PSI - Issue 71

Ganesh Nigudage et al. / Procedia Structural Integrity 71 (2025) 218–225

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2.2. Small punch test In this study, SP tests were performed at ambient temperature on RPV steel of as-fabricated condition and in thermally aged condition. To determine DBTT of as-fabricated RPV base steel material, SPT was done at sub ambient temperatures of -130, -100, -60, -30, 0 o C. All SP tests were carried out at crosshead speed of 0.5 mm/min. Liquid nitrogen was used for testing at sub-ambient temperature. For SPT, 10x10 mm 2 square specimens of 0.6 mm thickness were cut from half Charpy specimens using slow speed cutting machine. The specimens were reduced to 0.5 mm thickness using manual polishing on 600, 800 and 1000 grit size emery paper. Fine polishing was done using 1000 grit emery paper to reduce surface roughness. The test setup used for SPT is shown in Fig.1. The specimens prepared for small punch test are placed in lower die of test setup. The upper die is clamped over lower die using screws. The specimen is centrally loaded using 2.5 mm diameter spherical steel ball and punch. The specimen gets fixed in between the dies so specimen is prevented from cupping upward during punching and therefore the plastic deformation is concentrated in region below the punch. The fixture is mounted on screw driven universal testing machine along with extensometer. The extensometer had knife edges which were used to attach to punch and fixture to record punch displacement during test. The punch displacement measured using extensometer is used for analysis in this study.

Fig.1. SPT setup and positioning of extensometer.

2.3. Microstructural examination For microstructural examination under optical microscope, samples were cut from as-fabricated and thermally aged half Charpy specimens using slow speed cutting machine prepared by polishing using emery paper of grit 320 to 4000. Samples were polished until mirror finish is obtained and then swab etched using 2% Nital +98%ethanol solution. The etched samples were also observed under SEM for carbide distribution study. 2.4. Hardness Examination Specimens for hardness testing from as-fabricated and thermally aged condition were polished using emery paper of 400 and 600 grit. The hardness of specimens was evaluated using Vickers hardness testing machine at 30kg load and 10 seconds dwell time. 3. Results 3.1. Small Punch Test The correlations used to estimate tensile properties of as-fabricated base and thermal aged base specimens are as follows =0.17× 0.1 , ℎ 02 +172.35 (1) =0.088× ℎ 02 +32.81 (2) = 16.98 × −13.23 (3)