welding, reviewer

8/9/2019 welding, reviewer

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WORKSHOP THEORY AND PRACTICE(MACHINE SHOP)

Welding

Welding is

a fabrication or sculptural pro

cess that joins materials,

usually metals or thermoplastic

s, by causing coalescence.

This is often done

by melting the workpieces

and adding a filler material to

form a pool of molten material

(the weld pool) that cools to

become a strong joint,

with pressure sometimes used

in conjunction with heat, or by

itself, to produce the weld. This

is in contrast

with soldering and brazing,

which involve melting a lower-

melting-point material

between the workpieces to

form a bond between them,

without melting the

workpieces.

Many different energy

sources can be used for

welding, including a

gas flame, an electric arc,

a laser, an electron

beam, friction, andultrasound.

While often an industrialprocess, welding can be done

in many different

environments, including open

air, under water and in outer

space. Regardless of location,

however, welding remains

dangerous, and precautions

are taken to avoid

burns, electric shock, eye

damage, poisonous fumes,

and overexposure

to ultraviolet light.

History

The history of joining metals

goes back several millennia,

with the earliest examples of

welding from the Bronze

Age and the IronAge inEurope and the Middle

East. Welding was used in the

construction of the iron

pillar in Delhi, India, erected

about 310 AD and weighing

5.4 metric tons.[1]

The Middle Ages brought

advances in forge welding, in

which blacksmiths pounded

heated metal repeatedly until

bonding occurred. In

1540,Vannoccio

Biringuccio published De la

pirotechnia, which includes

descriptions of the forging

operation. Renaissance crafts

men were skilled in theprocess, and the industry

continued to grow during the

following centuries.[2] Welding,

however, was transformed

during the 19th century. In

1802, Russian scientist Vasily

Petrov discovered the elect

arc[3] and subsequen

proposed its possible practic

applications, includi

welding. In 1881-82 a Russi

inventor NikolaiBenardos created the f

electric arc welding meth

known as carbon arc weldin

using carbon electrodes. T

advances in arc weldi

continued with the inventi

of metal electrodes in the la

1800s by a Russian, Niko

Slavyanov (1888), and

American, C. L. Coffin (189

Around 1900, A.

Strohmenger released

coated metal electro

in Britain, which gave a mo

stable arc. In 1905 Russi

scientist Vladimir

Mitkevich proposed the usaof three-phase electric arc

welding. In 1919,alternati

current welding was invent

by C. J. Holslag but did n

become popular for anoth

decade.[4]

Resistance welding was a

developed during the findecades of the 19th centu

with the first patents goi

to Elihu Thomson in 1885, w

produced further advanc

over the next

years. Thermite welding w

invented in 1893, and arou


2/20


that time another

process, oxyfuel welding,

became well

established. Acetylene was

discovered in 1836 by Edmund

Davy, but its use was notpractical in welding until

about 1900, when a

suitable blowtorch was

developed.[5] At first, oxyfuel

welding was one of the more

popular welding methods due

to its portability and relatively

low cost. As the 20th century

progressed, however, it fell out

of favor for industrial

applications. It was largely

replaced with arc welding, as

metal coverings (known

as flux) for the electrode that

stabilize the arc and shield the

base material from impurities

continued to be developed.[6]

World War I caused a major

surge in the use of welding

processes, with the various

military powers attempting to

determine which of the

several new welding

processes would be best. The

British primarily used arcwelding, even constructing a

ship, the Fulagar, with an

entirely welded hull. Arc

welding was first applied to

aircraft during the war as well,

as some German airplane

fuselages were constructed

using the process.[7] Also

noteworthy is the first welded

road bridge in the world,

designed by Stefan Brya of

the Warsaw University of

Technology in 1927, and builtacross the river Sudwia

Maurzyce near owicz,

Poland in 1929.[8]

During the 1920s, major

advances were made in

welding technology, including

the introduction of automatic

welding in 1920, in whichelectrode wire was fed

continuously. Shielding

gasbecame a subject

receiving much attention, as

scientists attempted to

protect welds from the effects

of oxygen and nitrogen in the

atmosphere. Porosity and

brittleness were the primary

problems, and the solutions

that developed included the

use of hydrogen, argon,

and helium as welding

atmospheres.[9] During the

following decade, further

advances allowed for the

welding of reactive metalslike aluminum and magnesium

. This in conjunction with

developments in automatic

welding, alternating current,

and fluxes fed a major

expansion of arc welding

during the 1930s and th

during World War II.[10]

