METALLOGRAPHIC ASSESSMENT OF THE DEGRADATION OF STRUCTURAL STEEL IN A TRANSPORT TANK FOLLOWING MANY YEARS OF SERVICE UNDER CONDITIONS OF INTENSE DYNAMIC LOADING
DOI:
https://doi.org/10.32782/3041-2080/2026-7-19Keywords:
transport tank, dynamic loads, metallographic assessment, 09G2S steel microstructure, pearlite spheroidization, heat-affected zone, residual life, mathematical modelingAbstract
The article presents the results of a metallographic study of the influence of intense dynamic loads (vibrations, shock pulses, hydrodynamic impacts, as well as cyclic pressure changes during shunting operations and braking) on the steel microstructure of a transport tank boiler after long-term operation (over 30 years). The aim of the work was to establish quantitative patterns of degradation of ferrite-pearlite steel 09G2S in the base metal and the heataffected zone of welded joints, as well as to develop scientifically substantiated criteria for predicting the residual life. A set of complementary methods was used: optical and electron microscopy, microhardness analysis, quantitative metallography to determine the volume fraction of pearlite and the ferrite grain size, as well as mathematical modelling of diffusion processes. It has been experimentally established that in the heat-affected zone, the pearlite fraction decreases by 35–50%, the microhardness increases to 175 HV, and the ferrite grain size increases to 28–35 μm compared to the base metal (20–24 μm). Mathematical models describing the relationship between dynamic parameters and microstructural changes have been developed: a modified Hall–Petch equation for estimating the yield strength depending on the grain size, a dependence of microhardness on the number of shock impulses, an equation for the diffusion redistribution of carbon (pearlite spheroidisation), and an integral criterion of structural degradation Sd, which takes into account changes in three key parameters: grain size, volume fraction of pearlite, and microhardness. A high correlation (R² = 0.91) between the Sd parameter and the residual life of the boiler, determined according to the standard procedure, is shown. An analytical expression for predicting the residual life Rres = R₀·exp(–k·Sd) is proposed, where R₀ = 32 years, k = 2.8. It is proved that macroscopically unchanged metal may have significant microstructural damage (Sd ≥ 0.3), which requires mandatory metallographic control when extending the service life of tanks. The obtained results provide a scientific basis for the transition from scheduled maintenance planning to a strategy of maintenance based on the actual condition of the metal, which makes it possible to increase the safety of railway transport, prevent sudden fatigue failures, and reduce unjustified operating costs for boiler replacement.
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