Abstract
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.
 |
Introduction
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.
Conclusion
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.
0 comments: