of abrasive that can tackle those carbides makes a big difference.

Re: Stones Can't Sharpen HAP40?

Wed Mar 19, 2014 9:30 pm

The Shapton stones will do fine. I sharpened my S30V spyderco today using a Shapton pro 1k and Naniwa green brick without a single issue. I've done it with King stones too.

The carbide formation will be low because of the low percentages of vanadium, Co aka cobalt does not form carbides, Cr aka chromium does form carbides but again the low percentages in HAP40 will not allow it to form them in abundance. The higher levels of tungsten will lead to the formation of carbides but it should not make it difficult to sharpen as they are not very hard carbides compared to vanadium.

So, overall a big fuss was made and it was all for nothing because the chemistry of the steel was not understood.

Re: Stones Can't Sharpen HAP40?

Wed Mar 19, 2014 9:49 pm

I'm happy to be proven wrong if it means less work. Thanks Jason!

Re: Stones Can't Sharpen HAP40?

Wed Mar 19, 2014 10:16 pm

Sometimes when people hear "super steel" or "particle metallurgy" there is sudden thoughts of high wear resistance and Vanadium carbides. It takes certain percentages of a given element for large changes to happen such as 11-13% chromium is needed to make a steel stainless with 11.5 of that needing to be free chromium and not locked in carbides. You need 4% or more Vanadium before the wear resistance reaches a tipping point in volume that it becomes difficult to sharpen. So you see it's a combination of elements and there amounts that make a steel not the hype of its name ;)

When you have been around the steel frenzy for a while it's called "flavor of the week" it happens with every new steel. Learn the elements and what they do to steel and it will give you a better understanding of a new steel when it comes out.

Re: Stones Can't Sharpen HAP40?

Thu Mar 20, 2014 2:17 pm

Thanks for the insight, Jason. I was having a hard time believing that a steel producer would encourage use of one of their products for knives that ultimately proved unsatisfactory for the end-user community. Especially in Japan!

Re: Stones Can't Sharpen HAP40?

Mon Oct 03, 2016 7:43 am

I have a kohetsu Hap40 Gyuto. I sharpened nicely. Finished on a 6k grit King stone and balsa with diamond past. It was a beast. But I thought I don't know what I had in mind that after a days work, it was still sharp, I put it on the king 1k again, and on the 6k again. Steel is really sharp but I can see the edge is changing sides. It wasn't before it was really strong. Do I need to ReSharper it or there is something I can do to correct it? Or this or normal? I can Sharp a knife to hair popping sharp but maybe I went too far? Can someone help out? Thanks.

Re: Stones Can't Sharpen HAP40?

Mon Oct 03, 2016 10:16 am

Changing sides?

Re: Stones Can't Sharpen HAP40?

Mon Oct 03, 2016 10:39 am

Walk us through your progression, I'm curious what you mean by changing sides as well. Did you mostly work one side of the blade to alter the geometry of the edge? Did you work up a burr that keeps switching from one side to the other that still needs to be removed?

Re: Stones Can't Sharpen HAP40?

Tue Oct 04, 2016 4:32 am

I did read that HAP40 is capable of holding a <3 micron edge not as good as HCS but I doubt you would notice the difference much. The Kohetsu HAP40 Gyotu is an asymmetric grind from the factory, take a look at it with a straight edge, and the feel of my one felt wrong with a 50/50 sharpen. So I sharpen it approximately 15 degrees on the LHS and 10 degrees on the RHS, not saying a 50/50 sharpen is wrong it is just wrong for me. I usually finish with a Chosera 3K or an 8K Snow White.
Not sure if I could see or feel a chunk of Vanadium Carbide missing from the edge so I don't worry about it. Agree diamond finishing would be the way to a very refined edge and I did go that route with a hard strop and diamond paste, subjectively the edge did seemed to last even longer doing this.
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