Page 13 - ЛІТЕРАТУРНИЙ ОГЛЯД

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is caused by microstructural peculiarities, namely, cohesion loss between inclusions

and the matrix or between adjacent layers of ferrite-pearlite mictostructure;

 Fragments of intergranular fracture on the operational fracture surfaces, and

also on the fracture surfaces of the specimens after fatigue testing (against the

background of transgranular fatigue relief) made of low- (straight sections and bends

of steam pipelines of TPPs) and high-alloy (rotor blades of steam turbines) heat-

resistant steels of thermal power equipment, caused by formation and coagulation of

carbides along grain boundaries with their segregation from the matrix, and

subsequent pore formation with their following coalescence.
5. The microfractographic feature of degradation has been revealed for the

weld metal of the factory longitudinal welded joint on pipes operated on the oil main

pipeline in the form of cleavage fragments against the background of generally

ductile relief in the specimens after the Charpy testing, whereas the main

fractographic feature of degradation for other zones of the welded joint are

delaminations. Basing on this, the conclusion has been drawn about more intensive

degradation of the weld metal in comparison with other zones of the welded joint.

6. It was shown that the ratio KCU / KCV can be used as the informative

indicator of degradation of long-term operated low-alloy heat-resistant steels of the

strait sections, different zones of a bend and a welded joint of TPP steam pipelines.
(the ratio KCU / KCV is 1.2–1.4 for unexploited steel, and 1.5–2.5 for exploited

ones). Besides, Charpy impact toughness (KCV) is more sensitive to the changes in
the structural-mechanical state of steels due to their long-term operational

degradation than KCU values. It was explained by the fact that the resistance to
crack propagation in degraded materials reduced more intensively than the

resistance to crack initiation.

7. The critical state of degraded steels after their long-term operation under

ambient and high-temperature conditions has been substantiated for the first time by

the changes of the fracture mechanism. In the case of low-temperature operation, it is

the replacement of delaminations by brittle intergranular cleavages on the fracture

surfaces of structural elements after the Charpy testing, and for high-temperature