Welding Operations & Defects

(Shielded Metal Arc Welding (SMAW :Process Definition .1 Commonly called stick or covered electrode welding and it is manual process whereby an arc is generated between a flux-covered consumable electrode and .the work piece

:Process Principles .2 The arc generated when the electrode touches "scratches" the base metal. The .resulting arc melts both the base metal and the tip of the electrode The molten electrode metal/flux is transferred across the arc to the base-metal .pool that results the weld covered by slag from the electrode flux

:Equipments .3 The equipments consists of a power supply, electrode holder, electrode , welding .cables and work piece as shown in the figure below

:Process Capabilities .4 :(Materials (alloys Carbon steel, stainless steel , low alloy carbon steel , cast iron , cupper, nickel & .aluminum

:Shape Limitation .Un limited shape

:Dimensions Range .Thickness from 1.6 inch to unlimited thickness

:Advantages .5 .Easily implemented. 2. Inexpensive. 3. Flexible. .Filler metal protect the weld because of flux .5 .can be used for many materials .6

4. Easy to use .1

:Limitations .6 .Low productivity. 2. Low Deposition rate. 3. Operator dependant .1 .Heat of welding are too high for lead , tin , zinc and their alloys .4

(Gas Metal Arc Welding (GMAW :Process Definition .1 The arc is generated between a continuous wire electrode and the weld pool. The electrode wire is automatically fed from a spool into the weld pool by a wire .feeding system

:Process variables .2 .(Welding current (electrode feed speed .1 .polarity. 3. Arc voltage (arc length). .electrode extension. 6. Electrode orientation.

4. Travel speed .2 7. Electrode diameter .5

3. Equipment: The arc (1) is struck between the work piece and a metal wire electrode (2) that is continually fed forward into the arc. The wire is supplied on a reel (3), and is fed to the welding gun by the drive rollers (4), which push the wire through a flexible conduit (5) in the hose package (6) to the gun (7). Electrical energy for the arc is passed to the electrode through the contact tube (9) in the welding gun. This contact tube is normally connected to the positive pole of the power source, and the work piece to the negative pole. Striking the arc completes the circuit. The gas nozzle (11) that surrounds the contact tube (9) supplies shielding gas (10) for protection of the arc and the weld pool (12).

4. Advantages: 1. Deposition rates higher than SMAW. 2. Productivity higher than SMAW. 3. No slag remove and continuous.

4. Easily automated.

5. Limitations: 1. 2. 3. 4.

More expensive than SMAW. The process more complex to control. Restricted access. Its gun is larger than electrode holder in SMAW.

( Flux Cored Arc Welding ( FCAW :Process Definition .1 The arc is generated by a tubular wire with a flux between the continuous wire .electrode and the work piece

:Process Features .2 Flux-cored arc welding has two major variations. FCAW uses an externally supplied gas to protect the arc from N2 , O2 in the atmosphere . • The core ingredients in gas- shielded electrodes are slag formers, deoxidizers, arc stabilizers, and elements. • •

3. Process Equipments: 1. Contactor control . 4.welding gun. .wire driver motor .7

2. wire feed control . 5. work piece.

3. voltage control. 6. shielding gas source.

:Advantages .5 • • • • • •

High deposition rates. Deeper penetration than SMAW. High quality. Less pre-cleaning than GMAW. Slag covering helps with larger out-of-position welds. Self-shielded FCAW is draft tolerant.

6. Limitations: • • • •

Slag must be removed. More smoke and fumes than GMAW & SMAW. Spatter. FCAW wire is more expensive and complex than for SMAW. • Equipments are more expensive and complex than for SMAW.

(Submerged Arc Welding (SAW :process Definition .1 Flux is fed into the joint around the tip of the welding torch by a flux hopper. The .arc struck between the wire and work piece and shielded by molten flux

:Process Variables .2 • Continuously-fed like FCAW.

• Higher deposition by using a larger diameter. • Higher current. • The process can be utilized such as multiple torches and arrow gap welding.

3. Process Equipments: 1. Automatic wire feed.

2. 3. 4. 5. 6.

