PSI - Issue 82

S.S.M. Tavares et al. / Procedia Structural Integrity 82 (2026) 192–198 Tavares et al. / Structural Integrity Procedia 00 (2026) 000–000

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(b)

Fig. 2. Microstructure of specimens: (a) CR and (b) ST.

3. Results Fig. 2(a-b) shows the microstructures of specimens AR and ST etched with Beraha’s etching, both containing only ferrite and austenite phases. The %ferrite measured by DIP are 47.8 ± 6.0% in the AR and 52.1 ± 4.3% in the ST specimen. However, the readings of the ferritoscope for these two specimens were 41.0 ± 3.0 (% d ) and 40.0 ± 4.0 (% d ) respectively. Discrepancies between the ferritoscope reading and the real ferrite content of SDSSs are expected, and one of the reasons for this is that standards for the ferritoscope calibration are not made of the same steel analyzed. Fig. 3(a-d) shows the microstructures of specimens AR-850-15, AR-850-30, ST-850-15 and ST-850-30, as observed in the SEM, without etching. Specimens aged for 15 minutes contain basically d , g and c phase precipitated in the grain boundaries. In specimens aged for 30 minutes, colonies of s phase are observed. Also, the amount of c phase increased, now including intergranular particles. Fig. 4(a) shows the microstructure of the specimen treated at 1100 o C and slowly cooled in the furnace. Fig.4(b) presents the thermal cycle of this sample, measured with a type R thermocouple welded to the piece, showing that the time interval between 1000 o C and 600 o C was 7403 seconds. The consequence of this thermal cycle was the intense precipitation of c and s phase. Table 3 exhibits the phase amounts measured by DIP, the ferritoscope readings, the hardness values, and Charpy impact toughness. Specimens aged for 15 minutes at 850 o C presents small amounts of c phase, which does not increase the hardness, but provokes a significant decrease of toughness in the Charpy tests at -46 o C. This is in accordance with previous works which states that only ~ 1% of deleterious phases caused an important decrease of toughness in austenitic-ferritic stainless steels (Calliari et al., 2009). Such small amounts of c phase were detected by the decrease of the ferritoscope reading. When the aging time is increased to 30 minutes the amounts of deleterious phases ( c and s ) increased to 3.6% (AR-850-30) and 5.8% (ST-850-30), and a further decrease of the ferritoscope reading was observed. Also, the toughness values continued to decrease.

Table 3. Phase amounts, ferritoscope reading (F), impact toughness (ECVN) and Vickers hardness (HV10). Specimen % c % s F (% d ) ECVN (J) HV10 AR 0.0 0.0 41.0 ± 3.0 64.5 365 ± 4 AR-850-15 1.4 ± 0.3 0.0 33.0 ± 2.8 22.5 312 ± 9 AR-850-30 1.8 ± 0.5 1.8 ± 0.4 28.3 ± 2.8 8.0 311 ± 1 ST 0.0 0.0 40.0 ± 4.0 76.5 285 ± 4 ST-850-15 2.2 ± 0.9 0.1 ± 0.1 32.5 ± 3.0 21.5 279 ± 7 ST-850-30 3.6 ± 0.9 2.2. ± 0.9 28.0 ± 2.7 8.0 297 ± 1 1100-SC 1.7 ± 0.7 21.8 ± 0.3 10.5 ± 1.1 4.5 350 ± 1