Knowledge of the long-term microstructural stability of a new material is a fundamental requirement for industrial applications operating at temperatures > 500°C. For this reason, microstructural investigations on samples of the newly developed Alloy 699 XA after long-term exposure at temperatures between 500 and 900°C have been performed by scanning electron microscope (SEM). One key application of the new alloy is in the petrochemical industry under metal dusting conditions at high temperatures, underlining the importance of material characterizations presented in this study.
In addition to experimental investigations, thermodynamic calculations have been performed to get an overview of the relevant phases. One focus of the study was on the γ'-precipitation behavior of Alloy 699 XA over durations up to 5000 h. The SEM results showed that the solvus temperature for the γ’-phase is slightly above 800°C, which is in good agreement with the results of mechanical tests on the samples after aging. However, this solvus temperature is higher than the value from thermodynamic calculations (770°C), highlighting the importance of long-term exposure experiments.
Another essential result was that no detrimental, incoherent phases have been found even after 5000 h of exposure at higher temperatures and no continuous occupancy of one of the phases on the grain boundaries, which could have a negative impact on the creep strength of the material.