Investigation of IGSCC behavior of sensitized and laser-surface-melted Alloy 600

An attempt was made to modify the surface of sensitized Alloy 600, without affecting the bulk properties, by a laser surface melting (LSM) technique in order to improve its resistance to intergranular stress corrosion cracking (IGSCC) in oxidizing environments. A surface layer of 150–200 μm was melted by a CO2 CW laser beam. The microstructures of the laser-surface-melted specimens were characterized using optical, scanning, and transmission electron microscopy. Slow strain rate tests (SSRTs) at a strain rate of about 4×10−7 s−1 were carried out in a 0.1 M sodium tetrathionate (Na2S4O6) solution at room temperature in order to evaluate the resistance to IGSCC of the laser-surface-melted specimens. The microstructure of the melted and resolidified layer had a cellular/columnar structure which consisted of cells 1–2 μm in diameter. The thin, solidified-surface layer was observed to act as an excellent barrier to the initiation and propagation of stress corrosion cracking in the experimental conditions. The fracture mode of the sensitized Alloy 600 changed from a brittle intergranular fracture to a typical ductile transgranular failure. The improved IGSCC resistance of the laser-surface-melted specimens can be attributed in part to Cr redistribution at the boundaries of the cells and grains and in part to the elimination of microstructural inhomogeneities such as precipitates, inclusions at the grain boundaries, during rapid melting and resolidification by the laser surface treatment.