オーステナイト系耐熱合金のクロマイズ処理法の検討

抄録

One of the most serious problems in gas turbines in which residual fuel oil is used would be the control of accelerated corrosion caused by fuel ash. In this paper, were studied chromium diffusion coating (chromizing) by powder method for austenitic heat resistant alloys used for gas turbine blades. The depth of chromium diffusion layer and surface chromium concentration of the layer were proved to be determined primarily by chemical compositions of the base alloy when the chromizing condition was fixed.
When super alloys such as S590 or S816 that have low iron contents were chromized by a mixture of chromium powder, alumina powder, and ammonium chloride, the thickness of the layer was limited to a small value mainly because of low diffusion constant of chromium in these alloys.
The maximum depth of the layer on S590 alloy obtained by powder method was 50 microns and surface chromium concentration was 84 wt per cent.
Addition of iron powder in the above chromizing powder showed a considerable effect on the growth of diffusion layer. This effect can be explained by a pronounced increase of chromium diffusion constant caused by dilution of chromium concentration in the layer. Chromium concentration of the layer decreases almost linearly with addition of iron powder. When 8590 alloy was chromized with powder consisted of 20% chromium-10% iron-70% alumina, the surface chromium concentration of the layer was 60 per cent.