FRACTOGRAPHIC SIGNS OF OPERATIONAL DEGRADATION OF HEAT-RESISTANT STEEL AND ELIMINATION OF ITS CONSEQUENCES BY HEAT TREATMENT

Abstract

It is shown that after ~ 21 × 104
h of operation of 25Kh2M1F steel in the elements of fastening fittings (nuts and studs) of the
inner case of the high-pressure cylinder of a steam turbine at a TPP, its mechanical characteristics (hardness, strength, plasticity, and
impact toughness) no longer correspond to the regulated values, which indicates operational degradation of the metal. It was found
that impact toughness was the most sensitive characteristic to degradation. It has been metallographically shown that the degradation
of 25Kh2M1F steel in the fastening elements is accompanied by an increase in the grain size of the former austenite and carbides
along their boundaries. Renovation heat treatment of the steel facilitates the refinement of both of these structural parameters, which
have a decisive influence on the mechanical properties of the steel.
Fractographic studies of specimens from exploited steel of both fastening elements after their impact toughness tests
revealed that in the vicinity of the notch tip almost immediately behind a very narrow strip of ductile relief of dimples formed
by the shear mechanism, their further fracture occurred brittle by the mechanism of transgranular cleavage, which is
consistent with critically low values of the impact toughness of steel. Secondary cracking along the boundaries of the former
austenite grains, which was especially pronounced in the area of spontaneous destruction of the specimens, was considered a
fractographic sign of the operational degradation of steel. This feature was considered as evidence of a decrease in cohesion
between adjacent grains, along which complex-alloyed carbides were precipitated, due to creep process during operation, lost
their connection with the matrix, forming pore chains. Small dimples on the surfaces of secondary cracks were considered not
a consequence of ductile fracture during specimen testing, but the result of visualization of damages along the grain
boundaries formed during the operation of the steel.
On the fracture surfaces of steel specimens after renovation heat treatment and subsequent impact tests, both macro
and micro signs of embrittlement disappeared. So, after two-stage normalization of the exploited steel on impact fracture
surfaces in the vicinity of the stress concentrator, a zone of typical ductile fracture was found, which extended over a
significant part of the specimen cross section, which is consistent with the high impact toughness of the renovatied steel. The
secondary cracking as the main sign of the operational degradation of the steel was not revealed on the specimen fracture
Міжвузівський збірник «НАУКОВІ НОТАТКИ». Луцьк, 2021, №71
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© Л. М. Свірська, Г. В. Кречковська, П. Р. Соловей, О. З. Студент
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surfaces. On the basis of the sizes of the transgranular facets in the area of spontaneous destruction of the specimens, it was
also confirmed that the grain was significantly reduced after the renovation heat treatment of steel. The loss of the cohesion of
non-metallic inclusions with the matrix was only sign of steel degradation, which could not be eliminated during renovation
treatment. But their relatively low density in the cross-section of the fastening elements neutralized their usually negative
effect on the performance characteristics of the steel after its renovation.
The results obtained give grounds to recommend a proven heat treatment to restore the mechanical characteristics of
the high-pressure cylinder studs and nuts of steam turbines and extend their service life