Thermal etching and smoothing of weld fracture surfaces at high temperatures

Abstract

When a fracture surface is exposed to high temperatures, its original features may be smoothed by surface diffusion, and new features such as thermal facets and grain-boundary grooves may form. Observations of these features on weld crack surfaces have frequently been taken to indicate crack formation at high temperature. However, when cracks form at low temperatures during multipass welding, the cracked material may be reheated to moderate temperatures during the deposition of later weld beads, and it has been suggested that this could also cause thermal etching. To investigate this possibility, the diffusional surface shape changes which can occur during weld thermal cycles are analysed. Calculated surface-feature sizes are then compared to those observed on fracture-surface replicas by transmission electron microscopy. These comparisons are used to discuss ‘chevron’ cracking in multipass submerged-arc welds, microcracking in a single-pass tungsten inert gas weld, and smoothing of fractographic features by heat treatment.