In-Service Degradation of Mechanical Characteristics of Pipe Steels in Gas Mains

The laws governing change in mechanical characteristics under tension in air of steels of different strength levels in the initial state and after long-term exploitation on gas mains have been established. Because of in-service degradation, ductility characteristics decrease, and strength characteristics increase. The higher the strength level of steel in the initial state, the weaker the effect of change in its characteristics. In spite of the longest-term exploitation, the ductility characteristics of Kh70 steel did not practically change, whereas those of 17G1S and Kh60 steels decreased greatly after less long-term exploitation. These effects are caused by the structural peculiarities of steels. Indeed, a texture was detected both in the axial and in the diametral section of pipes made of these steels. The length of almost continuous rows of pearlite grains in the axial direction reached 500 μm and in the transverse direction 40 μm. The increased etching of interfaces between ferrite and pearlite grains both along and across the pipe rolling direction was attributed to damages along these interfaces. Such damages were traps for hydrogen and hindered its diffusion redistribution in the cross-section of pipes. Hydrogen accumulated in them promoted lamination along interfaces and facilitated strain localization in the most weakened sections. Signs of the in-service degradation of steels of different strength levels have been detected fractographically. Firstly, it is the textured nature of specimen fractures at the macrolevel as laminations in the pipe rolling direction, which are caused by in-service damages of steels. We believe that hydrogen absorbed by the metal during long-term exploitation and accumulated in defects along interfaces gave rise to them. Secondly, in the central part of mode I fractures, large and flat lens-shaped areas with small dimples, which accumulate hydrogen at the bottom, which facilitated the destruction of partitions between them, were detected. Thirdly, within the boundaries of the conical parts of the fractures of all steels under investigation, amid shear mode small parabolic dimples, large flat dimples with the characteristic relief of the parallel traces of the rise of slip bands to their surface are observed. We suppose that this proves their existence in the section of specimens as early as before tensile test. The above structural and fractographic features of degradation are inherent in all steels, particularly in 17G1S steel, whose hardening was accompanied by a greater decrease in ductility due to degradation.