PSI - Issue 41

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Abdelmoumene Guedri et al. / Procedia Structural Integrity 41 (2022) 564–575 Abdelmoumene Guedri et al. / Structural Integrity Procedia 00 (2022) 000–000

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Fig. 4. Evolution of the maximum stress as a function of temperature for different strain rates. The hardening effect observed on the curves of evolution of the maximum stress as a function of temperature is due to the presence of the precipitates in the matrix and at the grain boundaries. These precipitates dispersed on the matrix obstruct the plastic deformation and thus promote an increase in the tensile strength of our material. At temperatures below 850°C, we do not observe a great dispersion of the values of the maximum stress, since the rise in temperature causes no additional precipitation. The difference in stress obtained can be explained by the fact that in this temperature range the precipitation conditions are identical and that only the proportion of phases  and  changes as a function of the temperature, thus causing a concentration of the stress at the of the most ductile phase. 3.2. Ductility The evolution of the ductility criterion Z%, which is the reduction of the area at the break deformation as a function of temperature, is represented in Figure 5. This figure confirms the existence of a particular ductility pocket for each cycle of heat treatments imposed on the specimens before deformation. The ductility drop is not very important. At least the ductility trough and for a strain rate equal to 1.96 x 10 -3 s -1 , it reaches 71.5%. The variation of the ductility as a function of the temperature seems to be characterized by two zones. The first is in the two-phase domain where a minimum of 800°C is recorded. On the other hand, a second zone is observed in the austenitic domain at 900°C. The mechanisms of embrittlement are in this case quite distinct, because they are related to two-phase effects in the two-phase domain (Zhang, et al., 2017) or to synergistic effects, that is to say of precipitation and segregation in the austenitic phase. Indeed, in the  -  domain, the ductility in the temperature range concerned varies directly with the ferrite content. The embrittlement mechanism in this area is based on the relationship between the presence of primary ferrite at the grain boundaries and necking, and the presence of a few ferrite islands at the austenitic grain boundaries slightly lowers the ductility.