High temperature oxidation resistance improvement in an FeMnSiCrNi alloy by Mn-depletion under vacuum annealing

High temperature cyclic oxidation behavior of a low manganese Fe12Mn9Cr5Si4Ni-NbC shape memory stainless steels

“…Furthermore, the oxides predicted by Thermo-Calc here, as also already reported for three Fe-Mn-Si-Cr-Ni alloys [5,6] are in accordance with the increase of MnCr 2 O 4 phase as p(O 2 ) decreases (Figure 4). Regarding the other phases predicted by Thermo-Calc simulation (Olivine, Rhodonite, Halite and so on) none of them were found here or reported during previous characterization of Fe-Mn-Si-Cr-Ni alloys oxidized [4][5][6][7][8]13]. Silica oxide was also never observed in these alloys after oxidation at 800 °C, although it was reported after oxidation at 600 °C [9].…”

“…The internal oxidation observed here in the ferrite layer was more evident and intense after oxidation at 1000 °C, fact explained by the higher O activity and diffusivity. This behaviour was never reported in previous studies for Fe-Mn-Si-Cr-Ni alloys [4][5][6][7][8][9]13], but in all cases the amount of Mn was higher (12.32 to 17 wt.%) than here (12 wt.%) and/or the oxidation periods were in several studies limited to a maximum of 120 h. The internal oxidation is caused normally by a selective oxidation of an element (as is the present case for Mn), which does not diffuse quickly to the surface while the region beneath the oxide scale becomes depleted in it [27]. The exposure time to oxidizing atmosphere can also influence as mentioned by Douglass et al [28], which not observed internal oxidation for short periods of oxidation in an Fe-Mn-Cr.…”

“…However, in Figure 10, literature values of k p are put together for comparative purposes in an Arrhenius plot, where open symbols accounts for cyclic tests and filled symbols for isothermal ones. A hypothetical linear fitting is presented (R²=0.87), excluding the points regarding the alloy with Co [4] and the alloy oxidized after vacuum annealing [8], showing that for all the other alloys, independently of the Mn content there is a similar and high mass gain. Clearly, the only successful attempts for decreasing mass gain during oxidation in air at high temperature in these SMSS alloys were, therefore, through the Co addition or vacuum annealing.…”

“…They observed that during oxidation at high temperature the oxide layer presents Mn-rich oxides (Mn 2 O 3 and Mn 3 O 4 ) and Mn-Cr-spinel, causing depletion of Mn and generating a ferrite layer near the metal/oxide interface. After that, several studies were developed in order to try improving the oxidation resistance of these alloys, named as shape memory stainless steels (SMSS) [4,[6][7][8].…”

“…One effective way proposed in the literature for an Fe-17Mn-5Si-10Cr-4Ni-VC alloy [8] to decrease mass gain is the application of a vacuum pre-treatment to promote Mn volatilization aiming to form a ferrite layer (170 m in thickness), depleted in Mn upon the austenite substrate. In this way, the oxidation rate at 800 °C, during 24 h exposure, was two 4 orders of magnitude smaller than that in samples without vacuum pre-treatment.…”