X-ray diVraction (XRD) studies were carried out on 2•25Cr-1Mo steel boiler tubes from a thermal power station with an objective of developing a faster method of health assessment of this steel at elevated temperatures on a routine basis. Carbide phase transformations in virgin and service exposed boiler tubes (up to ~100 000 h) were determined for plate specimens using high power X-rays. Iron and chromium rich carbides were detected by XRD at various stages of transformation towards final equilibrium. Compositional variations of equilibrium precipitate M 23 C 6 with service life or overheating in the specimens studied were monitored by a shift in the lattice parameter. T he XRD results were validated by optical and electron microscopy, hardness measurements, and particle size measurements.MST /4296