PSI - Issue 13

V. Pokhmurskii et al. / Procedia Structural Integrity 13 (2018) 2190–2195 V.Pokhmurskii, M.Khoma, V.Vynar, Ch,Vasyliv, N.Ratska, T.Voronyak I. Stasyshyn / Structural Integrity Procedia 00 (2018) 000–000 3

2192

oriented ferrite grains. After two days the concentration of hydrogen in Armko-iron is leveled. Macrocracks are generated and propagated under the hydrogen pressure and hydrogen also releases from the traps. In this case the stress relaxation takes place, so microhardness reduces and stabilizes. The movement of hydrogen in the surface layers of metal results in changes of its local concentration, the magnitude of the stress and micromechanical properties [Choo (1982), Tau (1996)]. The similar tendency of microhardness change is observed in the steel with pearlitic structure. In the initial state the perlite microhardness is 1.850 ± 0.130 GPa, in 15 minutes after hydrogenation it increases to 1.890 ± 0.210 GPa and after 24 hours to 2.270 ± 0.510 GPa (Fig.1). Under hydrogen desorption the scattering of microhardness values also increases. This is related with different orientation of the cementite plates, which affects the hydrogen diffusion coefficient under sorption-desorption [Motomichi (2014)]. Thus, the concentration of hydrogen in different metal microvolumes is different that effect on microhardness. When basic amount of diffusible hydrogen releases from the pearlite, the scattering of microhardness decreases and is in average 2.100 GPa.

ferrite; pealite

3.0

2.5

2.0

  GPa

H 0.02

1.5

1.0

50

5

0.5

1

10

t, h

Fig. 1 - Change of microhardness of hydrogenated Armco-iron (○) and У8 steel (●) during the hydrogen desorption.

Changes in the surface layers under desorption are visible in the pictures of the difference interferograms obtained as a result of interference of coherent laser irradiation with a wavelength of 450 nm on the surface of ferrite and pearlite. Almost parallel interference strips were observed on the surface of Armco-iron in the initial state. Numerous blisters were formed on its surface after hydrogenation (Fig.2). A large number of concentric rings appears on the interference pattern. As a rule, 1-2 interference rings are formed on the blister surface. The additional rings appear in the blister centers after 2 hours after hydrogenation. It can be an evidence of their growth. In 24 hours after hydrogenation the number of rings in separate blisters decreased, what was related with reduction of their height, due to the appearance of cracks on the surface and release of the excessive gas from the metal. Interferograms of steel surfaces had been analyzed. Steel У8 surface is deformed and some blisters appear on it after hydrogenation. With hydrogen release the level of deformation decreases (it is most intensive during the first 2 hours after hydrogenation) and stops after 2 weeks. The size of the blisters increases (according to 2-week observation). The height of blisters ranges from hundreds of nanometers to a micrometer or more. Figure 3 shows the changes in ordinates of local points on the surface of Armco-iron (curve 1) and У8 steel (curve 2) during desorption of hydrogen within 12 days after hydrogenation.

a b Fig. 2 – Inte r fe r og r amms on the su r face of A r mco-i r on afte r hy dr ogenation: a - afte r 15 min; b- afte r 2h .