The effect of alloying additions on the oxide morphologies of super-heater tubes at elevated temperatures

Abstract

This paper considers the oxidation of boiler tube alloys; viz. 321H which contains titanium, and E1250 which contains niobium and vanadium, at temperatures of 650 and 750°C, under laboratory conditions, for 720 hours. The resulting oxides have been characterised using advanced techniques including scanning electron microscopy, energy dispersive X-ray spectroscopy and site specific crosssections using focused ion beam milling.

The results have shown that small concentrations of titanium and vanadium have a pronounced effect on the morphologies of the oxide layers. Where titanium is present, as in alloy 321H, titanium rich nodules form leading to the development of “whisker” growths from the nodules. On the E1250 alloy, vanadium rich, gem like, crystals form on the surface of the chromium/manganese rich scale. Cross-sectional analysis has shown a possible relationship between the surface features and underlying niobium particles.

Keywords:

• stainless steel
• oxide features
• titanium
• vanadium
• super-heater tubes