Chapter 17 GMAW and FCAW: Flat Welding Position 255 Copyright Goodheart-Willcox Co., Inc. 2. Clean one edge of one piece and one surface of the other piece. 3. Select the desired welding electrode, machine settings, and shielding gas. Set up the machine for the metal transfer method you will be using. Adjust the flow rate. 4. Tack weld the clean edge to the clean surface in three places on each side. 5. Place the weldment so the weld is in the flat position, Figure 17-12. 6. Make a fillet weld on each side of the T-joint. Watch the weld pool as it forms. Keep a C-shaped pool. Inspection: The weld should be straight with evenly spaced ripples. Undercutting or overlap should not be visible. Welding a Butt Joint A butt joint can be formed with a square groove, or the edges can be prepared with a groove, such as a V-groove. Steel that is 3/16″ (4.8 mm) or thicker usually requires a prepared groove. Grooves used in GMAW and FCAW have a smaller angle than grooves used in SMAW. Full penetration of butt joints is usually required. A root opening (gap) is left between the two pieces of base metal when tack welding them. The arc must melt the edges of both pieces when welding. The root of the weld pool looks like a keyhole when welding. The keyhole is seen only on the root pass (fi rst weld pass) of a multiple-pass groove weld. A keyhole at the root opening indicates that both pieces of base metal are being melted. It also indicates that the molten metal is fl owing through the joint, resulting in full penetration. See Figure 17-13. To produce consistent penetration, the size of the pool and the keyhole must be kept constant as the weld progresses. A backing may be used to help produce consistent penetration. Backing is attached to the root side of the joint to control penetration. Backings can be used in all welding positions. Some backings become part of the completed joint, but most are removed after welding. Removable metal backings can be fl at or have a radius machined in them. See Figure 17-14. Removable metal backings are not Goodheart-Willcox Publisher Figure 17-13. Notice the keyhole at the leading edge of the weld crater. The keyhole shows both pieces have been melted. Keyhole Goodheart-Willcox Publisher Figure 17-14. This backing has a round groove milled into it. The groove ensures uniform penetration. A copper plate is used for welding steel. The metal to be welded is shown in gray. Weldment Machine penetration groove Backing