Page 9 - ЛІТЕРАТУРНИЙ ОГЛЯД

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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

th
th
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
br
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
brit

of brittle fracture resistance KCV / KCV . The fractographic criterion for reaching
in
op
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,