During the middle of t

century, many new weldi

methods were invented. 19

saw the release of st

welding, which soon becam

popular in shipbuilding a

construction.Submerged a

welding was invented t

same year and continues

be popular today. In 1932

Russian, Konstantin

Khrenov successfullyimplemented the f

underwater electric a

welding. Gas tungsten a

welding, after decades

development, was fina

perfected in 1941, and g

metal arc welding followed

1948, allowing for fast weldi

of non-ferrousmaterials b

requiring expensive shieldi

gases. Shielded metal a

welding was develop

during the 1950s, using a f

coated consumab

electrode, and it quic

became the most popu

metal arc welding process.1957, the flux-cored a

welding process debuted,

which the self-shielded w

electrode could be used w

automatic equipme

resulting in greatly increas

welding speeds, and th


3/20


4/20


GMAW are relatively low and

are therefore suitable for thin

sheet and sections less than

inch.

GMAW may be easily

automated, and lends itself

readily to robotic methods. It

has virtually replaced SMAW in

present-day welding

operations in manufacturing

plants.

Gas Tungsten-Arc Welding

Click to view larger JPEG. Gas

Tungsten-Arc Welding (GTAW),

also known as Tungsten InertGas or TIG welding, uses

tungsten electrodes as one

pole of the arc to generate

the heat required. The gas is

usually argon, helium, or a

mixture of the two. A filler wire

provides the molten material if

necessary.

The GTAW process isespecially suited to thin

materials producing welds of

excellent quality and surface

finish. Filler wire is usually

selected to be similar in

composition to the materials

being welded.

Atomic Hydrogen Welding

(AHW) is similar and uses an

arc between two tungsten orcarbon electrodes in a

shielding atmosphere of

hydrogen. Therefore, the work

piece is not part of the

electrical circuit.

Plasma Arc Welding

Click to view larger JPEG.

Plasma Arc Welding (PAW)

uses electrodes and ionized

gases to generate an

extremely hot plasma jet

aimed at the weld area. The

higher energy concentration is

useful for deeper and

narrower welds and increased

welding speed.

Shielded-Metal Arc Welding

Click to view larger JPEG.

Shielded-Metal Arc Welding

(SMAW) is one of the oldest,

simplest, and most versatile

arc welding processes. Thearc is generated by touching

the tip of a coated electrode

to the workpiece and

withdrawing it quickly to an

appropriate distance to

maintain the arc. The heat

generated melts a portion of

the electrode tip, its coating,

and the base metal in the

immediate area. The weldforms out of the alloy of these

materials as they solidify in the

weld area. Slag formed to

protect the weld against

forming oxides, nitrides, and

inclusions must be removed

after each pass to ensure a

good weld.

The SMAW process has the

advantage of being relativelysimple, only requiring a power

supply, power cables, and

electrode holder. It is

commonly used in

construction, shipbuilding, and

pipeline work, especially in

remote locations.

Submerged Arc Welding

Click to view larger JPE

Submerged Arc Weldi

(SAW) shields the weld a

using a granular flux fed in

the weld zone forming a th

layer that completely cov

the molten zone and preve

spatter and sparks. It also a

as a thermal insulat

permitting deeper he

penetration.

The process is obviously limit

to welding in a horizon

position and is widely used

relatively high speed sheetplate steel welding in eith

automatic or semiautoma

configurations. The flux can

recovered, treated, a

reused.

Submerged Arc Weldi

provides very high weldi

productivity....4-10 times

much as the Shielded MeArc Welding process.

MIG Welding

MIG (Metal Inert Gas) or a

even is called GMAW (G

Metal Arc Welding) uses

aluminium alloy wire as

combined electrode and fi

material. The filler metal

added continuously awelding without filler-mate

is therefore not possible. Sin

all welding parameters a

controlled by the weldi

machine, the process is a

called semi-automa

welding.


5/20


The MIG-process uses a direct

current power source, with the

electrode positive (DC, EP). By

using a positive electrode, the

oxide layer is efficiently

removed from the aluminium

surface, which is essential for

avoiding lack of fusion and

oxide inclusions. The metal is

transferred from the filler wire

to the weld bead by

magnetic forces as small

droplets spray transfer. This

gives a deep penetration

capability to the process and

makes it possible to weld in all

positions. It is important for the

quality of the weld that thespray transfer is obtained.

