Cryogenic Hardening


Cryogenic hardening is a cryogenic heat treating process where the material is cooled to approximately −185 °C (−301 °F), usually using liquid nitrogen. It can have a profound effect on the mechanical properties of certain steels, provided their composition and prior heat treatment are such that they retain some austenite at room temperature. It is designed to increase the amount of martensite in the steel's crystal structure, increasing its strength and hardness, sometimes at the cost of toughness. Presently this treatment is being practiced over tool steels, high-carbon, and high-chromium steels to obtain excellent wear resistance. Recent research shows that there is precipitation of fine carbides (eta carbides) in the matrix during this treatment which imparts very high wear resistance to the steels.

X-Ray Diffraction Analysis

The volume fraction of retained austenite is plotted against the subzero treatment temperature in Fig. 4.4 for specimens austenitized at 1293K and 1373K. The volume fraction of retained austenite is 12% for as quenched specimens after austenization at a 1293K and approximately 6% for specimens after cold and cryogenic treatment. However, it decreases with treating temperature going down for specimens’austenitized as 1373K.

Stress relieves

The other major reason for the improvement is stress relief. The densification process leads to an elimination of vacancies in the lattice structure by forcing the material to come to equilibrium at –320°F and lowering the entropy in the material. This lower entropy leads to the establishment of long range order in the material which leads to the minimization of galvanic couples in the material thus improving the corrosion resistance of materials including Stainless Steels. Besides, there is some amount of grain size refinement and grain boundary realignment occurring in the material.



          Heat treating is what gives metals its hardness as well as its toughness, wear resistance and ductility. Even performed properly, heat treating cannot remove all of the retained austenite (large, unstable particles of carbon carbide) from steel. Proper heat treating is a key part in increasing a part's toughness, durability, wear resistance, strength and hardness.  The beneficial changes that occur as a result of the heat treat process do not actually take place during the heating, but, rather from the cooling or "quenching" from the high temperature.  


Cryogenic hardenings can produce not only transformation of retained austenite to martensite, but also can produce metallurgical changes within the martensite. this offers many benefits where ductility and wear resistance are desirable in hardened steels.