kbuzbee wrote:It's a good question. As I'm coming to understand there are two factors at work here.
The size of the largest carbides
The total volume of carbides
Increasing either reduces edge stability as your edge gets thinner and more refined. Increasing both even more so.
Yes, what happens is that as the volume increases the carbides simply form so close to each other they aggregate into super-carbide structures which are far larger than the individual carbides themselves. This is no different for example than increasing the population of people in a court yard for example. At some point the population will be so large that you will see groups of people form which are far bigger than the individual people. If you keep increasing it the groups will merge and at some point, even though the people themselves never get any bigger, there will be that many people there is almost no free space (i.e., 121 REX).
I would love to find a carbide size chart to help some of this analysis.
The carbide sizes are effected by heat treatment as well as volume, so it isn't trivial. For example there is nothing stopping you from actually dissolving almost all of the carbide in S30V into the steel if you wanted, you would just have to soak the steel hot enough. However that would be a rather odd thing to do as what did you put it in there in the first place for then?
In short, if you want to see what it looks like carbide wise, you need to look at a properly prepared micrograph - but again, realize (and this is critical), the thermal processing effects this greatly. You can easily process AEB-L and make it very coarse both in carbide and aus-grain, though again I don't know why you would do that, but you could.
And I have "weird" thoughts, like iirc Niobium carbides are quite small and very hard, so what would happen if you made a partical steel with only iron, carbon and Niobium, would that work? What kind of edge would that have?
All of the steels made to cut things and retain a sharp edge are made with that perspective in mind. As you no longer focus on the sharp part you simply ramp up the carbide volume and as well usually the cobalt and moly to dramatically increase the hot hardness as usually such tools are ran at extreme speeds as they are roughing cuts.
Niobium, and nitrogen are becoming more used now for a number of reasons, one of which is that they form very hard and very small carbides, but how and when they form is also critical. But thermal processing as well is also advancing by changing how steels are rolled/normalized to effect the size of the martensite lathe packets inside the aus-grain.
This has a dramatic effect to increase all of :
-wear resistance (at the edge, not grossly)
Note it increases strength and toughness at the same time. The only downside of course is cost so it is mainly done now in parts of extreme service demands (aerospace is one such market).
captnvegtble wrote:... maybe it's better not to have a thinner grind?
It isn't the thickness but the angle which is critical because even what we would consider to be very thin edges are very thick compared to carbides. To be specific, 0.001" is 25 microns, so even a very thin edge of 0.005" is > 100 microns. The only steel which has aggregates that size is D2 (assuming normal steels, there are others but they are rarely if ever used in knives). The cobalt alloys also have very coarse carbide aggregates as well, similar to D2.