Influence of Prior Quenching and Tempering Treatment on Cementite Formation during Nitriding at 913 K for SCM440 Steel

This study investigated the mechanism of cementite formation in JIS-SCM440 low-alloy steel subjected to high-temperature nitriding (NH) at 913 K, followed by quenching and aging. Specimens with and without prior quenching and tempering (QT) treatment were analyzed to understand the influence of the initial microstructures on cementite precipitation. Comprehensive characterization using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), and electron probe microanalysis (EPMA) revealed that NH treatment promoted significant nitrogen diffusion into the steel matrix, resulting in the formation of a nitrogen compound layer and a hardened nitrogen martensite layer beneath the compound layer. In the NH specimens following QT treatment (QTNH), accelerated carbon diffusion to the surface led to cementite precipitation around the surface pores owing to the dissolution of fine carbides. Cementite enhanced the surface hardness of the porous regions compared to that of the NH specimens.