Abstract: Apparatus and method for cryogenically heat treating steel work-pieces to cause the super-strengthening (higher strength, ductility, better fatigue properties, etc.) of same is disclosed. The apparatus includes a retort having a cavity containing a fluidized bed of particles. A cryogenic liquid cools the fluidized bed of particles and a fluidizing gas activates the fluidized bed. Steel work-pieces placed within the fluidized bed are rapidly and continuously cooled from their austenite phase through their martensitic phase transformation causing the work-pieces to have a finer martensitic structure, with higher dislocation density, than that which occurs in work-pieces that are hardened by other heat treating apparatus and/or methods, and which exhbit better physical properties (e.g., higher strength, ductility, better fatigue properties, etc.).