Strengthening a fine-grained low activation martensitic steel by nanosized carbides

The significance of Ti in promoting the nucleation of MX-type carbides and further refine the MX carbides is currently unknown. Thus, we study here by high resolution transmission electron microscopy (HRTEM) and electron back scattered diffraction (EBSD), the determining role of titanium on the orientation of grains, texture, deformation and fracture of low activation martensitic steel with different Ti-content and determine the phase diagram. Strengthening mechanisms played by grain boundary and nanosized precipitates are discussed in detail. As the Ti content increased from 0.02 to 0.10 wt%, grain size of the LAM steels decreased significantly from 12 μm to 7 μm, accompanied by an increase in the volume fraction of MX carbide from 0.2 to 0.3% and decrease in the volume fraction of M23C6 carbide from 3.5 to 1.2%. Yield strength (σy) decreased with increasing Ti-addition. At room temperature, σy decreased slightly from 630 MPa to 590 MPa while it decreased from 315 MPa to 260 MPa at 600 °C for 0.02 and 0.10 wt % LAM steels. In contrast, elongation significantly increased from 37% to 47% at 600 °C with increase of Ti content. Boundary strengthening and MX-type carbide strengthening increased while M23C6 strengthening decreased greatly with increasing Ti-addition, which promoted the precipitation of fine MX-type and depressed M23C6. Excessive Ti-addition retarded the precipitation of large M23C6 carbides, which was responsible for significantly increasing elongation of 0.10 wt % Ti LAM steel, especially at 600 °C.