There are two different MIG-

welding processes,

conventional MIG and pulsed

MIG:

Conventional MIG uses a

constant voltage DC power

source. Since the spraytransfer is limited to a certain

range of arc current, the

conventional MIG process has

a lower limit of arc current (or

heat input). This also limits the

application of conventional

MIG to weld material

thicknesses above 4 mm.

Below 6 mm it is

recommended that backing is

used to control the weldbead.

Pulsed MIG uses a DC power

source with superimposed

periodic pulses of high current.

During the low current level

the arc is maintained without

metal transfer. During the high

current pulses the metal is

transferred in the spray mode.

In this way pulsed MIG is

possible to operate with lower

average current and heat

input compared to

conventional MIG. This makes

it possible to weld thinner

sections and weld much more

easily in difficult welding

positions.

TIG Welding

TIG-welding (Tungsten Inert

Gas) or GTAW-welding (Gas

Tungsten Arc Welding) uses a

permanent non-melting

electrode made of tungsten.

Filler metal is added

separately, which makes the

process very flexible. It is also

possible to weld without filler

material.

The most used power sourcefor TIG-welding generates

alternating current (AC).

Direct current can be used,

but due to high heat

generation on the tungsten

electrode when DC-EP

(electrode positive) welding,

that particular polarity is not

feasible. In some cases DC-EN

(electrode negative) is used,however, this requires special

attention before welding, due

to the arc's poor oxide

cleaning action.

AC TIG-welding usually uses

argon as a shielding gas. The

process is a multi purpose

process, which offers the u

great flexibility. By changi

the diameter of the tungst

electrode, welding may

performed with a wide ran

of heat input at differe

thicknesses. AC TIG-welding

possible with thicknesses dow

to about 0,5 mm. For larg

thicknesses, > 5 mm, AC T

welding is less economic

compared to MIG-weldi

due to lower welding speed

DC TIG-welding with electro

negative is used for weldi

thicknesses above 4 mm. T

negative electrode gives

poor oxide cleani

compared to AC-TIG a

MIG, and special cleaning

joint surfaces is necessary. T

process usually uses heliu

shielding gas. This gives

better penetration in thic

sections. DC TIG-welding

applicable for weldi

thicknesses in the range 0,

12 mm. More and mo

popular is also pulsed DC T

welding, which makes

possible to weld uniform we

with deeper penetration

the same heat input. Pu

frequency is usually in t

range 1 - 10 Hz.

http://www.ajeepthing.com

welding.html

WELDING SYMBOLS


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Special symbols are used on a

drawing to specify where

welds are to be located, the

type of joint to be used, as

well as the size and amount of

weld metal to be deposited in

the joint. These symbols have

been stan dardized by the

American Welding Society

(AWS). You will come into

contact with these symbols

anytime you do a welding job

from a set of blueprints. You

need to have a working

knowledge of the basic weld

symbols and the standard

location of all the elements of

a welding symbol.

A standard welding symbol

(fig. 3-43) consists of a

reference line, an arrow, and

a tail. The reference line

becomes the foundation of

the welding symbol. It is used

to apply weld symbols,

dimensions, and other data to

the weld. The arrow simplyconnects the reference line to

the joint or area to be welded.

The direction of the arrow has

no bearing on the significance

of the reference line. The tail

of the welding symbol is used

only when necessary to

include a specification,

process, or other reference

information.

Weld Symbols

The term weld symbol refers to

the symbol for a specific type

of weld. As discussed earlier,

fillet, groove, surfacing, plug,

and slot are all types of welds.

Basic weld symbols are shown

in figure 3-44. The weld

Figure 3-44.-Basic weld

symbols.

Figure 3-45.-Weld symbols

applied to reference line.

Figure 3-46.-Specifying we

locations.

Figure 3-47.-Arrowhe

indicates beveled plate.

symbol is only part of t

information required in t

welding symbol. The te

welding symbol refers to t

total symbol, which includ

all information needed

specify the weld(s) required

Figure 3-45 shows how a we

symbol is applied to t

reference line. Notice that t

vertical leg of the weld symb

is shown drawn to the left

the slanted leg. Regardless

whether the symbol is for


7/20


fillet, bevel, J-groove, or flare-

bevel weld, the vertical leg is

always drawn to the left.

