SLAG INCLUSIONS PDF Intermediate Weld Discontinuities

WSS là Guide WD2.1 Intermediate Weld Discontinuities Stray arc strikes, either with the electrode or holder, are more serious than expected. They may create a quenched and embrittled condition in the weld zone of alloy steels and are inadvisable even on mild steel, where high static or normal fatig...

WSS là Guide WD2.1 Intermediate Weld Discontinuities Stray arc strikes, either with the electrode or holder, are more serious than expected. They may create a quenched and embrittled condition in the weld zone of alloy steels and are inadvisable even on mild steel, where high static or normal fatigue stresses may be encountered. Repair of such damage may be difficult and costly, involving grinding and probably preheating in the case of alloys. 2.16 Slag Inclusions Slag Inclusions are oxides and other nonmetallic solids that are sometimes found as elongated or multifaceted inclusions in welds. Slag is always produced when welding with covered electrodes, tubular flux cored electrodes and submerged arc electrodes/flux, serving as scavengers of impurities in the molten metal pool. In addition, it forms a blanket over the weld to control cooling rate and exclude atmospheric oxygen and nitrogen from the hot metal surface. During the welding process, fluxes from slag are forced below the surface of the molten metal by the stirring action of the arc. While the weld metal remains molten, slag generally has time to float to the surface of the weld due to its lower density. A number of factors may affect the ability of the slag to rise to the surface of the weld resulting ïn slag that is trapped subsurface in the weld metal. Some of these factors are: $ $ High viscosity weld metal Rapid solidification of weld metal (heat input too low for the application) $ $ $ Improper manipulation of the electrode Improper interpass cleaning Weld discontinuities from a previous weld pass Rapid solidification of the weld metal sometimes does not allow enough time for slag to float to the surface of the weld metal. This is usually due to incorrect setting of welding parameters, resulting in a heat input that is too low for the application. High-speed welding applications result in low heat inputs and fast cooling, and procedures need to be developed to ensure that slag has adequate time to escape. Page 45 Copyright © 2015 CWB Group Industry Services An Iniative øf thế WSS Study Guide WD2.1 . cwbgroup Intermediate Weld Discontinuities z —- Covered electrode \ Slag rising to HN surface of slag solidifles weld pool Plus Core wire Droplets of filler metal coated with flux or slag entering molten pool through arc Base metal Previously deposited weld bead MPRenGiog an. riG.36| Formation oƒ slag m In multipass welding, insufficient cleaning between weld passes can leave portions of the slag coating in place, which is then covered by subsequent Dasses. Such slag inclusions are often characterized by their location at the edge of the underlying metal deposits, where they tend to extend ^ longitudinally along the weld. Slag lines can be either intermittent or continuous. lí the prior pass produces a bead that is too convex, or ïf the arc has undercut the joint face, ït will be difficult to remove the slag between the groove and the ^ = deposited metal. When the slag is left in place, it is covered by subsequent passes (see Figures 37). Trapped Slag Convex bead Undercui FiG.37) Trapped slag | WSS là Guide WD2.1 Intermediate Weld Discontinuities The proper choice of welding electrode is also important as some electrodes are specially formulated. For example, flux cored electrodes designed for out-of-position welding have a fast freezing slag system. The use of these electrodes for high-speed welding applications may result in slag inclusions. The proper choice of electrodes for high-speed welding are those designed for welding in the flat and horizontal positions with more fluid slag systems. Selecting the wrong size of electrode may also result in slag inclusions. In making a root pass, the electrode may be so large that the arc strikes the side of the groove instead of the root. The slag will roll down into the root opening and become trapped under the root layer. This is because the arc failed to heat the root area to a sufficiently high temperature to allow the slag to float to the surface. Slag inclusions are usually trapped below the surface and require NDT methods capable of detecting below-surface discontinuities. The majority of slag inclusions may be prevented by proper preparation of the groove before each bead is deposited (including sufficient cleaning) and using care to correct any contour that would be difficult to fuse fully with the arc. Page 4 Copyright © 2