P. 11


                         It  was  established  that  intergranular  fracture  fragments  revealed  on  the

                  fracture surfaces of heat-resistant steel elements (linear sections and different bend

                  zones of TPP pipelines, blades of steam turbine rotors) as well as on the fracture

                  surfaces  of  specimens  made  of  these  operated  steels  after  cyclic  crack  growth

                  resistance tests, are a common fractographic feature of cohesion weakening between

                  adjacent grains in low- and high-alloy heat-resistant steels.

                         The  relationship  between  the  effective  threshold  of  cyclic  crack  growth

                  resistance  and  the  area  of  fractographically  visualized  intergranular  fragments  S

                  against  the  background  of  transgranular  fatigue  relief  on  the  fracture  surfaces  of

                  heat-resistant steels of steam pipelines and steam turbine blades. It was revealed that

                  blade steel reached the critical structural-mechanical state under lower fraction of

                  intergranular fracture elements (S  = 0.07) than that for steam pipeline steel (S  ≈
                  0.27). This could be explained by the fact that turbine blades operated under cyclic
                  loading  are  more  sensitive  to  small  structure-induced  stress  concentrators  (pores

                  around  carbides  along  grain  boundaries  and  their  chains)  comparing  to  steam

                  pipelines operated mainly under static loads.

                         The  methodology  of  the  current  structural-mechanical  state  assessment  of

                  long-term operated structural steels has been elaborated, and the critical state for a

                  number  of  structural  steels  of  various  objects  operated  under  the  influence  of
                  ambient and technological conditions has been substantiated on the basis of obtainec

                  relationships between metallographic, fractographic and mechanical indicators.
                         Scientific novelty of obtained results

                         1. A  deviation  from  linearity  in  the  dependency  hardness–  grain  size  (Hall–
                  Petch-type) НВ – d    –1/2  has been revealed for the first time for low-alloy heat-resistant

                  steel 15Kh1MF after its long-term high-temperature (up to 540  С) operation on the

                  main steam pipelines of TPPs. On this basis, the structural criterion required for the

                  assessment of the current state of operated steel has been substantiated. It was shown

                  that  the  most  significant  changes  in  both  structural  (grain  size)  and  mechanical

                  (hardness  HB)  indicators  of  degraded  steel  state  due  to  shut-downs  of  the

                  technological process took place in the vicinity of the pipe surface. Thus, the current
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