Arc Welding

Chapter 2 Notes

Introduction
In arc welding, two metals are joined together by generating an electric arc between a covered metal electrode. The heat produced by the arc melts the metal which mixes with the molten deposits of the coated electrode.

Arc is powered by either 1) Direct current or 2) Alternating current The molten metal from the electrode is deposited on the base metal, and as the molten metal is deposited, a slag forms over the weld which insulates it against contaminants of the air while it cools. See figure 24, page 52

Power Supply
For effective arc welding, a constant current is required. Other electrical appliances, the demand for current is fairly constant, with arc welding the power fluctuates a great deal. When the arc is struck with an electrode, a short circuit results which immediately induces a sudden surge of electrical current.

Open circuit voltage- the voltage when the welding machine is running and no welding is taking place (Ranges from 50-100) Arc Voltage- the voltage after the arc is struck (Ranges from 18-36) Arc voltage will also vary with the length of the arc. It is difficult even for a skilled welder to maintain a perfectly uniform arc length. The amount of current required for any welding operation is governed by the thickness of the work metal.

Duty Cycle- the amount of time a power supply can deliver its rated output in ten minute period. Ex: 30% duty cycle three minutes of welding and seven minutes of cooling. Discussion: Why do you suppose welders are designed this way? In what situations would you require a higher duty cycle?

DC Generator
Direct Current- power supply is a generator driven by an electric motor or gasoline engine. Polarity- indicates the direction of current flow in a circuit Straight Polarity- the electrode is negative and the workpiece is positive and the electrons flow from the electrode to the workpiece.

Reverse Polarity- the electrode is positive and workpiece is negative and the electrons flow form the workpiece to the electrode. Changing polarity is as simple as flipping a switch on the welding machine. Alternating Current- The transformer welding machine takes current directly from a power supply and transforms it into the voltage required for welding.

An AC welder transformer has a primary and secondary coil to regulate current. The primary coil receives the alternating current from the power source and creates a magnetic field. Creates a burst of current to the arc then for welding. When the arc is struck, a sudden burst of current is sent to start the arc. You will hear the welding machine bear down when this happens After arc is struck, the machine automatically returns to the current amount set for the job.

One advantage of the AC welding machine is the freedom from Arc Blow. Arc Blow- this causes the arc to wander while welding. Arc blow can be corrected by changing position of ground clamp, changing position of the weld piece on the workbench. Porosity- when impurities get into the welding area while the molten metal cools causing tiny holes in the weld. Significantly reduces the strength of the weld and the asthetic value of the weld.

Rectifiers- a rectifier is a transformer which contain an electrical device that changes alternating current into direct current. We have these in our shop. Rectifier materials:
Selenium Silicon- provides operation at higher temperatures CD Rom on Arc Welding

Electrode Holder
Used to hold the electrode and guide it over the seem to be welded A good holder should be light to reduce excessive fatigue while welding, receive and dispose of electrodes easily, and be properly insulated. Always make sure the holder is connected firmly and properly to the cable. Never lay the cable or holder down on the work piece while the welder is running.

A 30 lead cable of a given size may be satisfactory to carry the required current, but if another 30 of cable is added, the combined resistance of the two leads reduces the current output of the machine. If then you turn the power output up, it could possibly cause the cable to overheat. This concept is why OSHA is so tough on schools for extension cords etc. Ex: No growlights in classroom anymore, fridge has to be unplugged , shop lights hooked direct.

Ground Clamp
Without proper grounding, the full potential of the circuit will fail to provide the required heat for welding. The place where the ground clamp is connected should be clean and free of rust. You should not have arcing between the work and the ground Never start the arc on the ground clamp The closer the ground clamp to the work, the better the connection for welding. Rule of thumb: Ground clamp should never produce steam if a small drop of water is placed on it.

Protective Shield
A suitable helmet or handshield is required for all arc welding. An electric arc produces a brilliant light and gives off invisible ultraviolet and infrared rays which can burn the eyes and skin. Never look at the arc from within a distance of 50 feet. Better yet, never look at it at all.

Lens Shades
Shade Shade Shade Shade Shade Shade 5 6/7 8 10 12 14 Light spot welding Welding up to 30 amps Welding from 30-75 amps Welding from 75-200 amps Welding form 200-400 amps Welding over 400 amps

Welders Clothing
Gloves- leather gloves are acceptable, and welding gloves are fine as well. However, use clamps, pliers, etc to pick up hot metal instead of your gloves. Use long sleeves that fit tightly so they do not get in the way of moving equipment Aprons- aprons shield the welding sparks from burning through onto your clothing/skin. Helmet of appropriate lens shade

Welding Safety Rules

Be sure the welding area has a cement or masonry floor Remove all combustible materials from the work area Do not use gloves or other clothing which contain oil or grease. Be sure all power wiring is correctly installed and maintained. Dont overload the welding cables. Always be sure your machine is properly grounded. Never work in a damp area

Welding Safety Rules cont.

