Fig. 1. Heat-treatment sequence of 15Co-12Ni secondary hardening steel.

Fig. 2. SEM micrographs of (a,c) as-quenched and (b,d) heat-treated 15Co-12Ni secondary hardening steels.

Fig. 3. EBSD analysis results: (a,e) band contrast maps, (b,f) grain orientation maps. (c,g) grain orientation maps of lenticular martensite and (d,h) corresponding misorientation profiles across the lenticular martensite.

Fig. 4. Kernel average misorientation (KAM) maps (a,c) and the corresponding KAM values histogram (b,d). Here (a,b) and (c,d) were analyzed in the as-quenched and heat-treated samples, respectively.

Fig. 5. (a-c) as-quenched and (d-e) heat treated high Co-Ni secondary hardening steels. Here (a,d) are band contrast EBSD maps overlaid with parent austenite grain boundaries (black), interpacket boundaries (red) and intrapacket boundaries (green). (b,e) are reconstructed grain orientation maps and (c,f) are boundary misorientations as one of the V1-V24 type misorientations [43].

Fig. 6. FE-SEM micrographs showing the precipitates and dislocations in inter- and intra-martensite; (a,c) as-quenched and (b,d) heat-treated samples. Here (c) and (d) are electron channeling contrast images, and P1 and P2 are the corresponding EDS point spectra in (a) and (b).

Fig. 7. (a) TEM image and (b) EDS line profile results in (a) of heat-treated high Co-Ni secondary hardening steel.

Fig. 8. (a) Tensile stress-strain curves, and (b) tensile properties of present steels compared with those of other reported high strength steels.

Fig. 9. Load-displacement curves for fracture toughness of heat-treated sample.

Fig. 10. Tensile fractographies of heat-treated sample; (a) low-magnification, (b) radial region of (a), (c) fiber region of (c) and (d) high-magnification of the image in (c).

Fig. 11. SEM fractographies of fractured compact tension specimen; (a) low mag. image showing the shear lip, (b) magnified image of fatigue pre- and post-crack interface and (c) high mag. image showing the dimple and carbides.