Carbides - The Nitty Gritty

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cm_bushman

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Oct 22, 2012
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This one is for the steel junkies and engineers among us!

So, after years of researching steels, it has become clear that carbides play a major role in the edge-retention and wear resistance of modern alloys, and even some legacy steels. I have also seen how the size of formed carbides supposedly determine the achievable fineness of the edge, which makes sense on the surface. I have also read all about the different carbide-formers employed in modern steels, as well as the use of Nitrogen to replace carbon, in which case nitrides form.

So after years of forumites name-dropping the likes of Vanadium, Chromium, Tungsten, Niobium, and sundry other carbides, I would like to know more about them in relation to one another, things such as:

Which is hardest?
What are the hardnesses of the common carbides, in HRc or otherwise? I've seen Tungsten carbide advertised as 71HRc, I don't know if that's accurate.
Which is largest?
Which is smallest?
I know Niobium has the greatest affinity to form carbides, what does this mean to an alloy employing Niobium?
How do Nitrides compare, (in hardness, size, technical difficulty to produce, etc.), to carbides, and do the same elements yield the same results?

I know carbides aren't everything, but after years of reading, I've never seen a genuine discussion of carbides beyond composition charts that tell us the relative potential of the number of different varieties, not their properties.
 
That's interesting, I always thought Vanadium carbides were quite large, but now I know!

Any specs or numbers on the size compared to other carbides? Is a Vanadium carbide a baseball whereas a Tungsten Carbide (for example) is a Softball? Is there a resource for this stuff?
 
cm_bushman said:
That's interesting, I always thought Vanadium carbides were quite large, but now I know!

Any specs or numbers on the size compared to other carbides? Is a Vanadium carbide a baseball whereas a Tungsten Carbide (for example) is a Softball? Is there a resource for this stuff?
Click to expand...

Yes there is.
 
I cant contribute to some of your questions, but I can provide some information.

Here is a comparison.

Knife Edge diagram by R. Landes (Metallurgist)

Metallurgy of Steel for Bladesmiths & Others who Heat Treat and Forge Steel - By John D. Verhoeven (2005) (Metallurgist)

CORROSION RESISTANT NITROGEN ALLOYED STEELS - The advantage of having Nitrogen in a steel can be seen here.

Other Nitrogen references:

Nitrogen Containing Austenitic Stainless Steels

InTech-Corrosion_resistance_of_high_nitrogen_steels.pdf

The odd thing is Nitrogen containing steels and their advantages have been known for some time, yet we have not really seen more of it available in knives, but price is high.

CARBIDE DESIGN IN WEAR RESISTANT POWDER MATERIALS

D2

index.php


AEB-L

index.php


To my knowledge Vanadium is not the smallest carbide former.

Here are fine carbide steels without Vanadium

Sandvik fine-carbide-steels/

Here is Devin Thomas on fine carbide steels: Devin Thomas FAQ
 
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Thanks guys, I guess that's as good as it gets, I'll have to take some advanced engineering classes before I can get a better idea of carbide formation and metallurgy. It's amazing what info is right here on the forum, thanks to a community effort.
 
marthinus said:
The odd thing is Nitrogen containing steels and their advantages have been known for some time, yet we have not really seen more of it available in knives, but price is high.
Click to expand...
Nitrogen steels have been available in knives for years. Spyderco uses H1, Benchmade uses N680, Sandvik 14C28N has a small amount of nitrogen.

Chuck
 
Carbide size is controlled by processing and alloy content. Iron carbide (cementite) can be extremely small to large enough to cause complete brittle failure in steels below 50 HRc. Tungsren carbide is the hardest I've seen.
 
My reaction was chromium carbides are largest, niobium (columbium) the smallest. Tungsten the hardest. Vanadium a versatile compromise. (Purely theoretically- since the point made by me2 is right.) As for the "legacy steels" those mostly hold an edge because of iron carbides, which are less wear resistant. Although, a theory of the alloy composition of some of the crucible wootz steels of antiquity is that the ore used naturally contained some vanadium, which enhanced the watering and the cutting ability. That's my $.02.
 
