SERVICE WELDING GUIDE Caterpillar

Stick Electrode Shielded Metal Arc Welding (SMAW)

Usage:

Fundamentals of the Process

Definition

Shielded Metal Arc Welding is an arc welding process wherein coalescence is produced by heating with an electric arc between a covered metal electrode and the work. Shielding is obtained from decomposition of the electrode covering. Pressure is not used and filler metal is obtained from the electrode.

Slang Names

1. Stick/manual metal arc.

Process Principles

1. Heat source - heat of the arc.

2. Shielding gas - slag formed by the decomposition of the flux.

3. Filler metal - comes from core wire.

4. Flux - contains deoxidizers/slag formed/ionizing elements to stabilize the arc/iron powder for higher deposition.

Figure 1

Methods of Application

1. Manual - most widely used - approximately 90%

2. Semiautomatic - not used

3. Machine

4. Automatic - gravity/massive/firecracker

Electrical Requirements

Welding Circuit

Figure 2

Welding Current types

1. A.C.

2. D.C.E.N. (straight) polarity.

3. D.C.E.P. (reverse) polarity.

Power Source Types and Characteristics

1. Generator (DC)

2. Transformer (AC)

3. Rectifier (DC)

4. Transformer Rectifier (AC/DC)

5. Alternators (AC)

Other Electrical Requirements

1. Multiple Operators

Other Equipment Requirements

Additional Equipment Necessary

1. Helmet/gloves/leathers

2. Chipping hammer/wire brush

3. Grinder for repair

Setup of Equipment

The manner in which equipment is set up in preparation for welding is an important consideration, both from the safety standpoint and for the speed and ease with which the weld can be made.

Welding should be done in a specially prepared welding area if at all possible. It is usually quicker and easier and always safer and more convenient to carry the work to the welder rather than carry the welder to the work. If welding must be done in places other than the regular welding areas, extra attention should be given to safety precautions, and other workmen in the area should be notified that welding will be going on.

Adequate ventilation and lighting and a source of sufficient power must be provided. The welding machine should be located close enough to the work to assure free and easy manipulation of the electrode holder and cable. Cable connections must be made soundly and the ground cable clamp must be attached securely to the work to assure an uninterrupted flow of current. The ground clamp should always be placed on the piece being welded; current must never be allowed to flow through bearings, hydraulic cylinders or other precision surfaces. If this occurs, minute pits will form at the points of contact and cause excessive wear.

Whenever possible, the work should be positioned so the face of the weld being made and the axis of the weld will be within 10° of horizontal. In this position, higher amperages and deposition rates can be used, the puddle of molten metal is easier to control, and the welder is in a more comfortable position. In addition, more welding skill is generally required for welding in positions other than flat and horizontal.

Striking an Arc

An arc is struck by momentarily touching the work with the electrode. When the arc is struck, there is a tendency for the electrode to stick to the work because of the sudden rush of current caused by the short-circuiting of the welding machine. To avoid this sticking or freezing of the electrode, a motion similar to that used to strike a match should be employed (Figure 3). After the arc is struck, a slightly longer than normal arc should be held until the parent metal is heated to the melting point and the proper size puddle of molten metal is formed. When it is necessary to restrike an arc to continue an uncompleted weld, the recommended procedure is to strike the arc at the forward or cold end of the crater and move the electrode rearward over the crater and then forward again to continue the weld. This procedure fills the crater and avoids porosity in the weld and trapping of slag.

Figure 3 - Arcs must be struck carefully to keep electrode from sticking.

Controlling the Arc

The two primary factors involved in manipulating the arc are the length of the arc and the amperage of the welding current being used. The arc must be long enough so the diverging arc stream covers the width of the puddle and a good gas shield is maintained. An arc that is too long will tend to swing sideways, causing excessive spattering, and will result in an imperfect gas shield, allowing elements in the air to come into contact with the molten metal. This gives rise to a very irregular weld bead with poor penetration (Figure 4). Too short an arc, on the other hand, will not heat the parent metal sufficiently and may short out and cause the electrode to stick to the work. An arc of the proper length produces a steady transfer of metal from the electrode to the puddle with little spatter and tendency for the electrode to stick. An arc length approximately equal to the core diameter of the electrode will usually produce these characteristics.

