INVESTIGATION OF IGSCC BEHAVIOR OF SENSITIZED AND LASER-SURFACE-MELTED ALLOY-600

An attempt was made to modify the surface of sensitized Alloy 600, wit hout affecting the bulk properties, by a laser surface melting (LSM) t echnique in order to improve its resistance to intergranular stress co rrosion cracking (IGSCC) in oxidizing environments. A surface layer of 150-200 mu m was melted by a CO2 CW laser beam. The microstructures o f the laser-surface-melted specimens were characterized using optical, scanning, and transmission electron microscopy. Slow strain rate test s (SSRTs) at a strain rate of about 4 x 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-mel ted specimens. The microstructure of the melted and resolidified layer had a cellular/columnar structure which consisted of cells 1-2 mu m i n diameter. The thin, solidified-surface layer was observed to act as an excellent barrier to the initiation and propagation of stress corro sion 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 resistanc e of the laser-surface-melted specimens can be attributed in part to C r redistribution at the boundaries of the cells and grains and in part to the elimination of microstructural inhomogeneities such as precipi tates, inclusions at the grain boundaries, during rapid melting and re solidification by the laser surface treatment. (C) 1998 Elsevier Scien ce S.A. All rights reserved.