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The ductile-brittle transition temperature (DBTT) of 9Cr-1Mo steel was characterized by an RT NDTbased K IR curve approach and a reference temperature (T 0 )-based master curve (MC) approach. The MC was developed at a dynamic loading condition (loading rate of 5.12 m/s), using precracked Charpy V-notch (PCVN) specimens, and the reference temperature was termed T 0 dy . The RT NDT and T 0 dy were determined to be Ϫ25 °C and Ϫ52 °C, respectively. The T 0 dy was also estimated from instrumented CVN tests, using a modified Schindler procedure to evaluate K Jd ; the result shows close agreement with that obtained from the PCVN tests. The ASME K IR -curve approach proves to be too conservative compared to the obtained trend of the fracture toughness with temperature. The cleavage fracture stress, , estimated from the critical length, l*, shows good agreement with that estimated from the load-temperature diagram (2400 to 2450 MPa), which was constructed from the CVN test results. The crack initiation mechanism has been identified as decohesion of the particle-matrix interface, rather than as the fracture of the particles. s f *

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