Microstructure Development during Creep Deformation of 9Cr–1Mo–V–Nb Steel with Excess Nitrogen Introduced by Solution Nitriding – Multidimensional Scatter Diagram Analysis of STEM-EDS Maps by Machine Learning –

Creep deformation and precipitation behavior of 9Cr–1Mo–V–Nb steel with excess nitrogen introduced by solution nitriding were investigated. Precipitation of Cr2N phase was confirmed in addition to M23C6 and MX phases in the tempered microstructure. The creep strength of the steel was significantly reduced by solution nitriding, while the creep rupture elongation was increased. To characterize the complex precipitation behavior of the nitrogen-added steel, a machine learning-based clustering method of the multidimensional scatter diagram of the X-ray intensity of the alloying elements in each pixel of a STEM-EDS map was developed. Reduced number density of precipitates and enhanced coarsening kinetics of both Cr2N and MX were proposed as the mechanism of weakening caused by excess nitrogen.