Chapter 6 Shielded Metal Arc Welding Objectives Describe

Chapter 6 Shielded Metal Arc Welding

Objectives • Describe how and why electricity flows in an electrical circuit and list the three variables in Ohm’s law. • Demonstrate your ability to correctly complete a safety inspection on an arc welding station. • Plan and list all the hand tools, personal safety equipment, and clothing required for a given welding task. • Demonstrate your ability to select the correct electrode, current, and polarity for welding with the SMAW process in a given welding position. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Objectives • Demonstrate your ability to strike a welding arc and produce acceptable stringer and weaving beads. • Demonstrate your ability to correctly restart the arc and blend the new and old beads. • Demonstrate your ability to use the SMAW process to create acceptable welds on all five basic joints in all positions. • Describe all types of weld defects and demonstrate your ability to inspect and differentiate between acceptable and unacceptable welds. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Objectives • Successfully pass a safety test as related to arc welding work areas, equipment, clothing, tools, and correct welding procedures in all positions. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Shielded Metal Arc Welding • Shielded metal arc welding (SMAW) is a commonly used welding process • Both alternating and direct current can be used for SMAW • With the correct size electrode, the heat generated can melt any weldable metal • SMAW is used in many industries and applications © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Direct Current Arc Welding Fundamentals • Arc welding is a group of welding processes that melt the base metal using heat from an electric arc • SMAW is done by producing an arc between the base metal and a consumable, flux-covered metal electrode • Dc arc welding power sources are called welding machines © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Direct Current Arc Welding Fundamentals • Ohm’s law for electricity states that voltage in a closed circuit has a constant relationship to the current and the resistance of the current • This illustration shows a diagram of a DCEN arc welding circuit © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Constant Current Power Sources • Constant current power sources are drooper-type power sources • A welder using SMAW must try to keep a constant arc length • The current changes slightly as the arc length changes • To make a good quality weld, the welder must find and keep the proper arc length © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Constant Current Power Sources • The welding arc is seen to be divided into two parts: the stream and the arc flame • The vaporized metal in the arc stream appears yellow • Liquid metal in the arc stream appears green © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

DCEN and DCEP Fundamentals • In a direct current electrode negative (DCEN) circuit, the electrons flow from the negative terminal of the machine to the electrode • This process was formerly called direct current straight polarity (DCSP) • It is sometimes necessary to reverse the polarity in the arc welding circuit © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

DCEN and DCEP Fundamentals • When the electrons flow from the negative pole of the arc welding machine to the base metal, this circuit is direct current electrode positive (DCEP) • This was previously called direct current reverse polarity (DCRP) • The choice of when to use DCEN or DCEP is determined by the electrode being used © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