Figure 3-46 shows the

significance of the positions of

the weld symbols position on

the reference line. In view A

the weld symbol is on the

lower side of the reference

line that is termed the arrow

side. View B shows a weld

symbol on the upper side of

the reference line that is

termed the other side. When

weld symbols are placed on

both sides of the reference

line, welds must be made onboth sides of the joint (view

C).

When only one edge of a joint

is to be beveled, it is

necessary to show which

member is to be beveled.

When such a joint is specified,

the arrow of the welding

symbol points with a definitebreak toward the member to

be beveled. This is shown in

figure 3-47.

Figure 3-48 shows other

elements that may be added

to a welding symbol. The

information applied to the

reference line on a welding

symbol is read from left to right

regardless of the direction ofthe arrow.

Dimensioning

In figure 3-48, notice there are

designated locations for the

size, length, pitch (center-to-

center spacing), groove

angle, and root opening of a

weld. These loca tions are

determined by the side of the

reference line on which the

weld symbol is placed. Figure

3-49 shows how dimensions

are applied to symbols.

Figure 3-48.-Elements of a

welding symbol.

Figure 3-49.-Dimensions

applied to weld symbols.

Figure 3-50.-Dimensioning

welds.

Figure 3-51.-Supplementa

symbols.

Figure 3-50 shows the meani

of various welding dimensi

symbols. Notice that the s

of a weld is shown on the l

side of the weld symbol (fig.

50, view A). The length a

pitch of a fillet weld a

indicated on the right side

the weld symbol. View B sho

a tee joint with 2-in

intermittent fillet welds th


8/20


are 5 inches apart, on center.

The size of a groove weld is

shown in view C. Both sides

are 1/2 inch, but note that the

60-degree groove is on the

other side of the joint and the

45-degree groove is on the

arrow side.

Supplementary Symbols

In addition to basic weld

symbols, a set of supple

mentary symbols may be

added to a welding symbol.

Some of the most common

supplementary symbols are

shown in figure 3-51.

Contour symbols are used with

weld symbols to show how the

face of the weld is to be

formed. In addition to contour

symbols, finish symbols are

used to indicate the method

to use for forming the contour

of the weld.

When a finish symbol is used, it

shows the method of finish,

not the degree of finish; for

example, a C is used to

indicate finish by chipping, an

M means machin ing, and a G

indicates grinding. Figure 3-52

shows how contour and finish

symbols are applied to a

weldng symbol. This figure

shows that the weld is to beground flush. Also, notice that

the symbols are placed on the

same side of the reference

line as the weld symbol.

Figure 3-52.-Finish and contour

symbols.

Figure 3-53.-Specifying

additional welding

information.

Another supplementary

symbol shown in figure 3-51 isthe weld-all-around symbol.

When this symbol is placed on

a welding symbol, welds are

to continue all around the

joint.

Welds that cannot be made

in the shop are identified as

field welds. Afield weld symbol

is shown in figure 3-51. Thissymbol is a black flag that

points toward the tail of the

welding symbol.

Specifying Additional

Information

It is sometimes necessary

specify a certain weld i

process, a type of electrod

or some type of refer en

necessary to complete

weld. In this case, a note c

be placed in the tail of t

reference line. (See

Figure 3-55.-Example

welding symbol in use.

fig. 3-53.) If additioninformation is not neede

then the tail is omitted.

Multiple-Weld Symbols

When you are fabricating

metal part, there are tim

when more than one type


9/20


weld is needed on the same

joint; for example, a joint may

require both a bevel groove

weld and a fillet weld. Two

methods of illustrat ing these

weld symbols are shown in

figure 3-54. Note that in each

welding symbol, the bevel

groove weld is to be

completed first, followed by

the fillet weld.

http://www.tpub.com/steelwo

rker1/29.htm

JOINT DESIGN AND

PREPARATION OF METALS

4-1. JOINT TYPES

Welds are made at the

junction of the various pieces

that make up the weldment.

The junctions of parts, or joints,

are defined as the location

where two or more nembers

are to be joined. Parts being

joined to produce theweldment may be in the form

of rolled plate, sheet, shapes,

pipes, castings, forgings, or

billets. The five basic types of

welding joints are listed below.

a. B, Butt Joint. A joint

between two members lying

approximately in the same

plane.

b. C, Corner Joint. A joint

between two members

located approximately at right

angles to each other in the

form of an angle.

c. E, Edge Joint. A joint

between the edges of two or

more parallel or mainly

parallel members.

d. L, Lap Joint. A joint

between two overlappingmembers.

e. T, Tee Joint. A joint between

two members located

approximately at right angles

to each other in the form of a

T.

