Abstract: Two heats of low activation martensitic (LAM) steels with Ti and Ta (denominated as 9Cr-Ti and 9Cr-Ta), respectively, developed as candidate structure materials for nuclear reactor were characterized. This paper was focused on the effect of titanium on the microstructures and mechanical properties of 9Cr LAM steel in as-received condition (normalized at 950℃ for 30 min with water quenching plus tempered at 780℃ for 90 min with air cooling). Chemical analysis and microstructure observation were conducted on 9Cr-Ti and 9Cr-Ta with optical microscopy, X-ray diffraction analysis, scanning electron microscopy and transmission electron microscopy. Impact properties and tensile strengths were measured with Charpy impact experiments and tensile tests. The results indicated that 9Cr-Ti and 9Cr-Ta were fully martensitic steels in as-received condition. MX type and M₂₃C₆ type precipitates were observed distributing along boundaries of prior austenite grains and martensite laths in 9Cr-Ti. The addition of titanium accelerated the precipitation of TiC and TiN, and produced much finer grains in 9Cr-Ti than 9Cr-Ta at the same normalization temperature. Mechanical properties tests showed the ductile brittle transition temperatures of 9Cr-Ti and 9Cr-Ta were about -90℃ and -85℃, respectively. The ultimate tensile strengths at room temperature and 600℃ were 680 MPa and 365 MPa for 9Cr-Ti, and 660 MPa and 335 MPa for 9Cr-Ta, respectively. The favorite impact toughness and tensile properties of 9Cr-Ti could be attributed to the fine grains in as-received condition.

Key words: 9Cr low activation martensitic steel, Alloying effect, Microstructure, Mechanical properties