PSI - Issue 39

Gonzalo M. Domínguez Almaraz et al. / Procedia Structural Integrity 39 (2022) 281–289 Author name / Structural Integrity Procedia 00 (2019) 000–000

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After the heat treatment of solution annealing, allowing the specimens machining, a double aging is carried out at 720° C for 8 h, followed by cooling inside the furnace decreasing 100° C in 1 hour to reach 620° C. Once reached 620º C inside the furnace, specimens are maintained for 8 hours and finally cooled in air down to room temperature, Figure 2(b) [25-27]. This second heat treatment allows to limit hardness, which must do not exceed 40 HRC [28], or 430 HV10 [16], in order to avoid stress-induced corrosion, corrosion by sulfide cracking and/or embrittlement by hydrogen.

Fig. 2. (a) Heating treatment of solution annealing, quenched with water (W.Q) and (b) Double aging heat treatment, cooling with air (A.C.).

3.2 Microstructural Characterization Micrographic analysis was carried out on specimens as-received by scanning electronic microscopy, using the Jeol model JSM-5910LV. The results of these observations are shown in Figure 3: the high content of delta phase δ, which is distributed both within the grain and in the grain boundaries, is illustrated in Figure 3(a); some of the observed carbides are of NbC type, detected in different shapes and sizes. In previous work [24], it is demonstrated that NbC carbides are needle ortho-shaped, globular, amorphous and with block morphologies. After the solubilization heating treatment, most of the precipitates were dissolved in the matrix γ, including the δ phase. In addition, it is observed that the average grain size does not have significant changes, nor do the NbC carbides, as shown in Figure 3(b). During the heat treatment of solubilized the δ phase is dissolved, meaning the decrease of mechanical properties such as: yield stress, tensile stress and hardness decrease, improving the machining of specimens of this aeronautical material. After double aging treatment, it is observed a typical austenitic microstructure in the material, as depicted in Figure 3(c).