Correlating the weld-bead's ‘macro-, micro-features’ with the weld-pool's ‘fluid flow’ for electron beam welded SS 201 plates

Abstract

The phenomenological modeling, a CFD-based simulation tool, was implemented to study the nature and magnitude of fluid flow within the weld zone (WZ) at varying heat input conditions, and further, analyzed its effect on the macroscopic (bead geometry and spiking) and the microscopic (hardness and ferrite arm spacing) features of the WZ during electron beam welding of SS201 plates. Upon calculation of the fluid flow parameters, the mean flow velocity ‘Vm’, Reynolds number, Peclet number, and Grashof number of the molten fluid in the weld-pool were observed to increase by around 3 times with the increase in heat input. The fluid convection was found to be a dominant factor in determining the mode of heat transfer. The weld-bead geometrical features, namely depth and width, were determined by the fluid flow velocities ‘(Vf)depth or width’ along the respective axes. The micro-hardness (MH) and secondary ferrite arm spacing (SFAS) were observed to have an inverse relationship. The increment in SFAS by approximately 1.3 times, due to the (a) decrease in the weld zone cooling rate by around 2.5 times and (b) increase in the coarsening coefficient by approximately 2 times because of Cr diffusion at the higher mean flow velocity ‘Vm’, was observed. Upon microstructural investigation, it was found that the slower cooling rate through the transformation temperature zones favored the formation of vermicular δ-ferrite in the austenite ‘γ’ phase matrix in the WZ, and thus, MH of base metal was observed to be more than that of the weld zone. The spiking amplitude was observed to be influenced by the gap generated between the capillary tip and the weld-pool bottom, and the velocity of approach ‘Va’ of the molten fluid, and the same was found to increase by around 2.5 times with the increase in the melting efficiency by approximately 1.4 times.