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.