I must admit to being a bit in the middle of two directions being approached here. First I must say that although multiple quenching is not my cup of tea I can live with folks doing what they feel works, but the 24 hours in between is nothing less than nonsense that undermines whatever credibility the technique has to begin with, this is not directed at you Erik as I doubt you would claim this as your own invention, but instead a criticism of the popular information pool you are doing your best to work with, and being victimized by. Mete or I or any number of folks with a even minimal grasp of the mechanisms at work could give good and sound reasons why there is nothing there, those on the other side for the most part can only offer that this is what they heard they should do, and the ones they heard it from mostly only have "well I did it one time and it seemed to make a difference". You know one day I ate oatmeal and juice for breakfast and that day all of my forge welds came out perfect, perhaps the secret to perfect damascus is oatmeal and O.J. ? Or perhaps, if I make the effort and take the time to look a little deeper and analyze it, I would see that I normally skip breakfast, and that day I finally had the energy to be at full speed in my work. After realizing this I can dispense with turning every little thing I did that day into my firm knifemaking ritual and focus on the exact things I did that day that resulted in a tighter solid state weld.
Now I am going to flip flop on you folks so I can p!$$ everybody off. On the other hand I have pointed out before that I do not see the benefits in tempering untransformed austenite, so why the rush before Mf? The time to temper simple steels (stainless alloys not included) is when Mf is reached, doing it before could get you more retained austenite than what you need. "hand warm" is a feeling not a temperature, hand warm to you will definitely be different for me. Every steel has its own Mf so picking a given temperature to stop the quench at and applying it to all would be as inappropriate as using one austenitizing temp for all. If you are going to interrupt the quench before Mf do it at Ms and get the benefits of martempering. Spec sheets recommend this because much of industry could work with huge varying cross sections with a bazillion little splines comming off, not a simple flat piece of steel with an edge, under the former circumstances to heck with retained austenite, lets keep that $$$$ part in one piece for now.
Simple steels should not see any significant gain in hardness from freezing since they are not prone to retained austenite, I have often said that if you are seeing noticeable jumps in hardness from freezing 10XX something was definitely not right in your initial heat treat, tempering untransformed austenite before Mf could be reached would be one of those "somethings".
I work mostly with O1 and L6, if ever there were two simple alloys that will come apart by waiting on the temper these are it, in martempering I always let them go to room temp and then often cool them even more in cold water before the first temper, I have never cracked a blade doing this and I can give the actual reasons as to why if needed.
Flip flop again and I can say that I have blown blades apart by quenching all the way to Mf and then waiting overnight to temper, the poor little blades simply cannot handle the forces exerted by all that BCT (body centered tetragonal) martensite for extended periods. If you got all your carbon into solution and locked it there, you have forced atoms to assume positions they do not want, history has shown us how much power can be involved when man starts forcing atoms to do things they don't want to do. No hammer, no fire, no press or rolling mill can compete with strain of that magnitude and if you wait to relieve it something will have to give. To the folks that regularly get away with waiting days to temper and think this paragraph is hogwash, I could give the standard line that you have been lucky, or I could point out that a very good explanation is within this paragraph if you reason it out. Insufficient austenization (too cool or too short) will not get all available carbon into solution. Insufficient austenization done repeatedly will effectively take carbon out of solution by segregation so of course the blade can handle the wait to temper, not much BCT stacking to convert to begin with. I could go into what happens when plate martensite habit planes impinge on each other, or the effects of increased dislocations, but I think we have more than enough to get the point already.
I guess what I am saying is that we knifemakers need to abandon the comfort of over simplifying this stuff, it is never as simple as it may seem, as this two pager post demostrates all too well
724wd, I would preheat to let the oven even out and stop it heavy fluctuations to regulate before putting the blades in, better yet as mete pointed out with the brick or a couple larger piece of metal for the blades to go between preheated with the oven and would providing a nice thermal mass to even things out. Temper for at least an hour and a half to two hours. Take the blades out and let them cool, heck I quench them to get to work quicker. I personally wouldn't turn the oven off because I will want to temper them again as soon as the blades are cool or I have done some hardness tests. I often do three tempers and have very consistent observations and records to show the homogenizing benefits of it, on the last temper just shut the oven off and walk away. Extended times at reasonable temperatures seems to be more beneficial than shorter times and higher temperatures.
I'm going to reread it right now.