It's worth noting that the carbides in the micrographs above for D2 and AEB-L are primarily Cr carbides of one form or another. So just with that 1 element, we have a size range of less than 1 micron to over 0.001 inches. Most here know, but for those who don't, ~25 microns = 0.001". The largest carbide in that D2 picture is larger than the 30 micron scale at the bottom. The difference is when they form. The large carbides in D2 form from the liquid as it cools. There is enough Cr and C that you'll get some carbides floating around in the liquid before it solidifies, and the only thing to stop them from growing is the amount of Cr and C available. Many modern cutlery steels are like this, and it's one reason for the use of PM technology.
 
me2 said:
It's worth noting that the carbides in the micrographs above for D2 and AEB-L are primarily Cr carbides of one form or another. So just with that 1 element, we have a size range of less than 1 micron to over 0.001 inches. Most here know, but for those who don't, ~25 microns = 0.001". The largest carbide in that D2 picture is larger than the 30 micron scale at the bottom. The difference is when they form. The large carbides in D2 form from the liquid as it cools. There is enough Cr and C that you'll get some carbides floating around in the liquid before it solidifies, and the only thing to stop them from growing is the amount of Cr and C available. Many modern cutlery steels are like this, and it's one reason for the use of PM technology.
Click to expand...

It seems to me that those large carbides (as shown) in the D2 would cause a lot of problems, especially with finishing, sharpening, etc. Is this example fairly common or more a result of poor heat treatment?
 
Bo T said:
It seems to me that those large carbides (as shown) in the D2 would cause a lot of problems, especially with finishing, sharpening, etc. Is this example fairly common or more a result of poor heat treatment?
Click to expand...

It's fairly common; D2 is widely known for its "orange peel" look when finished; that's why you rarely see a highly-polished D2 blade, and why it's known best for "toothy" edges. Some say "it takes a lousy edge but holds it forever" - that's an exagerration on both points.

None of this means it's no good for knives - folks who really have their HT nailed down make blades with it that reportedly exhibit outstanding performance and edge-holding. Lots of people love 'em for dirty work like field-dressing, where a toothier edge is a good thing. CPM-D2 addresses the issue of those large clumps of carbides, just as CPM-154 is cleaner and finer-grained than 154CM.
 
Something else to consider: the size range of the carbides.

Manufacturers will usually quote the average size of the carbides but some will be smaller and some will be larger. Some may aggregate together during HT forming even larger chunks, especially common in steels that have high carbide volume... there isn't enough room to stay separated.
 
Knoop Hardness Scale.

Chromium Carbide 1735
Molybdenum Carbide 1800
Titanium Nitride 1800
Tungsten Carbide 1880
Titanium Carbide 2470
Silicon Carbide 2480
Vanadium Carbide 2660
 
One reason CPM-S35VN is so tough and makes such a good fine cutting/slicing blade is the hard and reasonably small vanadium and the tiny Niobium carbides. This 1-2 punch gives it one of the highest toughness ratings for common blade stainless steels. The edge retention is excellent, and the corrosion resistance is very high. This steel with a good HT at 2000F and cryo can make an Rc 60-61 knife that will out perform most of the normal blade stainless steels commonly available. It puts D-2 and 440-C to shame.
 
Anyone know the relative affinities for carbon and if those are temperature dependent? Does the affinity simply follow electronegativity?

-Sandow
 
Ankerson said:
Knoop Hardness Scale.

Chromium Carbide 1735
Molybdenum Carbide 1800
Titanium Nitride 1800
Tungsten Carbide 1880
Titanium Carbide 2470
Silicon Carbide 2480
Vanadium Carbide 2660
Click to expand...

Interesting. The values I have seen show Tungsten Carbide (W2C) higher than VC. W2C is a very fine carbide found after tempering, so it's not too common.

Sandow, the carbide forming tendency is in rough order from greatest to least below. The top ones are close and might be different depending on sources. I don't know about electronegativity, but it'd be interesting to check. Different carbides do form at different temperatures, so that has to be taken into account.

Tungsten, Niobium, Vanadium, Titanium (these 4 are close, niobium may be higher than vanadium, most steels don't have Ti in them, so it's hard to find data. I'll double check my source to be sure.)

Molybdenum
Chromium
Iron
Manganese (very low tendency to form carbides, and will be found in the (Fe,Mn)3C carbide instead of it's own formula.
 
Electronegativities are:

Iron 1.8
Molybdenum 1.8
Silicon 1.8
Tungsten 1.7
Vanadium 1.6
Niobium 1.6
Chromium 1.6
Titanium 1.5

Not really seeing much of a pattern there. What I've found discussing niobium supports it binding carbon sooner than most.

Is s35vn trying to achieve extremely fine carbide grains by have stoichiometric limits on a variety of carbide formers? Would make sense that the .5% Nb would bind to near saturation and have the rest soaked up by the 3% V, but why not limit the V to .5 as well and let a third carbide former soak the surplus carbon. Wouldn't you end up with a much finer grain structure with more stoichiometricly limited carbide formers?

-Sandow
 
I'd guess that the electronegativity has less of an effect than the orientation of the bonds due to the valence electrons, the size of the metal ions, and the kinetics of bond formation vs the equilibrium states of the different carbides.
 
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