The Influence of Wet-Steam Flow Electrization on the Surface Strength of Turbine Blade Materials

Abstract

The phenomenon of droplet impingement erosion of a wet-steam turbine’s last-stage blades is conditionally considered in the aspect of two “opposing” processes: the force impact of droplets on metal and “resistance” of the metal to this impact. The article places its focus on “resistance” of the metal surface layer to the force impact under the influence of electrified steam and concomitant electrophysical phenomena on its mechanical properties. It has been established that the electrophysical phenomena accompanying wet-steam flow electrization facilitate saturation of the metal with hydrogen (6–10 times higher in comparison with a neutral flow) and have a significant effect on its microhardness (the changes toward its decreasing may be as much as 50%). These changes take place in a thin surface layer, and their extent and nature depend on the flow polarity and also on the frequency and polarity of the electric field induced on the turbine blade. The decrease of microhardness observed in the thin surface layer can be attributed to the phenomenon of local microplasticization, which occurs as a result of facilitating the incipience of microshears when hydrogen cations interact with the dislocation nuclei generated in introducing an indenter. Such plasticization at the microlevel facilitates the development of hydrogen brittleness at the macrolevel. It has been established that, as far as the maximal erosion-induced strength degradation of blade materials is concerned, a positively charged wet steam flow is the most dangerous factor, which most frequently takes place in the case of using water chemistry with pH > 9 (ammonia water chemistry). It is shown that it is possible to alter the development kinetics of destruction caused by droplet impingement erosion by changing the physicochemical properties of thin surface layers of metals.