To evaluate the oxidation resistance of Alloy 617 and Haynes 230, oxidation tests were performed at 900°C and 1100°C in air and helium environments. Scale characterizations were assessed on specimens exposed to air using thin-film XRD, XPS, SEM and EDX. Oxidation resistance was dependent on the stability of the surface oxide layer, which can be affected by minor alloying elements such as Ti and Mn. At 900°C, for Alloy 617, a mixture of the extensive NiO-Cr 2 O 3 double layer and isolated NiO-NiCr 2 O 4 -Cr 2 O 3 triple layer were observed at a steady-state condition. For Haynes 230, a MnCr 2 O 4 layer was formed on top of the Cr 2 O 3 layer, resulting in a lower oxidation rate. At 1100°C, both alloys showed a double layer consisting of an inner Cr 2 O 3 and outer MnCr 2 O 4 or TiO 2 . The spallation of outer layer and subsequent volatilization of the Cr 2 O 3 layer produced a rugged surface and interface as well as internal oxidation.

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The effect of grain size on creep properties of type 316LN stainless steel has been investigated at 600 o C under different stresses. The initial strain at the beginning of creep tests decreased with the decrease of grain size. This was confirmed by the Hall-Petch relationship. The steady state creep rate decreased to a minimum value at the intermediate grain size (dm=80~130 μm) and then increased with the further increase of grain size. This result agreed with Garofalo's model stating that grain boundaries act simultaneously as both dislocation sources and barriers to dislocation movement. The rupture elongation at the intermediate grain size was minimal due to the cavity formed easily by carbide precipitates in the grain boundaries.

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