Non-destructive evaluation of aging in welded pipeline X60 and X65 by thermoelectric power means

Abstract

In this investigation, the sensibility of the thermoelectric power (TEP) to the aging condition of API X60 and API X65 steel pipeline welded joints was evaluated. Specimens of both API welded joints were artificially aged at 300 °C for 3, 7, 9, 18, 30 and 45 h, the microstructural evolution was assessed via optical microscopy (OM) and scanning electron microscopy (SEM). Image analysis revealed no significant changes in the microstructure with respect to the aging time. Vickers micro-hardness evaluation showed that both welded joints reach a maximum hardness value after 3 h of artificial aging followed by a reduction in its value in the next aging conditions. TEP was measured using the hot tip technique applying different hot tip temperatures (55, 60, 65, 70 °C), where higher temperature values showed to increase the sensibility of the technique to detect microstructural changes that affect micro-hardness and where not possible to observe via OM and SEM. TEP and micro-hardness exhibit an opposite correlation, which was confirmed calculating the Pearson's correlation coefficient (r). The correlation coefficient decreases when higher hot tip temperatures where employed to measure TEP. The good correlation between TEP values and micro-hardness allows to use TEP measurement as a prediction tool of material micro-hardness.