13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Figure 3 Influence of temperature on fracture in duplex stainless steel, (a). At -75°C, impact toughness 46J, (a). At -130°C, impact toughness 14J, (c). Enlarged cleavage fracture at -75°C, (d). Enlarged cleavage in weld metal at -100 °C, a cracking inclusion is a Griffith crack. 3.2 Influence of cluster on DBT behavior Duplex stainless steel can suffer from a spinodal decomposition at temperatures between 300-500°C where the ferritic phase undertakes a miscibility gap that gives rise concentration variations with Fe-rich (α) and Cr-rich (α’) regions (Fig. 4a), which leads to the formation of Fe-rich and Cr-rich clusters or phases [1, 2, 5]. Spinodal decomposition leads to an increase in the hardness of the ferritic phase, but not the austenitic phase (Fig. 4b). It was found that an increase in hardness of the ferritic phase promotes the occurrence of cleavage or DBT (Fig. 4c). Using Eq. 1, the hardness at T50 can also be determined as shown in Fig. 4d. This type of curve provides useful information for a quick evaluation of the influence of spinodal decomposition on the brittleness of the material. The fracture structure in Fig. 4c is different from that in Fig. 3b. In Fig. 4c, the ferritic phase has a cleavage fracture, but not the austenitic phase. The austenitic phase is still ductile, and dimples can be observed. This indicates that the stress concentration at the cleavage crack in the ferritic phase is not high enough to initiate a quasi-brittle fracture as that in Fig. 3b since the fracture toughness in the austenitic phase in this case is still high enough. The above discussion shows that local approach to Al oxide
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