# Same Rockwell Hardness - why is one knife harder to sharpen than another?



## hmansion (Feb 6, 2017)

I'm not a metallurgist but am curious as to why, say, a Masamoto KS gyuto (white steel HRC 62) is so easy to sharpen versus, say, a Hattori gyuto (VG-10 HRC 61), which isn't quite as easy (still not difficult). I'm clearly mistaken in equating hardness to ease of removing steel...


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## foody518 (Feb 6, 2017)

Abrasion resistance and other factors that aren't HRC


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## jklip13 (Feb 6, 2017)

Yup, hardness is not really a very precise assessment. HRC measures the resistance to a diamond probe being pushed into the metal. It doesn't measure the hardness of individual particles in the steel such as carbides or impurities. I think wear resistance is the factor that equates to ease or difficulty of sharpening. You can look up material spec. sheets from the steel manufacturer, but it ultimately all comes down to the heat treatment. It's very difficult to get empirical with knife steel.


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## chinacats (Feb 6, 2017)

Not so sure where hardness pays into this? My suggestion is that it may play in but only when comparing the same steel at different hardness. White steel is some of the easiest to sharpen regardless of hardness...vg10 reported to be some of the more difficult.


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## Jovidah (Feb 6, 2017)

A few things that come to mind.

First off... steels aren't necessarily a homogeneous 'blend'. Mixing different parts (for example adding chromium, or vanadium) isn't like adding lemonade to water. Sometimes it's more like adding chuncks of chocolate to cookie dough, or adding hazelnuts to chocolate. So some elements will create carbides, that can sometimes be much stronger than the matrix they're in. For example in a VG-10 steel you'll find elements such as vanadium which (after baking the dough / heat treating) will be much harder than the HRC 60 matrix they're in. Some lower-alloy steels (like white steel; it's a very pure steel with only carbon added) don't have these fancy carbides in them, thereby making them much easier to sharpen. 
As you can imagine there are both advantages and disadvantages to having harder carbides in the end-product and how you bake your steel dough (the heat treat) makes the big difference here.

Second, some additions will change the behavior of the matrix. So some steels can be tougher (chewier) thereby making them a lot harder to abrade. When you're sharpening on a stone, you are basically grating metal off your knife. Things that are tougher (have a little give / deform a bit) can be harder to grate. Compare for example grating soft cheese to hard cheese.

There's probably a lot more but these were what came to mind.


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## DaveInMesa (Feb 6, 2017)

As others have said, hardness is not directly related to sharpening ease or difficulty. In general, it is the measure of resistance to pressure, as jklip13 said. In the knife world, hardness determines how thin a bevel can be used without being overly susceptible to folding over. Softer steels must be limited to relatively thick bevels, in order to be perceived as sharp for any length of time used. Harder steels can be much thinner, and therefore perceived as sharper, and they can retain that thin edge much longer. But, sharpening is based on wear resistance, which is not related to hardness. 

Wear resistance is related to the other elements that make up the steel alloys. White steel has very little wear resistance, so it sharpens extremely easily, but it can be hardened to a high level, so it retains its edge well. VG10 has lots of other elements that are not present in white steel, such as Chromium, Vanadium, Molybdenum, and Cobalt. Chromium makes steel stainless, softer, yet more wear resistant. Cobalt is another wear-resistant addition. So, VG10 is harder to sharpen, but has other good qualities.


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## HHH Knives (Feb 6, 2017)

Jovidah said:


> A few things that come to mind.
> 
> First off... steels aren't necessarily a homogeneous 'blend'. Mixing different parts (for example adding chromium, or vanadium) isn't like adding lemonade to water. Sometimes it's more like adding chuncks of chocolate to cookie dough, or adding hazelnuts to chocolate. So some elements will create carbides, that can sometimes be much stronger than the matrix they're in. For example in a VG-10 steel you'll find elements such as vanadium which (after baking the dough / heat treating) will be much harder than the HRC 60 matrix they're in. Some lower-alloy steels (like white steel; it's a very pure steel with only carbon added) don't have these fancy carbides in them, thereby making them much easier to sharpen.
> As you can imagine there are both advantages and disadvantages to having harder carbides in the end-product and how you bake your steel dough (the heat treat) makes the big difference here.
> ...



