Issue 42

A. Strafella et alii, Frattura ed Integrità Strutturale, 42 (2017) 352-365; DOI: 10.3221/IGF-ESIS.42.36 362 Morphological and microstructural characterization Steel specimens were observed by Optical and Scanning Electron Microscopy to characterize the raw materials and highlight the microstructure induced by 20% cold-working. Before the analysis, the steel samples were glyceregia etched for the examination of grains structure [9]. Glyceregia is indeed recommended for revealing the austenitic microstructure, as grain structure, and outlines carbides [10]. Slip produced by cold-working is shown in Fig. 13. a) b) Figure 13 : Glyceregia etched as received steel: a) Optical and b) SEM micrograph. Moreover, the specimens were examined after rupture by scanning electron microscopy. Secondary electrons are used to obtain the fracture surface morphology. Typical SEM images of fracture surfaces obtained from the creep specimens tested in air are shown in Fig. 14. Fig. 14-a concerns a low magnification and also includes an insert macroscopically displaying the specimen immediately after rupture. Both the specimen necking, identifying the ductile fracture, and the typical changes of fracture direction, characterizing the transgranular fracture, are pointed out. The fracture surface has a wrinkled appearance of mixed or ductile fracture. The micrographs of Figs. 14-b,c,d showed a combination of a cup-cone and transgranular fracture modes. The first one is characterized by its typical dimples, while the second one by the river pattern. The typical equiaxed dimples, characteristic of ductile fracture in tension, and then dominant cup-cone mode, are characteristic of surfaces close to central area of specimen (Fig. 14-b). Instead a transgranular fracture mode is dominant in the central part of specimen, where local facets of quasi-cleavage are noticed (Fig. 14-c). The other regions of the fracture surface are a combination of shear rupture (transgranular rupture) and cup-cone fracture (coalescence by shear of voids caused by plastic deformation). The transgranular rupture is incomplete, showing a mixed rupture mode, as in the case of external surface corresponding to the specimen necking. The deformation bands with the elongated dimples on the fracture surface are clearly displayed in Fig. 14-d. These considerations move to the conclusion that the specimen rupture mode is mixed, transgranular and cup-cone mode. These observations are consistent with the expectations: the addition of Ti induces the precipitation of carbide particles and consequently the improvement of the high temperature mechanical properties, as creep strength, as reported by Zahra and Schroeder (1982) [11]. They studied creep properties of an austenitic steel with a chemical compositions similar to that of 15-15Ti(Si), finding that the fine TiC precipitates in Ti-stabilized austenitic stainless-steel cold-worked alloy form preferentially on the intergranular dislocations and they are more stable [11]. This justifies transgranular or ductile fracture. Even for specimen tested in lead, morphological features of fracture surface was analyzed by Scanning Electron Microscopy (Fig. 15), but a chemical polishing was necessary to differentiate steel surface fracture from eventual lead residues and to reveal the fracture mode. The Fig. 15-a also includes an insert macroscopically displaying the specimen immediately after rupture. The loss of steel ductility and the typical brittle fracture are evident from the brilliant and polished fracture surface, along 45°, with negligible plastic deformation. It can be observed that the fracture is brittle, as showing by the typical intergranular fracture, where the decohesion occurring along a weakened grain boundary is evident (Fig. 15-b). Grains are clearly observed in their three-dimensional structure, within a polished fracture surface with no wrinkled appearance, unlike ductile materials (Fig. 14).