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 degrees C was cellular Cr2N, and its precipitation depended heavily on the aging temperature. The most sensitive precipitation temperature of cellular Cr2N was 750 degrees C. At 750 degrees 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 sigma 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.

Automated summary

The precipitation behavior of P550 steel for non-magnetic drill collars was investigated, with cellular Cr2N being the main precipitate formed at 650-900 degrees C. The precipitation of cellular Cr2N at 750 degrees C was found to be the most sensitive, with short-range intergranular diffusion of Cr controlling initial precipitation and long-range bulk diffusion of Cr controlling growth during long-term aging. Additionally, increasing nitrogen content promoted the nucleation and driving force for cellular Cr2N precipitation.