PSI - Issue 71

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

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and depth of attack shall increase with exposure time causing the mechanical properties to decrease with increasing exposure time. Further, the statistical distribution of defect depth is studied in subsequent sections. 1.7. Tensile tests carried out in air environment (under stress only) To assess whether the observed changes in mechanical properties under molten glass environment are not due to exposure under stress, a test was performed under air at 900 °C and 20 MPa for 95 hours. As shown in Fig. 5, exposure to stress in air led to a moderate reduction in ductility from 68% to 52% while strength remained largely unaffected. In contrast, specimens exposed to the molten glass environment exhibited a significant decrease in both strength and ductility indicating a pronounced detrimental effect of the glass medium on mechanical properties.

100

Exposure duration - 95 hrs Air environment

Air environment

Molten glass environment

80

60

40

20 Stress (MPa)

Test temperature 900deg.C

0

0

15

30

45

60

75

Strain (%)

Fig. 5: Comparison of stress strain curve of alloy690 material after exposure to molten glass and air environment

4. Evaluation of defect size and density under different test conditions After testing the specimen under different test conditions, the specimens were cut along the cross-section at a distance of around 5 mm from the broken region. The specimens were polished using very fine grit size paper in the polishing machine. All the specimens were seen under the optical microscope to determine the depth of attack of the molten glass on alloy 690 material. The optical microscopy image of the cross-section of the specimen tested at 800 °Cunder air environment (Fig. 6(a)) and molten glass environment (Fig. 6(b)) is shown below. It can be observed that there is very minimal attack in the air environment as compared to the molten glass environment. Whole periphery of the specimen gets attacked by the molten glass due to its intrusion into the grain boundaries of the material. The distribution of defects along the periphery is shown in Fig. 6(c) by aligning all the edges of the specimen along a single line. This is done to determine the distribution of defect size along the periphery of the specimen.

(a)

(b)

(c)

Fig. 6: Optical microscopy of cross-section of tested specimen at 800 °C under (a) air environment; (b) exposure of 95 hrs and stress level of 30 MPa in molten glass environment (c) periphery of the cross-section of specimen.

Comparison of defect size with distance along the periphery is shown in Fig. 7. It can be observed that for exposure time of 95 hrs, the maximum defect size increases from 78 μm to 102 μm as the temperature is increased from 800 °C to 900 °C. It indicates that more damage shall occur at higher temperature for the same exposure time. This is in line with the observation discussed in section 3 where more changes in mechanical properties were observed at higher