Issue 46

A. Deliou et alii, Frattura ed Integrità Strutturale, 46 (2018) 306-318; DOI: 10.3221/IGF-ESIS.46.28 309 E [GPa] ν  Y [MPa]  U [MPa] A (%) k n 221 0.3 500.3 573.14 33 840 0.25 Table 3 : Tensile properties of X70. Material Y σ [MPa] U σ [MPa] Elongation (mm) WM 603.44 642.80 6.33 Table 4 : Tensile properties of weld metal Microstructure analysis In order to carry out the microstructural study of our material, we performed optical microscope observations on polished sections (1 μm) then etched with 3% Nital at the transverse surface of the welded joint (Fig. 4). Examinations performed on unaffected sections reveal the presence of numerous alignments of MnS inclusions in the base metal and globular oxides in the welded metal. Optical micrograph shows an increase in grain diameter from the base metal to the weld joint. According to Fig. (5.a), the microstructure of the base metal consists as expected essentially of polygonal ferrite grains (white) and pearlite (black) organized in strips. This structure is produced by the segregation of Mn and P during rolling [34-37]. At the approach of the weld we find a more homogeneous organization of the grains The microstructure of the region near base metal consisted by equiaxed very fine grains of ferrite and pearlite.. For the heat affected zone (HAZ) (Fig. 5b), they appear acicular ferrite grains in the form of needles with coarse polygonal ferrite grains. The microstructure of the weld metal (WM) consists of dominant polygonal ferrite, acicular ferrite around the inclusions, and islands of pearlite (Fig.5c). The microstructure obtained is agreed with a number of studies oriented towards the analysis of the structure [26], [38-41]. Figure 4 : Transversal section for micrographic examinations and hardness measurement Figure 5 : Welded joint microstructure as observed through optical microscopy: (a) base metal (M: X200) ( b) heat affected zone (M: X200) and the weld metal (M: X500) (c)