Edge wear in high carbide steels as influenced by the carbide size

[Cliff Stamp]

I have long noticed that in very high carbide steels the following can happen :

-the edge retention can at times act like there was little carbide or even worse (blunt faster than a low carbide steel)

-the blunting is highly random (both in each use and even along the edge in any given use)

-the edge at times would get get "sharper" during the cutting but it was obviously getting more worn

However this did not correlate at all to grinding the steel or the materials data on wear resistance which was very consistent. You don't find anyone saying D2 is very easy to grind for example but you can easily find people saying Elmax has poor edge retention and some people say it is great even when they were all doing the same thing (cutting rope of cardboard).

Now you can just say this is all due to people not knowing what they are doing and introducing random variation, and this is true, but it happens so often and I saw it myself that I knew it had to me more than that and specifically that there was a very different mechanism happening between the steel and the belt when grinding the knife than in the steel and material being cut (rope, cardboard, etc.) when the knife was used.

I had concluded that what was happening was a combination of the following effects (all of these I have wrote on before over the years) :

-the amount of carbide in the edge is random and is just a consequence of the random distribution of the carbide in the steel and thus when you form an edge some parts of it have a lot of carbide and some don't.

This means that you get very random behaviour along the edge as it reacts in a very inconsistent manner. This gets worse with very large aggregate carbides (D2 is much more extreme for example than MBS-26).

-when very large particles hit an edge they are hitting basically a chunk of matrix + carbide and thus they always have a consistent effect

That is why abrasive grinding is always the same result, everyone says D2 is hard to grind, S60V is more difficult to grind, 121 REX is extremely difficult to grind, etc. .

-however when very small particles hit an edge sometimes they hit the carbides and some times they hit the matrix

This means the wear can be both very random and at worse often dramatic as if they tear out the steel between the carbides the carbide can be torn out and thus you have a steel which acts (in those spots) even worse than if it had no carbides at all.

Now again while I thought these were true, and the data supported it, and it made sense I didn't have direct evidence (you have to etch steel and do high mag shots to directly confirm carbide behaviour).

A few years back Roman wrote about edge stability and talked about many of these issues indirectly. However very recently doing a search on the material properties of Vascowear this paper turned up :

http://kau.diva-portal.org/smash/get/di ... FULLTEXT01

It has a wealth of data, but among it it specifically talks about the difference between :

-grinding a flat piece of steel to determine wear resistance and wear at the edge of a cutting tool

-the difference between large abrasive particles and small abrasive particles hitting the edge of a cutting tool

It is a very interesting read and covers a lot of ground, but in particular it was nice to finally get independent confirmation of what I had held to be true.

Now what does all of that mean to a regular ELU, well basically this - assuming you are cutting something which has very fine abrasives then you can expect to see :

-parts of the edge will blunt much more than others

-sometimes the edge will blunt much faster than other times

These effects get stronger as :

-the angle gets lower

-the carbide volume increases

-the carbide size increases

This is one of the reasons why you can find people doing rope cutting tests on S90V, Elmax, D2 etc. and finding average to poor performance and then other people do the same thing and find extreme performance. It doesn't matter how well you try to control the "variables" there is going to be inherent variation due to the random way that small abrasive particles can interact with a composite system of carbides + matrix in the edge of a steel knife.

(Ironically this means that the people who report the "odd" findings are most likely presenting real data, the people who always match the wear resistance tables are most likely making up data or having it be seriously perception biased.)

What is the basic lesson here - simple, don't be do quick to judge and use and sharpen the knife a few times and look at the average performance, especially with high carbide steels.

[Blerv]

Very interesting analysis Cliff. It certainly makes sense to me and further cements the difference between edge retention and grindability.

Steels like VG10 seem to have a pretty fine grain structure although with the high levels of chromium I would expect fairly large chromium carbides mixed with the carbon. Just as an example, if Hitachi increased the carbon content besides higher brittleness would it take on a more "toothy" nature?

I recall you saying Tungsten and Chromium carbides form quite big compared to carbon and vanadium. I'm just trying to figure why it seems cleaner than numbers would predict since those carbides (in theory) should be painfully obvious. I'm guessing this is simply my lack of molecular empathy.

I flirted with getting a Superblue Caly3 but the budget isn't agreeing at the moment. It would be great to try something else down the road in the FRN line if it happens.

Thanks in advance!

