I do all of my tempering in low temp salts, the thermal conduction is many times faster than an oven and the heat is completely uniform. I do most of my "walking in tempers" to determine my desired HRC for 30 minutes, which would be impossible with any type of oven, and then my long soaks. After studying 52100 inside and out, most only guess from the outside, I am convinced that most of the fantastic performance gains many makers have gotten from it with oddball heat treatments is a matter of contrast and elation at overcoming all the issues that you can get with that steel if you are not very careful. I have often joked that 52100 scares me that there could be some element in it that burns out and gets into a smiths brain because it seems that whenever I hear of some of the strangest and convoluted heat treating practices 52100 is most often involved somehow.
But joking aside 52100 is just steel but like all modern steels it has its own unique chemistry and heat treating parameters based on its application. It was designed and meant for bearings, so not only do you have to realize its unique heat treating requirements, but you need to adjust what industry gives you to make a cutting tool out of it, not so for something like say O-1 which was designed for may cutting and slitting type applications. For bearings compressive strength is very important and abrasion resistance is a close second, heat treating for such deals with retained austenite a little differently than what we do and 52100, for a simple steel, is very prone to retained austenite. As Ed mentioned the heating cycles in forging can disperse and refine the carbides (not the magic hammer mind you, but the heat cycles). Avoiding slow cools from above critical, such as many of the traditional bladesmithing anneals is essential. When heating the book will tell you 1550F, but that is under the assumption that you are making bearings and will have different approaches for dealing with retained austenite. If you keep your austenitizing temp below 1500F you can expect to reach as much as 67 HRC as you reduce the amount of RA.