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The resistance of 20 Cr/25 Ni steels to carbon deposition. II: Internal oxidation and carburisation
Author(s) -
Holm R. A.,
Evans H. E.
Publication year - 1987
Publication title -
materials and corrosion
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.487
H-Index - 55
eISSN - 1521-4176
pISSN - 0947-5117
DOI - 10.1002/maco.19870380403
Subject(s) - materials science , chromia , metallurgy , oxide , grain size , alloy , intergranular corrosion , non blocking i/o , chromium , carbon fibers , tin , deposition (geology) , diffusion , metallography , microstructure , composite material , chemistry , paleontology , biochemistry , thermodynamics , physics , sediment , composite number , biology , catalysis
Metallography has been carried out on specimens of 20 Cr/25 Ni/Nb, 20 Cr/25 Ni/Ti, 20 Cr/25 Ni/TiN steels which had been exposed at 923 K, 1023 K and 1123 K in CO 2 /CO gases with CO contents varying from 2 ↪ 100%. The variation of surface oxide thickness with all test parameters was determined. It was shown that the oxide formed on the 20/25/Nb steel at short exposure times in high CO environments at 923 K was absent after more prolonged exposure. This is a consequence of the coarse surface grain size retarding chromia formation and of the proximity of the test conditions to the Fe/Fe 3 O 4 /Fe 3 C triple point. The finer surface grain size of the Ti‐bearing steels ensured the formation of a Cr‐rich oxide in all test atmospheres. Coinciding with the absence of a surface oxide in the 20 Cr/25 Ni/Nb steel at 923 K, pitting oxidation occurred which developed parabolically with time. The data were used to obtain an estimate of the oxygen diffusion coefficient at this temperature. Severe carburisation was also observed under these conditions. Additionally, measurements were made of the depth of intergranular oxidation. This was found to be most severe in the 20 Cr/25 Ni/Ti alloy and increased with temperature and environmental CO content. The 20/25/TiN steel also exhibited sub‐surface voidage (globular attack) but its rate of development was insensitive to gas composition.