*THIS^^ *

I do appreciate the way you used food stuff to describe this. It is accurate As well it gives a great visual. 

If you dont mind. I may use the analogy in some form at some point to help answer similar questions. and Advise clients on steel choices.

Blessings


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## Jovidah (Feb 6, 2017)

Thanks for the compliment.  
Feel free to use (or correct!) anything I say in any way you see fit. Any knowledge I have on the subject is entirely indebted to this forum and all the knowledgable people here spreading their wisdom. I just try to regurgitate and spread the word.


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## LifeByA1000Cuts (Feb 6, 2017)

I find a 64+ HRC AS is childs play compared to what happens if even a mid-level maker (let's say we are talking Tojiro, not 58 HRC damascus wally special) *means* VG10


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## hmansion (Feb 6, 2017)

VERY helpful replies here and I understand now that a Rockwell rating only scratches the surface :groucho: at describing the "character" of any particular steel.

@Jovidah Your baking analogy is wonderful and so clear!! Thank you!!


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## TimoNieminen (Feb 6, 2017)

Jovidah said:


> So some elements will create carbides, that can sometimes be much stronger than the matrix they're in.



A traditional heat treatment for wootz was just to air-cool the blade, giving a soft body but loaded with carbides so the final product had good edge retention. (At 1.5% to 2% carbon straight out of the crucible (forging would reduce this somewhat), without thermometers or clocks, or what we know about optimum heat treatment, attempting a quench-and-temper might not produce a good result. I've known people to break antique wootz swords by dropping them on the floor. But 1.5+% carbon gives a carbide bonanza.)


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## milkbaby (Feb 7, 2017)

TimoNieminen said:


> A traditional heat treatment for wootz was just to air-cool the blade, giving a soft body but loaded with carbides so the final product had good edge retention. (At 1.5% to 2% carbon straight out of the crucible (forging would reduce this somewhat), without thermometers or clocks, or what we know about optimum heat treatment, attempting a quench-and-temper might not produce a good result. *I've known people to break antique wootz swords by dropping them on the floor.* But 1.5+% carbon gives a carbide bonanza.)



Like dropped them on purpose?! :bigeek:


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## panda (Feb 7, 2017)

masamoto ks = graphite pencil
hattori = crayon


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## marmozet (Feb 9, 2017)

VG10 stainless steel vs white 2 carbon steel. The stainless steels with vanadium and chromium are the main difference and it seems that these elements make it harder to sharpen. I know with vanadium tools, they have a very hard grittiness to them as does tungsten. At a guess they make stainless steel sharpening feel different and more tricky.


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## Jovidah (Feb 9, 2017)

marmozet said:


> VG10 stainless steel vs white 2 carbon steel. The stainless steels with vanadium and chromium are the main difference and it seems that these elements make it harder to sharpen. I know with vanadium tools, they have a very hard grittiness to them as does tungsten. At a guess they make stainless steel sharpening feel different and more tricky.



On that matter. If I remember correctly, aluminium oxide - the abrasive used in most sharpening stones - has a hardness somewhere in the higher 60s (lets just say 70). Vanadium carbides can be harder (80ish?), so your stones actually cannot abrade them. All they can do is abrade the matrix so the carbides pop out. 
Molybdenum and tungsten are somewhere in the 70s as well.
This explains why some (most) stones become so much slower grinding certain steels. 

I can imagine this also plays a role in how some knives / steels retain rather toothy edges off certain stones, but I'm not sure if there's any truth to that.
In theory, the difference between steels - both in speed and finish - should be a lot smaller when using diamonds, as they're actually hard enough to abrade the fancy carbides. Although I've never used them. Anyone who can chime in?


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## Jovidah (Feb 9, 2017)

Minor addition: http://www.crucibleservice.com/eselector/general/generalpart1.html
This seems to explain most of it pretty well, and has info on the hardness of the carbides. They use HRC measurements so it's actually quite readable.


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## TimoNieminen (Mar 11, 2017)

milkbaby said:


> Like dropped them on purpose?! :bigeek:



Accidently. IIRC, while cleaning it in one case.


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