Abstract A comprehensive study of the structure, mechanical properties including impact toughness of 09Mn2Si steel used for constructing main gas pipeline” Mastakh-Berge-Yakutsk” after 37 years of operation was carried out. A comparative analysis of the results obtained for the emergency stock steel was conducted. It is shown that long-term operation does not give rise to a substantial redistribution of cementite. Deformation aging is slightly expressed and is manifested through the precipitation of finely dispersed carbides in the grain bulk. The revealed structure degradation does not practically make sense under static tension or hardness measurement. At the same time, under cyclic testing the microstructure degradation being occurred during long-term operation results in fatigue lifetime decrease especially at the stage of crack initiation. The most pronounced manifestation of structure changes is related to decreasing impact toughness of 09Mn2Si steel which reaches ~ 2 times under negative temperatures. The reasons and mechanisms of the observed phenomena are discussed.