Research of the structural degradation features of long-term operated pipe steels

Abstract

The operation of the main pipelines is influenced by a number of factors, causing the steel strain ageing that
shows up only in case of its long-term operation (over 20 years). The analysis of studies on the pipe steels features
in the initial state and after a certain period of operation showed the presence of the steel mechanical properties
changes, which are expressed in the strength increase and reduction of the viscous and plastic properties in
comparison with their nominal values. But the valid methods for research of the steels mechanical properties
changes are often valid for only one brand or type of material, and ignore the irreversible changes processes in the
microstructure. Therefore, to determine and study the obvious signs of microstructural changes in the pipe metal
and determine their relationship with the changes of the metal mechanical changes. The research complex of
physical and mechanical properties of the pipe steel samples was carried out to solve this problem. The samples
were cut out from the operated pipeline and also from the emergency stock pipes. The metal wall thickness crosssectional
hardness, toughness, coercive force, microhardness, macro- and microstructure of the pipe steel were
studied. Comparison of the operated pipe metal data and the emergency stock pipe metal data with the
corresponding certificate values indicated the presence of pipe metal ageing signs, which is manifested in the main
mechanical properties degradation predefined by the deformation ageing and the microstructure phase components
restructuring.