PSI - Issue 17

Flavio Pereira de Moraes et al. / Procedia Structural Integrity 17 (2019) 131–137 Flavio Pereira de Moraes et al. / Structural Integrity Procedia 00 (2019) 000 – 000

136

6

3.2 Mechanical tensile tests, impact and hardness

Results of Charpy impact, tensile and Vickers hardness tests (2 kgf), all performed at room temperature, are presented in Table 3, along with typical results of AISI 316 L steels in the initial solution annealed condition.

Table 3. Results of the impact, tensile and hardness tests performed in samples of the AIA316L pipe after exposure at 640ºC and typical values for solution annealed condition of the same steel. Material Yield strength (MPa) Ultimate tensile strength (MPa) Total elongation (%) Absorbed energy (J) Vickers hardness (kgf/mm 2 )

Pipe after 640ºC exposure After solution annealing

333 205

650 515

27.5

57

376 220

40

100

The results of Table 3 clearly indicate that the material suffered, in comparison to the solution annealed material, an increase in strength and a decrease in ductility and toughness due to the long exposure at 640ºC. Calculation of life under flow conditions performed using the two available methods of the API 579-1/ASME FFS-1 standard (Larson Miller and MPC Omega Project), report values over three million hours. Furthermore, no evidence of macroscopic deformations has been detected in the pipe. Hence, the observed changes detected in the mechanical properties should be attributed to the microstructure degradation that occurred during the pipe exposition at high temperature. Fig. 4, Fig. 5 and Fig. 6 present the micrographs of the fracture surfaces of the Charpy samples, using SEM with secondary electrons. The micrographs reveal at least three interesting aspects: i) intergranular fracture; ii) presence of dimples, i.e. evidence of ductility and iii) presence of sigma phase at grain boundaries, which justifies the losses in ductility and toughness measured in the mechanical tests. 3.4 Intergranular corrosion resistance (practice “A”) Practice “A” of the ASTM A262 (9) standard is a test that makes possible the evaluation of the intergranular corrosion, also called sensitization in austenitic stainless steels. Fig. 7 shows the images obtained with optical microscopy (Fig. 7a) and with SEM (Fig. 7b) of the surface after testing. 3.3 Fracture analysis of Charpy samples

Fig. 7. Et ched surface using practice “A” observed in different sample positions by optical microscopy (a) and by SEM (b).

Etching of the grain boundaries is accentuated and has been classified, according to the ASTM A262 (2008) standard: one or more grains surrounded by ditches. It is relevant to outline that no M 23 C 6 carbide particles have been