PSI - Issue 16

Ihor Dmytrakh et al. / Procedia Structural Integrity 16 (2019) 113–120 Ihor Dmytrakh et al. / Structural Integrity Procedia 00 (2018) 000 – 000

115

3

, 

 ppm ,

(1)

C

0 128 .

  A

6 m 10   



H

where A and m are some constants that depend on system “material – environment” and testing conditions. Based on the formula (1) the specimens were hydrogen charged to assigned level of C H , before testing.

3. Effect of hydrogen concentration on strain behaviour of pipeline steels under static loading

Standard cylindrical specimens with a diameter of 5 mm were hydrogenated to a specified level of the volume concentration C H and subjected to uniaxial tension up to fracture. By using the “applied load vs. specimen elongation” curves, we plotted the “stress σ vs. strain ε ” dependences for different concentrations C H in the metal. In finding stresses σ , we took into account the true values of the cross-sectional area of strained specimen and, on this basis, determined the true values of the yield point σ y , ultimate strength σ u , and fracture strength σ f for steel for different values of C H . The trends of changes in the cross-sectional areas of the specimens with different concentrations C H are identical in all stages of their deformation, i.e., for σ = σ y , σ = σ u and σ = σ f . First, as the hydrogen concentration increases (up to C H ≅ 0.227 ppm), the cross-sectional area of the specimens S rapidly decreases. This fact shows that, within the indicated concentration range, the deformation of the specimens is accompanied by high plastic strains. As the values of C H increase further, the area S increases. Hence, for the indicated conditions, the specimens are deformed under lower plastic strains. Therefore, we can assert that, for low-alloyed steels, there exists a characteristic level of concentration C H = C H * , for which the mechanism of the influence of hydrogen on the deformation and fracture of steels changes. After the tensile testing of specimens, their fracture surfaces were subjected to fractographic investigations using a scanning electron microscope Zeiss EVO-40XVP. The results of comparative assessments enabled us to reveal the following specific features of fracture depending on the concentration C H . The surface of non-hydrogenated specimen ( C H = 0) exhibits the signs of mixed fracture. In this case, parallel with the “pitting” topography that corresponds to the typical ductile mechanism of fracture, we observe the presence of the surface topography typical of quasicleavage, which is an indication of brittle fracture. As the hydrogen concentration increases, the density of pits on the fracture surface becomes higher and, at the same time, the elements of quasi-cleavage disappear, i.e., the intensity of plastic strains accompanying the fracture processes in steel increases. This occurs up to a concentration C H ≅ 0.227 ppm, inclusively. As the hydrogen content increases further, the elements of quasicleavage topography appear on the fracture surfaces, and the mechanism of fracture of steel gradually becomes mixed again.

Fig. 2. Generalized diagram of the influence of hydrogen concentration C H on the deformation of the 20 steel.

Fig. 1. Dependences of the yield point σ y of the 20 steel on the volume concentration of hydrogen C H in the metal.

By using the true values of the cross-sectional areas of specimens in different stages of their deformation, we computed the corresponding true values of the yield point σ y depending on concentration C H . It was discovered (Fig. 1) that hydrogen leads to the plasticization of the material for C H ≤ C H * and causes its embrittlement for C H ≥