Effects of Intergranular Segregation and γ-Grain Size on Low Temperature Toughness of Tempered Martensite in Low Alloy Steel

With the growing demand for carbon capture and storage (CCS) technologies to achieve a carbon-neutral society, the low-temperature toughness of casing materials used in CO2 injection wells has become increasingly important. This study investigates the effects of phosphorus (P) segregation at grain boundaries and prior-austenite grain size on the toughness of tempered martensitic steel API 5CT L80 Type1. Steels with different P contents and grain sizes were fabricated and evaluated through Charpy impact testing, fractography of Charpy fracture surfaces, and Auger electron spectroscopy. To comprehensively evaluate toughness-controlling factors, the Hyodo model based on the Brechet-Louchet framework was employed to calculate intergranular fracture stress. This calculation incorporated not only yield strength and grain size but also grain boundary cohesion energy estimated from the measured segregation levels of P and C. The results revealed that higher intergranular fracture stress correlates with lower ductile-to-brittle transition temperature (vTrs), demonstrating that intergranular fracture stress is an effective indicator of toughness for CCS applications. This approach enabled the combined effects of grain size and grain boundary segregation to be integrated into a single parameter.