Effects of cold work on the oxidation behavior and carburization resistance of Alloy 800

Abstract

The structure of the oxide layer formed on Alloy 800 at 600°C in superheated steam markedly indicates the role of the grain boundaries as easy diffusion paths of Cr and Mn to the alloytoxide interface. Increasing the number of grain boundaries by lo-%% cold work leads to increasing Cr-and Mn-content in the scale and to decreasing oxide growth rates. Variation of the grain size by different annealing treatments leadssince the Cr-content in the scale is decreasing with the grain sizeto a linear relation of growth rate and grain size.The effect of cold work was also demonstrated on the protectiveness of the oxide scale towards carbon uptake and carburization of Alloy 800. After preoxidation of differently deformed specimens at 9OO"C, these were exposed to a CO-C02-H20-H2 mixture at 700°C for long time. The gas mixture was tagged with "C so that the C-ingress into the oxide scale and into the alloy could be sensitively monitored by autoradiography and (upon stepwise polishing) radioactivity measurements of the carbon penetration. The carbon uptake is effectively reduced with cold working; in contrast a non-deformed, electropolished and preoxidized specimen shows relatively high C-content after exposure.The investigations prove the highly favorable effect of mechanical pretreatment on the formation of the oxide scale on an austenitic Fe-Ni-Cr alloy. Cold work and other methods of surface deformation (grinding, polishing, sand blasting, shot peening) generate easy difision paths for fast Cr-diffusion to the surface and sufficient supply of Cr to form a protective oxide layer.Der Aufbau der Oxidschicht, die sich auf Alloy 800 bei 600°C in iiberbitztem Dampf bildet, zeigt deutlich die wichtige Rolle der Korngrenzen als schnelle Diffusionswege des Cr and Mn zur Grenzflache LegierunglOxid. Eine 1040% Kaltverformung fiihrt aufgrund der zunehmenden Zahl an Korngrenzen zu erhohtem Cr-und Mn-Gehalt der Oxidschichten und zu abnehmenden Oxidationsgeschwindigkeiten. Bei Veriinderungen der KorngraSe durch verschiedene Gluhbehandlungen erhalt manbei mit der Korngro6e abnehmendern Cr-Gehalteine lineare Beziehung zwischen Oxidationsgeschwindigkeit und KorngroBe.Die Auswirkung der Kaltverformung wurde ebenfalls fib die Schutzwirkung der Oxidschicht gegen das Eindringen von Kohlenstoff und die Aufkohlung von Alloy 800 gezeigt. Nach Voroxidation verschieden verformter Proben bei 900°C wurden diese liingere Zeit einer CO-COz-Hz-H20-Atmosphtire bei 700 O C ausgesetzt. Das Gasgemisch war mit I4C indiziert, SO da5 die C-Aufnahme in die Oxidschicht und in die Legierung durch Autoradiographie und Radioaktivitats-Messungen (unter schrittweisem Abpolieren) sehr empfindlich nachgewiesen werden koMte. Die C-Aufnalune wird mit zunehmendem Verformungsgrad wirksam vermindertim Gegensatz hierzu zeigt eine unverformte, elektropolierte und dann voroxidierte Probe relativ hohe C-Gehalte nach der Auslagerung.Diese Untersuchungen belegen die giinstige Auswirkung einer mechanischen Vorbehandlung auf die Bildung einer schiitzenden Oxidschicht auf einer aust...

Role of Nanostructure in Electrochemical Corrosion and High Temperature Oxidation: A Review

“…A common high-Cr alloy (such as 18Cr-8Ni stainless steel) forms an inner layer of Fe/Ni oxide, during initial stages of oxidation, which eventually converts into a protective oxide, i.e., Cr 2 O 3 when sufficient Cr diffuses from the alloy bulk to the oxide scale-alloy interface. [202] The kinetics of transition of Fe/Ni oxide into protective layer of Cr 2 O 3 depends on the supply of chromium by diffusion in the alloy matrix, which is governed profoundly by grain size of the alloy. As clearly demonstrated in the literature, [203] a relatively fine grain (~17 lm or less) 18Cr alloy easily developed a uniform layer of Cr 2 O 3 .…”

Long-term behaviour of Nb and Cr nitrides nanostructured coatings under steam at 650 °C. Mechanistic considerations

“…Cr, Mn, Fe spinels are known to be protective in particular, if they are rich in Cr and Mn [23,24]. In the present case, this oxide formed on the lateral surfaces of the crack/void could have been further enriched with Cr by diffusion from the substrate.…”

Oxidation Resistance of Nanocrystalline Alloys

“…It was established in the literature that a fine grain (~17 µm or less) stainless steel easily developed a uniform layer of Cr 2 O 3 . For an alloy with grain sizes greater than ~40 µm, this protective layer of Cr 2 O 3 was difficult to form due to insufficient grain boundary diffusion and inadequate chromium supply [98][99][100]. In nanocrystalline materials where grain size is very fine and diffusion coefficients are high, such Cr oxide formation should be facilitated by a large extent.…”

“…They reported that the oxide film nucleated at the grain boundaries needs to grow laterally for the formation of a uniform oxide layer and reported that the finer the grain size, the better the uniformity of oxide should be. Later it was shown that above a critical grain size, formation of a uniform Cr layer was not possible which could be understood based on the combined role of diffusion and nucleation site densities in the alloys [44,45,[98][99][100]. However, these studies were performed on the material where minimum grain size was a few microns.…”