PSI - Issue 60

B Shashank Dutt et al. / Procedia Structural Integrity 60 (2024) 471–483 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

473

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blanks, compact tensions (CT) specimens of 20 mm thickness were fabricated. The CT specimens were tested at temperatures of RT (room temperature), 380 and 550 °C. Tensile testing and fracture was carried out in as weld condition and after aging.

Table 1: Chemical composition (wt %) of SS 316LN

C

N

Mn 1.7

Cr

Mo 2.49

Ni

Si

S

P

Fe

0.027

0.08

17.53

12.2

0.22

0.0055

0.013

Bal.

Table 2: Details of aging conditions

Aging temperature (C)

Aging durations, h

370 475 550

1000 1000 1000

10,000 10,000 10,000

20,000 20,000 20,000

The CT specimens were subjected to pre-cracking under fatigue loading conditions. The initial crack length after pre-cracking was targeted as ~ 25 mm. Fatigue loading for pre-cracking was done under decreasing stress intensity factor ( ∆ K ) range. During pre-cracking, loads applied were in the range of 12-5 kN. After pre-cracking, CT specimens were side-grooved by 10% of the original specimen thickness on both sides. Fracture testing ( J -R curves) was carried out using a suitable servo-hydraulic machine at load line displacement rate of 0.01 mm/s. During testing, loads, crack opening displacement and crack lengths were monitored and recorded. The fracture testing was carried out as per ASTM 1820 standard (ASTM 2017). After fracture testing, CT specimens were broken and initial and final crack lengths were determined optically by nine-point average method.

3. Results and Discussion

3.1 Tensile properties and fracture results

The tensile results of the welds are shown in Figures 1-3. For specimens tested at RT (Fig.1), decrease in yield strength was observed after 20,000 h duration for all the aging temperatures, compared to as weld condition (unaged). No significant change in ultimate tensile strength was observed (RT tested) after various aging conditions. Decrease in total elongation (%) was observed after aging at 475 and 550 °C for 20,000 h durations, compared to as weld condition (tested at RT). No significant changes in ultimate tensile strength was observed after all aging conditions and tested at 380 and 550 °C, as observed in figures 2 and 3. From figures 2 and 3, it is observed that decrease in total elongation (%) occurred after aging at 475 and 550 °C for 20,000 h durations, compared to as weld condition. Similar decrease in ductility was observed previously for SS 316L welds, subjected to various aging (350, 400 C and 4000-16,000 h durations) conditions and tested at RT and 288 °C (Youn et al. 2021). J -R curves of aged specimens tested at RT, 380 and 550 C are shown in Figure 4. Decrease in slopes of J -R curves was observed after 20,000 h aging durations for all the aging temperatures, compared to as weld condition (RT tested). Decrease in slopes of J -R curves was observed after various aging conditions and tested at 380 and 550 °C. Decrease in slopes of J -R curves was also observed for SS 316L welds subjected to aging (350, 400 °C and maximum of 15,000 h) and tested at RT and 288 °C (Youn et al. 2021). From J -R curves (Figure 4), J 1c values were determined (Figure 5). All the J 1c values (all aging conditions and test temperatures including as weld condition) satisfied the plane strain condition (minimum thickness or B) for valid material fracture toughness. Minimum thickness for valid material fracture toughness was estimated using equation 1. From J 1c values, equivalent K j1c values were determined using equation2.