Fig.
3: Photomicrographs of α-P steels - NR - S355J2+N
In Fig. 3(a), the optical
micrograph shows that the microstructure is composed of P (dark
etching) and the rest α (light etching). Microstructural
examination of S355J2+N shows that, 80% of the microstructure is
allotriomorphic α and others P , with traces of acicular or
Widmanstätten α [37]. The P colonies in the J2N- steel
are large, blocky and dense. This is a consequence of its chemical
composition, heat-treatment and deformation or manufacturing method.
Fig. 3(b) is the SEM micrograph of the microstructure. The blockyP and the proeutectoid α , are noted accordingly. The
proeutectoid α is the α phase that formed before the
formation of P at the eutectoid temperature
(723oC). Fig. 3(c) is the part denoted with a yellow
box in Fig. 3(b) and the micrograph shows differently oriented Pcolonies as demarcated by the yellow lines. Fig. 3(d) shows what would
have been the image of the part in the yellow box in Fig. 3(c) on a
typical transmission electron microscopy (TEM) micrograph. The structure
that appears as parallel rods in that colony is the cementite
(θ ). Note that the θ plates are not generally continuous.
The α that formed in between the θ rods are referred to as
eutectoid α – because they are produced during the Pformation at the eutectoid temperature. Fig. 3(d), is extracted from Ref
[33]. Fig. 4 shows the microstructures of G10, G8. These
microstructures are found to be similar, but it is evident that they are
distinct from that of J2N in Fig. 3. Fig. 4(a) is the optical image of
G10 while Fig. 4 (b) is the SEM image. Similarly, Fig. 4(c) is the
optical image of G8 while Fig. 4(d) is the SEM image. It is evident that
the SEM image shows more details of the microconstituents. In both
microstructures, the P nodules are very small compared to that of
J2N and the volume fraction of the P is also less than that of
J2N.
Fig. 5 shows the morphologies of the phases present in J2N in the
rolling direction. The ferrite phase tends to have three forms – high
relief (HR ), low relief (LR ) and ribbon like structure
(HA ) as shown in Fig. 5(a). Note that the low and high reliefα phase variants could not be distinguished under optical
micrographs (see Fig. 3(a)). These features are important in analysing
the work presented in this paper. For the purposes of description, letαHR denotesα with high relief and αLR is low relief
while the ferrite ribbon is denoted αHA . The
yellow arrow shows the P , the blue arrow indicates theαHA , the red arrow is theαHR and the black arrow is theαLR . To the extent of information available to
the authors, the α phase morphology observed in Fig. 5(a) is
novel and has not been described in any literature elsewhere. Fig. 5(b
to d) show the Energy-Dispersive X-ray (EDX) analyses of the chemical
composition of the phases present in the J2N at locations 1 to 31 and
the results are shown in Table 3.