PSI - Issue 41

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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retarding effect of the precipitates and elements in solid solution promotes the appearance of the dynamic recrystallization and by the same slowed the hot damage of our material. At high temperature, the softening effect due to dynamic recrystallization decreases stress concentrations at grain boundaries and triple points. At the same time, it prevents the nucleation of new cavities as well as the coalescence of micro-cavities appearing on these sites during the first stages of plastic deformation.

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Fig. 7. Microstructure of samples deformed at 800 °C.

The migration of grain boundaries under the effect of dynamic recrystallization is at the origin of the isolation of the cavities and microcracks inside the grains, which slows their growth and their coalescence. It can cause the closing of microcracks and make their migration very difficult. It globally decreases the rate of segregation of sulfur and phosphorus at the grain boundaries and at the precipitated interfaces / matrix. 3.3. Influence of Temperature on Ductility The temperatures at which ductility is restored are all the higher as the three important factors contributing to the abrupt loss of ductility are non-predominant. These factors are :  the decline in the level of segregation of sulfur and phosphorus;  the progressive dissolution of the precipitates as a function of the rise in temperature;  the proportions of ferritic and austenitic phases in the two-phase domain. The presence of the first two factors increases the number of potential cavity initiation sites and prevents migration of grain boundaries by delaying dynamic recrystallization. The loss of ductility observed at temperatures between 800 and 900°C can be attributed to these factors. In this temperature range, segregation and precipitation of the elements present in our material are preponderant see Figure 8: Presence of several precipitates on the matrix (TiC) and inside the cavities (MnS) and Figure 9: Presence of several matrix precipitates (MnS) aligned and (TiC). It is therefore obvious that the austenization temperature of 1200°C, the temperature practiced in the rolling mills and in our study, cannot dissolve the aluminum nitride particles which are one of the causes of the ductility drop in the presence of segregated sulfur. Other works on precipitates of Nb, V, and Ti in the presence of sulfur have led to the same conclusions (Trombert, 1988).