PSI - Issue 14

T. Narayana Murty et al. / Procedia Structural Integrity 14 (2019) 664–667 Narayana Murty et al/ Structural Integrity Procedia 00 (2018) 000 – 000

666

3

2.3. Metallography

Ring was subsequently ground with increasingly fine emery papers including 220, 320, 500 and 800 grit. For optical microscopy, hydrided Zr-2.5% Nb alloy specimens were swab etched with cotton soaked in a solution of HF: HNO 3 : H 2 O:10:45:45 for ~35 s. 3. Results and Discussion The typical pattern of hydride formation near a punch mark is shown in figure 1. As shown in the figure 1, the micrograph is divided into different regions containing circumferential hydrides, mixed orientation and radial hydrides. The threshold stress for reorientation ( σ th ) of hydrides is reported to be around 180 MPa- 210 MPa (Singh et al. (2004)) for reactor operating temperature. These values are reported for uniaxial tensile case. As shown in the figure 1, the pattern of hydride formation near punch mark is symmetric about the axes, which passes through the centre of punch mark. This may be due to symmetry in the stress field generated due to punching. A detailed stress analysis has to be performed to find the values and directions of these stresses generated. The region of hydride formation is divided into different regions based on orientation of the hydrides as shown in figure 1. Fully circumferential hydrides are formed in the regions A and B. This may be due to stresses generated in this region are either compressive or less than the value of threshold value ( σ th ). Mixed orientation of hydrides is observed to be formed in the regions E, F, G and H. Hence, the values of stresses generated in these regions is around the value of σ th . Radial hydrides are observed to be formed in the regions C and D. The stresses generated in these regions should be tensile and their values are expected to be above the threshold value. To investigate the effect of punching load on the formation radial hydride formation, punching load was varied from 100 N to 800 N. The micrographs showing the hydride formation for different punch loads is shown the figure 2. The punching area is increasing due to increase in the punching load as expected. The pattern of hydride formation is same as explained in above paragraph and not varying much with the punching load. But region containing the radial hydrides is increasing with increase in punch load. Radial hydrides were observed to be formed through thickness for the punching loads having value more than 400 N. Fracture toughness of Zr-2.5% Nb PT material is reported to be observed to depend on the fraction of radial hydrides and decreases significantly with increase in fraction of radial hydrides (Singh et al. (2004); Sharma et al. (2018)). Hence, amount of punching load may affect the integrity of PT of PHWR through the formation radial hydrides near the punch marks. Due to anisotropic nature of PT material (Singh et al. (2005)), the mechanical behaviour of this material will be different in different directions. Present paper deals with effect of punching in radial- circumferential plane only. It is necessary to conduct the similar experiments on axial- circumferential plane for the complete assessment of PT integrity. Detailed stress analysis has to done to quantify the stresses generated due to punching.

Fig. 1. Typical hydride orientation near the pumch marks.