Structure-property relationship of 2.25Cr-1Mo pressure vessel steel under intercritical and conventional heat treatments

The widespread use of 2.25Cr-1Mo steel in nuclear reactor pressure vessels is attributed to its excellent combination of mechanical properties, including high strength, good toughness, superior creep resistance, and weldability. Now, these properties primarily depend on the microstructure, which consequently depends on the heat treatment schedule. The conventional heat treatment for this steel involves austenitization above the upper critical temperature to form a single-phase austenite and then quenching, followed by tempering. However, localized exposure to intercritical temperatures (between upper and lower critical temperatures) during fabrication or repair of pressure vessels can alter their microstructure and affect performance; yet the underlying structure-property relationship remains insufficiently understood. Therefore, this study systematically investigates the structure–property relationship of 2.25Cr–1Mo steel subjected to conventional quenching (from 1000°C) and tempering (QT), and intercritical quenching (from 800°C) and tempering (IQT). Phase analysis performed using X-Ray Diffraction (XRD) technique, confirmed the presence of ferrite in IQT and tempered martensite in QT samples. Moreover, a thorough microstructural analysis, conducted using a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDX), revealed a dual-phase microstructure (44% ferrite + 56% tempered martensite) with Cr-Mo-enriched carbides for the IQT condition compared to a single-phase (tempered martensite) microstructure for the QT condition. These variations in microstructures resulted in a 23% loss in yield strength, a 31% decrease in the ultimate tensile strength, and a 19% reduction in toughness of IQT samples. Moreover, fractographic analysis revealed 27% larger dimples and 24% lower shear fracture portion with higher surface roughness for IQT samples. These findings establish a strong link between phase constitution, carbide chemistry, tensile properties, and fracture behavior and are relevant for pressure-vessel steels used in nuclear and petrochemical components, where unintended intercritical exposure during welding or fabrication may compromise structural integrity.