Kinetic and Metallography Study of the Oxidation at 1250 °C of {Co+Ni}-Based Superalloys Containing Ti to Form MC Carbides

Six conventionally cast chromium-rich titanium-containing alloys based on cobalt and nickel with various Co/Ni ratios were considered. They were tested in oxidation in air at 1250 °C for 70 h in a thermo-balance. The mass gain curves were exploited to specify different types of kinetic constants as well as several parameters characterizing the oxide spallation occurring during cooling. The obtained results show that, the higher the Ni content, the slower the mass gain and the better the quality of the protective external chromia scale. Secondly, no dependence of the oxide spallation characteristics on the Co content was clearly noted. Globally, the isothermal oxidation behavior becomes better when Ni is more and more present at the expense of Co. Titanium seems to be playing a particular role in the process of oxidation. It notably leads to the presence of an external thin TiO2 continuous scale beyond the chromia scale. The thermogravimetry records were numerically treated to determine the parabolic constant and the chromia volatilization constant. The values of these constants evidenced a double tendency: chromia growth acceleration and chromia volatilization slow-down. These trends are to be confirmed and further investigated.

Oxidation and Microstructural Behaviors of Ni-Based Alloys Strengthened by (Ta, Hf)C Carbides at 1250 °C in Air

Two alloys based on nickel and designed to be reinforced by MC carbides thanks to the presence of Hf and Ta were produced by casting. They were subjected to 50 h-long isothermal exposure at 1250 °C in synthetic air with thermogravimetric monitoring of the oxidation progress. In the as-cast state, they contain both significant quantities of (Hf,Ta)C carbides. Their verified melting start temperatures, close to 1300 °C, allowed performing the planned oxidation test. The two alloys demonstrated a chromia-forming behavior with limited mass gain rates. However, they also showed a rather low resistance to oxide spallation at cooling, which is in proportion with the Ta/Hf ratio. After 50 h at 1250 °C, the morphology of the carbides had significantly evolved, from their initial script-like shape to a fragmented and coalesced state. The results are promising, but the use of these alloys at 1250 °C needs further improvements on the mechanical level.

On the spallation of oxide scales in high-strength low-alloy (HSLA) hot-rolled steels

In this work, results on the causes that could promote the abnormal spallation of the oxides formed on the surface of high-strength low-alloy (HSLA) steels are presented. By means of Rietveld refining of X-ray diffraction spectra, scanning electron microscopy analyses and calculations, it was found that the value of the thermal stress experienced by the oxide scale reached a maximum when the oxide scale was comprised by 65% wt magnetite Fe3O4 and 24% wt wustite FeO this, due to the incomplete transformation of the latter phase to Fe3O4 and α-Fe from cooling from 670 °C to ambient temperature. Contrarily, it was found that when a balance in the amount of Fe3O4 and FeO was 46.4 and 46.5%wt respectively, the calculated thermal stress was reduced, and oxide spallation was not that severe. The reasons for oxide scale detachment from the surface of the steels are explained in terms of the adhesion energy of the bulk oxide scale, the amount of magnetite Fe3O4 present in the oxides and the chemical composition of the steel particularly the elements chromium and titanium.

Oxidation Behavior of Alumina-Forming Austenitic Steels in Superheated Steam at 700 °C

In this study, three alumina-forming austenitic (AFA) steels (Fe-18Ni-12Cr-AlNbC, Fe-18Ni-12Cr-Al, and Fe-18Ni-16Cr-AlNbC) were exposed to superheated steam (SHS) at 700 °C for 1000 h to study their oxidation behavior. The Fe-18Ni-16Cr-AlNbC alloy showed the best performance as it had the least weight gain due to a denser surface oxide layer and the lack of internal oxidation. The Fe-18Ni-12Cr-AlNbC alloy suffered from internal oxidation and had a greater weight gain than Fe-18Ni-16Cr-AlNbC, while Fe-18Ni-12Cr-Al experienced weight loss due to perhaps external oxide spallation, leaving a less protective Fe-oxide on the external surface.