[Cliff Stamp]

Steels like VG10 seem to have a pretty fine grain structure although with the high levels of chromium I would expect fairly large chromium carbides mixed with the carbon. Just as an example, if Hitachi increased the carbon content besides higher brittleness would it take on a more "toothy" nature?

As with most of the commentary here they are general rules so be careful in applying them in a very rigid and absolute perspective.

If you increase carbon in a chromium rich steel you will cause two things to happen :

-the as quenched hardness will increase
-the corrosion resistance will go down
-the primary carbide volume will go up

As an extreme example of this, the largest difference between D2 and AEB-L is the huge difference in carbon. What this does is cause more of the chromium to be tied up in primary carbide (this is carbide that remains from the initial melt). This difference is so huge that the steels do not even look anything alike :



and :

I recall you saying Tungsten and Chromium carbides form quite big compared to carbon and vanadium.

Carbides are carbon+X, iron carbide (cementite) is very small as on the order of 0.1 micron. Vanadium, Niobium and Tungsten form in general very small carbides, on the order of 1 micron. Chromium in general will rapidly form huge aggregates when it is present in large quantities.

I'm just trying to figure why it seems cleaner than numbers would predict since those carbides (in theory) should be painfully obvious.

VG-10?

VG-10 has a significant amount of Cobalt, one of the many advantages of Cobalt is to allow the raising of the aus-temp of the steel. This allows more of the alloy to be put in solution (reduce the size/amount of the primary carbide). This allows the steel to be :

-harder
-more corrosion resistant
-higher secondary hardening response

The downside is a reduction in impact toughness and wear resistance. The other thing could simply be that the VG-10 was rolled more significantly, or subjected to repeated rolling/normalization cycles to better distribute the carbides. The latter is likely because it is a steel often bought for blades.

The best way to really understand these types of differences (or any actually) is to look at an extreme case which are otherwise similar. D2 for example and Buck's 420HC are a good pair as the hardness will be similar so you can clearly see the difference between a very high volume and very segregated carbide steel and a very low volume and very well distributed carbide steel.

[Blerv]

Thanks for the information Cliff. :)

I know much of the talk describing steels on the web is general. As mentioned my own use of VG10 and notice of grain structure is laden with odd comparisons and bias from what I've read. Conclusions have been far from lab testing.

Moving from general to specific. That's the goal! :D

[The Mastiff]

-the amount of carbide in the edge is random and is just a consequence of the random distribution of the carbide in the steel and thus when you form an edge some parts of it have a lot of carbide and some don't.

This means that you get very random behaviour along the edge as it reacts in a very inconsistent manner. This gets worse with very large aggregate carbides (D2 is much more extreme for example than MBS-26).

-when very large particles hit an edge they are hitting basically a chunk of matrix + carbide and thus they always have a consistent effec

Cliff, does Niobium show any improvement over the other carbides in this respect. You stated they tend to not form in the corners and are more spread out or in the middle. I don't recall exactly what you said but I got the impression there was a difference. Niobium carbides have about the same size and hardness as vanadium so it would seem like a decent improvement and worth making. Just a thought.

Joe

[Cliff Stamp]

Niobium has many benefits in steels which is why you are seeing it more and more both as a micro-alloy and as a significant carbide former. As a micro-alloy the main benefits is that it has extremely low solubility in austenite which means it has a very strong grain refining effect so you get increases in toughness and strength (or you can increase hardness and retain toughness). As a carbide former it has a large advantage in that it tends to form direct niobium carbide. Vanadium however tends to much more readily be dissolved into the chromium carbide and while it makes it harder it is *much* softer than the solid niobium carbide. This is why you will see modifications to steel which decrease vanadium content and increase niobium content. This then refines the grain, allows higher soak-temperatures, increases wear resistance and toughness (two different ways).

[chukar8]

So if I am understanding this correctly, The smaller and better distributed the carbides are, the better the edge retention etc is? It seems it would be more consistent at the very least. Great thread BTW. Thanks for the magnified pics of the steel too.

[Cliff Stamp]

So if I am understanding this correctly, The smaller and better distributed the carbides are, the better the edge retention etc is?

Again, you want to take care with absolutes, but as a general rule yes. The simplest way to understand this would be to compared the two steels :

a) All of the carbide is just centered in one big carbide in the middle of the bar

b) The carbide is extremely small (less than one micron) and all evenly distributed through the entire bar

Just imagine making a knife out of those two bars.

