Deep Cryogenic Tempering Process

Heat treating is what gives steel 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 a steel. Proper heat treating is a key part in increasing a part's toughness, durability, wear resistance, strength and Rockwell 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. The benefits of the quench do not stop at room temperature, as many alloys will continue to show significant improvements if the quench temperature is reduced further to subzero levels approaching absolute zero. While it is impossible to actually achieve -460°F, (a molecular zero movement state that eliminates all stress), deep cryogenic temperatures are very efficient and cost effective in increasing dimensional stability, increasing wear resistance and performance of most alloys.

Deep Cryogenic Process:

The process is based on a predetermined thermal cycle that involves cooling of the tools/parts in a completely controlled cryogenic chamber. The material is slowly cooled to -300F and "soaked" at that deep cryogenic temperature for 20-40 hours. The material is then allowed to return very slowly to ambient temperature. The complete cryogenic cycle can take up to 70-75 hours to complete. This procedure of precisely controlled temperature profiles avoids any possibility of thermal shock and thermal stress that is experienced when a tool or part is subjected to abrupt or extreme temperature changes. In this process liquid nitrogen is used as a refrigerant.

Cryogenic processing is not a substitute for heat treatment, but rather an extension of the heating / quenching / tempering cycle. In most instances the cryogenic cycle is followed by a heat tempering procedure. As all alloys do not have the same chemical constituents, the tempering procedure varies according to the materials chemical composition, thermal history and/or a tool’s particular service application.

Microstructure Changes:

Two main changes in the microstructure of the steel occur as a result of Cryogenic Treatment. These changes are the principal reasons for the dramatic improvement in wear resistance.

Retained Austenite :

Retained austenite is a softer grain structure always present after heat treatment. By applying cryogenic treatment, retained austenite is transformed into the harder, more durable grain structure - martensite. The range of retained austenite in a material after heat treating may be as high as 50 % or as low as 3 %. The amount depends on the heat treating operator and the accuracy of the heat treating equipment. Cryogenic treatment simply continues the conversion initiated by heat treatment, whereby almost 100 % of the retained austenite is converted to martensite. As greater amounts of retained austenite are transformed, and wear resistant martensite is increased, the material obtains a more uniform hardness.

Fine Carbide Precipitates:

Fine eta(h ) carbide particles (precipitates) are formed during the long cryogenic soak (chromium carbides, tungsten carbide, etc., depending upon the alloying elements in the steel). These are in addition to the larger carbide particles present before cryogenic treatment. These fine particles or "fillers", along with the larger particles, form a denser, more coherent and much tougher matrix in the material.

Results and Benefits of Cryogenic Process:

Cryogenically treated materials show a marked increase in wear resistance without any discernable change in dimensional or volumetric integrity.

Machining treated material is easier and cleaner.

Redressing or regrinding treated tools removes less stock material resulting in longer tool life.

The material shows little or no change in yield or tensile strength.

The treated material becomes less brittle, without a change in original hardness.

The most significant and consistent change is the increased toughness, stability and wear resistance.

The Process is also used extensively to relieve residual stresses.

The surface energy of martensite is higher than the surface energy of austenite due to the differences in their atomic structures.In potential adhesive wear situations, the martensite is less likely to tear out than is austenite. The probability of wear particles forming in a steel in which the austenite has been transformed to martensite is less than for the steel containing some retained austenite. The adhesive wear coefficient is decreased, and the wear rate is decreased.

In abrasive wear situations, both the martensite formation and the fine carbide formation work together to reduce wear. The additional fine carbide particles help support the martensite matrix. This makes it more difficult to dig out lumps of the material.

Almost any kind of tool steel or dynamic part, for whatever application, will exhibit some kind of life increase. As less tools, or parts are needed, there is substantial savings in dollars. Additional savings include less downtime and short runs, less maintenance and change-over, which allows for lower production costs.

Applications:

Industries include: Paper and corrugated board industries, metal working and fabrication, plastic industry & rubber industry, Oil & Gas, Petro-Chemical, Forest Industries Mining, Agriculture, Music, Stamping Houses, Forging Companies, Heavy Equipment Industry, Machining, Automated Industry, Aerospace & Aircraft, Manufacturing, Furniture, Garment Industry, Food Industry, High Performance Engines, Sporting Goods and more. (Mill cutters, form tools, lathe tools, broaches, ballscrews, all types of dies, punches, gear cutters, arbors, drills , molds)

Welding applications: Copper electrodes exhibit longer life, show less wear and deformation and they can be used with less power input. Cryogenic Treatment reduces tip burn-off and carry reduced amperage on heliarc tungsten electrodes.

LINKS:

The Cryo-Bond Tempering Process

History of Cryogenics

What is Deep Cryogenic Tempering?

Comparison between shallow quenching vs. deep cryogenic tempering

Cryogenic Society of America, Inc. (CSA)

Back to Table of Contents

Last Update: November 11, 1999

By: Serdar Z. Elgun