Materials Science Forum Vols. 522-523

Abstract: In order to study the nucleation and growth of cracks in the outer oxide scale which expand into the underlying alloy, deformation in creep in oxygen or in vacuum of zirconium and Zircaloy-4 has been studied mainly at 500°C. Influence of applied stresses, atmosphere and alloy’s grade on the deformation and oxidation processes are especially analyzed. The results underline the presence of two distinct deformation domains for both alloys grades, depending on the applied stress value. The presence of the oxide scale leads only to slight modifications on the deformation mechanism but it induces an increase of the deformation rate. This enhancement is especially observed in the case of the pre-oxidized Zircaloy-4 whose cracks remain mainly located in the outer part of the oxide. In opposite, the pre-oxidized zirconium shows cracks located down to the underlying metal. Acoustic emission is used to follow, in situ, in temperature the damage process of the outer zirconia layer during creep, and precisions about the oxidation mechanism and the effect of applied stress on oxygen diffusion and oxide growth rate are obtained thanks to the use of 18O as a marker.

Abstract: Strains in thermally grown oxides have been measured in-situ, as the oxides develop and evolve. Extensive data have been acquired from oxides grown in air at elevated temperatures on different model alloys that form Al2O3. Using synchrotron x-rays at the Advanced Photon Source (Beamline 12BM, Argonne National Laboratory), Debye-Scherrer diffraction patterns from the oxidizing specimen were recorded every 5 minutes during oxidation and subsequent cooling. The diffraction patterns were analyzed to determine strains in the oxides, as well as phase changes and the degree of texture. To study a specimen's response to stress perturbation, the oxidizing temperature was quickly cooled from 1100 to 950oC to impose a compressive thermal stress in the scale. This paper describes this new experimental approach and gives examples from oxidized β-NiAl, Fe-20Cr-10Al, Fe-28Al-5Cr and H2- annealed Fe-28Al-5Cr (all at. %) alloys to illustrate some current understanding of the development and relaxation of growth stresses in Al2O3.

Abstract: Adhesion energy values for thermal oxide scales cyclically grown at 850 and 950°C in air on ferritic and austenitic stainless steels were obtained using an inverted blister test and a tensile test working in the SEM chamber. The blister test used water pressure for debonding the metalscale interface, whereas the tensile test led to transverse compression generating scale spallation by buckling. Adhesion energy, defined by energy for interface crack propagation by unit area, was shown to be in the range 10 to 650 J.m–2 for the chromia-rich scales with thickness in the micrometer range. Ferritic grades gave less adherent scales than austenitic ones, and a great influence of titanium was evidenced, greatly increasing scale adhesion; niobium was less operative. Adhesion was well connected with nature and morphology of Ti and/or Nb-containing precipitates at the metal-scale interface.

Abstract: The oxidation behaviors of several commercial low and ultra low carbon steels in air at 700-950°C were studied. Under isothermal oxidation conditions, the oxidation kinetics, scale structure and scale-steel interface adherence were significantly affected by the steel composition.

Abstract: The behavior of the surface oxide scale on steel products during hot rolling process influences the surface properties of final products. To investigate the deformation and the fracture behavior of surface oxide scale of Fe-13Cr alloy, a hot rolling test was carried out. The oxide scale rolled out was observed in detail by using TEM. The specimen was hot-rolled after oxidation at 1100
for 90 minutes in air. The hot rolling tests with two conditions (. The hot rolling test of the outer scale {=whole layer scale} , . The hot rolling test of the inner scale that removed the outer scale) were carried out. The rolling reduction rate was 25, 44, 58, and 68%. The outer scale was composed of Fe2O3 and F3O4, and the inner scale was composed of Fe3O4, FeCr2O4, and a small amount of Fe2SiO4. Fe2SiO4 formed along the grain boundaries of the other oxides (Fe3O4, FeCr2O4) was observed by TEM. In the test , Fe2O3 of the outer scale was pulverized to fine particle that looks like red powder, and Fe3O4 of the outer scale was cracked by hot rolling. A ductility-like behavior was observed in the inner scale (Test ). That is, it was found by the SEM observation that porosity and micro cracks of the surface oxide disappeared gradually according to the increase in the rolling reduction. It was thought that the porosity and the micro cracks eased the compression stress caused by hot rolling. In the case of high reduction rate, FeSi2O4 ,which is a low melting point oxide, formed on grain boundary caused grain boundary slipping. When the rolling reduction is very high, plastic deformation by dislocation occurred in Fe3O4 and FeCr2O4. However, these oxides were broken, when their plasticity would not be able to accept considerably high rolling reduction.

