Corrosion-mechanical characteristics of the materials of nonmagnetic shroud rings of turbogenerators. I. 8Mn−8Ni−4Cr and 18Mn−4Cr steels

Abstract

On the basis of the comprehensive analysis of the corrosion-mechanical characteristics of austenitic nonmagnetic 8Mn−8Ni−4Cr steels, we show that this type of steel is susceptible to corrosion cracking even in low-aggressive aqueous solutions, including humid working environments at a temperature of 80°C. The application of aluminum coatings and mechanical surface treatment increase the time to failure of specimens. Only deep drying of hydrogen used to cool the shrouds eliminates the causes of corrosion cracking of these types of steel. The procedure of cold tensile hardening cannot be applied to 40Kh4G18 steel because it is characterized by high susceptibility to the formation of α-martensite in the process of plastic deformation due to the insufficient stability of austenite. The optimal vanadium contents of steel guaranteeing its high strength and plastic characteristics lie within the range 1.3–1.5%. In the absence of water, 18Mn−4Cr steel does not exhibit any noticeable susceptibility to corrosion cracking. For this reason, the main condition that should be realized to prevent corrosion cracking and corrosion fatigue of shroud rings is the maintenance of the dry state of working environments in the processes of manufacturing, storage, and transportation of these rings, in service, and during downtimes. The presence of Cl− ions, hydrogen sulfide, halides, and nitrates in the working environments significantly enhances the fatigue crack growth rates in the entire investigated range. These ions also promote crack growth under the conditions of long-term static loading.