The first one would have a section of edge which behaved like a piece of solid carbide and the rest of it was pretty much like carbide free steel. Now what would happen if that carbide tore out, it would leave a jagged hole and the knife might even break around it.

The second one would be very consistent in all ways.

The goal then is to move from the first to the second.

However, there are some that argue the exact opposite. As an example David Boye is a long standing advocate of extreme carbide segregation so much so that he uses the steel in the as-cast (dendretic form) where the carbides are not even broken apart by the normal rolling. This leaves them in large networks were they are literally 100X the size.

Boye argues then that you will get extreme edge holding because the steel wears around the large carbides and exposes them which leaves a jagged edge which will keep cutting. This is enhanced by using a very thin edge which is very acute and left at a rather coarse finish. Now this actually does work as he describes but you end up with edge retention of a really low sharpness, but for people who are very adverse to sharpening this is a good choice. It is basically D2 taken another step further so if you know how D2 compares to say 52100, then jump up that again and you have Dendretic carbide behaviour.

However, and this is a big however - when most people use Boye's knives the reason they see a huge performance increase over their other knives has nothing to do with the dendretic carbides is it because :

a) His edges are extremely thin, less than 0.005" is not uncommon

b) He uses a more coarse than average finish

c) His edge angles are lower than the normal standards

Thus the cutting ability is so highly magnified that they cut exceptionally well even when they have blunted. In is too bad he doesn't make his drop point hunters any more because they were really exceptional.

To give you some perspective, Phil Wilson got his style of edge grinding from Boye, but Boye was even thinner.

Boye also made some very interesting performing knives because of the extremes. You would get a dive knife with a 3/8" spine for prying, but with an edge 0.005"/15 dps which had extreme cutting performance and ease of sharpening.

[chukar8]

Thanks, I see the point in how not only are these tests effected by the variations of procedures used to test, but also by the steel structure as well which will vary from bar to bar. So is it the heat that causes these carbides to form, or is it a chemical process (like cement)? At what point do these carbides stop forming in the steel making process?

[Cliff Stamp]

The carbides in steel form during the cooling after the initial melt for a number of reasons, primarily because the steel simply can not dissolve them as it cools just like you can dissolve more sugar in hot water but as it cools it comes back out. The carbides that form during this stage are called the primary carbides, they form in large networks and they are broken up during the rolling.

When the steel is heat treated, it is soaked at a very hot temperature (but not enough to liquefy it), then it is quenched (cooled), and then it is tempered. Carbides will come out again during the quenching, and again during the tempering. During the quenching they come out for similar reasons (lower solubility) and in the tempering they come out due to physical changes in the steel.

However the carbides that come out after the melt are called secondary carbides and they are tiny compared to the primary carbides. Primary dendritic carbides are on the order of 100 microns, the secondary carbides are 1000 times smaller.

[chukar8]

Thanks for taking the time to explain, This is what I love about forums like this where people share their knowledge, definitely a positive atmosphere.

[The Mastiff]

Thanks for taking the time to explain, This is what I love about forums like this where people share their knowledge, definitely a positive atmosphere.

I agree. I like to learn and can get much more info out of a website when it has less drama. This website is about the best for that with the occasional heated dispute more often than not settling or stopping itself without need of moderation.

There are also some pretty talented and experienced people willing to share their knowledge with those of us who are asking.

Many company websites are mostly dedicated to selling product. Sal gives me the impression he's not just about the money when he talks to us. Even the product line with it's mule team, for example isn't there to make money so much as to give us all a learning experience with steels we may never see elsewhere, or at a much more affordable price than buying expensive knives just to try to see if the steel suits our needs.. The mules and sprints have been a large part of my whole Spyderco experience, along with the community atmosphere here with many of the designers available to answer questions to us. Even metallurgists from some of the steel producers stop by, as well as the great people from Niagara Specialty Metals who buy huge lots of steel ( heats from the foundry can be 80,000 plus lbs.), roll it to different thicknesses, widths, lengths etc. so it becomes able to be used by production companies, knife makers, etc. They are going to be a driving force behind the CPM 154 clad S90V and hopefully others to come in the future.

All in all this is a place like no other. Read the posts and pay attention. Despite my almost 3k posts I still do far much more reading and learning than talking.

Oh yeah, almost forgot. Thanks Cliff for the info on Niobium carbides. I still need to do some reading on them.

