Page 12 - ЛІТЕРАТУРНИЙ ОГЛЯД

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state of steam pipeline steels is proposed to be estimated measuring hardness on the

pipe external surface during their scheduled inspections (the critical value of hardness

is 133 HB).

2. It has been proved for the first time that the sizes of microstructure

elements (carbides and grains) measured across pipe wall in steam pipelines well

correlate with hoop tensile stresses acting permanently in the cross section of the

pipe wall under steady-state operating conditions or occurred for a short time during

planned or forced shutdowns. It was shown that the pipe outer surface is subjected to

tensile stresses at all stages of their operation, and this stimulates the grain growth,

intensifies the coagulation of carbides along grain boundaries, and accelerates the

pipe fracture due to creep.

3. The dependency has been built for the first time between the structural С /
p
С (area of inclusions delaminated from the matrix on the unit area of a
mi
metallographic picture) and fractographic S / S (area of brittle fragments on the unit
cr
area of a fractographic image of Charpy specimens after the impact test) indicators

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of the state of long-term operated steels produced in the late 19 and early 20
centuries. The fractographic indicator is revealed to be more sensitive (more than

twice as much) to steel degradation than the structural one. The possibility of its use

in the assessment of the degradation degree and also for the evaluation of the critical
state of other operated steels has been substantiated.

4. Fractographic features concerned with the changes in the fracture
mechanism of degraded structural steels and occurred as a result of their long-term

operation under ambient and high-temperature conditions have been distinguished
and classified for the first time:

 Delaminations at the macro- and microscale and intergranular cleavages on

the fracture surfaces of the laboratory specimens after the tensile, impact and stress

corrosion cracking tests made of carbon and low-alloy rolled steels produced in the
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late 19 and early 20 centuries (components of bridges, structures, water towers
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and a lighthouse), in the second part of 20 century (gas- and oil pipelines) and
long-term operated under ambient conditions. The occurrence of these delaminations