Uniform Quench Renewal Rates (UQRR)
UQRR involves the proper application of quench cooling during heat treatment. Agitated air, inert gas, or other types of quenchants, create a uniform rate of cooling in the “contiguous grains” of austenite as the grains cool to the martensite start temperature and harden. The rate of cooling must be fast enough and uniform enough at the hot part shell to “miss the nose of the curve” for the martensite start temperature for a particular alloy’s hardenability. At the same time, the quenchant must be “renewed” to not overheat the quenchant at the part surface.
The key to controlling distortion and avoiding part cracking is to apply the quenchant with a Uniform Quench Renewal Rate (UQRR). The problem with oil, polymer water and traditional water or brine quenching is at the initiation of quenching, there is a film boiling (a steam blanket of interconnected large bubbles) that insulates the hot part surface. As the quench cooling continues, the film boiling gas vapor blanket will begin to chaotically collapse, and the film boiling gives way to another phase of boiling called nucleate boiling.
A UQRR that simultaneously “instantly-impacts” the entire hot part shell can “set” the part shell geometry, prevent part cracking and provide a uniformly higher as-quenched surface hardness (for the given material’s hardenability).
Once the shell of the part is hardened uniformly, the part geometry is set, and the layers below will cool by uniform conduction. The combination of the UQRR at the part surface, and the uniform cooling of the core by conduction through a part’s given mass, transforms the part into its final hardened shape and virtually eliminates the problem of heat treating distortion.
As long as the quench cooling rate is UNIFORM at the part shell, whether the quenchant is gas or water, the part will not crack.