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

665

2

of Zr-2.5%Nb alloy (Bajaj and Gore (2006)). Hydrogen embrittlement of Zr alloys is primary concern over the structural integrity of these pressure tubes (Northwood and Kosasih (1985); Singh et al. (2002)). These pressure tubes are manufactured in such a way that it gives a particular texture with elongated grains in the axial directions (Srivastava et al. (1985)). Due to this initial texture, there are two hydride orientations possible in the pressure tube i.e. circumferential and radial hydrides. Formation of hydrides is reported to reduce the fracture toughness of Zr alloys (Singh et al. (2013)). Formation of radial hydrides is even more deleterious as apart from causing significant reduction in fracture toughness, they may facilitate crack growth by Delayed Hydride Cracking (DHC) (Northwood and Kosasih (1985); Singh et al. (2002)) phenomenon to form through thickness crack in the pressure tube. But due to the microstructure, only circumferential hydrides forms in the material. However, radial hydrides can form when material is cooled under stress above a threshold values (Singh et al. (2004)). Mechanical handling of these pressure tube during manufacturing, commissioning or at service may cause some localized deformation due to indents and punch marks, which will create localized residual stresses in the material. Radial hydrides may form in this localized stressed region near the indents. Therefore, it is very important to assess the possibility of radial hydrides formation near these regions of localized deformations and recently it is reported that the fracture toughness values of PT reduces significantly in the presence of radial hydrides and it varies with the fraction radial hydrides to the total hydrides present (Sharma et al. (2018)). In this study, Pressure Tube material was deformed locally by using an I shape punch indenter with a predefined load there after samples were hydrided to 50 wppm using gaseous charging and metallographically examined for identifying hydride orientation. Indentation load was also varied to study the effect of load on the hydrides orientation. DHC Delayed Hydride Cracking PHWR Pressurized Heavy Water Reactor PT Pressure Tube σ th Threshold stress for reorientation

2. Experimental procedure

2.1. Indentation

Small ring of 20mm length was cut from the Zr-2.5Nb alloy pressure tubes to carry out the experiment. Ring was polished to get the flat surfaces on both ends. An ‘ I ’ shape indenter was connected to universal testing machine (UTM). Indentation was performed using UTM with predefined load on the radial-circumferential plane of the pressure tube.

2.2. Hydriding

Desired amount of hydrogen in the Zr-2.5%Nb pressure tube spool piece was charged by gaseous hydrogen charging method without altering its microstructure and texture by exposing the freshly polished metal surface to high purity hydrogen gas at high temperature. In this technique, autoclaved oxide layer on both sides of Zr-2.5%Nb pressure tube spool piece was removed by grinding and polishing successively up to 1200 grit emery paper and heated in high purity hydrogen gas atmosphere in a modified Sievert's apparatus. The system is evacuated to a dynamic vacuum of the order of 10 -5 torr using an turbo molecular pump to prevent oxidation of the sample as the oxide layer acts as a physical barrier and prevents hydrogen ingress. The system is heated up to 360 0 C before it is exposed to high purity hydrogen gas. The system is isolated and in a constant volume system, the amount of hydrogen charged is directly proportional to the difference between initial and final partial pressure of hydrogen in the control volume. Ring of diameter 83.3 mm, wall thickness 3.5 mm and length 20 mm was charged with 50 wppm of hydrogen.