Carbide tear-out questions

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I have read numerous comments on hard steels having vanadium carbide tear-out during sharpening. I had never heard of this until recently. Are these carbides unmelted steel, pieces added later that aren't melted, or formed during the heat treating? I always assumed that blades are made from homogenous steel.
 
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formed during the heat treating? I always assumed that blades are made from homogenous steel.

I'm no big expert and owe BladeForums for most all I know but yes; formed during heat treat. They are extremely fine (hopefully . . . if the heat treat is done right).
Homogenous steel is mild steel (not heat treatable). Google some microscope metallurgical photos of high vanadium carbide steel. It looks like a jigsaw puzzle or a mosaic.
 
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Vanadium & chromium carbides are actually first formed in the manufacture of the alloy, before it's ever heat-treated. Subsequent heat treat can change their properties somewhat (size, hardness to some small degree), but they exist in the steel from the time of it's manufacture, formed at temperatures up around 1000°C.

Different manufacturing processes, like the powder metal (PM) process, can make a difference in how large they get and how uniformly distributed ('desegregated') they are in the manufactured steel. PM-process steels produce carbides that are typically pretty small, usually 2 - 4 microns or smaller, and they're more uniformly distributed throughout the steel, which helps it remain strong and tough overall. Non-PM processes would tend to allow the carbides to cluster together in large masses, the effect called 'carbide segregation', during manufacture. In such cases, the carbides may end up being 50 microns or larger in size. So, they'd still be there, but at potentially much larger size, which makes grinding such steels more difficult, and also can negatively alter steel properties such as toughness (resistance to breakage). As an example, the difference in ease of grinding or sharpening can be seen in comparing conventionally-manufactured 'ingot' D2 with it's huge chromium carbides (making it harder to grind easily), versus CPM-D2 and it's much smaller chromium carbides (easier grinding/machining), made by the powder-metal process.
 
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David Martin

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I cannot recall the members name but he is educated in this area and has access to SEM and has taken very detailed photos of high
vanadium steels sharpened at various stages. His conclusions are that vanadium carbide tear out does not occur. DM
 

FortyTwoBlades

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It's something that seemingly can occur in some rare circumstances, but is extremely unlikely in most of the situations it's commonly thought to occur in.
 

David Martin

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Most of us do not sharpen the vanadium steels fine enough for the grit to begin touching the fine carbides. DM
 

FortyTwoBlades

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Not just that, but even when doing so, it doesn't generally result in carbide tearout, just blunting of the carbides.
 
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I cannot recall the members name but he is educated in this area and has access to SEM and has taken very detailed photos of high
vanadium steels sharpened at various stages. His conclusions are that vanadium carbide tear out does not occur. DM

I think Todd would disagree that your statement is equivalent to his. I think a more correct statement would be that carbide tear-out did not occur under the exact parameters of his test.

And I have seen it happen under certain circumstances quite frequently - namely at very high hardness levels with HSS. How much this happens and how severe it is will be pretty dependent on the circumstances - steel type, hardness and abrasive used.
 
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I think Todd would disagree that your statement is equivalent to his. I think a more correct statement would be that carbide tear-out did not occur under the exact parameters of his test.

And I have seen it happen under certain circumstances quite frequently - namely at very high hardness levels with HSS. How much this happens and how severe it is will be pretty dependent on the circumstances - steel type, hardness and abrasive used.

This is what I wonder about too. It seems, since a steel's toughness and hardness directly impact how easily it might break or be damaged, it could also impact how easily (or not) the carbides might be torn out of the matrix. In other words, with the matrix steel being variably tough or strong, according to specific manufacture and heat treat, it may in some cases behave like the binders in sharpening stones, having more or less tendency to break down and 'release' the carbides, as a stone releases grit from the binder.

I've read at least one reference mentioning this type of wear (carbide tear-out) in tungsten carbide tools specifically, and how the 'matrix' material used in them (cobalt, usually) can be engineered differently in terms of it's toughness and strength, and therefore it's ability to hold the carbide grains themselves. And in the context of those tools, the size of the carbides themselves are mentioned as making a difference too, with smaller carbides less-prone to being pulled out.

It just seems to me, there are probably too many variables to say definitively if it actually happens or not. Or put another way, proving that it NEVER happens, I think, would be exceptionally hard, with so many possible variables affecting it.
 
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Oh it definitely DOES happen. I posted a series of microscope photos some time ago in a "super-steel" stropping thread showing a progression on a HSS blade where it was very clearly evident they were being torn out on lower grit Shapton stones. At higher grit they stayed put but were prominently exposed. HeavyHanded HeavyHanded will probably remember that thread.
 
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Oh it definitely DOES happen. I posted a series of microscope photos some time ago in a "super-steel" stropping thread showing a progression on a HSS blade where it was very clearly evident they were being torn out on lower grit Shapton stones. At higher grit they stayed put but were prominently exposed. HeavyHanded HeavyHanded will probably remember that thread.

I do remember seeing at least one such thread, with a lot of input from HH. I'm inclined to believe it can happen, myself.
 

ToddS

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It seems that some people think of carbides at analogous to smooth stones in cement. We have all seen broken concrete with those stones "pulled out" leaving a hemispherical void. The problem with this analogy is that the carbides in steel are actually securely bonded to the surrounding iron.

Carbides are more brittle than the surrounding iron, so they will crack if they are subjected to the bending that a thin apex or burr experiences during sharpening.

These images (S30V) show the spheroidal vanadium carbides (typically 1-2 microns in diameter)near the edge.

The first one shows a couple of vanadium carbides right in the apex, thinned to a keen edge with no evidence that "tear out" can occur.

s30v_06.jpg


The second image shows the vanadium carbides around a micro-chip with some cracking and one is possibly separating from the surrounding matrix. This is the closest I have seen to "tear out."

s30v_03.jpg
 
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I think that the amount of sharpening on the lower hardness abrasives is a large determining factor too. Heavy bevel setting for a long while might tend to remove them. Of course the steel type and hardness (high hardness is more brittle, would tend to shed them easier). And the included angle of the blade as well. If one were to just kiss the edge after diamond sharpening I'm sure it would be just fine. I also think it's pretty self-evident that quite a few people are reporting worse edge longevity when some of these steels are sharpened on non-diamond abrasives. Size and variety of carbides is another factor - the ones in my images were about 4-5 microns, and I think they're tungsten carbides. An aside: many people use the term "tear-out" but it could well be just carbide fracture/breakage. Either way, the net result seems the same. Voids with no carbide and a ragged edge. The fine edge gets wiped out and the edge radius becomes larger until the carbides still in place are reached. The edge may still continue to cut quite well with a "sawing" motion, but push cutting is degraded.
 
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I have read numerous comments on hard steels having vanadium carbide tear-out during sharpening. I had never heard of this until recently. Are these carbides unmelted steel, pieces added later that aren't melted, or formed during the heat treating? I always assumed that blades are made from homogenous steel.
Hi,
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https://knifesteelnerds.com/2018/08/27/what-is-edge-stability/





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