CRYOGENIC HARDENING

The process of cryogenic hardening involves cooling the material to about 185 °C (301 °F), typically with liquid nitrogen. If the composition and previous heat treatment of a given steel allow for the retention of some austenite at room temperature, it can have a significant impact on the mechanical properties of that steel. It is intended to enhance the quantity of martensite in the crystal structure of the steel, boosting its toughness occasionally at the expense of strength and hardness. To get superior wear resistance, this treatment is currently applied to tool steels, high-carbon, high-chromium steels, and in some circumstances, cemented carbide.?According to recent study, this process precipitates very fine carbides (eta carbides) in the matrix, giving it a very good wear resistance.

The transformation from austenite to martensite is mostly accomplished through quenching, but in general it is driven farther and farther toward completion as temperature decreases. In higher-alloy steels such as austenitic stainless steel, the onset of transformation can require temperatures much lower than room temperature. More commonly, an incomplete transformation occurs in the initial quench, so that cryogenic treatments merely enhance the effects of prior quenching. However, since martensite is a non-equilibrium phase on the iron-iron carbide phase diagram, it has not been shown that warming the part after the cryogenic treatment results in the re-transformation of the induced martensite back to austenite or to ferrite plus cementite, negating the hardening effect.

The phrase "cryogenic tempering" is technically erroneous because the transition between both phases occurs instantly and is not dependent on diffusion, as well as because this treatment induces more complete hardening rather than reducing extreme hardness.

It is not necessary for hardening to result via martensitic transformation; it can alternatively be achieved through cold work at cryogenic temperatures. The materials alter as a result of the flaws generated by plastic deformation at these low temperatures, which are frequently quite distinct from the dislocations that typically form at ambient temperature and in some respects resemble the effects of shock hardening. Although this method is more efficient than conventional cold work, it mostly serves as a theoretical testing ground for more practical methods like explosive forging.