The multistage cryogenic process is an evolution from the conventional cryogenic treatments of materials. It needs shorter process time achieving the same or even better results.
This paper introduces some basic fundamentals of these treatments, their effects and applications, the equipment, etc. Some examples with different materials and from different industrial sectors are also presented as well as some brief comments about R&D and future trends of the technology.
People usually relates heat treatments with high temperatures, but thermal treatments can also involve cooling. Although it has been traditionally considered that deep cold temperatures have no permanent effect on the materials, it is not true at all.
Heat treatments were already known and used centuries ago but the access to really low temperatures was only possible in relatively recent days. Although the first experiences took place at the beginning of last century, it is not possible to properly speak about industrial cryogenic treatments of materials until the 70’s when the liquefied gases became more affordable and the treating equipment had more accurate process control systems.
During the 80’s and 90’s the use of this technology increased and some treating facilities were opened, mainly in the US. Nowadays it is possible to find cryogenic processing companies in many countries all over the world.
Although still hardly known and used in Europe, the cryogenic treatment of materials is a technology that is slowly getting acceptance in industry. It basically consists in submitting the materials to deep low temperatures for increasing some of their performance characteristics like wear resistance or fatigue life.
1. THE PROCESS
Cryogenic treatments basically consist in submitting the materials to deep cryogenic temperatures (below 120K) following predetermined time-temperature curves in order to enhance some of their physical or structural properties.
According to the previous definition, it must be noticed that the subzero processes (at about -80 ºC) that are used in many traditional heat treating facilities to reduce the austenite content in some tool steels cannot be considered cryogenic treatments.
1.1 Conventional cryogenic treatments
There is no a standard process for cryogenic treatments but most of them are quite similar. In conventional cryogenic treatments the materials are slowly cooled down to a temperature
around -180 ºC and maintained for a period of time that lasts from eight hours to two or even more days. After the soak, the materials are slowly heated up to ambient temperature. Sometimes the treatment is completed with a soft tempering. The entire process typically needs two to three days to be completed.
There is a conventional process sub-category called “wet process“ where the soak is made by submersion in liquid nitrogen. Anyway, the previously described “dry process“ (no liquid nitrogen in the chamber) is more widely used.
The cryogenic treatments are performed in chambers designed for this purpose. The material is usually cooled using liquid nitrogen that is introduced in the processor through solenoid valves controlled by computer. Most of the modern chambers have heaters that also allow to control the temperature during the heating phases of the process.
1.2 Multistage cryogenic process
The multistage cryogenic treatment is a more advanced process that has been developed as an evolution from the conventional ones. In this treatment the isothermal soak at cryogenic temperature is substituted by several cryogenic cooling/heating phases. This process is more effective but its main advantage is that it is much faster (an average of fifteen hours for the whole process) than the conventional ones.
The cryogenic chambers that are used to apply a multistage cryogenic treatment are specially designed to perform this type of process.