Joe

[buckthorn]

Cliff- This is fascinating material, indeed. Thank you. Two questions: In the two photomicrographs a few posts back which steel is which image? (Sorry if I should already know that- the top one has the greater carbon content, does it not?) and; Sandvik claims fine microstructure for their knife steels <http://www.smt.sandvik.com/en/products/ ... ife-steel/> I realize their steels have lower carbon content than most discussed on this forum but are their compositions/processes helpful to our discussion? Thanks.

[chukar8]

The science behind the tools we love. I'm lovin it.

[Josh623]

Thanks for the post cliff! I always learn a lot from you. So I have a few questions... Please forgive my lack of knowledge.

How do you identify carbide in any given steel structure? In other words, what what are carbides and what are examples? Secondly, can you explain how one could tell what the grain /carbide structure size is in a certain steel? Ie. How do I know if I have a large quantities of small carbides or a low content of large carbides?

[Cliff Stamp]

How do you identify carbide in any given steel structure? In other words, what what are carbides and what are examples? Secondly, can you explain how one could tell what the grain /carbide structure size is in a certain steel? Ie. How do I know if I have a large quantities of small carbides or a low content of large carbides?

To clarify do you mean from looking at a micro-graph (image) or looking at the elemental composition?

Cliff- This is fascinating material, indeed. Thank you. Two questions: In the two photomicrographs a few posts back which steel is which image?

D2 is the top one with the large carbide aggregates.

Sandvik claims fine microstructure for their knife steels <http://www.smt.sandvik.com/en/products/ ... ife-steel/> I realize their steels have lower carbon content than most discussed on this forum but are their compositions/processes helpful to our discussion?

They make steels for razor blades so they are the limit in that regard, their steels are to fine edges what Sal is to professionalism in customer response/management for example. 13C26 the Sandvik steel and AEB-L (and a bunch of other names) are all the same steel. The reason it has a very fine carbide size and distribution is not the carbon content specifically but the carbon/chromium ratio.

I wrote this a few years back : http://www.cliffstamp.com/knives/articl ... ation.html . I wrote that six years ago and some of the terms could have been a little clearer in retrospect and especially some of the discussion on hardness should be clarified in regards to micro-structure but it steel deals decently well with the issue of carbon and chromium and carbides in stainless.

[Josh623]

I guess I'm referring to the elemental composition.

[buckthorn]

Thanks Cliff-another cup of coffee and I'll use the link to read your early information. By the way, I live about four miles from the Crucible plant. It's an ancient plant but they certainly do make some good stuff. When I'm carrying something with S30 or S30VN I smile a bit as I drive past the place. By the way, I've got a custom folder by Steve Mullin (long gone from knifemaking) that's at least sixteen years old made with the early 440V. It's a nice knife but a bit difficult to sharpen!

[Cliff Stamp]

I guess I'm referring to the elemental composition.

There are two ways to approach it :

a) Use approximate formula's which come from basic metallurgy and rules of composition/regression. You can find these in the patent information.

b) Calculate the isothermals

In general you need fairly specialized knowledge to do that, for the general user then your approach is going to be more practical to :

a) find the data, you can often get this from the manufacture if you just ask

b) use some general rules (but take some care here)

Here are some general rules :

1) as you increase carbon content then you will get a higher volume of carbides (as carbides are all carbon + something else)

2) as you increase chromium you increase carbide volume and size (as chromium forms among the largest size carbide with highest tenancy to aggregate as for example molybdenum will dissolve into chromium carbide)

2) as you increase other carbide formers (vanadium, tungsten, are the major ones) you increase carbide volume

As an example here is AEB-L / 13C26 :



and here is 19C27 :



Now it should be obvious that there is a huge difference in the carbides. If you look at the elementation composition the reason is clear, there is a 50% increase in carbon content so you would expect to see a huge difference in carbides formed. Since the carbides are mainly chromium, you would expect to see large chunky carbides when you have a lot of them and indeed this is exactly what you see.

By the way, I've got a custom folder by Steve Mullin (long gone from knifemaking) that's at least sixteen years old made with the early 440V. It's a nice knife but a bit difficult to sharpen!

Thanks. 440V was certainly ambitious as a knife steel considering the huge difference over the steels at the time in regards to carbide volume.

[chukar8]

So increasing carbide volume i.e. adding carbon, tungsten, vanadium etc, the carbides stay smaller because there are more?