Technical Diagnostics and Non-Destructive Testing #4, 2019, pp. 50-57
Analysis of behaviour of dissimilar steel welded joint in the structure of reforming process pipeline of a reactor block
G.P. Pisarenko Institute for Problems of Strength of the NAS of Ukraine. 2 Timiryazevskaya Str., 01014, Kyiv.
The paper deals with the features of fracture of reforming process pipeline of a reactor block transporting working product with up to 85% hydrogen content, with operating time of 200 000 h at 520 C and medium pressure of 5.3 MPa. In the pipeline structure pipes from steel 15Kh5M of martensitic class were joined by austenitic welds to each other and to a flange from steel 20 of pearlitic class. It is shown that microstructure of the pipe base metal, weld and HAZ metal did not undergo any essential changes from the impact of hydrogen-containing medium during long-term service. However, hydrogen corrosion occurred in the flange material, which was manifested in decarbonization and degradation of metal properties that led to welded joint fracture. Decarbonized zones which formed around the pure graphite chains (cementite decomposition products) and coagulated cementite particles weakened the matrix and caused brittle fracture. It is found that the crack propagated through the flange metal in the areas of normalizing and incomplete recrystallization of the HAS. 23 Ref., 5 Tabl., 6 Fig.
Keywords: hydrogen-containing medium, process pipelines, long-term service; metallographic studies, metal microstructure, mechanical testing, characteristics of strength, ductility and impact toughness, brittleness temperature, property degradation, fracture
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