Chapter 6 Advanced Shielded Metal Arc Welding 2012

Chapter 6 Advanced Shielded Metal Arc Welding © 2012 Delmar, Cengage Learning

Objectives • Discuss how metal must be prepared before welding • Describe the process, and demonstrate making the root pass, filler weld, and cover pass in all positions and techniques • Explain the purpose of a hot pass • Tell what should be checked with a visual inspection, and describe the appearance of an acceptable weld © 2012 Delmar, Cengage Learning

Objectives (cont'd. ) • Demonstrate how to make: – A root pass on plate in all positions – A root pass on plate with an open root in all positions – An open root weld on plate using the step technique in all positions – A multiple pass filler weld on a V-joint in all positions using E 7018 electrodes – A cover bead in all positions © 2012 Delmar, Cengage Learning

Objectives (cont'd. ) – A single V-groove open root butt joint with an increasing root opening – A single V-groove open root butt joint with a decreasing root opening – SMAW welds of plate to plate – SMAW welds of pipe to pipe © 2012 Delmar, Cengage Learning

Introduction • SMAW process produces high-quality welds – Welders frequently make welds in difficult situations to a code or standard • Metal edges must be prepared – 100% joint penetration – Code-quality welds – Metal thicker than ¼ inch • Preparation improves strength – Prepared joints require more than one weld pass © 2012 Delmar, Cengage Learning

Introduction (cont'd. ) • Root pass – Fuses and seals parts together • Hot pass – Improves weld contour • A test for one company may not qualify a welder for another company – AWS Certified Welder program • Available from AWS's office in Miami, Florida © 2012 Delmar, Cengage Learning

Root Pass • First of a multiple pass weld – Fuses two parts together – Establishes depth of weld metal penetration – Needed to obtain a sound weld – May be open or closed – Can use a backing strip or backing ring • Backing strip used in a closed root can remain as part of the weld or be removed • Removable backup tapes have been developed • Tape can be peeled off after weld is completed © 2012 Delmar, Cengage Learning

FIGURE 6 -2 Root pass maximum deposit 1/4 in. (6 mm) thick. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning

Root Pass (cont'd. ) • Widely used in plate and pipe designs – Face side is not as important as root surface on back or inside – If root surface is correct: front side can be ground, gouged, or burned out • Weld is evaluated from root side only – Root face for most grooves will be about the same size © 2012 Delmar, Cengage Learning

FIGURE 6 -4 Using back gouging to ensure a sound weld root. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning

Root Pass (cont'd. ) • Control penetration on joints with varying root gaps – Stepping electrode manipulation – Key hole • • Electrode is moved in and out of molten weld pool Metal flows through key hole to root surface Key hole ensures 100% penetration Process requires more welder skill © 2012 Delmar, Cengage Learning

FIGURE 6 -7 Electrode movement to open and use a key hole. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning

Hot Pass • Surface of a root pass may be irregular • High-strength code welds – Root pass and each filler pass must be ground • Grinding is important when high-strength, low alloy welding electrodes are used • Hot pass – Cleans out trapped slag – Makes a root pass more uniform – Uses high amperage and a fast travel rate – Rapidly melts a large surface area © 2012 Delmar, Cengage Learning

Hot Pass (cont'd. ) • Small amount of metal should be deposited – Resulting weld is concave • Concave weld is easier cleaned • Failure to clean a convex root leaves wagon tracks • Can be used to repair or fill small spots – Incomplete fusion or pinholes • Normal weave pattern – Straight step or “T” pattern © 2012 Delmar, Cengage Learning

FIGURE 6 -19 Slag trapped between passes will show on an X ray. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning

Hot Pass (cont'd. ) • Key points – Do not allow molten weld pool to cool completely – Do not blow shielding gas covering away from the molten weld pool – Penetration of the molten weld pool must be deep • Free all trapped slag • All porosity must be burned out © 2012 Delmar, Cengage Learning

Filler Pass • Fills groove after root pass – Made with stringer or weave beads • Characteristics – Weld beads must overlap • Stringer beads overlap 50% • Weave beads overlap 25% – Finish bead is smooth – Each weld bead must be cleaned before the next bead is started © 2012 Delmar, Cengage Learning

FIGURE 6 -24 Filler pass buildup sequence. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning

