Abstract
Abstract— Creep Crack Growth (CCG) tests were performed at 405°C on specimens cut out of the cold bent extrados of five tubes of a C‐Mn‐Mo steel. Intergranular fracture and grain boundary cavitation was less in the C‐Mn‐Mo than in the C‐Mn steels, in accordance with better CCG resistance of the former material. The dimensions and hardness variation across the crack tip process zone were measured by microhardness profiles performed on metallographic sections of the broken samples. TEM analysis of the dislocation patterns close to the fracture surface confirmed the presence of temperature‐ and stress‐induced plasticity phenomena. A significant enrichment of N at grain boundaries (GB) inside the process zone was detected by Auger spectroscopy; N not only inhibits dislocation motion and stress field relaxation at the crack tip but also causes a decrease in GB cohesion ahead of the crack tip. These results help in understanding the micromechanisms which reduce the creep ductility of C‐Mn‐Mo and C‐Mn cold bent tubes and the role of chemical composition in improving CCG resistance.
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