Interesting. Thanks.
While I don’t have a reference immediately available to support this, my recollection is that cycling through the cold temperature phase change observed in some alloys will convert more of the retained austenite than a single cooling cycle. It’s my recollection that A2 exhibits such behavior; going to liquid nitrogen temperatures causes it to pass through a phase change and corresponding density shift that, as I recall, not only converts some of the retained austenite but also reduces the size of the carbides (i.e. if I am remembering correctly, some of the carbon from the carbides goes into martensite formation).
Now, it’s been quite a while and I may be misremembering or conflating information here; but there might be a worthwhile experiment to test O1‘s response (if any) to cold cycling.
I don’t know if O1 exhibits such a phase change, let alone at what temperature that might be observed, but have you experimented with multiple freeze-thaw cycles prior to tempering?
I recall seeing pictures of distribution of carbides - I don't know if they were necessarily on average smaller, but for lack of a better way to put it, they were "thinner" and tied together to each other better or tied into the matrix better.
Carbides are sort of poorly understood, and the most complicated thing that I do is a quick heat with XHP, which gets things done about 80% as well as commercial could (the highest reaches of hardness aren't accessible to me). But stuff like ingot stainless and D2s, A2s, etc, I don't have a way to normalize them and re-dissolve the chromium carbides (PM you can cheat by not enlarging them from the state that they're delivered - in ingot, they're not so well distributed from the start).
At any rate, big carbides crack, then the crack propagates and they break up and then fall out of the matrix leaving an unsupported area. I thought they left the matrix whole until sometime last year when someone pointed me to pictures on the science of sharp and you could see the actual breaking carbides..
...or maybe better put, I read it on larrin's page first that cracks threatening toughness start in carbides and then propagate to the matrix, but seeing pictures of it solidified what larrin said more clearly - the cracking and leaving the matrix is drastic, and early. So, to have smaller carbides or more even distribution is a good thing.
I saw this problem with blue steel, which has odd tungsten carbides distributed unevenly. It's just not that good of a steel if you like uniformity and I haven't seen a sample in a plane of blue or super blue that doesn't do this. I get the origin of it (tungsten carbides melt during forging, but keeping them away must require something that modern processors don't do, or maybe nobody ever did - like continuing to forge until risky temps).
notice the pocks in the edge. Compare this to O1
this pocking starts right away and when you can compare the surface finish, the pocks aren't big enough to create visible large scratches, but the surface is more dull.
And for comparison, A2 - LN cryo treated irons (they are the best I've seen - in my limited testing IBC's irons wore less evenly and I didn't test LV's but beach shows that).
Having not really experimented with these, I wonder if the change in structure that's drastic in pictures is all cryo.
Regardless if it isn't, I've seen schedules for high vanadium and chromium PMs that are crazy long (like several days in total) and have several cryo treatments).
I often wonder how many commercial woodworking products in the turner's arena made of stuff like 10V actually get proper attention (and there's a whole bunch of other problems I never see in simple steels, like carbide coarsening).
