Fracture toughness and tensile properties of nano-structured ferritic steel 12YWT

The oxide dispersion strengthened (ODS) steels are being developed and investigated for fission and fusion structural applications in Japan, Europe, and the United States. In this paper, the fracture toughness and tensile properties of an ODS steel with nominal composition Fe-12Cr-2.5W-0.4Ti-0.25Y₂O₃ (designated 12YWT) were investigated and compared to commercial reduced-activation ferritic/martensitic (RAFM) steels. Small, 1.6-mm thick and 3.2-mm wide, 3-point bend specimens were used for fracture toughness characterization of this steel. Specimens were fatigue pre-cracked to initial crack length (a) to width (W) ratio of 0.45 and tested quasi-statically in the temperature range from -50 °C to 550 °C. Specimens tested up to 50 °C exhibited elastic-plastic cleavage fracture that was typical for the transition region in ferritic steels. The master curve transition temperature, T₀, for the 12YWT alloy was determined to be 102 °C. Specimens tested at 100 °C and higher exhibited ductile stable crack growth. In these cases, the J-integral at the onset of stable crack growth (J_Ic) was determined from the J-R curves. Their equivalent values in terms of stress intensity factor, K_JIc, were about 93 MPa√m at 100 °C and decreased to 53 MPa√m at 550 °C. This study showed that oxide dispersion strengthening resulted in significant decreases in the toughness properties compared to commercial RAFM steels, although appreciable level of toughness was still retained. Tensile tests were performed at temperatures between room temperature and 800 °C. As expected, this material exhibited very high yield strength, ∼1300 MPa, at room temperature. For comparison, the yield strength of commercial RAFM steels is about 550 MPa. Yield strength of 12YWT decreases as test temperature increases and at 800 °C it is about 323 MPa.