On the reasons for the premature failure of safety valve springs in the equipment of the primary oil refining

The reasons for the premature failure of a spring made of steel 50KhFA used in the safety valve of the column head part of the flare facility were 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, chemical analysis of the steel composition, energy-dispersive X-ray spectroscopy (EDS) of metal in local areas, macro- and microstructural analysis, macro- and electron fractography, phase chemical and X-ray structural analysis, hardness and microhardness tests and reductive heat treatment were used in the research. The data on the characteristic external signs, typical micro damages, and the mechanism of destruction under low-temperature hydrogen sulfide corrosion of steel 50KhFA with hydrogenation, the most dangerous accompanying process, were gained in the study. According to the results obtained, the chemical composition and hardness of the spring metal meet the requirements of the standards for steel 50KhFA. The microstructure of the studied metal is tempered martensite with a decarburized layer up to 0.158 mm in thickness present on the surface. The analysis of the results showed that the premature failure of the spring is attributed both to technological heredity and contact with the working environment unauthorized by the project. An ulcerative damage of the metal surface and penetrating of corrosion products into the depth of metal due to violation of the integrity of the spring coating were revealed, which indicates a low resistance of 50KhFA steel to low-temperature hydrogen sulfide corrosion. The failure occurs predominantly near non-metallic inclusions along the boundaries of primary austenitic grains, where the largest precipitates of chromium carbides are located, as well as along the interphase boundaries of oriented carbide plates.