4-2. WELD JOINTS

In order to produce

weldments , it is necessary to

combine the joint types with

weld types to produce weld

joints for joining the separate

members. Each weld type

cannot always be combined

with each joint type to make

a weld joint. Table 4-1 shows

the welds applicable to the

basic joints.

4-3. WELD JOINT DESIGN AN

PREPARATION

a. Purpose. Weld joints adesigned to transfer t

stresses between t

members of the joint a

throughout the weldme

Forces and loads a

introduced at different poi

and are transmitted

different areas throughout t

weldment. The type of loadi

and service of the weldme

have a great bearing on t

joint design required.

b. Categories. All weld joi

can be classified into tw

basic categories:

penetration joints and part

penetration joints.


10/20


(1) A full penetration joint has

weld metal throughout the

entire cross section of the

weld joint.

(2) A partial penetration joint

has an unfused area and the

weld does not completely

penetrate the joint. The rating

of the joint is based on the

percentage of weld metal

depth to the total joint; i. e., a

50 percent partial penetration

joint would have weld metal

halfway through the joint.

NOTE

When joints are subjected to

dynamic loading, reversing

loads, and impact leads, the

weld joint must be very

efficient. This is more important

if the weldment is sub jetted to

cold-temperature service.

Such services require full-

penetration welds. Designs

that increase stresses by theuse of partial-penetration

joints are not acceptable for

this type of service.

c. Strength. The strength of

weld joints depends not only

on the size of the weld, but

also on the strength of the

weld metal.

(1) Mild and low alloy steelsare generally stronger than

the materials being joined.

(2) When welding high-alloy or

heat-treated materials,

special precautions must be

taken to ensure the welding

heat does not cancel the

heat treatment of the base

metal, causing it to revert to its

lower strength adjacent to the

weld.

d. Design. The weld joint must

be designed so that its cross-

sectional area is the minimum

possible. The cross-sectional

area is a measurement of the

amount or weight of weld

metal that must be used to

make the joint. Joints may be

prepared by shearing, thermal

cutting, or machining.

(1) Carbon and low alloy joint

design and preparation. Theseweld joints are prepared

either by flame cutting or

mechanically by machining or

grinding, depending on the

joint details. Before welding,

the joint surfaces must be

cleared of all foreign materials

such as paint, dirt, scale, or

must. Suitable solvents or light

grinding can be used forcleaning. The joint surface

should not be nicked or

gouged since nicks and

gouges may interfere with the

welding operation.

CAUTION

Aluminum and aluminum

alloys should not be cleaned

with caustic soda or strongcleaner with a pH above 10.

The aluminum or aluminum

alloy will react chemically with

these types of cleaners. Other

nonferrous metals and alloys

should be investigated prior to

using these cleaners to

determine their reactivity.

(2) Aluminum and aluminu

alloy joint design a

preparation. Weld jo

designs often unintentiona

require welds that cannot

made. Check your design

avoid these and similar erro

Before welding, the jo

surfaces must be cleared of

foreign materials such

paint, dirt, scale, or oxid

solvent cleaning, lig

grinding, or etching can

used. The joint surfaces shou

not be nicked or goug

since nicks and gouges m

interfere with weldi

operations.

(3) Stainless steel alloy jo

design and preparation. The

weld joints are prepar

either by plasma arc cutti

or by machining or grindin

depending on the allo

Before welding, the jo

surfaces must be cleaned

all foreign material, such paint, dirt, scale, or oxid

Cleaning may be done w

suitable solvents (e.

acetone or alcohol) or lig

grinding. Care should

taken to avoid nicking

gouging the joint surfa

since such flaws can interfe

with the welding operation.

4-4. WELD ACCESSIBILITY

The weld joint must

accessible to the welder usi

the process that is employe

Weld joints are often design

for welds that cannot

made. Figure 4-2 illustra


11/20


several types of inaccessible

welds.

http://arcraftplasma.com/wel

ding/weldingdata/jointdesign.

htm

Weld Types and Positions

WELD TYPES AND POSITIONS

a. General. It is important to

distinguish between the joint

and the weld.

Each must be described to

completely describe the weld

joint.

There are many different types

of welds, which are best

described by their shape

when shown in cross section.

The most popular weld is thefillet weld, named after its

cross-sectional shape.

Fillet welds are shown by

figure 6-24.

