In the field of material treatment, it is usually
considered that cryogenic temperatures are those below 120 K (-153°C). Consequently,
conventional subzero treatments, often referred to as shallow cryogenic
treatments and usually performed at temperatures around -80°C, cannot be
regarded as real cryogenic processes.
Cryogenic temperatures couldn’t be achieved until the
late 19th century and, therefore, the emergence of cryogenic
treatments in industry is relatively recent. The development of this technology
has been based mainly on empirical results. The basic research of the
transformations produced in the materials when exposed to cryogenic
temperatures is usually conducted with significant delay with regard to
development of practical applications.
In general, cryogenic treatments have been considered
as separate operations, added to the conventional heat treatments. This is
something that has conditioned the development of knowledge in this field, and
also the reliability of the results obtained with these processes. Maybe this
happens because, very often, this technology is used in tools and finished
components, without paying much attention to the previous operations. This
approach doesn’t enable a good control over the process results since these
depend on the material history before the cryogenic treatment. And, obviously,
the previous heat treatments play a crucial role.
In this regard, the consideration of cryogenic
treatments as independent operations is a mistake. The right way to contemplate
them is not as a supplementary step, but as an integral part of the overall
heat treatment process. Only in this way its full potential will be exploited,
selecting the route that is most adequate in each case depending on the
material considered and the application in which it will be used.
We will try to illustrate it with an example. Let’s
consider a case hardening steel like 18NiCrMo5, which is commonly used in
applications where high yield strength and good wear resistance are required (shafts,
gears, cams, etc.). The heat treatment process of this steel starts with a
cementation step in order to increase the carbon content in the surface of the
component. The subsequent quenching, followed by a tempering cycle at not more
than 200°C, provides a very hard surface while the core remains soft and tough.
When considering the cryogenic treatment of a
component made of case hardened steel, two basic strategies could arise. One is
to apply it to the already heat treated part, that is, after tempering. The
other one is to perform the cryogenic process after quenching but before
tempering.
Several investigations focused on studying the effects
of cryogenic treatments in this steel grade have been carried out in recent
years, but the results seem confusing and sometimes even contradictory.
Actually, this happens because in most of these studies only one of the two
approaches has been considered, not taking into account that the results that
are obtained with each of the treatment strategies are significantly different.
Taking the standard heat treatment as a reference, the
results of the studies based on the first route mentioned (cryogenic treatment
after tempering) show increased hardness, improved wear resistance and slight
increases in tensile strength, but also worse toughness. Moreover, greater
fatigue resistance is achieved, accompanied by a marked decrease in the dispersion
of the results, which is a very interesting data.
Moreover, the results of the studies conducted on
18NiCrMo5 cryogenically processed between quenching and tempering are somewhat
different. Here the cryogenic treatment increases the hardness (as a
consequence of a smaller amount of residual austenite), the wear resistance and
the dimensional stability. In this case it also increases the toughness but,
however, the fatigue resistance worsens significantly.
These considerations concern the cemented layer, since
no significant changes were observed in the core, regardless of the approach
used for the cryogenic treatment.
In both cases the overall result is clearly positive.
Nevertheless, to properly choose the most suitable process route to be used in
a specific application, it must be considered the component, the application
and, consequently, the most determining failure mode to be faced during
operation. If it is foreseen that the material will suffer impacts, it seems
that a cryogenic
treatment between quenching and tempering is the most appropriate option. However, if fatigue is a determining factor, a
cryogenic treatment after tempering appears like the best choice. Moreover, if
wear is the main concern, both alternatives are, in principle, valid. In any
case, the interest of using cryogenic treatments for increasing the performance
of cemented steels seems beyond dispute.
Recent studies conducted in IK4-Azterlan with other
steel grades confirm the remarkable influence that the process route has on the
results induced by cryogenic treatments. Indeed, cryogenic treatments could
even lead to a rethinking of conventional heat treatment schemes for certain
materials, thank to innovative and more efficient processes leading to better
results.
All this becomes really
complex in practice since, for each material, countless combinations of factors
such as temperatures, times, number of cryogenic steps, tempering cycles, etc.
could be considered. Before subjecting a material to cryogenic treatment, it is
convenient to take some time to think about the requirements of the application
and how the different process alternatives could satisfy them. Fortunately,
very often few simple field tests are enough to assess the results but,
sometimes, a more comprehensive study will be required to get more out the huge
potential of this novel technology.
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