Operating Degradation of Gas-Main Pipeline Steels

Mechanical properties of pipeline steels in the initial. state and after long term service are investigated. Long term service of gas trunk pipelines is revealed to be accompany by in-bulk steel degradation. It reflects firstly by the decrease in the reduction of area, fracture toughness, crack growth resistance. The peculiarities of mechanical behaviour of exploited steels are shown: the opposite change of the plasticity parameters, the decrease in the RA accompanied by the increase in elongation, the decrease in the resistance to brittle fracture simultaneously with the decrease in hardness. The decrease of the crack growth resistance parameters is the most pronounced. For the exploited steels the properties of down parts of the pipes are worse than upper parts and near the inner surface especially. The main factor of pipe degradation in the long term service has been assumed to be the microdamage developed during exploitation.