Correlation of mechanical properties with microstructure of Alloy 800 after annealing at 800 - 1000 0C

The microstructures of so called high temperature alloys, which have been developed for service temperatures up to 8000C, are not necessarily stable at higher temperatures. The mobility of alloying elements is very high in FeCrNi alloys at 950 0C e.g. iron, chromium or nickel can diffuse up to 0.1 mm distance in one year, which is about a grain diameter. Interstitials like carbon or nitrogen show a four orders of magnitude higher diffusivity than the alloying elements. In addition, the carbon solubility in this type of alloy is reported to be very low. Therefore the alloys are supersaturated with carbon after heat treatment above 1100 0C and water quenching although the absolute carbon content is very low. At service temperatures around 8000C the solubility of carbon is still about one order of magnitude lower than at heat treatment temperature. This will lead, together with the high mobility of elements, to precipitation of carbides even after short times and will cause microstructural changes. The work presented was performed in order to investigate the influence of temperature on structural stability of the alloys and to establish a correlation between structural changes and changes of mechanical properties. (Auth.)