Microscopic investigation of sensitized Ni-base alloy 600 after laser surface melting

Abstract

A study using a transmission electron microscope (TEM) equipped with an energy-dispersive X-ray spectrometer was carried out to investigate microstructural and compositional changes in sensitized Ni-base Alloy 600, the surface of which had been melted by a CO 2 laser beam. Cr-rich carbides, having formed along grain boundaries during sensitization treatment, were dissolved completely or partially in the heat-affected zone (HAZ) close to or far from the laser-melted zone (LMZ), respectively, accompanied by the disappearance of Cr depletion at the boundaries. The microstructure of the LMZ consisted of fine cells in the grains having epitaxially solidified from the HAZ, and Cr concentration was observed to increase along the cell and grain boundaries in the LMZ. In addition, the cell walls in the LMZ were decorated with dislocations, and very fine particles, found to be a TiN type, were distributed randomly along the cell walls with tangled dislocations around them. The high dislocation density in the cell walls is attributed to the residual tensile stress developed during the rapid solidification.

Double loop electrochemical potentiokinetic reactivation test of Nickel-base Alloy 600 surface-melted by a CO2 laser beam

“…2(c). These were identified as Crrich M 23 C 6 and Cr 7 C 3 [8], the same found in the case of the SA+SEN Alloy 600 [5]. The average size of the Cr-rich carbides precipitated on the grain boundaries in the LSM+SEN Alloy 600 was much smaller than that in the SA+SEN Alloy 600.…”

“…This result originates from the fact that pre-existing Cr-rich carbides of the sensitized Alloy 600 were completely melted/dissolved due to the high energy density of a laser beam. Moreover, Cr-rich carbides were not re-precipitated due to the high cooling rate during the LSM process [5]. Therefore, low DOS of the LSM Alloy 600 is caused by the de-sensitization (or, disappearance of the Cr depletion zones) due to the LSM process.…”

“…The degree of Cr depletion on the grain boundaries was, therefore, not severe. The minimum Cr concentration on the grain boundaries was measured as 12 wt% [8], which was considerably higher than the 7.3 wt% of the SA+SEN Alloy 600 [5]. Therefore, it can be concluded that the laser treatment suppressed the precipitation of grain boundary Cr-rich carbides in Alloy 600, and this led to the low DOS value of the LSM+SEN Alloy 600.…”

“…A continuous CO 2 laser beam was used; details of the LSM procedure are described elsewhere [5]. Some of the LSM specimens (hereafter LSM Alloy 600) were also subsequently sensitized under the same conditions as for the SA+SEN Alloy 600 (hereafter LSM+SEN Alloy 600).…”

“…LSM can be applied to repair the tubes degraded by IGA/IGSCC during the normal operation of nuclear power plants, since the laser beam can easily be directed to the failed parts through a beam transmission system such as an optical fiber. From previous studies, it was demonstrated that LSM improved the resistance to IGA/IGSCC of the sensitized Alloy 600 in sulfurbearing environments [4], mainly due to the metallurgical changes induced by the laser treatment [5]. During failure by IGSCC, a crack starts at the free surface and propagates through a connected pathway of the susceptible grain boundaries.…”