(A) study on the microstructure and corrosion characteristics of laser surface melted Alloy 600

Abstract

Surface melting by a continuous $CO_2$ laser beam was performed on the Ni-base Alloy 600 (or Alloy 600), which has been used as a steam generator tubing material. The microstructural changes of the laser treated Alloy 600 were investigated with various microscopic equipments including a scanning electron microscope and a (scanning) transmission microscope equipped with energy dispersive X-ray spectroscopy. The susceptibility to intergranular corrosion of the laser treated Alloy 600 was evaluated from the results of the C-ring and DL-EPR tests. By laser surface melting (LSM) on the sensitized Alloy 600, the Cr-rich carbides having precipitated along the grain boundaries during sensitization treatments were dissolved completely or partially in the heat affected zone (HAZ), close to or far from the laser melted zone (LMZ), respectively. At the same time, the disappearance of Cr depletion at the boundaries were accompanied (i.e., de-sensitization occurred). The microstructure of the LMZ, having epitaxially solidified from the HAZ, consisted of a very narrow plane front solidification region and a cellular solidification region with fine cellular-dendrites in the grains. Chromium was enriched along the cell boundaries in the LMZ due to the microsegregation during solidification. In addition, very fine particles were distributed along the cell boundaries with tangled dislocations around them. The solidification behavior in the LSM Alloy 600 was analysed with respect to the laser treatment conditions employed in the present study. Most of the tiny particles in the cellular-dendritically solidified regions of the LMZ were identified as TiN type particles. Another type of particle, MgS was also found as being attached on the TiN type particles. The tiny particles were located along the cell boundaries in the cellular-dendritic structure, but not observed in the regions solidified by a plane front mode in the LMZ. The number density of TiN type particles was measured to...