PSI - Issue 13

V. Pokhmurskii et al. / Procedia Structural Integrity 13 (2018) 2190–2195 2 V.Pokhmurskii, M.Khoma, V.Vynar, Ch,Vasyliv, N.Ratska, T.Voronyak I. Stasyshyn / Structural Integrity Procedia 00 (2018) 000–000

2191

the equilibrium concentration during electrochemical processes, under gas discharge or implantation of ions [Fromm (1980)]. Hydrogen is irregularly distributed over the cross section of samples under the influence of concentration gradients and physical fields stresses. Non-stationary hydrogen is hypermobile: at room temperature its diffusion coefficient is by 12 orders of magnitude higher than of the other impurity atoms and the activation energy is close to the high-temperature (≈12.6 kJ/mol). Diffusible hydrogen has a considerable destructive effect in metals and causes initiation and propagation of microcracks, formation of blisters. Structure defectiveness promotes gas effusion from the places of local oversaturation for a relatively short period of time. After its withdrawal the traps of crystal lattice structures contain hydrogen with a three-time higher activation energy (~ 37...42 kJ/mol) - the so-called residual hydrogen that is inactive and is released from the metal only after heating to a temperature over 200°C [Fromm(1980), Smialowski (1962)]. Investigations of the diffusible hydrogen in metals, its influence on the structure and properties have not been completed, especially under non-stationary conditions [Zhang (1997), Murakami (2008), Hirth (1980)]. In many papers the time after hydrogenation to testing the properties of materials is not included. However the amount of diffusible and residual hydrogen strongly depends on this time [Hagi H.(1994), Tau (1996)]. Investigation of the changes in the metal under hydrogen desorption and the time for which they occur, are very important to establish the regularities of the hydrogen influence to the metals. The aim of this work is to investigate the effect of hydrogen desorption on change of microstructure, topography and some properties of the surface layers of iron alloys with ferrite and pearlite structure. 2. Experimental procedure The plate specimens (50×20×2 mm) has been prepared from Armco-iron (ferrite structure) and У8 steel (pearlitic structure). The surface of the specimens was grinded and polished to a roughness of Rz = 2.5+0.065 µm, then annealed in a vacuum at a temperature of A C3 + 30ºС. Specimens were hydrogenated electrolytically in 1 N solution H 2 SO 4 + 10 mg/l As 2 O 3 during 1 h, at 1.0 A/dm 2 . The concentration of diffusible hydrogen was determined by the method of vacuum extraction and the concentration of residual hydrogen was determined using the BRUKER Galileo G8 analyzer. Metallographic investigations were performed on the optical microscope "Neophot-2", scanning electron microscope EVO 40XVP with a system of microanalysis on energy-dispersive X-ray spectrometer INCA ENERGY 350. The method of two-step phase-shifting interferometry were used to control the surface relief of the specimens. It allows us to reproduce the surface relief with parameters Ra<0.04+0.005μm, Rz < 0.1+0.005μm) [Muravsky (2012)]. The tribological behaviour of hydrogenated material under reverse movement by the chart “ball-plane” was studied. Corundum ball (Ø 9 mm) was used as a counterbody. The applied load was 2 N, indenter sliding velocity – 1.6 mm/s, test duration – 2000 s. 3. Results and discussion After cathodic polarization of Armco-iron (1 A/dm 2 , 1h) the content of diffusible hydrogen was 8.1 ppm while of residual – 4.49 ppm. Residual hydrogen in the ferrite structure stored in micro and macro defects (blisters, microcracks). At the same hydrogenation conditions У8 steel absorbs the diffusible hydrogen 10.1 ppm, and the residual – 2.33 ppm. This is related with highly-defective interphase boundaries between ferrite and cementite plates in the pearlite [Hagi H.(1994)]. Moreover, hydrogen almost not dissolves in cementite, and this reduces its diffusion [Karpenko (1962), Smialowski (1962)]. The influence of these factors determines the amount of sorbed hydrogen by the material. Therefore, the concentration of residual hydrogen in steel with pearlitic structure is almost twice less than in ferrite. Concentration of hydrogen after electrolytic hydrogenation changes as a result of its desorption and redistribution in microvolumes of metal. Dislocation density, stress value and micromechanical properties of metal undergo changes in a result of hydrogen movement [Choo (1982), Tau (1996)]. During hydrogen desorption the microhardness both of ferrite and pearlite changes. Thus, the ferrite microhardness in the initial state is 0.91 ± 0.07 GPa and in 15 minutes after hydrogenation its value increases to 1.02 ± 0.18 GPa (Fig. 1a). When diffusible hydrogen release ferrite, its microhardness increases, reaching the maximum after 24 hours, which is 1.37 ± 0.42 GPa. A significant scattering of values is associated with different rates of hydrogen desorption from differently