British metallurgist Graham Clarke shared his thoughts on how to get good grain size. There’s a good discussion of quenchants starting at the 4:47 mark. 

https://youtu.be/Yv5Xz8rI6Fs

 

Dr. Larren Thomas of Knife Steel Nerds also has a good video about thermal cycling and grain size:

He makes the very interesting point (which he expands on considerably in his book) that while multiple cycles do improve grain structure, if the initial normalization has been done properly, there’s no measurable improvement in toughness.