Effect of Al–Si coating layer on the penetration and microstructures of ferritic stainless steel, 409L GTA welds

In this study, commercial 409L ferritic stainless steels and Al–8 wt% Si alloy-coated 409L ferritic stainless steels were subjected to gas tungsten arc welding and the effect of the coated layer on the penetration properties and microstructure were investigated. Full penetration was obtained with a welding current greater than 90 A and a welding speed lower than 0.52 m/min. At the full penetration condition, the bead width of Al–8 wt% Si alloy-coated 409L ferritic stainless steels was narrower than that of commercial 409L ferritic stainless steels. In these samples, a narrow and deep weld pool was considered to have formed under gas tungsten arc welding due to Marangoni convection with existing oxides in the Al–Si coating layer. The grain size of both materials was the largest in the fusion zone and decreased from near the heat-affected zone to the base metal. Especially, the grain size of the heat affected zone and the fusion zone of Al–8 wt% Si alloy-coated 409L ferritic stainless steels were small compared to those of commercial 409L ferritic stainless steels. The hardness was maximized in the fusion zone and decreased from the bond line to the base metal. The maximum hardness in the fusion zone was attributed to the re-precipitation of the TiN and Ti(C, N), which were present in the base metal with the finer grain size during the melting and solidification processes. However, the maximum hardness of Al–8 wt% Si alloy-coated 409L ferritic stainless steels was relatively high due to the formation of oxides such as Al2O3 and SiO2 that originated from the dissolved elements from the coating layer.