7. 8. 9.

Flux hopper. Welded power. Flux feed tube. Flux shelf. Solid slag . Welding vee. Weld backing plate. Work connection.

5. Process Capabilities: Materials (alloys): Steels, low alloying steels, stainless steel, nickel, chromium, and molybdenum steels.

Shape Limitation: Not suitable for thin material.

6. Advantages: • • • •

High deposition rates. No arc flash. Easily automated. Joints can be prepared with narrow grooves.

7. Limitations:

• Slag removal required. • Flux handling equipments. • Flux obstructs view joint. • Flux is subject to contamination porosity.

Electro Slag Welding (ESW) 1. Process Definition: A large weld pool is supported between the walls of a thin plate. Weld pool shielding is provided ,by a molten metal slag bath.

2. Process Variables: 1. The non-consumable guide method. 2. Consumable guide method uses a guide tube to guide the wire.

3. The flux is small quantities.

3. Process Equipments: • • • • • • • • • •

Electrode lead. Power source. Wire feed drive. Wire reel. Oscillation. Molten slag. Molten weld pool. Work lead. Retaining shoe. Consumable guide tube.

4. Advantages: • Used for thick weldments. • Produces sound welds. • Increased cost effectiveness as thickness increase.

5. Limitations:

• Once started , the weld must continue. • Coarse grained heat affected zone.

Plasma Arc Cutting (PAC) 1. Process Definitions: It is rrosion processes that utilizees a constricted arc in the form of a highvelosity jet of ionized gas to melt & sever metal in narrow . lacalized area.

2. Process principles: A cool & inert gas, such as compressed air, is forced under pressure through a small orifice in the front of cutting torch . This torch is connected by leads to a dc power supply. In the torch, apportion of the inert gas is changed into plasma by heat created by the discharge of high-voltage arc from the power supply. This arc is created between an electrode (-ve) in the torch & the tip nozzle of the torch through which the gas flows.

3. Process Capabilities: Materials (alloys): Ferrous & non-ferrous metals.

4. Advantages: •

• • • •

High speed than oxyfuel cutting 5 times approximately. High productivity. Easy to be automated. Good control & more flexible. High energy that cuts higher penetration.

5. Limitations: • • • •

More complex torch. High maintenance cost. More parameters need to set up compared with oxyfuel cutting. High capital cost.

6. Applications: Automobile manufactures , auto body repair shops who must cut highstrength steels. • Cutting stainless steel in manufactures of food processing & kitchen equipments. • Special & subcontracted items from aluminum , brass , copper , carbon steel , stainless steel ,…..etc. •

Stud Welding (SW) 1. Process Definition: It a commonly method for joining a metal stud, or fastener , to a metal work piece. The process has been used as an alternative metal-fastening method.

2. Process Principles:

• Gun is properly positioned. • Trigger is depressed & stud is lifted , creating an arc. • Arcing period is completed & stud is plunged into molten pool of metal on base material. • Gun is withdrawn from welded stud & ferrule is removed.

3. Process Equipments: • • • • • •

Power cable to work. Power source terminal connections. Control unit. Stud welding gun. Power cable to gun. Control cable to work.

4. Advantages: • Portable. • Inexpensive method.

Symbolic representation of welds on drawings: A welding symbol on a drawing consists of: An arrow line (1) One or two reference lines (2) An elementary symbol (3) Possible supplementary symbols Dimensions of the weld

Figure 15.1 Symbols used on welding drawings.

Symbolic presentation of welds on drawings are given in IS0 2553:

Figure 15.2 Examples of elementary symbols.

Elementary symbols and supplementary symbols In general, the elementary symbol is similar in shape to that of the welded joint (i.e. before welding, indicating how the metal sheets are to be prepared for welding). Examples of elementary symbols are shown in Figure 15.2. If the unbevelled edge exceeds 2 mm, the joint is a single-V butt joint with broad root faces (Y). If not, it is a single-V butt joint. Supplementary symbols may be used, in combination with the elementary symbols: see Figure 15.3. Absence of supplementary symbols means that there are no specific requirements in respect of the shape of the weld surface.

