Hisao Fujikawa

Internal-external transition for the oxidation of Fe-Cr-Ni austenitic stainless steels in steam

Several Fe-Cr-Ni austenitic stainless steels (Cr wt.%: 13–25, Ni wt.%: 15) were oxidized in steam for 1000 hr at 500–900°C. The oxide scales were examined and categorized with respect to the chromium concentration and the grain size of the base metal. Experiments showed three conditions for the critical bulk Cr concentration and the oxidation temperature at which the oxidation behavior changed drastically. Metallographic examination showed that two of these three conditions resulted from the internal-external transition of Cr2O3 either on the metal surface or along the grain boundaries of the base metal. Attempts were made to interpret these conditions from the available oxidation theories. Atkinsons treatment was employed with some modification to incorporate the grain-boundary diffusion of Cr in the base metal. The calculation basically explained the internal-external transition for the oxidation of these steels.

The Effects of Small Additions of Yttrium on the High-Temperature Oxidation Resistance of a Si-Containing Austenitic Stainless Steel

The oxidation characteristics of a 19Cr–10Ni–1.5Si alloy with and without 0.03Y have been examined using scanning-electron microscopy, secondary-ion mass spectroscopy and transmission-electron microscopy. Y has been found to segregate to oxide grain boundaries and thereby to inhibit outward-cation diffusion and promote the internal oxidation of Si. The synergism between the beneficial effects of Y and Si is described, and the results are discussed in relation to the de-oxygenating and de-sulphurizing influences of Y in steel production.

The Effect of Carbon, Zirconium, Niobium, and Titanium on the Oxidation Resistance of Chromium Stainless Steel

The effects of C, Zr, Ti, and Nb on the oxidation resistance of 17% Cr steels have been investigated by means of isothermal heating and cyclic heating at temperatures up to 1000°C. It has been found that C has a detrimental effect on the oxidation resistance of 17% Cr steels. The oxidation behavior of steels containing Zr is different from that of steels containing Ti and Nb. Zr improves the oxidation resistance of 17% Cr steels to a great extent. Particularly, alloys containing Zr at a concentration which exceeds the stoichiometric equivalent of the total amount of C and N show excellent oxidation resistance up to 1000°C. Electron probe microanalysis shows the presence of a protective oxide layer consisting of Si‐rich oxide at the metal‐oxide interface, and no Zr is detected in the scales. With steels containing Ti and Nb, on the other hand, there is no protective Si‐rich oxide, but Ti and Nb are detected in the scales. These scales are not protective. As the effective factors of excellent oxidation resistance of Cr steels containing Zr, the behavior of carbide, grain growth, and phase transformation have been investigated.

Effect of Small Amounts of Ni on Liquid-Cu Embrittlement in Hot-Rolled Mild Steel After High-Temperature Oxidation

The effects of additions of as little as 0.02%Nito mild steel on the morphology of the scale-steelinterface and surface cracking were studied duringhigh-temperature oxidation and subsequent hot-ductility tests of 0.5%Cu-bearing steel. Liquid-Cuembrittlement occurs in a steel containing0.5%Cu-0.001%Ni because of concentrated liquid-Cu layerat the scale-steel interface over the temperature rangeof 1100-1300°C. The addition of as little as0.023%Ni to 0.50%Cu-containing steel recovers theductility in the oxidation temperature range of1250-1300°C. This is because liquid-Cu particles areoccluded into the scale. Thus, the amount of enriched Cuper unit surface area decreases at the scale-steelinterface. It is suggested that liquid-Cu particleocclusion, which has not yet been elucidated, results from heterogeneous enrichment of Ni andheterogeneous oxidation at the scale-steelinterface.

The oxidation and carbon transfer behaviour of Ni-27Cr-5Mo-5W based Alloys in an impure Helium atmosphere at high temperature

The corrosion behaviour of a series of Ni-27Cr-5Mo-5W alloys with various element additions in an impure helium atmosphere at 1273 K has been studied. Although the extent of oxidation was small, significant amounts of carbon transfer occurred. The carbon transfer behaviour was influenced by alloy composition with the effects of chromium and aluminium being particularly marked. The effect of Cr was interpreted by the carbon activity-oxygen potential equilibrium diagram for Cr and a Ni-Cr alloy and that of Al was interpreted by consideration of possible deterioration of surface scales due to the volume increase expected in the internal oxide zone where Al2O3 stringers precipitate.