Page 11 - ЛІТЕРАТУРНИЙ ОГЛЯД

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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

p
b
intergranular fracture elements (S = 0.07) than that for steam pipeline steel (S ≈
с
с
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

о
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