Abstract
Formation of spot defect is often observed on the surface of weld bead during the Tungsten inert gas welding of 18-8 stainless steel thin sheet.We investigated into the cause of this spot and discussed its formation mechanism from a viewpoint of non-metallic inclusion in the parent and weld metal.Results are summarized as follows:(1) The amount of inclusion in parent metal expressed as a cleanliness evaluated by point counting method has no effect on the occurrence of spot.(2) The inclusions in parent metal, which have high melting point and do not dissolve in molten pool, form the spot on the surface of weld bead.(3) On the other hand, the inclusions in parent metal, which have a large size, of more than 8-10 μ in mean dia., though with a low melting point, form the spot on the surface of weld bead.From these experimental evidences, we conclude that the cause of spot formation is related to the inclusions in parent metal, having a high melting point and/or a large size. And we illustrate the mechanism of spot formation: The inclusions in parent metal at first float from melt in molten pool, and then agglomerate on the surface of molten pool as a floater; when the floater grows to some extent, it is fixed at the end of the molten pool with tear drop type, and in situ the floater solidifies as a spot defect.
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