Grain boundary migration at surface of austenitic stainless steel weld metal

摘要

This paper describes an investigation of the grainboundary migration occurring at the weld metal surface,comparing the actually observed grain boundary migration and theresults predicted by the thermal grooving theory of grain boundarymigration based on the flow of surface atoms. The relationshipbetween the solidification grain boundary plane angle in relation tothe direction normal to the surface and the amount of grainboundary migration are discussed. The results obtained may besummarised as follows:1 The grain boundary migration occurring at the weld metalsurface differs in a number of important respects from that whichoccurs inside. Grain boundary migration is here basically observedto occur from the columnar crystals towards the axial crystals. Inthis case, the solidification grain boundary plane is inclined at lessthan around 40°in relation to the direction normal to the surface.The cross sectional shape of the grain boundary groovequalitatively resembles the analytical solution predicted by thethermal grooving theory of grain boundary migration based on theflow of surface atoms. The direction of grain boundary migrationalso agrees with the analytical solution. In the weld metal, grainboundary migration therefore occurs through being susceptible tothe effect of the flow of surface atoms depending on the angle atwhich the solidification grain boundary plane is inclined in relationto the direction normal to the surface.2 As the solidification grain boundary plane becomes increasinglyinclined in relation to the direction normal to the surface, theamount of grain boundary migration at the weld metal surfaceincreases. At a solidification grain boundary plane angle of lessthan around 40°, however, the amount of grain boundarymigration at the surface sharply decreases. The grain boundarymigration induced by the flow of surface atoms also only everoccurs near the surface. This is due to the fact that the grainboundary plane inside migrates in the wake of the grain boundarygroove migration at the surface. That is to say, the grain boundaryplane inside acts as a friction coefficient in relation to the velocityof migration itself. This friction coefficient is proportional to thethird powder of the grain boundary length (corresponding to thenumber of atoms migrating across the grain boundary plane).Incidentally, fewer atoms migrate across the inside grain boundaryplane as the solidification grain boundary plane becomesincreasingly inclined in relation to the direction normal to thesurface, the velocity of grain boundary migration consequentlyincreases and the amount of grain boundary migration increases.