Abstract: Hardness of oxide scales on Fe-(0, 0.5, 1.5, 3.0)Si alloys was studied at room temperature after oxidation at 1273 K for 18 ks in oxygen, and at 1073 and 1273 K for 180 and 1080 ks in dry air, by micro-Vickers hardness measurements. After oxidation at 1273 K for 18 ks, high-temperature hardness of oxide scales on Fe-(0, 1.5, 3.0)Si alloys was also measured at 1273 K. Oxide scales on Fe-Si alloys were mainly Fe2O3, Fe3O4, FeO and Fe2SiO4. Hardness of Fe2O3, Fe3O4 and FeO on Fe was 6.7, 4.0 and 3.5 (GPa), respectively, and hardness of Fe2O3 on Fe-Si alloys slightly increased with increasing silicon content at room temperature. At 1273 K, hardness of Fe3O4 and FeO on Fe was 0.08 and 0.05 (GPa), respectively, and hardness of Fe2O3 on Fe-1.5Si alloy was 0.32 (GPa), and that of Fe2O3 and Fe2SiO4 on Fe-3.0Si alloy was 0.53 and 0.63 (GPa), respectively.

Abstract: The adhesion and micro-structure of the primary scale formed in LNG combustion gas on steels containing Si, up to 3.0mass%, have been studied using a hot-compression test at 1273K, Raman spectroscopy, and X-Ray absorption fine structure analysis (XAFS). The Fe2SiO4 at the scale/steel interface is increased as the Si content increases, suppressing the diffusion of Fe ions from the steel and forming dense and highly-adhesive subscale, when oxidized below the temperature at which the Fe2SiO4 melts. The behavior of the secondary scale growth in air up to 1173K on these has been studied using in-situ XRD. It is apparent that the formation behavior of Fe2SiO4 ,FeO and Fe2O3 is influenced by the Si content.

Abstract: Raman spectroscopy was conducted to evaluate mechanical stress in growing α-Cr2O3 scale upon oxidation of austenitic 25mass%Cr-20mass%Ni and ferritic 17mass%Cr stainless steels at 1173 K in air for up to 24 h. Sintered α-Cr2O3 pellet was heated to 373-1173 K and examined in order to obtain the temperature dependence of the wave length of the major Raman peak. For 1.2 mm thick 25mass%Cr-20mass%Ni steel specimen, compressive growth stress was indicated for α-Cr2O3 scale right upon oxidation and the stress increased until oxidation for 3 h, but it saturated and remained constant thereafter. The growth stress of α-Cr2O3 scale was 0.7 ± 0.1 GPa at 1173 K. For 1.2 mm thick ferritic 17mass%Cr steel specimen, mechanical stress was compressive, but the saturated growth stress was around 0.2 GPa, considerably smaller than the “strong” 25mass%Cr-20mass%Ni steel specimen. For 0.1mm thick austenitic 25mass%Cr-20mass%Ni steel specimen, the test results were similar. These were attributed to the different high-temperature strength of the metal substrate. Hence, for high-temperature oxidation of thin foils and/or ferritic steels of which high-temperature strength of the metal substrate is relatively poor, stress relaxation of protective α-Cr2O3 scale can result and the growth stress of α-Cr2O3 scale may be lowered by the “weak” metal substrate. Raman spectroscopy can offer useful information on the mechanical stress of protective oxide scale even at high temperatures.

Abstract: Isothermal oxidation was carried out on Fe and Fe-(1.0 and 5.0mass%)Cr alloy at 800°C and 1000°C in dry (PH2O=1x10-4 atm) and wet (PH2O=0.035 atm) air. Fracture behavior of the scale was investigated using acoustic emission (AE) analysis during cooling. The water vapor content of the atmosphere has a major influence on the oxidation of the Fe-Cr alloys, whereas it has virtually no effect on the oxidation of Fe. During cooling, significant AE activity starts at about 450°C on the Fe and Fe-Cr alloys oxidized in wet air. This critical temperature is independent of sample composition, oxidation temperature and scale thickness. In the case of the oxidation of the Fe-Cr alloys in dry air, however, the critical temperature shifts to higher temperatures with increasing scale thickness and Cr content. Fracture behavior of the scale should be related to scale structure caused by sample composition and oxidation condition.

Abstract: The scale failure temperature (Tf) during cooling from 1173 K of low carbon steels containing Si of up to 2.1% was assessed by in-situ acoustic emission measurements and analyses including wavelet transform. In general, Tf lowers with an increase in the Si content for steels without S or P, indicating that the scales on higher Si-content steels are more resistant to thermal stress. This tendency becomes larger for higher cooling rate. Contrarily, Tf rises with an increase in the S, P or (S+P) content for 1% Si steels. This means that the scale failure is enhanced by the additives. S segregates at the scale/substrate interface and seems to enhance the partial scale separation. P is incorporated in the (FeO+SiO2) layer on the substrate and forms microspores at the interface to the FeO layer, and thus enhanced the crack initiation by providing the sites for stress concentration. Wavelet transform showed that the scale failure mode is mainly the following; local separation of the scale over a small area takes place first, and then cracking in the scale follows. For steels containing S or/and P scale cracking is the initial failure in many cases, probably because the stress concentration sites are already formed during the scale growth.