It is not possible to give a categorical answer to this question because there is no universal consensus about the threshold defined by the term cryogenic. When talking about materials, the reference value usually considered is 120 K, equivalent to -153ºC (often rounded to -150ºC).
Aside from these references, it should be clear that the treatments commonly known as shallow cryogenic treatments are nor the type of processes we are talking about. These relatively short subzero processes, quite common in the heat treatment industry, are performed (just after quenching at about -80ºC to get a better transformation of retained austenite in some steel grades and, thus, to improve the dimensional stability of certain components.
The true cryogenic treatments are performed at much lower temperatures (usually around -180ºC) and last significantly longer, triggering additional transformations that further increase the scope of the technology and the variety of its applications. Apart from better dimensional stability (the transformation of retained austenite is further improved) they provide additional benefits like increased wear resistance, extended fatigue life, improved conductivity, etc.
Although, in some circumstances, cryogenic treatments can provide a certain increase of the hardness, this property is not significantly altered. Therefore, they cannot be considered as an alternative to the conventional hardening (quenching + tempering) treatment, the standard process for many steel grades, even though they can often be indeed a very good complement.
However, when cryogenic treatments are applied to a previously hardened steel, this parameter may suffer slight variations. This is particularly evident when the steel contains a significant amount of residual austenite. The very low temperatures will transform part of this austenite into martensite and, as a result, the final hardness will be slightly higher. Moreover, it can be frequently observed that the resultant hardness distribution is more homogeneous.
In general, cryogenic treatments do not change the dimensions of the materials. In fact, the treated components usually increase their dimensional stability. Additionally, neither the appearance is altered nor corrosion is produced (the process is carried out in inert atmosphere). Thus, the treatment is virtually undetectable in practice.
Only in the case of steels containing a high percentage of retained austenite after hardening its transformation could result in a very slight growth of the part. This would happen because the volume of the crystal of martensite is slightly bigger than that of the austenite. Anyway, high austenite contents can be problematic in certain applications because this steel phase is metastable and can, uncontrolled, transform into martensite during service. Consequently, cryogenic treatments are very useful to detect and prevent this kind of problems, especially in those applications where dimensional stability is a must.
Yes, it is, although certain precautions must be taken when the part is formed by several elements tightly joined together, especially if said elements are made of different materials. In this case the expansion coefficients and, consequently, the dimensional variations generated during the treatment, will probably be different. This could lead to high stress levels in certain areas and, as many materials become brittle at low temperatures, certain risk of breakage could arise (special care has to be taken with plastics). Whenever possible it is recommended to separate the different elements before the treatment or, at least, to loosen the tight connections that could create excessive stress. In case of doubt, some previous tests might be advisable in order to verify that no problems will appear due to this effect.
Something that has to be clear is that this is neither a laboratory technology, nor an exclusive process only used for expensive and delicate components. On the contrary, cryogenic treatments are economic and efficient industrial processes that can be used in all kind of applications.
It is the experience of IK4-AZTERLAN and its ability to design and manufacture cryogenic processors what allows that practically any requirement of size or capacity is not a problem. If necessary, it will almost always be possible to design and manufacture the right equipment for meeting almost every requirement that a customer can pose.
Apart from this and from being more effective, the multi-stage cryogenic treatments have a major advantage if they are compared with other conventional cryogenic processes. Since the total process time is much shorter, the equipment is notably more productive. So, it is by far the best solution, especially when the treatment of large amounts of material is needed.