decreasing of fracture toughness value is occurred (Fig. 3). The values of HC for studied steels are given in Table 5. Fig 3. Scheme of determining of work for local fracture initiation Table 5 Critical hydrogen concentrations HC for API grade pipeline steels Steel 3 6 HC ,10 mol cm X52 4.3 X70 2.3 The steel X70 demonstrates very high sensitivity to notch effect even in air and the presence of hydrogen strengthens this tendency. The hydrogen concentration about 3 6 HC 2.3 10 mol cm can be considered as critical, because at this conditions the given steel losses in 2.5 times its local strength with comparison of test in air. Therefore, for assigned testing conditions, steel X52 is preferable than steel X70 from the point of view of local strength at notches under hydrogen embrittlement. Fig .4 Ductile-brittle transition with hydrogen concentration in API5L X52 steel. This critical hydrogen concentration at ductile brittle transition can be explained by a competition to the fracture process volume and the hydrogen affected fracture process volume. In an area approach, the effective distance Xef represents the diameter of the fatigue process zone assumed as Displacement, Load, P * Local fracture initiation detected by AE iU
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