Abstract: Two types of 9Cr low activation martensitic steels (named 9Cr-1 and 9Cr-2) were developed in University of Science and Technology Beijing. 9Cr-1 and 9Cr-2 were produced by vacuum induction melting method, and examinations of the microstructures were carried out with X-ray diffraction analysis, optical microscopy, scanning electron microscopy and transmission electron microscopy. The ultimate tensile strength and yield tensile strength were evaluated with tensile tests. The impact properties were characterized with Charpy impact experiments. The results indicated that 9Cr-1 and 9Cr-2 on as-received condition (95℃/30~min/water quenching plus 780℃/90~min/air cooling) were fully martensitic steels free of β-ferrite. The ultimate tensile strength of 9Cr-1 and 9Cr-2 were 695~MPa and 680~MPa, respectively. However, 9Cr-2 showed a fine grain size of 4.8 μm, and its value of ductile-brittle transition temperature (DBTT) was -90℃ under as-received condition. The additions of vanadium, titanium and boron accelerated the formation of MX precipitates and resulted in fine grains and precipitates. The fine grains effectively reduced the value of DBTT from -60℃ to -90℃ with identical upper shelf energy (USE). The decrease in silicon concentration of 9Cr-2 induced a slight reduction in ultimate tensile strength from 695 MPa to 680 MPa.

Key words: 9Cr low activation martensitic steel, Grain size, Mechanical properties