PSI - Issue 40

Vladlen Nazarov et al. / Procedia Structural Integrity 40 (2022) 341–347 Vladlen Nazarov / Structural Integrity Procedia 00 (2022) 000 – 000

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The destruction process begins on different boundary radii for variants (a) and (b) due to the difference in the mechanisms of origin of main cracks. The main crack is generated when the tangential stress equals break creep stress for variant (a). The main cracks are beginning formed for variant (b) when the methane pressure in the micropores equals the ultimate value. As the result of hydrogen diffusion and its chemical interaction with steel carbides, the high pressure of gaseous methane is created inside the pores. This explains why for pores of minimum size, the gaseous methane internal pressure assumes the maximum value, which leads to the main cracks generation on the inner boundary radius. To describe the dependence of the methane pressure on an arbitrary radius, one can use the approximate equation Nazarov (2014b) of state for the gas

h

  q Tc

h

(14)

,

0

p







The dependence (Fig. 4) of the methane pressure inside micropores decreases on increasing radius and increases on time to a certain equilibrium value.

Fig. 4. Dependences (14) of the dimensionless pressure of gaseous methane inside the pores on the dimensionless radius at different successive times. The accumulation of methane gas concentration h c in the pores of decarburizing steel is accompanied by an increase in pressure h p in these pores. The ratio 4 / 3 C eq h eq c c  of the equilibrium concentrations of methane CH 4 and carbon C follows from the equality of the molar masses of the reacting substances of the chemical reaction 4 C 4H CH   . The process of hydrogen diffusion into steel is very fast and can be ignored. The concentration h c of methane gas is determined from the parabolic equation with one initial condition and two boundary conditions. Boundary condition at the inner radius