The proper amperage to be used will vary with different welding situations depending on the type of material being welded, the type and size of electrode, etc. In general the recommendations of the electrode manufacturer should be followed. Too low a welding current will not generate enough heat to melt the parent metal sufficiently. This will result in excessive piling up of weld metal and an overlapping bead with poor penetration (Figure 4). Too high a current will give rise to excessive spattering, an irregular deposit of weld metal, and undercutting along edges of the weld (Figure 4).

Figure 4 - Arc length and welding current must be controlled for sound welding beads.

Deflection of the arc arising from concentrations of the magnetic field set up in the work can be controlled in a number of ways.

. Place the ground connection at the start of the weld if back blow is a problem; place it at the end of the weld if forward blow is a problem.

. Weld toward a heavy tack or a weld already made.

. Hold as short an arc as possible so the force of the arc offsets the arc blow.

. Reduce the current, reverse the polarity, or switch to AC current.

Manipulating the Electrode

Once a good arc is established and maintained, the electrode must be held and moved properly to assure a sound weld through the control of the puddle of molten metal. Primary factors here are electrode angle, travel speed, and weaving of the electrode. Travel speed should be such that a flat or slightly convex bead is deposited. If travel speed is too fast, a small, irregular bead with a concave surface will result. If travel is too slow a heavy convex bead with overlapping edges will result. Generally, travel speed has to be reduced for vertical and overhead welding.

The quality of a weld can be affected to a large degree by the angle at which the electrode is held to the work. For normal flat welds, the electrode should be perpendicular to the surface of the joint and inclined in the direction of travel. The amount of inclination will affect the penetration, with greater inclination giving less penetration. Fillet welds require different electrode angles for successive passes, and the position of the joint will further affect angle requirements. Specific recommendations for individual types of welds will be made in the following sections.

It is often desirable to lay a weld deposit that is wider than can be obtained with a single bead. This is accomplished by weaving the tip of the electrode back and forth over the joint as the electrode travels along the joint. There are many weave patterns used in welding; the important requirements are that the motion is uniform and the weaves are close enough together to assure good fusion at all points in the deposit. Again, specific recommendations will be made in the following sections.

In addition to widening the weld deposit, weaving helps to work impurities to the top of the puddle of molten metal and can be used to control contour and undercutting, as well as heat input and temperature in a joint. Control of heat input and temperature by weaving is especially useful in vertical and overhead welding. Disadvantages of weaving include the possibility of losing the gas shield and a possible increase in distortion from warping. Low hydrogen electrodes should not be weaved more than two times the diameter of the electrode in order to avoid breaking the gas shield.

Certain environmental factors affect the welder's ability to manipulate the electrode and control the puddle. The welding area should be orderly, well lighted and well ventilated. For welding in close or restricted quarters, such as inside the draft tube of a scraper, additional illumination and additional ventilation in the form of a small fan located near the work area is desirable. The welding helmet should be kept clean of smoke and weld spatter.

All welding should be done from a comfortable position so the welder can hold a steady arc and feed the electrode into the joint at a constant rate. Both hands should be free to grip the electrode holder and guide the electrode or steady the work as necessary. If the joint is in a hard-to-reach or awkward location, the electrode may be bent in such a manner that the proper electrode angle and arc length can be held from a more relaxed position (Figure 5.)

Figure 5 - Electrodes can be bent for welding in hard-to-reach locations

When the bead is started, a slightly longer than normal arc should be held until a sufficient amount of parent metal melts to form a puddle of the proper size. Then the arc length is shortened and travel is begun (Figure 6). At the end of the weld, the bead is stopped by shortening the arc and momentarily backing to fill the crater and then whipping the electrode sharply backward to break the arc. If the bead must be stopped along the length of the weld for any reason (to change the electrode, for instance) the electrode should be whipped to the side, leaving an unfilled crater in which to begin the bead. It is often useful to check for proper electrode selection, arc control and electrode manipulation by welding on a scrap before welding the joint. The scrap should be in the same position as the joint.

Figure 6 - Control of the arc is important at the start and finish of a joint.

Tack Welding

Tack welding permits accurate positioning of members without special fixtures or clamping devices and controls distortion arising from cooling welds. Tack welds should be made with the same electrode and in the same manner prescribed for the final weld. All members of a weldment should be tacked together before any final welds are made. The following procedure is recommended for making tack welds:

1. Hold the member in position and make the first tack weld on the end of the member so the member can be repositioned slightly, if necessary (Figure 7). The first tack should be strong enough to hold the member in position.

