The marine steels have microstructures consisting essentially of small volume fraction of P in a α matrix. The morphology of the phases varied from banded to unbanded depending on the production process and sample thickness. The general microstructures of the steels in Table 4 are shown in Fig. 8. The grade of steel and the reference sources are also given. Fig. 8(a, b, c, d) are microstructures ofNR steels. The black etching is a banded P forming in the rolling direction and the light etching is the α . Fig. 8(e & f) are microstructures of the same NR steels, but taken from directions that are un-banded [37]. Fig. 8(g) is TMCPmicrostructure of the EH36 steel studied by Tsay [57]. The microstructure of TMCP steels is greatly refined as compares to those of conventional or NR steels, resulting in a significant improvement in strength and toughness. Hardness was measured to be around 168HV0.1 with equiaxed grains of average size of 15µm, measured in the through-thickness direction.
Fig. 9 presents the plots of the FCGR studies on the NRand TMCP steels listed in Table 6, including the results of the present study (PS ) in Fig. 7. For the test IDs in the PS , each sample is denoted with material type (e.g. J2N, G8 and G10), followed by the environmental air (A), the maximum load applied, Pmax, on the sample (e.g. 10 kN), and the appliedf (e.g. F of 0.2, 0.3 and 0.5 Hz). The NR data points are denoted with circles in Fig. 9. The J2N data points in the PS are coloured yellow, the NR data points extracted from the literatures are green or black. The TMCP data points are denoted with squares. The same trend observed in Fig. 7 is also seen in Fig. 9. It can be observed that in the Paris Region taken in the interval 20≤ ΔK≤35, theFCGR for the two microstructures separated into two domains, but with diffused inter-boundaries. The dash line is drawn arbitrarily to show possible demarcating line. Occupying upper half is the fatigue data points of NR steels and the lower half the FCGR data points for the TMCP . In other words, the FCGR in NRare generally higher than that of TMCP in air.