Role of gaseous environment and secondary precipitation in microstructural degradation of Cr-Mo steel weldments at high temperatures

This study is an attempt to understand the combined role of variations in oxidizing environment and secondary precipitation, in the microstructurally different regions of a standard Cr-Mo steel weldment, on the intensity of internal oxidation during high-temperature oxidation in air and steam environments. Samples of the weld-metal, heat-affected zone (HAZ), and base-metal regions were separated from the weldment of 2.25Cr-1 Mo steel and oxidized in the environments of air and steam at 873 K. The oxide scales and underlying subscales were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, and electron probe microanalysis (EPMA). Extensive internal oxidation and oxidation-induced void formation in the subscale zone and grain-boundary cavitation in the neighboring region were found to occur during oxidation in the steam environment. However, the internal oxidation and void formation were much more extensive in the subscale regions of the HAZ than in the subscales of the weld-metal and base-metal regions. As a result, the alloy matrix in the area neighboring the subscale region of the HAZ specimen suffered extensive grain-boundary cavitation. This behavior has been attributed to a rather specific combination and complex interplay of the environment, alloy microstructure, oxidizing temperature, and nature of the resulting external scale in causing and sustaining internal oxidation. The article also discusses the role of internal oxidation-assisted microstructural degradation in deteriorating the service life of components of 2.25Cr-1Mo steel.