Figure 7

2. Check the alignment with a level or a combination square (Figure 8). Reposition the member if necessary.

Figure 8

3. Make a second tack weld in a place that will further restrain the member from being distorted when the final welds are made (Figure 9).

Figure 9

4. Recheck alignment.

5. Make additional tack welds as required to hold the member in position while the final welds are being made (Figure 9).

Tack welds must meet all of the requirements for soundness that apply to final welds. Cracked tack welds must be gouged out and remade. If a cracked tack weld is not completely removed, it will initiate future cracking in the final weld.

Flat and Horizontal Welding

A weld joint is in the flat position when both the axis of the joint and the face of the weld are within 10° of horizontal and the welding is performed on the top of the joint (Figure 10). If the axis of the joint is within 10° of horizontal but the face is between 10° and 90° from horizontal, the joint is said to be in the horizontal position (Figure 10).

Figure 10 - Flat and Horizontal positions should be used when possible.

For normal flat welds, the electrode should be perpendicular to the surface of the joint and tilted slightly in the direction of travel (Figure 11). For deeper penetration, the electrode should not be tipped in the direction of travel, but rather it should be held perpendicular to the face of the weld so the heat from the arc is directed down into the joint (Figure 11). The weave pattern is the same as for normal flat joints.

For wide gap joints or joints with a poor fit-up, where excessive heat in the joint would cause a melt-through, the electrode should be tilted slightly away from the direction of travel (Figure 11). This angle directs the heat of the arc away from the center of the puddle of molten weld metal and allows the metal to cool more rapidly. The wider weave pattern is used because it also helps the molten weld metal cool more rapidly.

Figure 11 - Several electrode angles and weave patterns cam be used for flat welding.

For horizontal fillet welds, the electrode should be held midway between the horizontal and vertical plates (45° work angle) on the first pass so an equal force from the arc is directed against each surface (Figure 12). The second pass should be deposited against the horizontal plate to form a flat ledge on which the third pass can be laid. The second pass and all subsequent passes that do not contact the vertical plate should be made with a 20° work angle (Figure 12). The third pass and all subsequent passes that do not contact the vertical plate should be made with a 60° work angle. On all passes the electrode should be tilted in the direction of travel (Figure 13) so it makes a 60° angle with the horizontal plate (30° lead angle).

Figure 12 - Different work angles are needed on various passes for horizontal fillet welds.

The weave patterns for horizontal fillet welds utilize the force of the arc to wash molten metal up onto the vertical surface. This technique permits accurate forming of the weld deposit without undercutting (Figure 13).

Figure 13 - A 30° lead angle puts the electrode 60° from horizontal

For horizontal butt welds, the electrode holder should be held somewhat below the joint so the electrode is directed slightly upward. The electrode again should be tilted in the direction of travel so it has a lead angle of about 10°. The electrode is directed upward so the force of the arc will hold the puddle of weld metal in position until it freezes. The weave patterns recommended for horizontal fillet welds (Figure 13) should be used for horizontal butt welds, also. This is done so molten metal can be washed up onto the upper surface to prevent undercutting.

Overhead and Vertical Welding

A weld joint is in the overhead position when the axis of the joint is within 10° of horizontal and the welding is performed on the lower side of the joint (Figure 14). If the axis of a joint is between 10° and 90° from horizontal, the joint is said to be in the vertical position (Figure 14). Even thorough a joint inclined at, for instance, 30°, is closer to the horizontal than vertical, it is classified as a vertical weld because the problems encountered are essentially the same as those of a joint that is completely upright.

Figure 14 - Vertical and overhead welding is called out-of-position welding.

When working in the overhead and vertical positions, the welder must constantly take steps to counteract the effect of the force of gravity on the puddle of molten metal. The size of the puddle must be limited by reducing welding current, arc length and deposition rate. This keeps molten metal from dropping from the weld or running down over the work. The amount of slag shield must be reduced through proper electrode selection so the weld metal will freeze more quickly and in the proper location. The electrode must be held and moved in such a manner that the force of the arc holds the metal in position until it solidifies. All of these factors reduce welding speed and make it more difficult to obtain a sound weld.