DCEN and DCEP Fundamentals • DCEP produces better penetration than DCEN • Base metal thickness will affect which polarity is required • This illustration shows a diagram of a DCEP arc welding circuit © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Alternating Current Arc Welding Fundamentals • Several types of arc welding machines are used – Transformer-type, as shown in the illustration – Inverter – Motor- and engine-driven generator (Century Mfg. Co. ) © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Alternating Current Arc Welding Fundamentals • The current’s frequency is measured in cycles per second, or hertz (Hz) • The current in the U. S. A. is 60 Hz current • In most parts of the world, 50 Hz current is used © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Alternating Current Arc Welding Fundamentals • A method used to stabilize the ac arc is to increase the ionization of the material in the arc • Arc welding requires electrodes designed for use with alternating current • Welds performed with ac electrodes show good penetration © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Selecting an Arc Welding Machine • Selecting the type of current to use should be done after considering the advantages and disadvantages • Characteristics of a dc constant current-type arc welding machine – The ability to choose DCEP or DCRP – Can be used in positions other than flat – Electrodes designed to weld nickel, aluminum, and copper generally use DCEP – The ability to choose DCEN or DCSP © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Selecting an Arc Welding Machine • A disadvantage is that a dc arc welder is generally more expensive than a similar ac arc welding machine • Advantages of ac constant current arc welders – Welds have moderate penetration – Greater filler metal deposition rates and faster welding speeds – Machines are generally less expensive than similar dc arc welding machines © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Selecting an Arc Welding Machine • The major disadvantage of ac arc welding machines is that not all SMAW electrodes can be used with alternating current • Welding machines capable of both ac and dc operation are available © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Inspecting an Arc Welding Station • The arc welding station includes several parts – – – Arc welding power source Electrode lead and terminals Workpiece lead and terminals Electrode holder Workbench Ventilation © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Inspecting an Arc Welding Station • Before beginning to weld, check all parts of the arc welding station • Check that the electrode and workpiece leads are tightly attached to the machine • Inspect the electrode holder and make sure the handle is not cracked • Turn on the ventilation system to see that it is working © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Safety, Protective Clothing, and Shielding • Arc welding should be performed using the proper safety equipment • There are several hazards to be avoided – – – Radiation, such as ultraviolet rays, from the arc Flying sparks and small pieces of molten metal Electric shock Fumes Burns © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Safety • Helmets or handheld face shields with approved lenses must be worn to protect eyes from arc radiation • Never look at an arc from any distance unless your eyes are protected by approved filter lenses • Wear gloves and protective clothing to shut out the rays from the arc © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Safety, Protective Clothing, and Shielding • The arc welding operation should be shielded so that no one can look directly at the arc • It is advisable to wear suitable clothing to protect against flying sparks • Electric shock hazards can be avoided by working on a dry floor and wearing dry gloves • The health hazard from fumes can be avoided by using proper ventilation equipment © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Safety, Protective Clothing, and Shielding • To protect against burns from hot metal, welders should wear leather gloves © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Safety • Do not carry flammable materials, such as matches or lighters, when welding • Wear high shoes with safety toes • Avoid using arc welding equipment in wet or damp areas • Perform welding in an area that is well ventilated • Handle hot metal with tongs or pliers © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Safety • Before beginning to weld, inspect the complete arc welding station to make certain it is safe for use © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Arc Welding Power Source • An arc welding machine should never be started or stopped under load • Arc welding machines powered by ac are easy to start and stop using an on-off switch or buttons • Constant current-type arc welding machines are used for manual arc welding processes • The desired current is set on the machine © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Arc Welding Power Source • Amperage controls vary in appearance, location, and operation on various machines • Some machines have both coarse and fine current adjustments © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Selecting the Proper Electrode • Several factors should be considered when selecting an electrode for SMAW – – – – – Weld groove design Required tensile strength Base metal composition Position of the weld joint Rate at which to deposit weld metal Type of arc welding current used Penetration required Metal thickness Experience of the welder Specifications for the weld to be made © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Selecting the Proper Electrode • When a groove weld is made, the electrode must be small enough to manipulate at the root of the weld • The metal composition of the base metal will determine the metal composition of the electrode used (American Welding Society) © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Selecting the Proper Electrode • The welding position will determine the electrode used • Electrodes are made to be most effective with one type of welding current • Whenever a qualified welding procedure specification is used, the diameter and type of electrode are specified © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Striking the Arc • To strike an arc, the electrode must first touch the base metal to complete the circuit • The electrode is then pulled back and the current jumps to gap to create the arc • First attempts to strike an arc may cause the electrode to stick • Another problem may occur from withdrawing the electrode too far © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Striking the Arc • There are two common methods of striking an arc – Use a glancing or scratching motion with the end of the electrode – Use a straight down-and-up motion or pecking motion with the electrode © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Running a Bead • The first skill that a welder must master is the ability to run (form) a bead • The arc length must be varied slightly as different electrode diameters are used • When small, solidified metal drops are seen on the base metal surface, spattering is occurring © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Running a Bead • Stringer beads are narrow weld beads made without oscillating the weld pool • Weave beads are wide weld beads made by moving the weld pool side to side as it progresses along the joint © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Running a Bead • The arc should always be struck about 3/8″ ahead of where the bead should begin • The speed of forward motion is judged by two factors when welding – The bead width – The bullet-nose shape of the ripples at the rear of the molten weld pool © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Running a Bead • Weld reinforcement is the amount of buildup above the surface of the base metal • The proper current setting is important to make a quality weld or bead • A practical application of weld beads is the rebuilding of worn surfaces • Another application of bead work is hard-surfacing or wear-resistant surfacing © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Restarting and Finishing a Bead • When a SMAW bead is stopped prior to completion, a deep crater is left in the base metal • Restarting the arc and completing the bead must be done with care • When a SMAW electrode is stopped, it may be easy or difficult to restart an arc with the electrode © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Restarting and Finishing a Bead • Low-hydrogen electrodes are more difficult to restart • There are two ways to finish a bead without leaving a crater – Use a run-off tab – Reverse the electrode direction as the end of the weld is reached • A weld bead can be started on a run-on tab and continued onto the weldment © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Cleaning the Bead • When shielded metal electrodes are used, a brittle slag coating is left on the weld bead • This slag must be removed prior to restarting a bead • If the slag is not removed, the resulting weld will have slag inclusions © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

DC Arc Blow • A dc arc may have a tendency to wander from the weld line • This wandering is called arc blow and is one of two types – Forward arc blow – Backward arc blow • If the arc blow is very strong, preventive or corrective measures can be taken © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

DC Arc Blow • A welder can use the backstep method to create a continuous weld by performing a number of short welds © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Arc Welding Joint Designs • Shielded metal arc welding can be done on any of the basic joint designs • Weld joints may be in any position – – Flat welding position Horizontal welding position Vertical welding position Overhead welding position © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Weld Flaws and Defects • Completed welds may have a variety of flaws or imperfections • If a flaw is large, it is called a defect • A weld may have a properly contoured face or an undercut condition © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Weld Flaws and Defects • If the bead has a number of small pit holes, this indicates porosity • Other surface flaws may be seen during a visual inspection – Spatter – Slag inclusions – Cracks in the weld bead or weld crater • If a defect is found, the weld must be repaired © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

