Kinetics of Z-Phase Precipitation in 9 to 12 pct Cr Steels

The Z-phase nitride is seen as a detrimental phase in 9 to 12 pct Cr steels as it is in competition with the beneficial MX particles. Two model steels, with 9 pct Cr and 12 pct Cr content, respectively, were designed to study the effect of Cr on Z-phase precipitation kinetics. The steels were isothermally aged at 873 K, 923 K, and 973 K (600 °C, 650 °C, and 700 °C) for up to 30,000 hours in order for Z-phase to replace MX. X-ray diffraction (XRD) analysis of extracted precipitates was used to quantitatively follow the evolution of the nitrides population. It was found that the 12 pct Cr steel precipitated Z-phase 20 to 50 times faster than the 9 pct Cr steel. Transmission electron microscopy (TEM) was applied to follow the Z-phase precipitation, using energy-dispersive X-ray spectroscopy (EDS) line scans and atomic resolution imaging.