Effects of Semi-spheroidized Cementite in Ferritic Matrix and Grain Boundaries on CO2 Stress Corrosion in Cold-Rolled and Stress-Relieved Pearlitic Rods

This study investigates the microstructural evolution and mechanical performance of a commercial cold-rolled and stress-relieved pearlitic steel rod (Fe-0.71C-1.50Mn) for flexible riser applications. Residual stresses were introduced by three-point bending, followed by 95-day exposure to CO 2 -saturated synthetic seawater (10 bar, 40 °C) to simulate offshore annular conditions. Key findings include significant microstructural heterogeneity with lamellar, fragmented, and semi-spheroidized cementite forming networks along prior-austenite grain boundaries. The material showed a predominant {111} texture, average grain size of 20.48 µm, KAM of 4.67°, and GND density increasing from 4.8 × 10 14 m −2 (as-received) to 5.6 × 10 14 m −2 after bending and CO 2 exposure. A compact FeCO 3 film (minimum thickness 26 µm) formed on the surface. Mechanically, the rod exhibited yield strength of 610-620 MPa, ultimate tensile strength of 870-880 MPa, elongation of 0.21-0.23%, and Charpy impact energy of 10.1 ± 1.4 J. Despite localized pitting, no stress-corrosion cracks were observed. The results highlight the dual role of semi-spheroidized cementite and intrinsic low-angle boundary networks in balancing strength, ductility, and SCC resistance under combined mechanical-corrosive loading.