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line pipe steel is commonly used for oil and natural gas transport in the petroleum industry. This type of line pipe is manufactured using microalloyed steel with elements such as Nb, Ti, and V. The thermomechanically controlled rolling process (TMCP) is used to control grain refinement and subsequent precipitation strengthening during rolling, cooling, and coiling.[1–5] Pipe longitudinal welding is achieved through double-pass submerged arc welding. As are most alloys, line pipe steel is subjected to natural aging phenomenon that is likely to be accelerated by the service conditions, capable of producing changes in the microstructure and mechanical properties (yield strength, tensile strength, hardness, ductility, and toughness), described by the peak-aged condition associated with the highest strength and overaging condition related to the decrease in the strength by long aging times and hence probability of failure after several years of service.[6–9] There is some research on aging of ferrous alloys such as API-5L line pipe steel that have aimed to produce a clearer understanding of the causes and effects of aging, such as the degradation of the microstructure and mechanical properties. Some works are related to 17GS line pipe steel that transported oil, which after 30 years of service displayed significant changes in mechanical properties (yield strength, tensile strength, and Charpy absorbed energy).[10] A second case is the JIS-SM490YB-0.17 wt pct C low-strength steel for welded structures, which shows susceptibility to aging at 250 °C for 1 hour resulting in B. VARGAS-ARISTA, Doctoral Student, and J.M. HALLEN, Professor and Research Scientist, are with the Departamento de Ingenierı´a Metalu´rgica, IPN-ESIQIE, Laboratorios Pesados de Metalurgia, UPALM Zacatenco, 07738 Me´xico D.F., Me´xico. Contact e-mail: [email protected] A. ALBITER and C. ANGELES-CHAVEZ, Research Scientists, are with the Instituto Mexicano del Petro´leo, Me´xico D.F. 07730. Manuscript submitted May 12, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A

deterioration of toughness. This heat treatment is equivalent to 2-year-old naturally aged steel.[11] A third case is the weld metal of multilayer steel aged at 550 °C for a time up to 30,000 hours; the thermally affected zone exhibits minimum hardness values and microstructural changes such as the agglomeration and coarsening of precipitates.[12] Another example is found in the weld metal of 308 stainless steel produced by SAW aged at 500 °C for 24 hours, which resulted in a raise of susceptibility to intergranular corrosion caused by chromium carbide precipitation.[13] Additionally, a study related with the artificial aging in API-5L line pipe steel is carried out at 250 °C, resulting in the increase of ferrite grain size as a function of aging time and a greater susceptibility to corrosion in aqueous chloridecontaining media;[14] however, this study does not show a relationship between aging and mechanical properties. There are also reports on carbon steels, indicating that artificial agi