Identification of the precipitates by TEM and EDS in X20CrMoV12.1 after long-term service at elevated temperature

The crystal structures, morphologies, and compositions of carbides in a martensitic pipe steel of type X20CrMoV12.1 after long-term service at 550°C have been carefully investigated using transmission electron microscopy (TEM) and dispersive x-ray spectrometry (EDS). Most of carbides are M23C6. The M23C6 particles in prior austenite grains and in martensite lath boundaries are extensively coarsened. Most of the finer carbides within the matrix are also identified as M23C6, and no other type of carbide was found except for some tiny plate-like V-rich carbides identified as MC. The V and O mass fractions in the M23C6 carbides have variable morphologies and locations, which might be due to their coarsening process. They are coarsening at different rates even after long-term exposure at elevated temperature. The formation of V-rich MC during the long-term service contributes to hardness maintenance and is good for microstructural stability.