Transcript Chapter 17
Chapter 17 Gas Tungsten Arc Welding of Pipe © 2012 Delmar, Cengage Learning Objectives • Describe how a pipe joint is prepared for welding • List the four most common root defects and the causes of each defect • Discuss when and why a backing gas is used • Explain the uses of a hot pass • Sketch a single V-groove and indicate the location and sequence of welds for each position • Make a single V-groove butt welded joint on a pipe in any position © 2012 Delmar, Cengage Learning Introduction • GTA welding of pipe – Used when the welded joint must have a high degree of integrity • One that is strong and free from defects • Industries such as oil, gas, and nuclear, require this kind of joint • Welders skilled in GTA welding process – Make consistently high-quality welds • Low rejection rate © 2012 Delmar, Cengage Learning Introduction (cont'd.) • GTA pipe welders – Among the highest-paid workers in the industry – Positions available are among the safest and most prestigious – Excellent working conditions – Requires high levels of skill and technical knowledge – Requires practice and study © 2012 Delmar, Cengage Learning Practices • Practices in this chapter – All performed on mild steel pipe • Most readily available material on which to learn – Skill and techniques learned can be easily transferred to any piping material © 2012 Delmar, Cengage Learning Joint Preparation • Pipe ends must be grooved before welding – Type of groove depends on the pipe material, thickness, and application • Most often used grooves on mild steel – – – – Single V-groove Single U-groove Single J-groove Single-bevel groove © 2012 Delmar, Cengage Learning FIGURE 17-2 Grooves used for pipe-to-pipe and pipe-to-flange welds. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning Joint Preparation (cont'd.) • Pipe end is prepared with a 37½ degree bevel – When both pipes are prepared in this manner, they form a 75 degree single V-groove – Grinding or machining the sharp edge on the pipe after beveling produces the root face – Welding surfaces must be cleaned and smoothed Before the joint is assembled © 2012 Delmar, Cengage Learning Joint Preparation (cont'd.) • Pipe should be tack welded together – Root opening of 3/32 inch to 1/8 inch – Make four or five tack welds evenly spaced around the pipe – Tack welds should not interfere with starting or stopping the root pass • Starting or stopping on a root pass can cause poor penetration © 2012 Delmar, Cengage Learning FIGURE 17-7 Location of tack welds on small diameter pipe. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning Root • Deepest point into the joint where fusion occurs – Root penetration • Distance between original surface and deepest point of fusion – Root reinforcement • Metal deposited on back side of a welded joint • Common root defects – Incomplete fusion and root contamination – Concave root surface – Excessive root reinforcement © 2012 Delmar, Cengage Learning Incomplete Fusion • Weld does not completely penetrate the joint – May be a lack of fusion on one or both sides of the root – Conditions caused by not having enough heat penetrating the back side of the work – Can cause a stress point – Incomplete fusion is discovered by a root-bend test © 2012 Delmar, Cengage Learning Concave Root Surface • Back side of root weld is concave in shape – Common causes • Insufficient filler metal • Excessive heat is used in overhead position – Results in reduced weld thickness © 2012 Delmar, Cengage Learning Excessive Root Reinforcement • Excessive buildup of metal on back side of the weld – Uneven buildup and burn-through are sometimes called grapes – Cause stress points that result in weld failure – Root of weld reinforcement should not exceed 1/16 inch © 2012 Delmar, Cengage Learning Root Contamination • Back side of molten weld pool can be contaminated causing: – – – – Porosity Embrittlement Oxide inclusions Oxide layers • Root contamination leads to faster corrosion, oxide flaking, leaks, etc. © 2012 Delmar, Cengage Learning FIGURE 17-12 Root contamination. Larry Jeffus © 2012 Delmar, Cengage Learning Backing Gas • Protects root from atmospheric contamination – Type of gas depends on type of pipe • Argon can be used to back any type of pipe – Gas must have enough time to purge the pipe completely of air • Flow time = volume ÷ flow rate © 2012 Delmar, Cengage Learning Filler Metal • Added by dipping or by preplacement – Dipping is easier if rod is bent to the radius of the pipe – Preplacing the filler rod allows the root and filler passes to be made without rod manipulation © 2012 Delmar, Cengage Learning FIGURE 17-15 Filler rod curved to surface of pipe for ease in welding. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning Consumable Inserts • Preplaced filler metal – Used for root pass when high-quality welds are required • Reduce repairs when welding in difficult conditions • Classified by cross-sectional shape – – – – – A-shape J-shape Rectangular shape Y-shape Rectangular shape (contoured edges) © 2012 Delmar, Cengage Learning FIGURE 17-17 Specialized backing rings. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning Cup Walking • Most often used by pipe welders – Reduced welder fatigue – Better welding torch control – Longer and higher weld quality • Setup – Tungsten length: set so it extends out of the nozzle no more than the nozzle opening diameter • Technique – Nozzle and filler metal are rested on the pipe joint © 2012 Delmar, Cengage Learning FIGURE 17-23 The maximum recommended tungsten extension for cup walking. FIGURE 17-24 Rest the nozzle on the groove edges. © Cengage Learning 2012 © Cengage Learning 2012 © 2012 Delmar, Cengage Learning A B C FIGURE 17-26 Moving the top of the torch in a figure-8 pattern (A) first lifts one side of the cup (B) and lets you move it slightly forward. Then the other side of the cup is lifted (C), and it is moved forward. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning Practice Welds • Practice welds in this chapter – Should be made using the traditional method of adding filler metal and the cup walking technique • Both techniques are important © 2012 Delmar, Cengage Learning Hot Pass • Corrects problems caused by a poor root pass – Concave root surfaces: used to add needed metal and heat • Watch molten weld pool surface to see when it sinks slightly – Correcting incomplete fusion: used without adding more filler metal © 2012 Delmar, Cengage Learning FIGURE 17-32 A hot pass can be used to correct incomplete root fusion or excessive concavity of the root surface. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning Filler Pass • Weld layer made after hot pass – Fills the groove quickly – Should be complete fusion • Little penetration – Often only one GTA filler pass is used to protect the thin root pass from burn-through © 2012 Delmar, Cengage Learning Cover Pass • Stringer bead can be continued to cap the weld – Bead should overlap the pipe surface • No more than 3/32 inch – Visually inspect weld • Uniformity in width and reinforcement • Defects © 2012 Delmar, Cengage Learning Summary • GTA welding of pipe – Considered the most prestigious of the welding processes • Obtained this status because of where much of the GTA welding of pipe is done – Constantly changing weld position is the most challenging part – Light touch is needed to learn to slide ceramic cup • Cup rocking technique can produce welds that appear machinelike in quality © 2012 Delmar, Cengage Learning