Influence of long-term cold climate operation on structure, fatigue durability and impact toughness of 09Mn2Si pipe steel

The microstructure and mechanical properties of 09Mn2Si steel used for the transmission natural gas pipeline “Mastakh-Berge-Yakutsk” have been investigated comprehensively after 37 years of operation. The results obtained for the specimens cut from the pipe were compared to those obtained using the preserved (reference) specimens. It is demonstrated that long-term operation in harsh climate does change the pearlitic microstructure appreciably. The strain aging however can be noticed through precipitation of finely dispersed carbides in the grain interior. The latter causes some embrittlement of the steel measured under impact loading. Revealed microstructural changes are not practically important for “quasi-static” tension or hardness properties. Nonetheless, under cyclic testing the microstructure degradation, which occurred during long-term operation, results in a considerable reduction of fatigue life, which is particularly pronounced during the crack initiation stage. The microstructure degradation is most clearly seen in sharply reduced impact toughness which dropped by a factor of two at temperatures below zero Centigrade. Reasons and mechanisms of the observed phenomena are discussed.