If you are referring to the steel grade specified in SAE AMS 6418, the answer is yes it will respond to austempering, however, the transformation time to bainite will likely be greater than 24 hours, which usually renders the process unsuitable. Highly alloyed steels, especially those with considerable amounts of nickel, are generally not used for austempering due to this phenomenon. These grades have been created specifically to stabilize the austenitic phase so that they can be quenched into martensite, and then subsequently tempered to produce high strength and high toughness.
From what I could find on AMS 6418 (aka as Hy-Tuff steel), and depending on size (thickness), I would say that it can be austempered. The Nickel content is in the range of 1.65 to 2%.
