PSI - Issue 16

Yaroslav Ivanytskyi et al. / Procedia Structural Integrity 16 (2019) 126–133 Yaroslav Ivanytskyi et al. / Structural Integrity Procedia 00 (2019) 000 – 000 c ij  is determined according to (6), – its critical value, considering the influence of hydrogen-containing environment, determined from the experiment. The concentration of hydrogen in the material is determined from the model of hydrogen diffusion, taking into account the field of mechanical stresses (Hembara et al. (2016)) 129 4

H DV C

 C D C t



 

 



,

(10)

  

    

h

RT



where , , x y z         – Hamilton operator; D – diffusion coefficient; R – universal gas constant; T – absolute temperature; V H – partial molar volume of hydrogen in metal; t – time. The boundary and initial conditions are taken in accordance with the conditions of hydrogenation. Consider that if   , , , 1 x y z t   , then the local element of model is destroyed. Time of destruction of element * t t  is determined from a condition   * , , , 1 x y z t   . For determination of constant H W c is necessary to have series of curves of stationary uniaxial creep with the initial area of the third stage at a few values σ=const in the elastic area of work of material . On these curves points ( * * , t  ) corresponding to a limit between the second and third stages of creep are determined. At this moment of time gradual fracture of local elements is begun, that is the reason of appearance of the third stage on the creep curve. Therefore value of H W c , that is the accumulated energy in this element to the moment of time * t t  , which corresponds to beginning of the third stage of creep, and establish the critical value H W c .   , , , C C x y z t  – hydrogen concentration;  

3. Experimental procedure

To construct creep curves and determine the energy of destruction, experimental investigations were carried out on the stretching of Bridgman cylindrical samples made of 0.5Cr0.5Mo0.25V steel. The chemical composition of the steel is listed in Table 1.

Table 1. Chemical composition of 0.5Cr-0.5Mo-0.25V steel. Standard Std. No. Designation

Chemical composition [wt%] C Si Mn

P

S

Cr

Mo

V

Al

EN

10216

14MoV6-3

0.15 0.10

0.35 0.15

0.70 0.40

≤ 0.025

≤ 0.020 0.60 0.30

0.70 0.50

0.28 0.22

≤ 0.040

Two batches of samples were investigated. The first batch of samples was in the as-received state, and the second one was hydrogenated in gaseous hydrogen at a pressure of 3 MPa and a temperature of 640 ° C for 240 minutes in a special chamber (Fig. 1a, b).

c

b

a

Fig. 1. Creep testing machine with a camera to determine the creep of a test specimen (a), (c); device for equilibrium diagram building (b).