A new 12%Cr steel integrating good creep properties, fabricability and corrosion resistance up to 650°C is being developed. For each step of this development, long term oxidation tests (about 6000h and 8000h) were made in pure water vapour in the temperature range 600-650°C. The laboratory and industrial heats were tested in comparison with two 12%Cr steels (X20CrMoV12-1), a 9%Cr steel (T91) and a fined grain austenitic stainless steel (TP347FG). Corrosion damage was measured using mass losses obtained after a reducing descaling process. Weight loss and metallographic results confirm the good corrosion resistance in steam of the new steel, VM12, and show either 2 different corrosion mechanisms or the same mechanism but with 2 different spreading rates: for the VM12, one X20 heat and TP347 steels, the time required for the whole surface of the samples being covered with corrosion products is definitely longer than for the other 9-12 % Cr.
A similar map is represented in Figure 6b, considering the KS relations. For the considered tolerance, only a small proportion of grain boundaries coincides with one or the other theoretical misorientation. This proportion is 2 % NW-type variants, respectively 5 % for the KS-type variants.By gradually increasing the tolerance angle x, the fraction of grain boundaries whose misorientation corresponds to theoretical misorientations between variants grows regularly. With a x tolerance of 8, these fractions are respectively 50 % and 77 % for the NW and KS relations. Generally, the KS relation is more frequently fulfilled that the NW relation. This could be partly linked to the fact that 24 distinct variants are generated by the KS relations and only 12 by the NW relations. Figure 6c displays such grain boundaries for a tolerance x angle of 8 for the KS-type variants. If the KS relations are fulfilled during phase transformation, the variants inherited from the same austenitic grain must verify the KS relations with the parent grain and misorientations of type Dg 0 in pairs. Moreover, the boundaries between grains which do not correspond to misorientations Dg 0 generally characterise the boundaries between parent austenitic grains. The boundaries characterising misorientations deviating by more than 8 to Dg 0 are drawn as red lines in Figure 6c. They do not form continuous closed red lines which would have been a clear indication of parent austenitic grain boundaries. This can be related to the fact that the austenitic microstructure of the grade obtained after rolling below recrystallization temperature presents a high density of crystal defects in particular grain boundaries and sub-grain boundaries. Furthermore, two neighbouring grains that are not inherited from the same parent might wrongly appear as being related through the KS relation owing to the specific orientations of their respective parents and the large value of the tolerance angle x.Two methods were presented in this contribution with the purpose of studying the orientation relations between the parent and the inherited phase observed in the phase transformation of an HSLA steel. Both methods use local orientation data obtained by EBSD technique.The first method is a direct method, based on the presence of a minor residual austenite phase in the material. It is therefore possible to determine the misorientation between ferrite and austenite from their corresponding orientations. This experimental orientation relation is then compared to theoretical ones (NW and KS). In the case of the steel examined, the experimental relations were close to NW or KS relations, but none of these relations could be strictly verified.The second method is based on the determination of the misorientations between neighbouring ferrite grains compared to theoretical misorientations between NW and KS variants. We showed that a very low fraction of the grain boundaries presented misorientations corresponding to NW and KS variants with a 2 tolerance angle. This value rose to 77 ...
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