高Si含有オーステナイトステンレス鋼の高温酸化挙動に関する考察

抄録

An austenitic stainless steel with high silicon, 13Ni-18Cr-3.5Si alloy has recently been developed and recongnized as a new alloy resistant to the high temperature oxidation. This alloy has been adopted for a thermal reactor in the automobile field because it has an excellent resistance to oxidation and both the weldability and formability are superior to those of SUS310S. The oxidation mechanism of 13Ni-18Cr-3.5Si alloy, however, is not so clear as those of 35Ni-20Cr and 80Ni-20Cr alloys with high silicon.
In order to clarify the oxidation mechanism and the effect of strong deoxidation process, a cyclic oxidation test at 1000°C in air was carried out on 13Ni-18Cr-3.5Si alloys made through deoxidation process.
The surface morphology, microstructure, and composition of oxide were examined by means of optical microscopy, X-ray diffraction, scanning electron microscopy, and electron probe microanalysis.
No internal oxide was detected by optical microscopy or electron probe microanalysis of sections, while the adherence of the surface oxide was retained in 13Ni-18Cr-3.5Si alloy. The formation of a very thin amorphous SiO₂ film was detected beneath innermost oxide Cr₂O₃ by scanning electron microscopy and electron probe microanalysis of the back of oxide chemically stripped from 13Ni-18Cr-3.5Si alloy. It was assumed that this thin SiO₂ film contributed to the improvement of the resistance to oxidation.
Protective and mechanical properties of the SiO₂ film were markedly affected by the strong deoxidation process. The highly convoluted oxide/alloy interface was observed on 13Ni-18Cr-3.5Si alloy made by strong deoxidation with rare earth metals. The good oxide adherence was attributed to the improvement in the plasticity of thin SiO₂ film formed at oxide/alloy interface by rare earth element as an inpurity. An optimum combination of Si addition and strong deoxidation process was concluded to further improve the resistance to oxidation at high temperatures.