On the Reasons for the Premature Failure of Safety Valve Springs in the Equipment of Primary Oil Refining

Abstract

The reasons for the premature failure of a spring (made of 50KhFA steel) used in the safety valve of the column head part are analyzed. The failure of the spring occurred after 7 years of operation at a temperature below 90°C in a working environment of light oil products (sulfurous gasoline). Visual and measurement control, general chemical analysis of the composition of the steel, energy-dispersive analysis of the metal in local areas, measurements of hardness and microhardness, microstructural analysis of the metal, macro- and electron fractography, phase chemical and X-ray structural analyses, and recovering heat treatment are used for the studies. The characteristic external signs, typical microdamages, and mechanisms of sulfide (hydrogen) corrosion cracking of high-strength 50KhFA steel with the most dangerous accompanying process, hydrogenation, are revealed. The analysis of the chemical composition and hardness of the metal meets the requirements of the standards for this steel. The microstructure of the metal under study is tempered martensite with the presence of a decarburized layer of up to 0.158 mm in the surface layers. The results of the study show that the premature failure of the spring is attributed both to the technological heredity and operational factors (contact with the working environment beyond the design basis). Ulcerative damage of the metal surface with the penetration of corrosion products into the deep layers due to the violation of the integrity of the coating of the spring are noted, which indicates low resistance of 50KhFA steel to low-temperature hydrogen sulfide corrosion. The failure predominantly occurs near nonmetallic inclusions along the boundaries of the primary austenite grains, where the largest precipitates of chromium carbides are located, as well as along the interphase boundaries of oriented carbide plates.