Introduction
Meantime Gazprom is operating over 170000 kilometers of main gas pipelines, in practice more than 70% of gas pipelines are worn out for 30 years and more of operation. During this time gas pipelines insulating coating has become completely degraded, pipe steel aging process is in progress, metal structure is changing. The number of corrosive and fatigue damages of pipes is repeatedly increasing. The changes in strength property and microscopic structure of pipe steel have been examined on one of the local pipelines, which is being operated in the Republic of Bashkortostan.
The main gas pipeline was placed in operation in 1958. One of the gas pipeline design features is that it was constructed of composite pup-joints (no more than 6 m) of «Ц» category, 529 mm diameter, manufactured according to VTU-06-ОС-58 and using the Czech production pipes, of 508 mm diameter.
The 529 mm diameter and 8 mm wall thickness pipes were made of 10G2C1 grade steel with the following characteristics:
– break point, 
MPa;
– yield limit, 
MPa;
– percent eleongation, 
%;
– impact elasticity, at t=
, 108 KCV
At that time technical specification had no provision for metal sensing with metallography or UT methods and no limits for nonmettalic inclusions availability. During pipeline technical diagnosis with UT tests, point defects (nonmetallic inclusion type) and extensive defects (lamination type) have been discovered, which are not acceptable under the current re