In this study, a duplex treatment combining carburizing, nitriding, and subsequent induction hardening (IH) was applied to JIS-SCM420 low-alloy steel. A comprehensive evaluation was conducted to assess surface characteristics, including microstructure, hardness, residual stress, and fatigue performance. The IH process successfully produced a high-nitrogen-content Ε-Fe2-3(N,C) compound layer (2–3 μm thick) and fine acicular martensite at the surface, significantly enhancing surface hardness (950 HV0.03) and inducing beneficial compressive residual stress (−477 MPa). The IH-treated material exhibited a plane-bending fatigue strength of approximately 775 MPa, notably higher than that of conventionally carbonitrided specimens (700 MPa). This improvement was primarily attributed to the formation of the hard Ε-Fe2-3(N,C) compound layer and refined martensitic structure resulting from induction hardening. Additionally, IH activated residual interstitial elements, promoting the precipitation of stable surface nitrides. These microstructural changes effectively suppressed fatigue crack initiation and propagation, thereby extending fatigue life under cyclic loading conditions. © 2025 by the authors.

This study examines the influence of carbonitriding followed by sub-zero treatment on the microstructure, hardness, residual stress, and fatigue strength of JIS SCM420 low-alloy steels. Carbonitriding, followed by quenching and tempering treatment (NS), effectively enhanced surface hardness and induced compressive residual stress. The subsequent sub-zero treatment followed by quenching (S) further decreased the retained austenite (γR) content, leading to increased surface hardness and residual stress. Microstructural analysis revealed that sub-zero treatment not only reduced the γR content but also facilitated the formation of martensite and fine carbides, contributing to higher compressive residual stress. Despite these microstructural improvements, the fatigue limits of the NS and S specimens remained identical, suggesting that the stability of γR is a more critical factor for fatigue resistance than its volume fraction. © 2024 Elsevier B.V.

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. © 2025 The Iron and Steel Institute of Japan.