Abstract
The development of advanced reactor pressure vessel (RPV) steels with improved strength and toughness is essential for ensuring the safety and reliability of nuclear power plants. In this study, a modified SA508 Grade 4N low-alloy steel with reduced Mn-Ni-Cr content was developed through induction melting, followed by austenitization, water quenching, and tempering. Microstructural characterization using optical microscopy, scanning electron microscopy, and X-ray diffraction confirmed the formation of a predominantly tempered lath martensitic microstructure without detectable retained austenite. XRD analysis further indicated recovery of the martensitic lattice after tempering, as evidenced by reduced peak broadening. The developed alloy exhibited a yield strength of 585 MPa, an ultimate tensile strength of 635 MPa, 24% elongation, and 55% reduction in area, satisfying the minimum mechanical property requirements for SA508 Grade 4N Class 3 steel. Charpy impact testing demonstrated the characteristic ductile-to-brittle transition behaviour, with an absorbed energy of 41 J at −82 °C and an upper-shelf energy of approximately 115 J at room temperature. Although the upper-shelf impact energy was lower than that reported for conventional SA508 Grade 4N steels, the developed alloy satisfied the specified toughness requirements. It achieved the required combination of strength and ductility. The results demonstrate that an induction-melted, cast SA508 Grade 4N model alloy with reduced Mn-Ni-Cr content can achieve the required mechanical performance after quenching and tempering. In contrast, further optimization of the nickel content may enhance impact toughness.

Key words
martensite, reactor pressure vessel (RPV), synergistic effect, lath martensite, irradiation embrittlement, dislocation

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