Isothermal Martensitic Transformation as operating
Mechanism for Deep Cryogenic Treatment of Steels
Volume 1 - Issue 3
Gavriljuk VG*
Received: March 06, 2019; Published: March 25, 2019
DOI: 10.32474/MAMS.2018.01.000112
Full Text
PDF
To view the Full Article Peer-reviewed Article PDF
Mini Review
Cryogenic treatment of tool steels was proposed about seventy
years ago and studied more or less intensively since the fifties see
e.g. [1,2]. One distinguishes between the shallow, SCT, and deep,
DCT, cryogenic treatments. The first one is used at temperatures
between RT and -100 oC. Its effect amounts to the increase of
hardness accompanied by a decrease in toughness. The second
one is performed usually in the liquid nitrogen and leads to the
increased wear resistance and toughness. It is remarkable that, in
this case, the hardness can be even decreased. Within the last three
decades, a number of results were obtained on the effect of DCT on
tribological properties, e.g. [3-10]. At the same time, a definite gap
exists between the scientific research and practical applications of
this treatment. A reason for that is some scattering of experimental
results and the absence of knowledge about phenomena taking place
in the as-quenched steels cooled down to cryogenic temperatures.
A number of hypotheses was proposed in the attempts to interpret
the nature of a favorable DCT effect on mechanical properties of
tool steels. Some of them ar based on the enhanced precipitation
of -carbide, as it was shown by Meng et al. [3]. It is claimed,
e.g. [4-6,8], that these nano-carbide particles are precipitated in the
course of DCT and the following heating to RT or during subsequent
low temperature tempering. At least two experimental facts are at
varience with with this idea. First, the -carbide is found to
precipitate in the high carbon martensite without any DCT. Second,
this intermediate carbide is dissolved at temperatures above 200
oC, whereas the tool steels are tempered at significantly higher
temperatures, e.g. at 500 oC. The hypotheses of “low temperature
conditioning” and contraction of martensitic solid solution, see
e.g., [1-8, 11-16] suppose the migration of carbon atoms towards
dislocations followed by formation of nanoclucters which, in turn,
serve as nucleation sites for nano-carbides. As mentioned above, no
detectable migration of carbon atoms occurs in the Fe-C martensite
at temperatures below -100 oC. A common point in the all available
hypotheses is the denial of any martensitic transformation during
holding at deep cryogenic temperatures because it is believed that
martensite is totally formed during cooling above -100 oC. In other
words, ignoring was the isothermal martensitic transformation.
This transformation has been first time discovered and studied by
Kurdyumov and Maximova in 1948 [17]. In contrast to the so-called
athermal one characterized by the burst kinetics, it proceeds during
long-time holding at temperatures below-100 ºC. As example, the
isothermal martensitic transformation is shown in (Figure 1) to
proceed in the tool steel during holding at-150 ºC. Pietikainen [18]
was the first to find that the isothermal martensite is rather soft
and acquires brittleness only starting from about -50 ºC during
subsequent heating. Moreover, even the cracking accompanying the
martensitic transformation in the carbon steels did not occur in his
low temperature experiments.
Mini Review|
References|