2018 | AFC ACCOMPLISHMENTS 144 processing of thin core components such as fuel cladding and duct. It was also considered that high toughness might be achieved only when the selected processing route can avoid formation of too much or coarse brittle phases.We were able to produce tempered martensitic structures with ultrafine laths and precipitates by a combination of rapid quenching, i.e., water quenching(WQ), and relatively low degree of (or nontraditional) tempering. SixteenTMTs were designed and applied to two HT-9 steels, heat-3 and heat-4, in search of such new processing routes that can yield exceedingly high strength and high toughness in the operation temperature range of 300–550°C. Summarized below are the key observations in the mechanical testing and relevant conclusions. Figure 1 presents plots for the temperature dependence of yield stress and fracture toughness in various processing conditions.A wide range of strengths were measured depending on the degree of tempering, in particular. It is notable that both HT-9 alloys before tempering or after single tempering below 600°C can achieve ultrahigh yield stresses above the one GPa mark. Both the yield strength and the tensile strength of HT-9 steels monotonically decreased with test temperature, regardless of different alloy compositions and TMT routes; the decreasing of those strengths was slow up to 500°C and became more rapid between 500°C and 600°C. Overall, the final tempering temperature turned out to be the single most important factor controlling the strength of HT-9 steels. Meanwhile, the ductility parameters showed rather complex behavior as their temperature dependence depended on processing route, and thus on the strength of the materials. In general, the rankings of room temperature (RT) strength parameters were approximately reversed in the ductility parameters. The fracture toughness at RT is generally quite high (i.e., > ~200 MPa√m) except for the samples after no or low-temperature (< 500°C) tempering and the temperature dependence of fracture toughness above RT is strongly dependent on the degree of tempering. It is notable that the two processes with the HT-9 heat-3,WQ with 500°C tempering and WQ with 600°C tempering, yielded increased fracture toughness at 600°C: 250 and 230 MPa√m, respectively, which can be positively compared to the typical K-range of 150–200 MPa√m for the typical HT-9 steels. In summary, the mechanical properties of HT-9 steels varied widely with their processing routes, particularly on the degree of tempering. Some tailoredTMTs, e.g., combination of a rapid quenching and a limited tempering (WQ-500°C for instance) yielded excellent strength and improved fracture toughness.