Metallurgical Analysis of Direct Aging Effect on Tensile and Creep Properties in Inconel 718 Forgings

Performing the double aging thermal treatment on Inconel 718 forged components directly after forging instead of the standard thermal treatment sequence including a solution annealing step before the double aging is known to be beneficial to mechanical properties. In this work, this so-called direct aging (DA) effect has been assessed for tensile properties at room temperature and for creep properties at 650 °C/750 MPa. Mechanical characteristics obtained on direct aged specimens are compared to those obtained on specimens submitted to the standard thermal treatment sequence (i.e., including a solution heat treatment) and tested in the same conditions. The classical effect of direct aging thermal treatment is obtained; better tensile and creep properties are reached for DA specimens. However, contrary to what is generally assumed, fine microstructural characterizations in specimen threads reveal that the direct aging effect cannot be attributed to a higher residual strain hardening level preserved in the microstructure. It is actually shown that its beneficial effect on mechanical properties is due to a lower δ phase content obtained in microstructures having undergone such a treatment rather than the standard sequence. On the one hand, it has indeed been demonstrated that, counter-intuitively, the δ phase content actually increased during the “solution” treatment performed at 955 °C for 1 h. On the other hand, the relationship between mechanical properties and microstructural features (grain size, δ phase content, residual forging strain hardening level) has been analyzed in detail, and differences in mechanical behavior are mainly controlled by this increased amount of δ phase which deteriorates the mechanical properties. On the contrary, no significant effect of residual forging strain hardening on tensile and creep properties has been found for the investigated conditions.