P. 10


                  which were manifested on the fracture surfaces of pipes during their hydrotesting

                  until through failure in the metal of heat affected zone. This indicates the increase of

                  susceptibility  of  long-term  operated pipe steel  to  stress  corrosion  cracking  due  to

                  cohesion weakening between adjacent grains. Analysis of fracture surfaces of impact

                  specimens showed  that  the  main  manifestations  of  degradation of base  metal and

                  heat  affected  zone  were  delaminations,  and  in  the  case  of  weld  metal  –  local

                  fragments  of  transgranular  cleavage  against  the  background  of  generally  ductile

                  relief.  The  evaluated  dependency  between  the  changes  in  the  fraction  of  brittle

                  elements versus the loss in brittle fracture resistance is useful for ranging different

                  sections of an oil pipeline regarding their degradation level.

                         Analyzing the fracture surfaces of prematurely destroyed drill pipes, the level

                  of  steel  impurity  due  to  little  (up  to  1  mcm)  corrosion-active  inclusions  was

                  estimated.  It  was  shown  that  the  presence  of  these  inclusions  determine  a

                  susceptibility  of  steel  to  corrosive  influence  of  technological  environment  and

                  therefore  the  lifetime  of  the  pipes  intensifying  local  corrosion  and  facilitating  a

                  deepening  of  corrosion  pits  on  the  pipe  internal  surface  which  are  the  origins  of

                  stress corrosion cracks.

                         On the basis of an experimental relationship between hardness and grain size

                  HB–d   –1/2   determined  on  the  pipe  surface  of  TPP  pipelines,  the  critical  structural-
                  mechanical state for long-term operated 15Kh1MF steel has been substantiated as a

                  deflection on the linear Hall–Petch-type relationship. It was shown that despite the
                  same service duration, the only steel subjected to more shutdowns of TPP blocks has

                  reached the critical state. The relationship was built between the effective threshold
                  of fatigue crack growth resistance of the operated steels and their hardness ΔK          th eff  –

                  HB, which enables to substantiate the level of safe stresses and the critical crack size

                  in pipes.

                         It was revealed that impact toughness KCV is more sensitive to operational

                  degradation as compared to KCU, thus their ratio KCU / KCV was proposed for an

                  evaluation of the current state of long-term operated heat-resistant steels of pipeline

                  constituents (linear sections, different zones of bends and welds) of TPPs.
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