Advanced Shielded Metal Arc Welding Chapter 6 Advanced Shielded Metal Arc Welding

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

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 E7018 electrodes A cover bead in all positions

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

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

Introduction (cont'd.) Root pass Hot 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

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

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

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

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

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

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

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

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

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

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

Filler Pass Fills groove after root pass Characteristics 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

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

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

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

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

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

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

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

Preparing Specimens for Testing Maximum allowable size for fissures in a guided-bend test Given in codes for specific applications Some standards are listed ASTM E190 AWS QC10 AWS QC11 Copies available from appropriate organization

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

Acceptance Criteria for Face Bends and Root Bends (cont'd.) Specimen cut out 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

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

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

Preheating and Postheating Application of heat to metal before welding Helps to reduce: Cracking Hardness Distortion Stresses

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

Preheating and Postheating (cont'd.) 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

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

Summary Grooved welds on one-half inch thick plate Grooved welds 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