Overhead welding requires the most skill on the part of the welder. For overhead butt joints and overhead fillet welds where the face of the weld is horizontal. The electrode should be held perpendicular to the face and inclined slightly in the direction of travel (Figure 15). For overhead fillets where the face of the weld is other than horizontal, the electrode should be held so most of the heat is directed onto the upper surface (Figure 15). In each case, a circular or triangular weave pattern should be used to agitate the puddle to remove slag and impurities. A series of small beads is easier to run than a single large bead; however, thorough cleaning is necessary after each pass because impurities will not float to the surface of the puddle with the overhead position.

Figure 15 - Overhead butt and fillet joints require considerable skill from the welder.

Vertical welds may be made either upward or downward. Heavy joints are usually welded in the upward direction, while small joints on thin material should be welded downward. Welding downward requires somewhat less skill than welding upward. For both upward and downward vertical welding, the electrode should be held perpendicular to the face of the weld and inclined downward (electrode holder below arc) regardless of direction of travel.

When making a fillet weld in the upward direction, it is advisable to build a ledge or shelf on which to start the weld (Figure 16). The shelf should be built to the desired width with a series of short passes. When the shelf is the desired size, the first pass of the weld should be made, using a weave pattern consisting of a series of inverted V's with the highest point of the pattern in the root of the joint (Figure 16). This pattern spreads heat evenly to control the size of the puddle and allows impurities and flux to float downhill to the brim of the puddle. The bead should be small enough so all flux is kept in a molten or semi-molten state. The optional weave pattern (Figure 16) gives a smoother bead, but greater care must be taken because slag can easily become entrapped in the center of the pattern.

After the first pass has been made, the bead can be enlarged by using a weave pattern consisting of horizontal passes with an upward swing at each end. The upward swing is used to tie the weld into the base metal without undercutting (Figure 16). When low hydrogen electrodes are used, the weave pattern should be kept as narrow as possible to assure a good gas shield around the arc.

Figure 16 - The proper weave pattern spreads heat and allows good removal of impurities.

Electrodes

Electrodes are usable within a range of amperages. With SMAW welding, the operator has great amount of process control. The operator controls the arc length and uses a manipulating motion to control the arc. Specific welding settings are given in broad ranges. Welders normally listen for a frying or crackling sound, as well as making a visual inspection to confirm proper setting or make adjustments. Learning these skills is what makes and experienced welder.

E7018 Electrode Storage

These electrodes are to be purchased in hermetically sealed containers. After hermetically sealed containers are opened or after electrodes are removed from baking or storage ovens, the electrode exposure to the atmosphere should not exceed 4 hours, maximum. Electrodes that have been wet should not be used. Electrodes exposed to the atmosphere for periods less than 4 hours may be returned to the holding oven maintained at 250°F (120°C) minimum; after a minimum holding period of 4 hours at 250°F minimum the electrodes may then be reissued.

Electrodes exposed to the atmosphere for more than 4 hours must be baked for at least two hours between 500°F (260°C) and 800°F (430°C).

All electrodes shall be placed in a suitable oven at a temperature not exceeding one half of the final baking temperature for a minimum of one half hour prior to increasing the oven temperature to the final baking temperature. Final baking time shall start after the oven reaches final baking temperature.

Nominal electrode size reflects the diameter of the metal rod or core of the electrode. The overall diameter of the electrode with the coating will vary. As the rod diameter increases, the amperage necessary to produce a good weld will increase. Amperage ranges for difference electrode types can vary, even if the diameters are the same. Recommended amperages for electrodes are sometimes printed on the box ends. If no recommended settings are found on the packaging, the manufacturer may provide literature with recommended amperages. If written help cannot be found, consult with an experienced welder to select a starting amperage and make adjustments to get the desired weld bead shape and appearance.

Low hydrogen type electrodes (5, 6, or 8 as the far-right digit) will absorb moisture from the air if not stored properly. Electrodes are stored in ovens at a minimum temperature of 250°F. Electrode ovens are not to be used for food warming or storage of any other items.

Electrodes left out of the heated storage ovens for longer than two hours must be reconditioned in the oven at least 1 hour at 500°F or 4 hours at 250°F. Porosity of hydrogen-induced cracking will result from improper storage and use of the low hydrogen electrodes.

Figure 17 - Electrode storage oven

Figure 18 - Label on electrode can

Figure 19 - SMAW electrodes showing classification numbers

For electrode numbers with four digits, the two digits on the left denote tensile strength in thousands of pounds. If the electrode number has five digits, the three digits on the left denote tensile strength. For both, the right hand digit indicates the type of coating, and the second digit from the right indicates the recommended positioning for use.