ISSN:
1573-4889
Keywords:
oxidation
;
Mn2O3
;
spinels
;
manganowustite
;
mechanisms
;
fusion reactors
Source:
Springer Online Journal Archives 1860-2000
Topics:
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes:
Abstract The oxidation in air of an austenitic Fe-Mn-Cr steel containing 17.8 Mn, 9.5 Cr, 1.0 Ni, 0.27 C, and 0.03 N was studied over the range 700–1000°C. Oxidation of surface-abraded samples at “low” temperatures, 700–750°C, resulted in only Mn 2O3 containing dissolved chromium, except at corners, where large nodules containing spinel and manganowustite formed. The Mn2O3 layer grew into the substrate forming a globular-type film. This growth mode was the result of slow interdiffusion in the alloy after the cold-worked surface layer had been recrystallized and/or consumed, as evidenced by the formation of a ferrite layer subjacent to the scale and by the instability of the planar interface. No internal oxidation was observed beneath the Mn2O3 film at either 700 or 750°C. Samples oxidized in the“high-temperature” region, 800–1000°C, exhibited vastly different behavior, forming thick stratified scales at long times (24 hr), the scales consisting of a very thin outer layer of Mn2O3 (with appreciable iron in solution), Fe-Mn spinel beneath the outer layer, and a thick inner layer of manganowustite and a chromium-containing spinel. No chromium was found in the outer two layers. A thin layer of nearly pure Fe2O3 formed between Mn2O3 and the outer spinel. Quasiparabolic kinetics were observed. The high-temperature rates were about 103 to 104 times greater than at low temperatures at the “transition” temperature. The rapid rates at high temperatures were attributed to manganowustite growth. However, oxidation of an electropolished sample at 750°C, from which the superficial cold-worked layer had been removed, formed scales similar to those observed at high temperatures at comparable rates. A difference by a factor of over 104 existed between the oxidation rate of the electropolished sample and the surface-abraded sample at 750°C. The much slower oxidation rate of the latter is attributed to greatly enchanced manganese diffusion through the high dislocation-density, cold-worked layer. Short-time tests at 800°C revealed an incubation period during which a thin protective layer of Mn2O3 formed. The incubation period corresponded to the recrystallization time of the cold-worked layer. Subsequently, nodular growth occurred which was associated with internal oxidation. The nodules, consisting of spinel and manganowustite, eventually linked up to form a thick, stratified scale. Comparison of the scale structures with calculated phase diagrams of composition versus oxygen activity (at constant temperature), showed that the protective films formed at low temperatures were due to kinetics factors, involving enhanced manganese diffusion through the cold-worked layer, rather than to thermodynamics. A model for the breakdown of protective films is proposed which involves internal oxidation.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00655899
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