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OK, I think you are saying that you are placing the blade in a hot forge that you turn off, allowing both to cool slowly overnight. Still a lot of ungiven info. Type of forge (gas,coal) and temperature at start.
Here is one procedure - With a gas forge, bring the blade up to critical temp and turn the forge off. Let the blade cool with the forge for six hours. Another (better) system is to bring the blade to critical and place in a container of vermiculite ( or wrap in a roll of kaowool) and let cool for 6 hours. The metal should be completely annealed with either method. The risk of the first is more decarb if the temperature stays too high for too long.
Perhaps one of the coal forgers will post as to the merits/ risks of placing the blade in the coals and turning off the air flow, letting the fire go out with the blade in the coals and ashes.
I use the gas forge or an electric over to anneal.
The perfect solution would be to put it in a programable electric oven and let it cool from 1500F to room temp at a rate of 50-100 F per hour.
Annealing is mostly a matter of where and how you want your carbide. As free carbon within the ferrite is what you are trying to eliminate in order to have soft steel, that carbon has to go somewhere. If not dissolved in the ferrite it will gather together in other ways. In simple allows it will make iron carbide, in the alloys discussed here it will find chromium atoms to snuggle up with. Slow cooling will allow simple alloys to make pearlite through the grains, how coarse will depend upon how slow you cool it. Richer mixes will have extra carbide that will want to go straight for the grain boundaries on slow cooling (actually they are sort of ejected there by the other processes). So what of it? Well, nothing as long as you have sufficient temperature and time to get those buggers out of the grain boundaries on the next heat treatment, or they are going to result in embrittlement, just as large, coarse spheroids in spheroidizing will result in inconsistencies if not dealt with in the final treatments.
In this case it is matter of stopping and asking why you are annealing instead of just doing it because that is the way it is done. Machining operations will be easier with one type of anneal, just grinding gives more freedom. Delbert pointed out the differences with his anneal, this is a function of the alloy, the carbides in those steels will be a bear if you dont ball them up and take them out of play. The chromium in the 52100 will give you some of the same headaches but not to the same extent so if you are just grinding, you have more options open to you. If you were working 10XX a very gentle normalizing could replace the anneal and eliminate the grain boundary carbide issues. In fact with this steel one could heat above critical, quench to room temperature and then normalize a few times at temperature the never exceed nonmagnetic and get a pretty workable blank.
Steel selection, subsequent tooling operations, final heat treating techniques etc will all determine what temp and how long you anneal.
Do I enjoy taking what has always been very simple processes in blade making and turning them into a convoluted technical nightmare? Yes, sometimes I do.
Looking out for the steel alone, our time personal considerations aside, the less time spent at high temperatures, the better. Unfortunately so many of our operations require some time, so we should look for the least amount of time to get the job done and call it good.
Lamellar annealing (heating above critical and slow cooling to transform to pearlite): The cooling operation only need concern those temperatures where pearlite will form the quickest, around 1250 to 950F for 52100. Simply air cooling from critical will form superfine pearlite with bainite and will not be very fun to work afterwards. The wood ash trick will give mostly fine pearlite, workable, but drilling it may be touch and go.
With your forge trick an isothermal procedure would maximize your time. If you want pearlite all you need is that lower temperature range, the higher in that range the coarser the pearlite (which is softer) but the more carbide that may be ejected into the grain boundaries. So an ideal way to do it would be to have the forge at 1200F. heat the steel to critical in another heat source and put it in and seal it up. The rest of the time is only what is necessary for the pearlite transformation to complete, after that you could quench the steel to room temperature and not worry about hardness.
If one has this kind of control the transformation requires minutes not hours to be complete around 1200F., the reason bladesmiths go with hours is because they are dealing with continuous cools from 1450F. or better, to below that range, if you interrupt the process at any point before the austenite is completely converted, there will be a mixed microstructure that will give you some issues. So we are back to not what the steel needs but what your equipment can give it. The real concern in decarb and coarsening is during the unnecessary period from critical to 1200F where the austenite is just sitting around with nothing to do but get into mischief. After it has completed the transformation, pearlite is about as stable as it gets for phases and you can do whatever you want, so as soon as your forge drops below 950F no progress of any significance is being made with your anneal, if you want to get back to work quicker pull it out and let it air cool. But on the other hand, since not much happens to pearlite below 950F you could just leave it there and go to bed without worrying about it.
Spheroidal annealing: The other option I mentioned would be to heat to critical and quench your blanks to fully harden them and then heat to just below nonmagnetic several times. Not only will this refine grain like crazy, but it will take that carbon that is trapped entirely in solution from the hardening and ball it up into tiny spheres evenly distributed throughout the matrix. Cutting time spent with nasty decarburization, no grain growth, no worries about excessive grain boundary carbide. And as soon as you heat it the last time you can set it in the air and walk away. If this sounds too good, there is a downside- you will need to soak a little longer than pearlite to get spheroidal cementite back into solution when you do the final hardening.
I did it again I took a whole page just to say that all you really need to worry about is the time spent in your forge from critical to 1200F after that, no harm done
Sticking with your current method, I am saying that after the forge drops below 950F it doesn't matter what you do, take it out or leave it in for a week. If you go to critical before the slow cool, it is important that you spend enough time from 1200 to 950F to get the job done and that you not allow the blade to cool too quickly from critical until it has spent that time at around 1200F. in order to turn all that austenite into pearlite, otherwise it will turn into something else. For the most part you are at the mercy of the cooling rate of your forge. Heck, other than decarb you can undo any messes made, or make whole new ones in the next phase of heat treating anyhow.
I see the purpose of these forums as one of the greatest ways in history for the maximum ammount of people to learn from one guys questions, that is why I have the horrible habit of giving so much more information than what the original questioner needs. Also I am not of the school of simply following odd recipes handed out by "gurus", that is fine for cults but I like the idea of knowing all the underlying reasons in order to have the ability to work independently of the guy handing out recipes. I hope this hasn't caused undue confusion