Filler Pass (cont'd. ) • Ways to remove slag between filler weld passes – Chipping, wire brushing, and grinding • Weld can be checked by ultrasonic or radiographic nondestructive testing – Most schools are not equipped to do this testing – Check soundness by destructive testing © 2012 Delmar, Cengage Learning

Cover Pass • Last weld bead on a multipass – May use a different electrode weave – Must be uniform and neat looking – Appearance might be the only factor in accepting or rejecting welds – Should not be more than 1/8 inch wider than the groove opening © 2012 Delmar, Cengage Learning

FIGURE 6 -26 The cover pass should not be excessively large. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning

Plate Preparation • Weld groove prepares the plate – Can be cut into one side or both sides – May be cut into one or both plates of the joint • Depth, angle, and location – Determined by a code standard • SMA welds on plate 1/4 inch or thicker that need to have a weld with 100% joint penetration – Plate must be grooved • May be ground, flame cut, gouged, or machined © 2012 Delmar, Cengage Learning

Plate Preparation (cont'd. ) • Bevels and V-grooves – Best if cut before parts are assembled • J-grooves and U-grooves – Can be cut either before or after assembly • Groove on both sides – Tee joints, welds with little distortion, and welds that will be loaded equally from both sides • Back gouging – Cuts a groove in back side of a joint that has been welded © 2012 Delmar, Cengage Learning

FIGURE 6 -29 Typical butt joint preparations. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning

Preparing Specimens for Testing • Maximum allowable size for fissures in a guidedbend test – Given in codes for specific applications • Some standards are listed – ASTM E 190 – AWS QC 11 • Copies available from appropriate organization © 2012 Delmar, Cengage Learning

Acceptance Criteria for Face Bends and Root Bends • Key points – Weld is uniform – No arc strikes on the plate other than the weld – Free of incomplete fusion and cracks – Penetration must be 100% or as specified – Weld must be free of overlap – Correct weld specimen preparation is essential © 2012 Delmar, Cengage Learning

Acceptance Criteria for Face Bends and Root Bends (cont'd. ) • Specimen cut of test weldment – Abrasive disc – Sawing – Cutting with a torch • Flame-cut specimens – Grind or smooth the edges • All corners must be rounded – Radius of 1/8 inch maximum – Grinding or machining marks must run lengthwise © 2012 Delmar, Cengage Learning

Restarting a Weld Bead • Welding bead must be restarted – After stopping to change electrodes • Weld bead near completion – Should be tapered • Increase the travel rate • Before restarting – Chip slag and clean weld crater • Restart the arc in the joint ahead of the weld – Electrodes must be allowed to heat up © 2012 Delmar, Cengage Learning

Restarting a Weld Bead (cont'd. ) • Movement to root of weld and back up on bead – Builds up weld – Reheats metal • Avoid starting and stopping weld beads in corners – Tapering and restarting are especially difficult in corners • Often results in defects © 2012 Delmar, Cengage Learning

Preheating and Postheating • Preheating – Application of heat to metal before welding – Helps to reduce: • • Cracking Hardness Distortion Stresses © 2012 Delmar, Cengage Learning

Preheating and Postheating (cont'd. ) • Preheating is often required: – On large thick plates – When plate is very cold – When temperature is very cold – When using small diameter electrodes – On high-carbon or manganese steels – On complex shapes – With fast welding speeds © 2012 Delmar, Cengage Learning

Preheating and Postheating (cont'd. ) • Postheating – Applies heat to metal after welding – Used to slow the cooling rate – Reduces hardening • Interpass temperature – Temperature of metal during welding – Given as a minimum and maximum © 2012 Delmar, Cengage Learning

Poor Fitup • Some welding must be done on joints that are poorly fitted – Requires a good welder – Skilled welders can watch the molten weld pool and knows how to avoid disaster • Considerations – Amperage setting may have to be adjusted – May be necessary to break and restart the arc – May need to change the electrode angle © 2012 Delmar, Cengage Learning

Summary • Grooved welds on one-half inch thick plate – Most common test plates • Grooved welds – Used by many companies in testing • Vertical and overhead positions – Most common positions used in testing • Visually defect-free welds – Assumed to pass destructive testing – Always make welds as uniform as possible © 2012 Delmar, Cengage Learning