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Nature of the slow-fracture phenomenon in quenched steels

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Metal Science and Heat Treatment Aims and scope

Conclusions

  1. 1.

    The phenomenon of slow cracking in quenched steels is a process of subcritical development of microscopic cracks (20–50 μm), which are generated during the loading of the samples in the macroelastic region. Upon increasing the loading rate, an effect of the deformation-rate retardation of crack nucleation (DRR effect) is observed which leads to the appearance of an incubation period of the crack nucleation at the tip of the stress concentrator. This point needs to be taken into account while performing the tests.

  2. 2.

    The slow cracking of quenched steels is a particular case of hydrogen embrittlement of materials and is caused by the internal, diffusionally mobile hydrogen, which is formed during the quenching of the steel. Desorption of hydrogen during the course of the postquenching rest eliminates the tendency of steels toward slow fracture. In the example of a model steel, it is shown that the brittle state of the material including grain boundaries is an insufficient condition for the onset of slow cracking and that hydrogen needs to be present.

  3. 3.

    Rest of quenched steels, which causes changes in the mechanical properties, exerts the greatest influence on the value of the crack-nucleation stress. The maximum effect of rest is found to be in the case of the 30KhGSA steel containing ∼0.3% C with a hardness of 50 HRC at a stress-concentrator radius of 0.1–0.2 mm. The kinetics of increase in the mechanical properties σn AE, σF of the quenched samples during the course of the rest is in agreement with the kinetics of desorption of the diffusionally mobile hydrogen to the atmosphere.

  4. 4.

    The cause for the cases of slow cracking of objects and semifinished products observed in practice is the formation and growth of defects (cracks) of a quenching or casting origin on top of a high hydrogen content in the structure of the steel.

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Authors
  1. V. V. Zabil'skii
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  2. V. V. Velichko
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Additional information

Fizikotechnical Institute, Ural Branch, Russian Academy of Sciences. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 9–15, April, 1993.

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Zabil'skii, V.V., Velichko, V.V. Nature of the slow-fracture phenomenon in quenched steels. Met Sci Heat Treat 35, 198–206 (1993). https://doi.org/10.1007/BF00775137

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