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ISSN: 2641-6921

Modern Approaches on Material Science

Mini Review(ISSN: 2641-6921)

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

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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.

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