PSI - Issue 60

Cyril Reuben Raj et al. / Procedia Structural Integrity 60 (2024) 709–722 Cyril Reuben Raj / StructuralIntegrity Procedia 00 (2024) 000 – 000

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Thermal aging of the similar austenitic stainless steel pipe weldments is shown in figure 1. The thermal aging were performed in a furnace at constant temperature of 400 ℃ . Two SS-SS 304LN pipe weldments were thermally aged for 10,000 and 20,000 hrs and remained cooled gently. The accelerated temperature and aging hours were chosen according to the potential life hours of Indian nuclear power plants.

Fig 1. Photographs of the austenitic stainless steel weld pipe (a) As-welded pipe, (b) 10,000 hrs thermally aged and (c) 20,000 hrs thermal aged.

2.2. Metallographic analysis Two set of block specimens were machined from the full-scale pipe weld for metallographic analysis i.e. macrostructure, microstructure of the weld section using wire cut EDM. The specimens for microstructural examination were polished with emery paper varying the grit size at 60-120, followed by alumina polish. In order to clearly identify the weld microstructure the specimens were etched by preparing the etching agent in the ratio (Demineralized water 10 ml + Glycerol 45 ml + Hydrochloric acid 15 ml + Nitric acid 30 ml) for 30 minutes and cleaned to analyze its weld pattern. With the help of optical microscope, the morphological investigation were carried out after fine grinding with silicon carbide papers and polishing to mirror finish using diamond suspension (of particle size 1 µm). Electrolytic etching of the polished samples was carried out in 10% oxalic acid solution at 10 V for 90 s to reveal the microstructure. 2.3. Hardness and ferrite test The hardness measurement were carried out at macro and micro scale at the weld zone for detection of change in hardness due to thermal aging. Specimens were located at the center of the SMAW (Top and Middle location) and GTAW (Bottom location) zones for the hardness measurement. The examinations were conducted on micro hardness indentation machine (Model no. Fischerscope Hm 2000) with a load of 200 gf for macro scale and 10 gf for micro scale for a loading time of 20 s followed by a constant creep time of 5 s and further to unloading time of 20 s. The indentation is featured by initial contact between the specimen and the indenter and then continuously increasing the indenter load to the specified value. During the entire process, the applied load and the indentation depth is continuously monitored. The mechanical analysis of the indentation curves allows to calculate the hardness and the other mechanical properties of the material for loads as low as 0.4 mN. Due to the low load, the method allows to accurately localize the indent within the ferrite or austenite phases. The hardness is calculated directly from the load and depth measurements as per the procedure described in the standards [Anon (2002), Anon (2007)]. Hardness measurement was done on the polished samples, which were lightly etched using electrolytic etching in 10% oxalic acid solut ion to distinguish between the δ -ferrite and the austenite phases. The ferrite content (%) was recorded at the weld zone i.e. SMAW and GTAW zone with the help of ferritoscope FGAB1.3-Fe which has a measuring range of 0.1 to 80% Fe. The measurement is carried out at five different locations of the block specimen.