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                         An important scientific and technical problem has been solved in this work

                  devoted  to  the  development, approbation  and  implementation  of  methodology  for

                  assessing the structural and mechanical state of structural steels after their long-term

                  operation  under  various  temperature  and  stress  conditions  in  corrosive  and

                  hydrogenative environments. The methodology is based on the comprehensive study

                  of the regularities of changes in structural, fractographic and mechanical indicators

                  sensitive  to  steel  degradation,  searching  of  the  relationships  between  them  with

                  further substantiation of the critical state of degraded steels.

                         It  was  established  that  a  common  feature  of  degradation  of  rolled  carbon

                  steels (components of structures produced in the late 19  and early 20  centuries)
                  and low-alloy steels (gas and oil main pipelines) operated at ambient temperature is

                  decohesion of nonmetallic inclusions (as delamination origins) from the matrix. A

                  structural  peculiarity  of  high-temperature  (up  to  540°С)  degradation  of  high-  and

                  low-alloy heat-resistant steels is redistribution of carbon and alloying elements at the

                  grain  boundary  due  to  diffusion  and  coagulation  of  alloyed  carbides  as  sights  of

                  intergranular bond weakening.

                         It was established that delaminations against the background of typical ductile

                  fracture relief and transgranular cleavage with classic river patterns at the fracture

                  surfaces of impact specimens are fractographic signs of degradation for low-alloy
                  and  carbon  steels operated  under  ambient  conditions. Their degradation degree  is

                  proposed to be estimated as a area fraction of brittle fracture elements S  on the unit
                  area S of the fracture surface of the Charpy specimens. A unified relationship was

                  built for the steels from bridges, a water tower, a lighthouse, oil- and gas pipelines,
                  portal cranes and drill pipes between the fractographic indicator S  / S and the loss

                  of brittle fracture resistance KCV  / KCV . The fractographic criterion for reaching
                  the critical state of steels is substantiated by the change of the crucial sign of steel

                  embrittlement from delaminations on the fracture surfaces of Charpy specimens to

                  transgranular cleavage.

                         Fractographic signs of metal degradation of a longitudinal joint on oil pipes

                  were  revealed  in  the  form  of  round-shaped  fragments  of  intergranular  fracture,
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