PSI - Issue 71

P.K. Sharma et al. / Procedia Structural Integrity 71 (2025) 126–133

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environment under different stress, temperatures, and exposure time durations. The material stress-strain curves under different test conditions are discussed in subsequent sections. 1.5. Tensile tests carried out in air environment The reference tensile tests were carried out at 800 °C and 900 °C in the air environment. The material stress strain curve is shown in Fig.4. The yield strength, ultimate tensile strength and ductility obtained after testing is shown in Table 3. It can be observed that the yield strength changes from 180 MPa to 90 MPa at 800 °C and 900 °C respectively. This indicates a reduction in strength by 50% with increasing the temperature by 100˚C. The ductility gets increased from 40% to 68% with subsequent increase in temperature from 800 °C to 900 °C respectively. More hardening was observed for specimens tested at 900 °C as compared to 800°C. The upper and lower yield phenomena were more prominent for specimens tested at 800 °C. 1.6. Tensile tests carried out after exposure to molten glass environment The exposure to molten glass was carried out at different temperatures and stress levels. The exposure durations were varied from 24 hrs to 95 hrs in order to determine the changes in mechanical properties with exposure time. Stress levels were fixed at di fferent temperatures depending on the ratio of σ appl. /σ ys , thus the stress level is fixed at 30 and 20 MPa at 800 °C and 900 °C respectively. The change in material stress-strain curve with exposure time at different temperatures is shown in Fig. 4. It can be observed that with the increase in exposure time, the strength and ductility continuously decrease indicating degradation in mechanical properties due to exposure to glass environment. Change in mechanical properties under different conditions is shown in Table 4. Table 4: Mechanical properties of alloy690 material under different test conditions. Test environment Temperature (°C) Stress level (MPa) Exposure duration (hrs) Yield strength (MPa) Ultimate tensile strength (MPa) Ductility (%)

800 900 900 800 900 900 900

- -

0 0

180 82.5

186

40 68 52 27 36 27

Air (Reference tests) Air (exposure under stress only)

90 89

20 30 20 20 20

95 95 24 48 95

81

165

172 80.5 76.5 68.6

75 70 63

Molten glass

14.5

100

200

Molten glass environment (Exposure duration)

Air environment

24 hrs 42 hrs 95 hrs

80

150

60

100

Test Temperature - 800 deg.C Air environment Molten glass environment (Stress - 30 MPa, Exposure duration - 95 hrs)

40

Test temperature 900deg.C

50

20 Stress (MPa)

Stress (MPa)

0

0

0

15

30

45

60

75

0

7 1421283542

Strain (%)

Strain (%)

(a)

(b)

Fig. 4: Comparison of stress strain curve of alloy 690 material after exposure to molten glass environment at (a) test temperature of 800 °C and stress level of 30 MPa; (b) test temperature of 900 °C and stress level of 20 MPa.

After exposure of 95 hrs under glass environment, the ductility gets reduced by 33% and 78% at 800 °C and 900 °C respectively and the corresponding strength was reduced by 8% and 22%. It indicates that at higher temperature, more degradation in strength and ductility of the specimen may occur. This is due to the inter-granular attack by corrosive molten glass into the grain boundaries of alloy 690 material forming micro-cracks due to which the local stresses become higher. This causes the strength and ductility to be reduced. More and more cracks shall be formed