Originally Posted by tkrussell

I'm not a scientific metallurgist, but my company makes iron fittings, so I know a fair amount about iron alloys (cast iron and steel).

All iron and steel are alloys with carbon, chromium, molybdenum, vanadium, nickel, silicon, etc. in specific proportions. As the alloys cool from the molten state, they crystallize in particular forms depending on the alloy and cooling rate. Using a microscope, one can see the crystalline structure. To simplify slightly, the alloy and cooling rate determine the crystalline structure, and the crystalline structure ultimately determines the hardness, toughness, brittleness, etc of the part.

After the part is cooled, the crystalline structure can be modified by reheating to a specific temperature, and cooling at a specific rate (annealing, tempering, hardening) The surface can be modified by chemical treatment to modify the structure chemically (case hardening or nitriding (Tufftriding). Finally all alloys evolve their crystalline structure over time even at room temperature. Aluminum alloys do it faster than iron alloys.

Cryotreating accelerates the crystalline evolution of the alloy to the ultimate structure, which as it happens is harder than the original. (The usual technical term in iron alloys is martensite transformation into pearlite) Pearlite is a submicroscopic arrangement of Ferrite (iron) and cementite (iron carbide). The iron carbide gives hardness, and the small grain structure gives toughness. Again, this will happen over long time at room temperature, but cryotreatment accelerates and makes uniform the process. So much for tech talk. It's actually much more complex. Some alloys benefit more than others, and some parts may be adequate without treatment.

The structure can be verified microscopically. This means that cryotreatment has an objective means of measurement. (One could also do strength tests). In other words, it's not just hocus pocus.

Hope this helps in understanding the process.