PSI - Issue 13

Milan Uhríčik et al. / Procedia Structural Integrity 13 (2018) 1571 – 1576 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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After measuring of the internal damping in the initial state, the test bars were subjected to plastic deformation, making the bar length shorter than the original length. Measurements of the internal damping of the plastically deformed samples again showed an increase of the damping value in a temperature range of 80-100 °C (Fig. 9, Fig. 10 and Fig. 11). It can be said that the increase of values is not so significant as compared to the starting state of steels, for each plastically-deformed test bar. In any case, it can be assumed that the microplastic deformation (which causes irreversible movement, increasing the density of dislocations in small volumes of the material and causing consolidation) resulted in formation of the deformation martensite and this local maximum of internal damping is related to transformation of the deformation martensite to austenite.

Fig. 10. Results of internal damping measurement of AISI 316L after deformation.

Fig. 9. Results of internal damping measurement of AISI 304 after deformation.

Fig. 11. Results of internal damping measu rement of AISI 316Ti after deformation.

Austenitic stainless steels are easily subjected to plastic deformation, especially the slip mechanism and the accumulation of dislocations also occurs in the molding process. Multiple slipping systems have been activated during the rolling. Austenite is not completely stable during the molding and partially is transformed into the deformation induced martensite, which in principle has a different lattice than the classical martensite produced after the hardening and even may have a lower hardness. Deformation-induced martensite causes hardening of steel and the density of dislocations is higher at the grain boundary than in its volume (Solomon et al. 2010; Hedström 2007). The reversible process starts with heating the steel, in which the elastic deformation is removed, then the stress is reduced in deformed grains and dislocations accumulated in the grains are relaxed. The transformation of deformation martensite into austenite is accompanied by moving and reducing of vacancies and creating of additional lattice failures (Dryzek et al. 2014). The reverse process, which reduces the accumulated surplus energy in the volume of the material, is accompanied by the formation of annealing twins (Danilchenko 2016; Jin et al. 2013). The plastic deformation causes an increase of internal energy in austenitic steels. The amount of accumulated internal energy depends on the amount of plastic deformation and the purity of the material. By measuring the internal damping of austenitic steels dependence on temperature, in the initial state, the internal damping peaks were noticed within the temperature range of 80-100 °C. Those internal damping peaks are related to the deformation-induced martensite, which is produced by plastic deformation of austenite, probably during the molding. During the heating, the deformation-induced martensite is converted back to the austenite. The highest internal damping value was measured for austenitic steel AISI 316Ti. This can be caused by increased internal energy near the titanium carbides contained in the structure, or by a higher ability to form a deformation martensite. The second course of measurement of internal damping on the same test bars did not detect the presence of a peak, so it can be assumed that all the relaxation processes took place during the first measurement. Subsequently, the test bars were plastically deformed and then they were subjected to measurement of internal damping. For each plastically deformed bar, the peak of internal damping again occurred within the temperature range of 80-100 °C, but in this case it was no longer as significant as in the starting state. It is likely that the deformation martensite did not develop intensely in the plastic deformation process. The increase of dislocation density occurs with the size of the plastic deformation. But there is a difference, whether the plastic deformation takes place at slow 4. Conclusions