Figure 15.3 Supplementary symbols.

The importance of the reference lines: The position of the elementary symbol on the reference lines indicates on which side of the arrow line that the weld is to be placed. The upper, solid line (which is recommended to be terminated by a tail showing that the representation refers to IS0 2553)indicates a weld on the arrow side. In this case, the elementary symbol is placed on the solid line. The lower,

interrupted line indicates a weld on the other side. In this case, the symbol 'hangs' below the interrupted line. See Figure 15.4 and Figure 15.5.

The interrupted reference line is not used for fully symmetrical welds: examples are shown in Figure 15.6.

The position of the arrow line In general, there is no significance in the position of the arrow line in relation to the weld, except in the case of single bevel butt welds and single-J butt welds where the arrow of the arrow line must point towards the plate that is prepared. See Figure 15.7.

Dimensioning of welds The dimensions of the cross-section of the weld are shown to the left of (before) the elementary symbol (e.g. penetration for butt welds, leg length or throat thickness of fillet welds). Write the length of the weld to the right of (after) the basic symbol. E.g. 511300 indicates a square butt weld, with 5 mm penetration and a length of 300mm. •

indicates a continuous fillet weld with a leg length of 10 mm.

5x200 (100) indicates an intermittent fillet weld with a leg length of 8 mm, divided up into five 200-mm-long welds, spaced 100 mm (end to end) apart.

Standard Location of the Elements of the Welding Symbol

:Welding Defects The performance of welded structures or components in service depend on the quality of fabrication , which in turn is based on the presence or absence of defects or discontinuities in . welded joints

:Classification of Weld Discontinuities :Discontinuities may be divided into three board classifications 1. Design related. 2. Welding process related . 3. Metallurgical .

: Design related discontinuities .1 Include problems with design or structural details , choice of the wrong type of . weld joint for a given application , or undesirable changes in cross section

:Process related discontinuities .2 : Undercut A groove melted into the base metal adjacent to the toe or root of a weld and left . unfilled by a weld metal

: Slag inclusions

Non metallic solid material entrapped in weld metal or between weld metal and . base metal

: Porosity . Cavity-type discontinuities formed by gas entrapment during solidifications

: Overlap . The protrusion of weld metal beyond the toe , face , or root of the weld

Tungsten Inclusions Particles from tungsten electrodes that result from improper gas-tungsten arc . welding procedures

: Backing peace left on Failure to remove material placed at the root of a weld joint to support molten weld . metal

:Shrinkage voids . cavity-type discontinuities normally formed by shrinkage during solidification

: Oxide Inclusions . Particles of surface oxide that have not melted and are mixed into the weld metal

: (Lack of penetration (LOP . A condition in which joint penetration is less than that specified

: Lack of fusion . A condition in which fusion is less than complete

: Craters . Depressions at the termination of a weld bead or in the molten weld pool

: Melt-through A condition resulting when the arc melts through the bottom of a joint welded from . one side

: Spatter . Metal particles expelled during welding that do not form a part of the weld

: (Arc strikes (arc burns Discontinuities consisting of any localized re melted metal , heat-affected metal , or change in the surface profile of any part of a weld or base metal resulting from an . arc

: Under fill A depression on the face of the weld or root surface extending below the surface of . the adjacent base metal

: Metallurgical Discontinuities .3 : Cracks Fracture-type discontinuities characterized by sharp tip and high ratio of length . and width to opening displacement

: Fissures Small crack-like discontinuities with only a slight separation (opening . displacement) of the fracture surfaces

: Fish eye A discontinuity found on the fracture surface of a weld in steel that consists of a . small pore or inclusion surrounded by a bright , round area

: Segregation The non-uniform distribution or concentration of impurities or alloying elements . that arises during the solidification of the weld

: Lamellar tearing A type of cracking that occurs in the base metal or heat affected zone (HAZ) of restrained weld joints that is the result of inadequate ductility in the through. thickness direction of steel plate

:References* Welding processes handbook , Klas Weman .