Cryogenic hardening
Encyclopedia
Cryogenic hardening is a cryogenic heat treating process where the material is cooled to approximately −185 C, usually using liquid nitrogen
. It can have a profound effect on the mechanical properties of certain steel
s, 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.
The transformation from austenite to martensite is mostly accomplished through quench
ing, 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.
The transformation between these phases is instantaneous and not at all dependent upon diffusion
, and also that this treatment causes more complete hardening rather than moderating extreme hardness, both of which make the term cryogenic tempering
technically incorrect.
Hardening can also be accomplished by cold work at cryogenic temperatures. The defects introduced by plastic deformation
at these low temperatures are often quite different from the dislocations that usually form at room temperature, and produce materials changes that in some ways resemble the effects of shock hardening
. While this process is more effective than traditional cold work, it serves mainly as a theoretical test bed for more economical processes such as explosive forging.
Many alloys that do not undergo martensitic transformation have been subjected to the same treatments as steels—that is, cooled with no provisions for cold work. If any benefit is seen from such a process, one plausible explanation is that thermal expansion
causes minor but permanent deformation of the material.
Liquid nitrogen
Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...
. It can have a profound effect on the mechanical properties of certain steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
s, provided their composition and prior heat treatment are such that they retain some austenite
Austenite
Austenite, also known as gamma phase iron, is a metallic non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of ; other alloys of steel have different eutectoid temperatures...
at room temperature. It is designed to increase the amount of martensite
Martensite
Martensite, named after the German metallurgist Adolf Martens , most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is formed by displacive transformation. It includes a class of hard minerals occurring as lath- or...
in the steel's crystal structure, increasing its strength
Strength of materials
In materials science, the strength of a material is its ability to withstand an applied stress without failure. The applied stress may be tensile, compressive, or shear. Strength of materials is a subject which deals with loads, deformations and the forces acting on a material. A load applied to a...
and hardness, sometimes at the cost of toughness
Toughness
In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing; Material toughness is defined as the amount of energy per volume that a material can absorb before rupturing...
. 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.
The transformation from austenite to martensite is mostly accomplished through quench
Quench
In materials science, quenching is the rapid cooling of a workpiece to obtain certain material properties. It prevents low-temperature processes, such as phase transformations, from occurring by only providing a narrow window of time in which the reaction is both thermodynamically favorable and...
ing, but in general it is driven farther and farther toward completion as temperature decreases. In higher-alloy steels such as austenitic stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
, 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.
The transformation between these phases is instantaneous and not at all dependent upon diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
, and also that this treatment causes more complete hardening rather than moderating extreme hardness, both of which make the term cryogenic tempering
Tempering
Tempering is a heat treatment technique for metals, alloys and glass. In steels, tempering is done to "toughen" the metal by transforming brittle martensite or bainite into a combination of ferrite and cementite or sometimes Tempered martensite...
technically incorrect.
Hardening can also be accomplished by cold work at cryogenic temperatures. The defects introduced by plastic deformation
Plasticity (physics)
In physics and materials science, plasticity describes the deformation of a material undergoing non-reversible changes of shape in response to applied forces. For example, a solid piece of metal being bent or pounded into a new shape displays plasticity as permanent changes occur within the...
at these low temperatures are often quite different from the dislocations that usually form at room temperature, and produce materials changes that in some ways resemble the effects of shock hardening
Shock hardening
Shock hardening is a process used to strengthen metals and alloys, wherein a shock wave produces atomic-scale defects in the material's crystalline structure. As in cold work, these defects interfere with the normal processes by which metallic materials yield , making materials stiffer, but more...
. While this process is more effective than traditional cold work, it serves mainly as a theoretical test bed for more economical processes such as explosive forging.
Many alloys that do not undergo martensitic transformation have been subjected to the same treatments as steels—that is, cooled with no provisions for cold work. If any benefit is seen from such a process, one plausible explanation is that thermal expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
causes minor but permanent deformation of the material.