Shut off the welding machine before making repairs or adjustments to avoid shocks. Follow the manufacturers rules on operating the switches, and making adjustments. Protect others with a screen and yourself with a protective shield. Flying sparks are a danger to your eyes. Arc rays can cause painful burns and blindness. Keep firefighting equipment handy at all times.

Types of Welds/Joints to be Welded

Flat position Horizontal Position Vertical Position Overhead Position Fillet welds Groove welds Lap welds T-fillet welds Butt welds

Starting The Arc

Scratching method- almost identical to the same motion as striking a match. Tapping method- simply tapping the electrode on the workpiece. As soon as the arc is started, immediately raise the electrode a distance equal to the diameter of the electrode. Usually 1/8-1/4 inch.

If an electrode does stick, it can be broken loose quickly by twisting or bending it. If it still does not unlodge, release the electrode from the electrode holder. You still of course can always turn off the power to the machine. Just dont panic over the situation

Setting the Current

Current depends upon: The thickness of the metal to be welded The actual position of the welding The diameter of the electrode The rule of thumb: higher currents and larger diameter electrodes can be used for welding in flat positions than in vertical or overhead welding.

Setting the Current

Whenever the current is too high, the electrode will melt too fast and the molten puddle will be too large and irregular. Whenever the current is set too low, there will not be enough heat to melt the base metal and the molten pool will be small. The result is not only poor fusion but the deposit will pile up and be irregular in shape and aesthetically look poor as well.

Setting the Current cont

Too high current may also produce undercutting, which leaves a groove in the base metal long both toes of the weld deposit. A current that is set too low will cause cold laps to form where the molten metal from the electrode falls on the work without sufficiently fusing or penetrating the base metal. Both these problems cause weak welds. For examples turn to page 24 for diagrams.

Length of Arc
If the arc is too long the metal melts off the electrode in globules which wonder from side to side as the arc wavers and does not fuse. An arc that is too short fails to penetrate sufficient heat to melt the base metal properly. Generally, the length of arc should be approximately equal to the diameter of the electrode.

A shorter arc is usually better for vertical welding and overhead welding because you have better control of the molten pool. When the arc is too long, heat from the arc stream is dissipated too rapidly, causing much metal spatter. When the electrode, current, and polarity are correct, a good short arc will produce a sharp, crackling sound. A long arc sounds like escaping steam.

Travel Speed
Travel speed refers to the rate the electrode travels along the weld seam. The maximum welding speed is influenced by: 1) the skill of the operator 2) the position of the weld 3) the type of electrode 4) required weld penetration

Travel Speed
Normally, if the speed is too fast, the molten pool will solidify or freeze quickly trapping impurities in the weld deposit. If the speed is too slow, the metal piles up excessively and the weld bead is high and wide. Page 25, figure 40

Forming the Crater

As the arc comes in contact with the base metal, a pool or pocket is formed which is called a crater. The size and depth of crater indicates the amount of penetration. General rule; the depth of penetration should be from 1/3 to the total thickness of the weld bead.

For a sound weld, the metal deposited from the electrode must fuse completely with the base metal. Fusion will result only when the base metal has been heated to a liquid state and the molten metal from the electrode readily flows into it. Thus if arc is too short, there will be insufficient spread of heat. If arc is too long, heat is not centralized enough to form the desired crater.

Single & Multiple Pass Welds

A single pass weld involves depositing one layer of weld metal. In welding light and moderate materials, a single pass weld is usually sufficient. Multiple pass welds are when a heavier plated metal is utilized. This may require two or more layers of weld passing lapping over the other. Whenever a multiple pass weld is used, the slag on each welded bead must be removed completely before the next layer is deposited.

Types of Electrodes
The type of electrode selected for arc welding depends upon: 1) quality of weld required 2) weld position 3) Joint design 4) welding speed 5) composition of the base metal

Electrodes classified into 5 main groups:

1) 2) 3) 4) 5) mild steel high carbon steel special alloy steel cast iron nonferrous

Electrode flux materials:

Electrodes are coated with:
Cellulose Titanium dioxide Ferro-manganeese Cilica flour Calcium carbonate

Purpose of the flux coating:

Assist with easier arc starting Stabilize the arc Improve weld appearance and penetration Reduce spatter Protect the molten material contamination

Identifying electrodes:
Electrode requirements and specifications are determined by: 1) AWS (American Welding Society) 2) ASTM (American Society for Testing & Materials)

Electrode Classification
Example Electrode number: ( E-6013) E= electric arc welding First two digits= minumum allowable tensile strength 60,000 lbs. Third digit stands for welding position 1=any position 2=horizontal or flat positions 3=flat position only Fourth digit represents some special characterisic: (weld quality, type of current, amount of penetration) See page 31

Test Preparation
Notes on internet web site Review guide on internet web site
Assignments Questions on page 33 of your packet will be due before the test.