Diagnostics of the Engineering State of Steam Pipelines of Thermal Power Plants by the Hardness and Crack Resistance of Steel

We propose a nondestructive method for the diagnostics of the current state of heat-resistant steels after long-term operation in the steam pipelines of thermal power plants (TPP). It is based on measuring the sizes of ferrite grains and the hardness of 15Kh1M1F heat-resistant steel directly on the pipe surface and on the use of the deduced relationship between these characteristics (of the Hall–Petch type). The plot of this relationship contains two rectilinear parts. The first part agrees with the physical ideas concerning the correlation between the grain size and the strength (hardness) of the material. The second part with a rapid decrease in the hardness of steel is connected not only with the subsequent changes in the microstructure of steel but also with the development of the in-service defects scattered in the volume of the metal. To determine the limiting state of degraded steel, we used one of the parameters of fracture mechanics, namely, the effective threshold amplitude of the stress intensity factor (SIF) ΔKth eff determined with regard for the effect of fatigue crack closure. The application of this parameter is substantiated by the detected inversion of the influence of hydrogen on ΔKth eff (from positive to negative) observed as the degree of in-service degradation of steel increases. The indicated inversion is caused by the transformation of the mechanism of fatigue crack growth in the subthreshold part of the fatigue crack growth diagram from transgranular (caused by microshears at the crack tip in each loading cycle) into a brittler mechanism accompanied by the formation of intergranular fragments of mode I fracture in the damaged areas. Therefore, the threshold of cyclic crack-growth resistance corresponding to this inversion is taken as the critical value $$ \varDelta {K}_{\mathrm{th}\;\mathrm{eff}}^c $$ below which the probability of brittle fracture of the exploited steel becomes much higher. By using the constructed correlation dependence between ΔKth eff and the hardness of the metal and the determined critical value $$ \varDelta {K}_{\mathrm{th}\;\mathrm{eff}}^c, $$ we estimate the critical value of hardness. Below this value, the hazard of uncontrolled brittle fracture noticeably increases due to the presence of defects scattered in the bulk of the metal.