There are probably better terms than Carbide and Matrix in steels, or at least the matrix part. Carbide is pretty universal - hard bits in steel, but what is referred to as the matrix here is the rest of the steel.
Carbides in the steels we use are generally some alloying element paired with carbon. You can read all kinds of things about the various carbides and their hardness and size if you want. I'd love to go on about it, but it would be unhelpful other than to say in cold work woodworking tools like plane irons and chisels, we generally do best if carbides are as small as possible and as uniformly distributed as possible.
To put it relatively simple, the matrix of steel with grain boundaries kind of looks like a parched earth scene where the earth comes apart, but imagine that look if you stuck ball-like shapes in the boundaries between each cracked up segment. Some of the shapes could be balls (spheroid) and some can be longer and thinner. We can manipulate this to some extent, but I think again, you're better off reading about that elsewhere as it will get in the way of practical heat treatment by hand and eye unless you're already patting yourself on the back for making better irons and knives than you can get commercially.
you can find copious pictures of micrographs on the internet, but they're categorized for us at knife steel nerds, an excellent site run by Larrin Thomas. Scroll down and use the search function to find things like "D2", "A2", "O1" - the lighter colored portions are carbides and the rest is the matrix. You will not usually find grain boundaries as described above in these pictures unless someone goes to special trouble to isolate them, but they do show up in some, anyway, such as 20CV, if you search for those letters or scroll down. you can see a good illustration at carbides generally residing at grain boundaries, though 20CV has an enormous volume of them.
You may not want to read further than this if you don't care. Carbides for a hand and eye heat treater can be seen in tested samples but for woodworking, an experienced woodworker generally wants to not notice that they're there. I'm going to discuss what they typically do here further, but if it's TMI, that's fine.
So, what about the carbides:
* they will theoretically increase edge life
* any carbide of note that we will discuss will be harder than the matrix with iron carbides (iron and carbon) being in the older simpler steels
* for carbides to form, there needs to be some excess of carbon to form carbides, and the alloying elements usually seen in woodworking tools are tungsten and chromium, and sometimes in cold work tools (not turning tools), vanadium.
* iron carbides don't seem to have any impact on edge life, but they do have an impact on edge stability. Lower carbon steels end up in things like shovels and axe heads, and higher carbon steels (1.25% carbon, for example) end up appearing in straight razor steels, file steels and things like Hitachi White steel. Higher carbon is usually associated with higher hardness.
* there is a class of steels that intentionally avoids any substantial development of large carbides, referred to as matrix steels. this is generally achieved by alloying not so there isn't carbon in the steel, but so that there isn't enough to prefer combining with something and coming out of the matrix
* Chromium carbide is common in steels like 52100, A2, PM V11, D2
* Iron carbides are the primary carbon in steels like 1084, 1095, W1, 26c3, Nicholson and others' file steel (Carbon 130 - good luck finding that), and Hitachi White steels. There are others
* Vanadium carbides typically appear in steels that tout their vanadium content, but surprisingly, they are not generally present in "chrome vanadium" steels used in woodworking. The chromium and vanadium additions are small amounts and most chrome vanadium or chrome manganese labeled steels are more plain than what's above
* what about O1? O1 has a composition with surplus carbon, some chromium and some tungsten with manganese in a fairly large amount to make it harden more easily. I have no idea what the mix of carbides is, but they tend to be small and a separate post will talk about plate martensite, which has a big hand in why O1 and 1095 steels look really fine under a microscope but aren't as tough as steels that look like they'd be less tough
* what about blue steel? Blue steel is an older alloy - all of the above are pretty old, actually - but the point in blue steel was to add tungsten carbide to improve toughness and wear. In theory, tungsten carbide is dealt with in forging. in reality, tungsten carbides past a small tungsten addition result in poor dispersion in practice. The addition of tungsten in O1 is small and doesn't seem to cause the same problem. In blue steel, both pictures of worn edges of my own with commercial blue steel irons and Larrin Thomas's work on Knife Steel Nerds show areas of large carbides spaced far apart. This is a characteristic not so great for woodworking tools or knives, and the edge life of blue steel is a little more erratic and not as long as one would guess.
