Grain boundary migration at surface of austenitic stainless steel weld metal

Abstract

Summary This paper describes an investigation of the grain boundary migration occurring at the weld metal surface, comparing the actually observed grain boundary migration and the results predicted by the thermal grooving theory of grain boundary migration based on the flow of surface atoms. The relationship between the solidification grain boundary plane angle in relation to the direction normal to the surface and the amount of grain boundary migration are discussed. The results obtained may be summarised as follows. 1. The grain boundary migration occurring at the weld metal surface differs in a number of important respects from that which occurs inside. Grain boundary migration is here basically observed to occur from the columnar crystals towards the axial crystals. In this case, the solidification grain boundary plane is inclined at less than around 40° in relation to the direction normal to the surface. The cross sectional shape of the grain boundary groove qualitatively resembles the analytical solution predicted by the thermal grooving theory of grain boundary migration based on the flow of surface atoms. The direction of grain boundary migration also agrees with the analytical solution. In the weld metal, grain boundary migration therefore occurs through being susceptible to the effect of the flow of surface atoms depending on the angle at which the solidification grain boundary plane is inclined in relation to the direction normal to the surface. 2. As the solidification grain boundary plane becomes increasingly inclined in relation to the direction normal to the surface, the amount of grain boundary migration at the weld metal surface increases. At a solidification grain boundary plane angle of less than around 40°, however, the amount of grain boundary migration at the surface sharply decreases. The grain boundary migration induced by the flow of surface atoms also only ever occurs near the surface. This is due to the fact that the grain boundary plane inside migrates in the wake of the grain boundary groove migration at the surface. That is to say, the grain boundary plane inside acts as a friction coefficient in relation to the velocity of migration itself. This friction coefficient is proportional to the third power of the grain boundary length (corresponding to the number of atoms migrating across the grain boundary plane). Incidentally, fewer atoms migrate across the inside grain boundary plane as the solidification grain boundary plane becomes increasingly inclined in relation to the direction normal to the surface, the velocity of grain boundary migration consequently increases and the amount of grain boundary migration increases.