Experiment A1 versus A2

Secondary electron micrographs of the surface of TP347H FG for experiments A1 and A2 are shown in Figure 5-1. Both samples were oxidised for a total of 100 hours (2 x 50 hours) at 923 K where the first 50 hours was in air-saturated steam. In the case of experiment A1 the second 50 hour exposure was also in air-saturated steam whereas the second thermal exposure for A2 was in deoxygenated steam. On cooling after the first 50 hour cycle, both samples, shown in Figure 5-1(a) and (b), exhibited significant spallation with approximately 95% of the pickled concave surface spalling, mentioned earlier in Chapter 4. Adherent oxide was only observed around the edges of the samples. Further exposure resulted in further spallation of approximately 2.6% in A1 but no further spallation was observed in A2, as shown in Figures 5-1(c) and (d), respectively. SEM analysis of the surface of the samples further exposed to air-saturated steam, A1, showed areas of spallation and two distinguishable oxide morphologies. In regions of adherent oxide cracks were seen in the outer oxide layer. Where a deoxygenated environment was used, the surface of the sample had a uniform morphology and did not alter significantly between cycles.

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Figure 5-1 Secondary electron (SE) micrographs of the inner pickled surface of TP347H FG where (a) is cycle 1 of A1, (b) is cycle 1 of A2, (c) is cycle 2 of A1 and (d) is cycle 2 of A2. All exposures were for 50 hours at 923 K.

Spalled particles were collected after cycle 2 of experiment A1, Figure 5-2(a). The SEM images obtained of the spalled particles subsequent to the second exposure indicate two oxide morphologies suggesting spallation of both haematite and magnetite. Whiskers observed on spalled particles were used to help identify each oxide layer since whiskers were only observed on unspalled regions of the outer surface during SEM examination, i.e. from the haematite layer. Whisker morphology was also reported in the literature as occurring on haematite [79].

(a) (b)

(c) (d)

Magnetite

Magnetite

Magnetite

Magnetite Spinel

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Figure 5-2 SE micrograph of spalled oxide from TP347H FG exposed to air-saturated steam for 2 x 50 hours at 923 K (experiment A1).

Optical microscopy allowed the difference in contrast between haematite and magnetite to be observed that was not otherwise seen with SEM examinations of polished cross sections. Haematite appears a marginally darker grey than magnetite, Figure 4-4. There was no evidence for haematite found in optical cross sectional analysis for cycle 2 of experiment A1 as indicated in Figure 5-3. Magnetite was observed at the outer surface and spinel was seen as an inwardly growing oxide adjacent to the alloy. A Cr-rich oxide found to be chromite with a thin layer of Cr2O3 at the interface but not resolvable in Figure 5-6 was also observed at the base of the spinel along the grain boundaries.

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Figure 5-3 Optical cross sectional micrograph of TP347H FG exposed to air-saturated steam for 2 x 50 hours at 923 K, experiment A1.

The STORME technique was used to monitor the samples during cooling in laboratory air for these tests. Thermographic images were obtained for cycle 2 in both steam environments. A snapshot image is shown in Figure 5-4 where one spall site was identified and the thermal history was tracked. It was not possible to observe the delamination event for this site as it occurred during transition from the furnace to the heat proof surface, i.e. ΔT < 181.1 K. The radius of the spalled site increased gradually over the 35 seconds it was measured, with a large increase within the last 5 seconds prior to complete spallation. Also included in the figure is the critical strain energy criterion (CSEC). The CSEC assumes that strain energy builds up during cooling from the time at which delamination occurred and is only released when the critical value of ΔT was reached which in this instance was 405.6 K. Observations from the thermal imaging camera however showed the strain energy to be released gradually with increasing radius of the buckled site suggesting unstable buckling as was observed during cycle one of Chapter 4.

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Figure 5-4 (a) Thermographic image of TP347H FG during cooling from the second cycle in air-saturated steam for 50 hours at 923 K and (b) the radius of the spalled site indicated in the thermographic image with cooling.

A thermographic snapshot image taken from a sample from experiment A2 is shown in Figure 5-5. The surface of the sample cooled more uniformly than the corresponding sample in experiment A1. There were some localised points for which the temperature was measured to be lower than the rest of the surface, as highlighted in the image, but no visible spallation occurred at these sites. The radius of the localised site indicated in Figure 5-5 was measured and was shown to remain constant during the cooling period.

Figure 5-5 Thermographic snapshot of TP347H FG on cooling after the second thermal cycle in deoxygenated steam at 923 K for 50 hours.

(b)

(a)

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Cross sectional micrographs of the second thermal exposure for experiments A1 and A2 are shown in Figure 5-6. For experiment A1 (Figure 5-6(a)) the cross sectional image indicated that spallation occurred at the magnetite/ spinel interface and the crack continued beneath the magnetite layer that did not completely spall. Cross sectional analysis of a sample from experiment A2 (Figure 5-6(b)) indicated cracking occurred in the outer Fe oxide. These cracks were observed sporadically during cross sectional examination. It is postulated that these cracks were the reason for the localised temperature drops seen in the thermographic image for experiment A2, Figure 5-5. A Cr rich oxide was also observed at the base of the spinel along the grain boundaries.

EDS analysis was carried out on each sample and showed that from cycle one to cycle two there was negligible change in the concentration of oxidising elements within the Fe-Cr-Ni spinel scale for experiment A1. For deoxygenated conditions on the other hand, there was a decrease in the Fe concentration within the spinel layer, Table 5-3.

Figure 5-6 BSE images of TP347H FG where cycle 1 was carried out in an air-saturated environment at 923 K for 50 hours and cycle 2 was carried out for 50 hours at 923 K in (a) air-saturated steam and (b) deoxygenated steam.

(a) (b)

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Oxide thickness measurements were taken for both the Fe-Cr-Ni spinel and where possible, the outer Fe rich oxide, Table 5-3. As spallation of the outer Fe oxide occurred on cooling from the second cycle in experiment A1, the oxide thickness was difficult to interpret since it was unclear whether spallation always occurred at the spinel/ magnetite interface or within the magnetite layer where voids were observed.

For this reason, Fe rich oxide thickness measurements were not taken for this experiment. Since spallation did not occur on cooling during cycle two of experiment A2 measurements were able to be made for the Fe oxide. Comparisons between the two steam environments indicated the Cr rich oxide, identified as predominantly chromite in Chapter 4 at the base of the spinel is not continuous under either circumstances but continued to grow during the second thermal cycle. Because the oxide layer was not continuous, oxide thickness measurements were not recorded.

For both experimental conditions considered the average Fe-Cr-Ni spinel oxide thickness increased from cycle one to cycle two. Figure 5-7 compares the Fe-Cr-Ni spinel oxide thickness measurements for air-saturated and deoxygenated steam after cycle two conditions and indicated that the spinel oxide grew to a greater extent under air-saturated conditions and increased by 60% compared to 45% for deoxygenated conditions. The increasing slope observed for both conditions is indicative of the wavy interface detected during cross sectional examination. This is similar to that seen by Osgerby et al. [36] for 9Cr-1Mo steel exposed to flowing steam at 823 K.

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Figure 5-7 Oxide thickness measurements of Fe-Cr-Ni spinel as a function of the number of measurements taken for the oxide grown on TP347H FG in steam environments at 923 K for 50 hour cycles.