In ferrite-pearlite steel pipelines manufactured using technologies from the mid-1960s, a steady increase in brittle fractures is observed after 15-20 years of operation in northern regions at sub-zero ambient temperatures. A number of publications propose to consider the influence of daily temperature changes on the bearing capacity of pipelines by algebraically adding the reduced number of thermal cycles to the total number of loading cycles resulting from operating pressure pulsations and thus determining the total number of loading cycles. This approach does not take into account the impact of sub-zero temperatures on the physical and mechanical properties of the structural metal during long-term operation. The article presents the analysis of the fracture of pipelines made of ferrite-pearlite steels structurally reinforced by increasing the volume fraction of pearlite during operation in areas with sub-zero mean annual temperatures and large-gradient daily changes in sub-zero temperatures. The combined effect of long-term cyclic loading, internal shrinkage strains of structural elements with different crystal lattices and sharp changes in sub-zero temperatures create the plasticity reduction effect and an increase in the brittle fracturing trend of ferrite-pearlite steels at ambient temperatures above the cold brittleness threshold. A methodology for assessing the influence of sub-zero temperatures on the properties of pipe steels is proposed. For ferrite-pearlite class steels, sub-zero temperatures causing either significant or, in some steels, extraordinary drops in impact strength are determined at levels substantially above the cold brittleness thresholds. These sub-zero temperature ranges are defined as «low temperature brittleness ranges».
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