Corrosion Behavior of Si Diffusion Coating on an Austenitic Fe-Base Alloy in High Temperature Supercritical-Carbon Dioxide and Steam Environment

In order to enhance corrosion resistance of stainless steel (SS) 316LN at high temperature environments, surface modification was carried out by Si deposition and subsequent heat treatment at 900 °C for 1 h. This resulted in the formation of Fe5Ni3Si2 phase on the surface region. The surface-modified alloy was exposed to high temperature S-CO2 (650 °C, 20 MPa) and steam (650 °C, 0.1 MPa) for 500 h and evaluated for its corrosion behavior in comparison to the as-received alloy. In S-CO2 environment, the as-received SS 316LN showed severe oxide spallation and thick Fe-rich oxide formation, while the surface-modified alloy formed a continuous and adherent Si- and Cr-rich oxide layer. In steam, as-received SS 316LN formed very thick duplex Fe- and Cr-rich oxide layers. On the other hand, surface-modified SS 316LN formed notably thinner oxides, which could be attributed to the formation of Si-rich oxide under outer Fe-rich oxides on the surface-modified alloy. Thus, in view of the weight changes, oxide thickness, and morphologies of the two conditions, it was found that Si diffusion coating was effective in improving the corrosion resistance of SS 316LN in both S-CO2 and steam environments.

Influence of the Base Element on the Thermal Properties of Non- Ferrous Chromium-Rich TaC-Containing Alloys Elaborated by Conventional Casting: Part 3: Surface and Cross-Sectional Metallographic Characterization of the Oxidized Alloys

The six alloys the thermal properties of which and the tendency to oxide spallation of which were studied in the first two parts of this work, were here characterized after oxidation for 70 hours at 1250°C. The external chromia scale, and also the CrTaO4 subsurface oxide, formed for all the alloys, almost independently of the Co and Ni proportions in the base element content. But, because of the formation of more CrTaO4 for the nickel-richest alloys probably due to the higher availability of Ta in the matrix and its easier diffusion towards the neighbourhood of the oxidation front, the adherence of chromia was weakened and spallation, suggested by the thermogravimetric curves in the second part of this work, is here really observed and the denuded part of alloys clearly seen. The degradation of the subsurface, which can be in a first time summarized by the development of a carbide-free zone and a {Cr, Ta}-depleted zone, depends on the Co and Ni proportions. The microstructure of the bulk is differently affected by long exposure at elevated temperature. The changes in carbide population characteristics are stronger for the nickel-based alloys than for the cobalt-based ones. Finally, the isothermal oxidation behaviour is best for the nickel-richest alloys but the oxide spallation behaviour and the potential mechanical properties are the best for the cobalt-richest alloys.

Isothermal Oxidation of René N5 at 1150°C

In this study, single crystal superalloy René N5 was exposed in air at 1150 °C for up to 16 hours to evaluate the alloy’s short-term oxidation behaviour and the potential for developing a no-bond coat TBC system. The results showed that after 1 hour of exposure, a three layered oxide developed on the surface, consisting of spinel, Ta-containing oxide and alumina just above the substrate. After 4 hours of exposure, oxide spallation occurred; this became more pronounced after 16 hours. The oxide spallation took place between the top spinel layer and alumina layer, where Ta-rich oxide was more abundant. All samples tested for 0.5, 1, 2, 4 and 16 hours developed alumina near the substrate while the occurrence of NiO, spinel and Ta-oxides varied, depending upon the exposure time.

Adhesion of Oxides Grown in Supercritical Water on Selected Austenitic and Ferritic/Martensitic Alloys

A series of austenitic alloys (800H, H214, I625, 310S, and 347) with different surface finishes were exposed to supercritical water (SCW) at 550 °C and 2.5 × 107 Pa for 120 h, 260 h, and 450 h in a static autoclave with an initial level of dissolved oxygen of 8 ppm. Indentation with a hardness indenter was used for assessment of oxide adhesion. This was compared with the results of a similar test on SCW-oxidized ferritic alloys. Delamination in all the tested ferritic alloys was insufficient for quantification of the results but allowed for qualitative comparison within this group. In the set of austenitic alloys, oxide on stainless steel (SS) 347 exfoliated during cooling from 550 °C, and from the remaining four alloys, only oxide on H214 delaminated, which made the qualitative comparison across the whole group impossible. Energy dispersive X-ray spectroscopy (EDX) revealed that under delaminated external Cr2O3 on H214 alloy, there was a submicron thick layer of Al-rich oxide. To investigate a possible oxide spallation on austenitic samples during exposure, mass loss obtained through descaling was compared with mass gain due to SCW exposure. The results indicated that the applied descaling procedure did not, in most cases, fully remove the scale. Apart from one case (SS 347 with alumina surface finish), there was no clear indication of oxide spallation.