Microstructural Characteristics of Modified 9Cr-1Mo Steel with Low Creep Ductility

Microstructures of the two heats of Modified 9Cr-1Mo steel that exhibit comparable creep strength but largely different creep ductility were analyzed to identify the factors that can reduce their creep ductility. The critical difference between the microstructures of the two heats was the size and distribution of prior austenite grains (PAGs). For the heat with higher creep ductility, the size of the PAGs was ordinary, approximately 20 µm. In contrast, the microstructure of the heat with low creep ductility was characterized as a mixture of regions with ordinary-sized PAGs and extraordinary coarsened PAGs of several hundred micrometers. The creep deformation of the heat with low creep ductility was found to localize in the regions with ordinary-sized PAGs, which led to the development of creep cavities and cracks, eventually reducing the creep ductility.