Page 10 - ЛІТЕРАТУРНИЙ ОГЛЯД
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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.