The second most popular is

the groove weld. There are

seven basic types of groove

welds, which are shown in

figure 6-25.

Other types of welds include

flange welds, plug welds, slot

welds, seam welds, surfacing

welds, and backing welds.

Joints are combined with

welds to make weld joints.

Examples are shown in figure

6-26. The type of weld used

will determine the manner in

which the seam, joint, or

surface is prepared.

b. Groove Weld. These are

beads deposited in a groove

between two members to

joined.

standard types of groo

welds.

c. Surfacing weld (fig. 6-28).

These are welds composed

one or more strings or wea

beads deposited on


12/20


unbroken surface to obtain

desired properties or

dimensions.

This type of weld is used to

build up surfaces or replace

metal on worn surfaces. It is

also used with square butt

joints.

d. Plug Weld (fig. 6-28).

Plug welds are circular welds

made through one member

of a lap or tee joint joining that

member to the other.

The weld may or may not bemade through a hole in the

first member; if a hole is used,

the walls may or may not be

parallel and the hole may be

partially or completely filled

with weld metal.

Such welds are often used in

place of rivets.

NOTE

A fillet welded hole or a spot

weld does not conform to this

definition.

e. Slot Weld (fig. 6-28).

This is a weld made in an

elongated hole in one

member of a lap or tee joint joining that member to the

surface of the other member

that is exposed through the

hole.

This hole may be open at one

end and may be partially or

completely filled with weld

metal.

NOTE

A fillet welded slot does not

conform to this definition. f.

Fillet Weld (top, fig. 6-28).

This is a weld of approximately

triangular cross section joining

two surfaces at approximately

right angles to each other, as

in a lap or tee joint.

g. Flash Weld (fig. 6-29).

A weld made by flash welding

(para 6-5 d).

h. Seam Weld (fig. 6-29).

A weld made by arc seam or

resistance seam welding

(para 6-5 b). Where the

welding process is not

specified, this term infers

resistance seam welding.

i. Spot Weld (fig. 6-29).

A weld made by arc spot or

resistance spot welding (para

6-5 a). Where the weldi

process is not specified, t

term infers a resistance sp

weld.

j. Upset Weld (fig. 6-29).

A weld made by up

welding (para 6-5 e).

Section IV. WELDIN

POSITIONS

GENERAL Welding is oft

done on structures in t

position in which they a

found.

Techniques have be

developed to allow welding

any position. Some weldi

processes have all-positi

capabilities, while others m

be used in only one or twpositions.

All welding can be classifi

according to the position

the workpiece or the positi

of the welded joint on t

plates or sections bei

welded.


13/20


There are four basic welding

positions, which are illustrated

in figures 6-30 and 6-31.

Pipe welding positions are

shown in figure 6-32. Fillet,

groove, and surface welds

may be made in all of the

following positions.

FLAT POSITION WELDING In this

position, the welding is

performed from the upper

side of the joint, and the face

of the weld is approximately

horizontal.

Flat welding is the preferred

term; however, the same

position is sometimes called

downhand. (See view A,

figure 6-30 and view A, figure

6-31 for examples of flat

position welding for fillet and

groove welds).

HORIZONTAL POSITION

WELDING

The axis of a weld is a line

through the length of the

weld, perpendicular to the

cross section at its center of

gravity. a. Fillet Weld.

In this position, welding is

performed on the upper side

of an approximately horizontal

surface and against an

approximately vertical

surface.

View B, figure 6-31, illustrates a

horizontal fillet weld.

b. Groove Weld.

In this position, the axis of the

weld lies in an approximately

horizontal plane and the face

of the weld lies in an

approximately vertical plane.

View B, figure 6-30, illustrates a

horizontal groove weld.

c. Horizontal Fixed Weld.

In this pipe welding position,

the axis of the pipe is

approximately horizontal, and

the pipe is not rotated during

welding. Pipe welding

positions are shown in figure 6-

32.

d. Horizontal Rolled Weld.

In this pipe welding positio

welding is performed in t

flat position by rotating t

pipe. Pipe welding positio

are shown in figure 6-32.

VERTICAL POSITIO

WELDING a. In this position, taxis of the weld

approximately vertic

Vertical welding positions a

shown in view C, figures 6-

and 6-31.

b. In vertical position pi

welding, the axis of the pipe

vertical, and the welding

performed in the horizonposition.

