Precipitation behavior of P550 steel for non-magnetic drill collars during isothermal aging at 650–900 °C

Precipitation behavior of P550 steel for non-magnetic drill collars was investigated by microstructure characterization as well as thermodynamic calculation. The results demonstrate that the main precipitate formed at 650–900 °C was cellular Cr₂N, and its precipitation depended heavily on the aging temperature. The most sensitive precipitation temperature of cellular Cr₂N was 750 °C. At 750 °C, the cellular Cr₂N exhibited fast-slow precipitation kinetics with the aging time prolonging. The initial precipitation of cellular Cr₂N was governed by the short-range intergranular diffusion of Cr. During long-term aging, its growth was controlled by the long-range bulk diffusion of Cr. In addition, cellular Cr₂N induced the precipitation of r phase ahead of the cell after long period of aging. Increasing the nitrogen content resulted in the increment of both the nucleation site and the driving force for the cellular Cr₂N, which jointly promoted its precipitation.