Once is not enough if we are making a knife. 1650F, if we didn't do anything after that, would cause large grains to be formed, however they will be uniform in size throughout the blade. However, large grains are not conducive to a good edge stability, so we need to reduce the grain size. (I think it is understood that large grains if uniformly distributed are preferable to mixed grain size not uniform). To reduce grain size, we do further normalzing, but with descending heats. Usually it is 1650F, equalize, cool in air. 1550F, same thing. 1450F same thing. Now the blade is normalized, our grains our evenly sized and small, and evenly dispersed. Our first higher heat evened out the internal structure, but made the grain large. Our successive heats helped to reduce the grain size (carbide refinement) for a good edge. Normalization is not usually needed, as steels eutectoids and hypereutectoids generally are received in the spheroidized condition....ready to machine...ready to harden. However, Aldo's particular 52100 batch had seen what is called "coarse spheroidization". While super soft and a joy to work....it must be normalized to get max hardness. Most other eutectoids and hypereutectoids are received in "fine spheroidized" condition....no need to normalize. This was for a particular batch of Aldo's 52100. While not necessary most of the time, I like to do it anyway...that way I KNOW exactly what condition my steel is in going in to hardening. Hitachi steels and the O-7 from Germany I do not normalize, as there are fine spheroidized and only require a 10-15 minute soak during austenitizing. Hope that helps explain why Kevin used this procedure for that 52100. He can certainly expound WAY beyond what I have written!
Kevin just gave you a "teaser" I guess you could say! That first heat of 1650F is critical to get the coarse spheroidized 52100 to behave correctly, the most important part of that normalizing process.