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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  /
                  С   (area  of  inclusions  delaminated  from  the  matrix  on  the  unit  area  of  a
                  metallographic picture) and fractographic S  / S (area of brittle fragments on the unit
                  area of a fractographic image of Charpy specimens after the impact test) indicators

                  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
                  late 19  and early 20  centuries (components of bridges, structures, water towers
                  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
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