Cryogenic treatment of materials has been present in the aerospace industry for over 30 years primarily to enhance the service life of the treated steel.
A number of key factors affect the success of sub-zero treatments including time, temperature profile and tempering practice but used in specific combinations can produce excellent results particularly for wear dependent materials such as tool steels.
The word cryogenics is derived from the Greek words Kryos (meaning cold), and Genes (meaning born). The Cryogenics Society of America defines cryogenic temperatures as temperatures below 120K (-244F, -153C). Generally speaking, cryogenic treatment is the process of submitting a material to subzero temperatures (below 0 °C) in order to enhance the service life through morphological changes that occurs during treatment.
Cryogenic processing, originally developed for aerospace applications, has been used for over 30 years to improve the properties of metals.
According to the laws of thermodynamics, there exists a limit to the lowest temperature that can be achieved, which is known as absolute zero. Molecules are in their lowest, but finite, energy state at absolute zero. Absolute zero is the zero of the absolute or thermodynamic temperature scale. It is equal to – 273.15 oC or –459.67 oF. In terms of the Kelvin scale the cryogenic region is often considered to be that below approximately 120 K (-153oC). The common permanent gases earlier change from gas to liquid at atmospheric pressure at the temperatures shown in Table 1, called the normal boiling point (NBP). Such liquids are known as cryogenic liquids or cryogens.
Table 1: Normal boiling points of common cryogenic fluids
Refrigeration of metals to improve performance is divided into two categories: cold treatment and cryogenic treatment. Common practice identifies -120 °F (-84 °C) as the optimum temperature for cold treatment at which parts are held (soaked) for 1 hour per inch of thickness, then subsequently warmed in ambient air. Typical cryogenic treatment consists of a slow cool-down of -5 °F per minute (-3°C per minute) from ambient to -320 °F (-196°C), a soak for 24 to 72 hours, and warm up to ambient temperature. The cryogenically treated parts are then subjected to a temper treatment (300 to 1000°F or 149 °C to 538 °C) for a minimum of one hour.
Numerous factors impact how sub-zero treatments affect an alloy. Processing factors like time, temperature profile, number of repetitions and tempering practice, in conjunction with material parameters such as prior heat treatment and alloy composition will alter the final results. Table 2 discusses three sub-zero treatment applications. Sub-zero treatment falls into the broad categories of shrink fitting, cold treatment, and cryo-treatment. Figure 1 shows the differences in the basic processes as regards the time-temperature process cycle.
Table 2: An overview of sub-zero treatment processes for metals
Figure 1: Sub-zero process cycle profiles
Table 3 shows the average useful life of particular tooling pieces with and without the benefit of sub-zero treatment. A parameter called Wear Ratio, defined as the ratio of life after sub-zero treatment/ average tool life without sub-zero treatment, gives a measure of the amount of improvement this process can impart when applied correctly.
Differences in wear life, shown in Table 3, between parts cold treated at about –80°C (–110°F), and parts cryogenically treated at –190°C (–310°F) using liquid nitrogen, raised questions about the causes of the improved wear resistance. However, the overall results from these studies could not be disputed and further research has been conducted to gain a better understanding of the underlying mechanisms.
Table 3: Examples of tool life improvements using cryotreatment
* Wear ratio = Life After Treatment / Life before treatment
In Table 4 is showed the comparison of the wear resistance of different materials after cold treatment and cryotreatment.
Table 4: Percentage increase in wear resistance after cold treatment and cryotreatment