The Effect of Trace Elements on the Formation of Slag Spots During Gas
The Effect of Trace Elements on the Formation of Slag Spots During Gas Tungsten Arc Welding of 316 L Stainless Steel Tube Systems
Presentation at Semi Stainless Steel Task Force Meeting July 14, 2003 – San Francisco, CA Jan Rau
Introduction Gas Tungsten Arc Welding (GTAW) is used in the manufacturing of longitudinally welded tubes and as a standard for the installation of tube systems in the semiconductor and phar- maceutical industries. During visual and/or boroscopic inspection of welds small grey spots might be conspicuous, especially in the downslope area of orbital welds. These slag spots are also referred to as oxide islands. Apart from arc instability, an irregular weld bead and reduced penetration [1] slag spots are often considered as potential initiation sites for pitting corrosion and potential sources of particulate contamination into a flowing medium.
1 Slag spots — formation and chemical properties 1.1 Chemical composition of slag spots Slags on welds of 304 and 316 stainless steels are generally composed of high melting point oxides of aluminium, calcium, magnesium, titanium, zirconium, cerium, silicon, chromium, manganese and iron [1]. In the majority of investigations carried out by the author slags on welds of 316 L stainless steel revealed high concentrations of aluminium, calcium and silicon and only small amounts of other metals. Fig. 1 shows a SEM1 image (magnification 100 ) of a typical small slag spot in the downslope area of a weld sample 38.1 1.65 mm (electropolished seamless tube, 316 L / 1.4404, heat 446612). Fig 2 shows the corresponding EDX2 spectrum. Evaluation of this spectrum led to the analysis as listed in table 1.