I don't feel like looking it up. Does, say 12c27 have much in the way of carbides at all? There is grain structure and there are carbides.
Blessings upon you for saying it before I did.
The sizes given are usually given for standard heat treatments or ones that are specifically trying to reduce carbide size, or for PM steels, which is more how the steel is made than how it's heat treated. If you want the finest carbide size possible, you need to use a steel with relatively low carbon content, and you'll likely have little to no carbide at all.
Something to keep in mind when discussing stuff like this is that steels have more carbide in them when they are annealed than when they are hardened. That sounds bass-ackwards I know, but that's the way it works. For example, 1095 has about 15% carbide in the annealed condition. When hardened, it drops down to around 3-5% depending on how it was heat treated. When heat treating simpler, low alloy steels, one has a great deal of control over the carbide size. More complicated steels have carbides that cannot be as easily controlled. This is why there are no CPM versions of 1095 and 5160.
CPM steels generally have carbide sizes in the range of 4-8 microns. These are kind of fixed, as the temperatures needed to reduce the carbide size are detrimental to other things, like grain size. In simpler steels like 10xx series, the temperatures are not as high and negative effects can be dealt with easier. Simpler plain carbon and low alloy steels (10xx, O1, 52100, etc.) will have smaller carbides than the CPM steels, generally 1 micron or less. AEB-L, 13C26, 420HC, 425M and maybe 12C27 are in the same category. 12C27 can be heat treated with either very small or no carbides. The others can too, but generally are left with some carbides for wear, where 12C27 is more concerned with corrosion and other things.
There has been a lot of research over the years in how to reduce carbide size in various alloys. If you're willing to do some intricate heat treating, it's pretty complicated/interesting, but you can get extremely fine carbides far outside typical sizes for standard heat treatments.
Now, I would be thinking "what non-standard treatments do people do?" Look around in Shop Talk for all kinds of home expedient heat treatments, particularly in older threads.
