Taishi Moroishi

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 aluminum and titanium additions to Fe-21%Cr-32%Ni on the oxidation behavior in an impure helium atmosphere at high temperatures

The effects of aluminum and titanium additions on the oxidation and carburization behavior of Fe-21%Cr-32%Ni in an HTGR-simulated impure helium environment at high temperatures were studied. Intragranular and intergranular oxidation were the principal forms of degradation, with the effect of aluminum being major and that of titanium relatively minor. As the aluminum content in the alloy increased, the mode of degradation changed from both uniform intragranular and intergranular oxidation, to one involving only uniform intragranular oxidation. This transition in the degradation mode was explained by the volume changes in the alloy resulting from a combination of both a volume increase due to internal-oxide precipitation and shrinkage due to the condensation of vacancies formed as a result of selective removal of alloying elements to the external scale. Intergranular oxidation was observed only when the resultant volume change was due to shrinkage. When the resultant volume change was positive, only uniform intragranular oxidation occurred and at the same time, extensive carburization was observed probably due to the deterioration of the surface scale caused by the deformation of the alloy substrate. A small amount of titanium, ca. 0.4%, appeared to modify the phenomena caused by aluminum additions, e.g., causing increased intergranular penetration for the 0.4% Al alloy and internal-to-external transition for the 2.1% Al alloy. External scale formation without any internal oxidation was observed for alloys containing more than 1.9% Al at 973 and 1023 K and for an alloy containing 2.1% Al and 0.4% Ti at 1073 and 1123 K. In these cases, carburization was almost completely eliminated.

Oxidation behaviour of alloy 800 in an impure helium atmosphere at high temperatures

Abstract The high temperature behaviour of Alloy 800 in impure helium (containing H 2 , H 2 O, CH 4 CO, CO 2 ) typical of the environment to be encountered in High Temperature Gas-cooled Reactors was studied, with particular emphasis on the carbon transfer and the selective oxidation behaviour. Under the conditions of the present study, only limited carbon transport was observed, as was expected from a thermodynamic study. The most important mode of damage was intergranular oxidation which was observed to develop relatively deeply into the substrate. The intergranular oxidation resulted from selective oxidation. The chromium depletion developed during the selective oxidation may have caused vacancies to be injected into the alloy. The vacancies may have diffused and finally condensed as voids in the grain boundary area, which may have provided a fast oxygen supply path, allowing oxide formation along them.

On the High Temperature Corrosion by Alkali Sulfates (I)

Serious corrosion was experienced on 18 Cr-Si-Al steel tubes used in the high temperature zone of an oil-firing furnace. In order to investigate the parameters for resulting corrosion, several corrosion tests were carried on thirteen types of steels including 18 Cr-Si-Al immersed in the fused salts of V2O5, K2SO4 and Na2SO4 at various temperatures. Chromium steels containing Al were found resistant to the vanadium-attack but badly corroded by a K2SO4 and