SMAW Welding Techniques • Shielded metal arc welding is a skill • A number of variables must be controlled to make a good weld – – Arc length Travel speed Travel and work angles Electrode motion © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Edge Joint in Flat Welding Position • An edge joint may be welded in any position • On thin metal, no edge preparation is needed • On thicker pieces of metal, the edge should be prepared to provide a bevel- V-, U-, or J-groove • A piece of the same metal being welded should be used for practice © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Edge Joint in Flat Welding Position • Run test beads and reset the arc welding machine current until the desired bead is achieved • A tack weld is a small, well-fused weld used to hold parts in proper alignment © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Lap Joints in Flat Welding Position • Lap joints are commonly made in flat or horizontal welding positions • Fillet welds are used with lap joints • To weld a lap joint, the electrode should have a 20° drag travel angle and about a 45° work angle © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Lap Joints in Flat Welding Position • The fillet weld on a lap joint is made on the edge of one piece and the surface of the other piece • The finished bead should have the proper contour, be straight, and have a consistent width © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Corner and T-Joints in Flat Position • Inside corner joints are often made by butting two base metals together to form a square-groove joint • An outside corner joint is similar to a butt joint © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Corner and T-Joints in Flat Position • A T-joint is formed by placing one piece of base metal on the other to form a T-shape • This joint can be welded from one or both sides • Be certain to melt both surfaces on an inside corner joint © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Butt Joints in Flat Welding Position • It is suggested that a butt weld be practiced on low carbon steel 1/4″ to 3/8″ thick • The root pass in a butt joint must melt both pieces of metal and obtain complete penetration • For thin base metal, only one pass is required • Thick metal requires additional passes © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Butt Joints in Flat Welding Position • Use the keyhole method to ensure complete penetration in a groove weld • Many weld joints require more than one electrode to complete the weld © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Butt Joints in Flat Welding Position • Some weld beads require re-melting the existing crater before resuming the weld • The welder must clean the bead before attempting to add another bead • The weld should have no small cavities and should have good fusion • A wandering arc will result in incomplete fusion and a nonuniform weld bead © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Butt Joints in Flat Welding Position • The final beads used to fill the joint should be built above the original top surface © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Safety • Always wear safety goggles when removing slag and cleaning metal © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

SMAW in Horizontal Position • A butt, edge, or outside corner joint can be made in the horizontal welding position • The electrode should be pointed upward at an angle of about 20° © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

SMAW in Horizontal Position • A short arc length and lower welding current should be used • Be sure to eliminate undercutting at the edge of the bead • For a T-joint or inside corner joint, the electrode is inclined 20° in the direction of travel © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

SMAW in Horizontal Position • When making a fillet weld on a horizontal lap joint, the electrode should point more toward the surface than toward the edge © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

SMAW in Vertical Position • Welding in the vertical position may be done in either or two directions – Uphill (also called vertically up) – Downhill (also called vertically down) • The weld must be made so that slag is not entrapped in the weld metal • The material should not run or drip © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

SMAW in Vertical Position • The uphill method of welding is generally preferred • With downhill welding, the slag has a tendency to run into the molten weld pool © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

SMAW in Vertical Position • To control the weld pool heat and allow the metal time to cool, a whipping motion is used • Vertical butt, edge, and outside corner joints can be prepared using square-, V-, bevel-, J-, or U-groove edges • In a vertical weld, the bead must be straight with a uniform width • The weld must be properly fused © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

SMAW in Overhead Position • Overhead arc welding is generally the most difficult • Most electrode holders have jaws designed to allow the electrode to be held in a variety of positions © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

SMAW in Overhead Position • A covered electrode should never be bent to change its angle for welding • Bending a covered electrode will crack the covering and cause it to fall off • It is important to keep metal in the molten weld pool from falling due to gravity • A whipping motion is used to keep an overhead weld pool cool enough to control © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Safety • A welder’s helmet, cap, leather cape or coat, and leather gloves should be worn when overhead welding • Coveralls must be buttoned at the collar • All pockets should have closed flaps on them © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Review of Safety in SMAW • Safety rules must be carefully observed to prevent welding accidents – – – Protect your eyes and face with an approved helmet Wear recommended clothing and shoes Avoid open pockets and cuffs Keep the floor of the welding area dry Work on electrical power connections only if you are an experienced electrician © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Review of Safety in SMAW • Safety rules continued – Wear heavy, gauntlet-type gloves – Cover all skin during arc welding to prevent burns – Protect against harmful fumes by having good ventilation or a source of filtered air – Always use equipment approved by NEMA – Never operate an ac welding machine with the welding cables wrapped around the welding machine © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Safety • Protect your eyes and face from harmful rays and sparks by wearing an approved helmet • The floor on which the welder stands should be kept dry to reduce the chance of an electrical shock • Never use homemade or unapproved transformer equipment, which may be dangerous © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.