Carbides in Stainless Steels

Spurred by a discussion on Knifedogs regarding cementite/iron carbide in AEB-L/13C27, I found this thesis on carbides in stainless steels. It is a long one, and a PhD thesis, so be prepared. I have not read it, but will be perusing it over the next week or so.

[iris.lib.neu.edu]

Even if you are not interested in the equations which are used to model the diffusion/growth of the carbides, the first half of the thesis is a very nice over view of many aspects of metallurgy which are very pertinent to stainless steels in knives. The nice thing about a thesis, vs a paper, is the in depth literature survey. There is a tremendous wealth of material there.

If you've read it, is there any mention of iron carbide in the steels they use, which are similar to AEB-L, just slightly higher carbon?

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me2
... is there any mention of iron carbide in the steels they use, which are similar to AEB-L, just slightly higher carbon?

I must have missed that part.

There is extensive discussion of the difference between cementite and the chromium carbides and specifically the difference in the behavior of carbon vs chromium as one of those is interstitial and the other is substitutional both both dissolve in the austenite. The second half of the thesis is based on modeling the behavior of the carbides and finding how the dissolution and coarsening have different dependencies on temperature. Interestingly enough the rate of coarsening is so strong that the carbides can be larger after the soak, especially at high temperatures.

Note for anyone reading it, the way they refer to carbides is slightly different. Often times in discussing knives these terms are used :

-primary carbides, which are in the steel when you get it
-secondary carbides, which form in the steel in the quench/tempering

In the paper they use different definitions :

-primary carbides, what forms in the initial cooling after the melt
-secondary carbides, what forms after the thermomechanical processing (rolling)

As an aside, chromium carbide isn't actually chromium carbide as in it is a big chunk of chromium carbide. While it is short hand written as M23C6 of M7C3, this doesn't mean all of the M23 is chromium, it will be a mix of chromium and iron. The carbides form as bunches of elements, it just means that they are mostly chromium and mostly vanadium. Some of them have higher/lower solubilities. Vanadium carbide has a higher solubility in chromium carbide than Niobioum for example as you noted awhile ago.

I've been reading the paper a little, and have found what I was looking for I think. I rarely discuss such things as various element solubility in carbides. As I've discussed here before, I am hesitant to do so for a couple of reasons. First, I am not terribly knowledgeable in such areas. The second is there is a lot of misinterpretation of information as it is. I think such information might make it worse.

What exactly were you talking about / referring to?

The discussion of carbides in steels reminds me very strongly of how forging used to be discussed in the 90's. It took a few makers like Cashen to consistently and clearly discuss how forging was a shaping process and the thermal cycling was the critical part in regards to the microstructure.

Unfortunately with heavy carbide steels there is no Cashen-like maker who extensively uses such steels but does so on a basis of actual metallurgy and speaks of them in that manner. Landes for example does the latter, but he is often disregarded as he simply doesn't use them.

I'm not sure what you're referring to, but I think it's my reluctance to discuss/bring up certain topics to a certain level. There is enough trouble getting people to discuss things at the current level on most forums without bringing up the notion that carbides themselves are a material and can be studied in depth just as steel can. For instance, how much, if any amount, of chromium dissolved in cementite will give the optimal wear resistance/hardness for the carbide? In the really complex steels, are different combinations of niobium, vanadium, and chromium dissolved in the carbides better/worse? This is explored in terms of corrosion resistance in the CPM 110V patent you've linked to, during the discussion of how niobium reduces the solubility of chromium in the vanadium carbides, thus increasing corrosion resistance. I'm sure it's not just as simple as MOAR vanadium. Now, to be fair, there is always a level beyond what is being discussed. If I can just get people to understand that carbides are variable, can be controlled, and more isn't always better, I will take that as a start.

I think you have to be careful as to who you are talking to and it is pointless to go beyond it. However I would take care in simpling things to the point they are misleading. But yes, if people are arguing that there is no carbide in 1095 it is a bit pointless to talk about the fact that manipulation of the carbide density is necessary to get ideal martensite structure. It would be like trying to talk about why genetic drift can be a stronger driver than natural selection with someone who denies that evolution is a valid theory.

Does anyone know where the notion that AEB-L/13C26 steels don't form chromium carbides comes from? I keep seeing this, and can't find a beginning point for it. I know the heat treatment can be done to remove all the carbides, but no one I've heard of does this. Guesses welcome.

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me2
Does anyone know where the notion that AEB-L/13C26 steels don't form chromium carbides comes from?

This shows one of the most extreme points of both ignorance and lack of integrity I have seen in a long time in steels. The level of carbide in steel is dependent on the heat treatment, any steel can have all the carbide dissolved or all of it precipitated (essentially). The standard heat treatment for AEB-L leaves ~3/5% chromium carbide. Now only is this in much public work (peer reviewed) it isn't even calculated or measured it can just be directly observed. Anyone who makes that claim not only has a fundamental lack of understanding of even basic metallurgy, they are unwilling to look at even basic evidence. It might as well be a claim of flat earth.

The only thing I can think of is that someone misunderstood Verhoeven's/Landes comment about the composition being close to the saturation line as the distance from the saturation line is proportional to the carbide volume. It could also be some kind of distortion of something Jim (Ankerson) has said as he constantly says things like 1095 has no carbides. Based on what he has written, it is almost as if he thinks only Vanadium forms carbides in steels.

Your last comment cliff.
I think the word carbide itself has been diluted to a specific meaning by some such as vanadium carbide.

It could also be they are misunderstanding carbide volume ie. D2 large carbide. Aebl small carbide size.

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Kyley,

That is one of the problems with discussion. If we don't use terms in well defined ways then miscommunication is almost guaranteed. In some cases it is obviously intentional, in other cases not so much. It isn't possible obviously to control what happens elsewhere, but here at least we can speak semi-intelligently and not talk about compacting steel molecules.