Nanoindentation measurements were conducted in order to evaluate the mechanical properties of the residual ferrite itself. The trace of a Berkovich tip can be placed within the interiors of the residual ferrite regions, while conventional micro-Vickers diamond tips using 100-mN loads cover 7 x 7 pm. Figure 10 shows the hardness change in the individual phases measured by this nanoindentation technique as a

Figure 10 Hardness change at room temperature as a function of tempering conditions for the residual ferrite and tempered martensite. NT: normalizing and tempering; FC: furnace cooling. Ukai, S.; Ohtsuka, S.; Kaito, T.; Sakasegawa, H.; Chikata, N.; Hayashi, S.; Ohnuki, S. Mater. Sci. Eng. A 2009, 510-511,115-120.

parameter of the tempering conditions.26 The decrease in hardness is significantly restricted in the residual ferrite as compared to that of the martensite phase in terms of increasing the tempering conditions. The overall hardness measured by the micro-Vickers tester is also shown by the broken line which covers both the residual ferrite and martensite, therefore, representing the average hardness of both phases. Hardness Hv is correlated with yield stress sy using the relationship provided by Tabor.27 For tempering conditions at 800 °C for 58 h, which is equivalent to tempering at 700 °C for 10 000 h based on the LMP (Larson-Miller parameter), hardness can be converted to yield stress at room temperature for the individual phases: 1360 MPa for the residual ferrite and 930 MPa for the tempered martensite. The yield strength of the residual ferrite is 1.5 times higher than that of mar­tensite at tempering at 700 °C for 10 000 h.

A full ferrite ODS steel and full martensite ODS steel were manufactured, and the oxide particle dis­tribution in both ODS steels was measured by TEM. The results are shown in Figure 11.28 It is obvious that a few nanometer-sized oxide particles are finely distributed in the full ferrite ODS steel, whereas their size is coarsened in the bi-modal distribution in the martensite ODS steel. Considering that the residual ferrite phase belongs to full ferrite ODS steel, resid­ual ferrite contains fine (nanosized) oxide particles which are responsible for higher strength in residual ferrite containing ODS steels. In regard to the bi­modal distribution of oxide particles in martensite

ODS steels, the a-g-phase transformation could induce the coarsening of oxide particles by disturbing the interface coherency between these particles and the g-phase matrix.