Study on the influence of microstructural defects of API 5L-B, X52, and X52QS pipeline steels on fracture toughness degradation under in-situ hydrogen environment

This paper systematically studies the hydrogen permeation behavior and hydrogen-induced cracking sensitivity of three typical pipe materials for medium and low pressure pipe networks, namely API 5L-B (banded structure), X52 and sulfur-resistant steel X52QS. In-situ gas-phase hydrogen permeation testing under 4 MPa H 2 and fracture toughness evaluation were conducted to systematically analyze the influence of microstructure on hydrogen-induced damage behavior. Results show that API 5L-B exhibits the highest hydrogen trap density, the lowest fracture toughness, and the most pronounced brittle fracture characteristics, indicating the greatest susceptibility to hydrogen embrittlement. X52 shows moderate sensitivity, while X52QS demonstrates the lowest trap density and the best resistance to hydrogen-induced cracking. Further analysis reveals that the increased susceptibility of API 5L-B is primarily attributed to the segregation of its ferrite/pearlite banded structure, higher pearlite content, high dislocation density, and the presence of Al 2 O 3 /MnS inclusions.