Such defects can be considered as typical stress concentrators and consequently the privileged sites where the further damaging and failure processes occur. In this paper, evidence on a critical hydrogen concentration for the ductile brittle transition of two pipe steels X52and X70 is presented. The strong influence of hydrogen embrittlement on fatigue crack initiationis the second point which focuses our attention. 2. OBJECT OF STUDY AND EXPERIMENTAL PROCEDURE The objects of study were two API grade pipeline steels, namely: X52 and X70. Nowadays, the steel X52 is the most usable in existed gas pipelines. The specimens for tests were machined from real pipes (Table 1). The chemical composition of steels and their mechanical properties in air are given in Table 2 and 3 respectively. Table 1 Size of pipes Steel grade Outer diameter D, mm Wall thickness t, mm X52 610 11.0 X70 710 12.7 Table 2 Chemical compositions of steels (wt %) Steel grade C Mn Si Cr Ni Mo S Cu Ti Nb Al X52 0.206 1.257 0.293 0.014 0.017 0.006 0.009 0.011 0.001 <0.03 0.034 X70 0.125 1.68 0.27 0.051 0.04 0.021 0.005 0.045 0.003 0.033 0.038 Table 3 Mechanical properties of steels in air at ambient conditions Steel grade MPa U, MPa Y, Elongation. % X52 528 410 30.2 X70 712 590 18.3 For tests (fatigue and fracture), the special “Roman tile” [8] specimens were used. The specimens were notched for modelling of the longitudinal external defects under operating internal pressure. The specimen view and notch geometry are given in Fig. 1. The study was conducted in special soil solution NS4 with pH=6.7 [9] Fig. 1 “Roman tile” specimen and geometry of notch t D 700 A t 0.2t r=0.15mm A 450
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