Carbides are predictive in terms of edge wear and toughness to a point, but looking at microscopic pictures only tells part of the story. In 2019 testing plane irons on here, we proved that V11 lasts twice as long planing agreeable wood as does O1. The volume of carbides in it is large, and they are (my interpretation, not confirmed by lee valley) well dispersed, and primarily chromium carbides. Lee valley and carpenter's suspected analogue both show a trace of vanadium, but my opinion is that it doesn't affect anything other than preventing grain growth when heat treating, which allows a higher terminal hardness. That's a good thing.
Vanadium is probably the wood turner's carbide, but large volumes of vanadium result in very hard but brittle carbides, and I've never realized any real benefit from them. I think they'd do well in a standard test, but in regular hand planing, they suck with two exceptions - CPM 3V and Magnacut steels. We won't be doing anything with these, but they both have carbides small enough that there's not much detriment at an edge. CPM 10V looks great under a microscope, but if I were chasing edge life, I'd never use it over V11 or Carpenter XHP (again with the latter, good luck finding it - it's probably stored somewhere in a big container with the last remaining Carbon 130). Vanadium is loved by coarse edge knife fanatics, and it does really well in standardized tests that aren't much like woodworking, like cutting cards full of sand well past what we'd consider dull. Vanadium is also excellent for metal on metal rubbing surfaces, but so are large carbides and we don't like them in woodworking tools, either.
3V is OK, and magnacut is what it claims to be - it's a novel way to keep vanadium carbides small at some volume and keep chromium in the steel matrix and out of large carbides. the result is a steel that wears pretty well, is very fine, and very stainless. it's novel in that it didn't exist until very recently when everything else mentioned did. I bought a magnacut plane iron, of course, just to test one. I think it makes an OK plane iron steel, but offers no real practical benefit beyond stainlessness, especially not in proportion to the cost. I would imagine it makes a dynamite kitchen knife steel for cork sniffing knife folks, but even there, it doesn't really solve any practical problems that would affect "some guy who uses a knife a lot and is good at refreshing the edge".
Looking at all of these things is interesting, but not necessary to make good tools. it's marginally necessary to chase results that challenge the best japanese smiths or identify problems with snapped samples or worn edges.
The only easy thing I've found to do is to put a chipbreaker on a plane iron, put it close to the edge, and plane a couple of hundred feet for a few minutes. The result is something like this .
In that picture, the chipbreaker has pressed the exiting shaving down into the wood, allowing it to rub the plane iron on the way out as it's held down. The tiny comet looking things are carbides. They are most likely round, owing the comet tail to the fact that the carbide is protecting the matrix of steel behind it. Before planing with the iron, it looks like this. The scale of the picture shows them to be a couple of microns, and they show up because they cast a shadow. this is my picture taken with visible light and nothing special - somewhere around 1 micron, they don't deflect light even if they're still there, and you can't see them. instead, the worn surface looks like fudge like this worn edge picture of an old English Mathieson plane iron.
Note that you don't see anything in the freshly sharpened iron. Chromium carbides aren't as hard as common synthetic abrasives, so they don't appear after sharpening - they're "cut down" to be level with the steel matrix around them.
Just keep in mind these pictures aren't definitive. O1 will show almost nothing compared to this orderly array in 52100 ( a ball bearing steel ), 52100 makes a much tougher knife and a better ball bearing, both last about the same amount of time in wood, and O1 picks up a shaving more easily while cutting.
Note, too, you can see why non-powder D2 steel isn't that well liked. Chromium carbides are enormous and widely dispersed.
All of this helps to put pictures to the word "tough carbides". there really aren't tough carbides - there are hard carbides. they're less tough than the steel around them, but it sounds good and the word toughness is often misused to describe hardness or strength in steel.
But, if you ignored everything below the dotted line above, I think it will not make any difference in your toolmaking. these types of edge pictures are confirmatory for a more serious amateur, like me, wanting to confirm that my forge heat treated samples have a structure at least as fine as commercially heat treated micrograph pictures. If you are pushing boundaries or solving a bad sample of steel purchased (more practical to just move on and find a better supplier), you might want to see something like this. 99% of the time, you won't.
And very lastly, you might think of the term carbide as the teeth on saws. Enormous surpluses of carbides aren't useful to us and I've never looked at close at what "carbide" is in a saw tooth. I suspect it's when the composition of the metal used in a tooth is so high in carbide content that it passes an arbitrary content, and maybe at some point. That would explain both hardness and why straight up carbide is brittle and needs a very steep angle in a cutting tool. And still will break or chip if conditions aren't great.
