Fracture Stages and Residual Strength of Pipe Steel after Long-Term Operation

The paper investigates the mechanical and physical properties of low-carbon pipe steel (analog of steel 20 corresponding to TU-28-FR-73) after 39 years of service life in a crude gas pipeline at the Orenburg oil and gas condensate field. It is shown that the standard mechanical properties of the gas pipeline steel after long-term operation correspond to the TU-28-FR-73 standard. The effect of preliminary cyclic loading on the residual tensile strength of steel, the acoustic emission characteristics, and the residual magnetic field intensity estimated by the metal magnetic memory method is studied. After preliminary cycling, the steel was hardened and the acoustic regime changed due to damage accumulation. Four stages of damage evolution in the tensile steel are identified both before and after preliminary cyclic loading, and informative diagnostic criteria for the stages are proposed. Relationships are derived to relate the relative number of preloading cycles with the cumulative acoustic count, acoustic activity, slope of cumulative count-strain curves, acoustic gap duration, and residual magnetic field intensity. The listed characteristics are shown to be promising as diagnostic criteria for the state of the material.