The pipe may or may not

rotated. Pipe welding positio

are figure shown in figure 6-3

OVERHEAD POSITIO

WELDING In this weldi

position, the welding

performed from the undersi

of a joint. Overhead positiwelds are illustrated in view

figures 6-30 and 6-31.

POSITIONS FOR P

WELDING Pipe welds a

made under many differe

requirements and in differe

welding situations.


14/20


The welding position is

dictated by the job.

In general, the position is fixed,

but in sane cases can be

rolled for flat-position work.

Positions and procedures for

welding pipe are outlined

below.

http://www.weldguru.com/w

eldtypesandpositions.html

Arc-Welding

Introduction

Arc welding is the fusion of

two pieces of metal by an

electric arc between the

pieces being joined the work

pieces and an electrode

that is guided along the joint

between the pieces. The

electrode is either a rod that

simply carries current between

the tip and the work, or a rod

or wire that melts and supplies

filler metal to the joint.

The basic arc welding circuit is

an alternating current (AC) or

direct current (DC) power

source connected by a

work cable to the work

piece and by a hot cable to

an electrode. When the

electrode is positioned close

to the work piece, an arc is

created across the gap

between the metal and the

hot cable electrode. An

ionized column of gasdevelops to complete the

circuit.

The arc produces a

temperature of about

3600C at the tip and melts

part of the metal being

welded and part of the

electrode. This produces a

pool of molten metal that

cools and solidifies behind the

electrode as it is moved alongthe joint.

There are two types of

electrodes. Consumable

electrode tips melt, and

molten metal droplets detach

and mix into the weld pool.

Non-consumable electrodes

do not melt. Instead, fi

metal is melted into the jo

from a separate rod or wire.

The strength of the weld

reduced when metals at hi

temperatures react w

oxygen and nitrogen in the

to form oxides and nitrid

Most arc welding proces

minimize contact betwe

the molten metal and the

with a shield of gas, vapour

slag. Granular flux,

example, adds deoxidiz

that create a shield to prote

the molten pool, th

improving the weld.

GAS TUNGSTEN ARC WELDIN

TIG WELDING

INTRODUCTION

The American Weldi

Societys preferred name

this arc welding process is G

Tu

ngsten Arc Welding As tname implies the process us

an external Gas supply and

Tungsten electrode

produce an arc that me

and fuses the metal to

welded with or without the u

of a filler wire.

The term TIG WELDING is

common shop term that

derived from t

name TUNGSTEN INERT GASWELDING. This nam

describes the same proc

and highlights the fact that

Tungsten electrode and

external inert gas are used

produce a weld.

In this text the term GTA

(Gas Tungsten A


15/20


Welding) will be used to

discuss this process.

The Gas Tungsten Arc Welding

process may be used to weld

most metals and alloys in any

position with or without the use

of filler wire. Because of the

smaller heat zone and weld

puddle with the excellent

shielding effect of the gases

used, the welds produced are

often stronger than welds

made with other processes.

Although GTAW is slower and

produces smaller weld beads

than SMAW or GMAW it is

often the process of choice

for welding thinner sections,Aluminum, specialty metals

and Stainless steels.

GTAW EQUIPMENT SET UP

Gas Tungsten Arc Welding is

done by setting up a Torch to

a Constant Current Welding

Machine and an external Gas

Supply to shield the weldarea. An optional Foot

Pedal may be used to

remotely control the

amperage during

welding. Filler Wire may be

added as necessary or

welding may be done by

fusing the parts with the

molten weld puddle.

The major component parts of

the welding circuit described

below are:

1. THE GTAW OR TIG TORCH

2. CONSTANT CURRENT

MACHINE

3. A GAS SUPPLY

4. A FOOT PEDAL

5. FILLER WIRE

GAS METAL ARC WELDING

(MIG)

INTRODUCTION

The term Gas Metal ArcWelding is the American

Welding Societys preferred

name for this semi-automatic

welding process that uses a

wire feeder to deliver the filler

metal to a hand operated

gun to produce the weld.

The process is also widely

known by the shop name MIG(Metal Inert Gas). The name

Metal Inert Gas was used

when the process was first

developed to weld Aluminum

using an inert (chemically non

reactive) gas supply. The

process has evolved to

become a favorite choice for

welding steel with gases that

are not inert.

When compared with Stick

welding, the mig welding

process is faster, easier, and

requires little cleanup of

welds. This makes Mig welding

cost effective for production

welding in fabrication shops.

The wire fed welding arc is

capable of joining t

sections and bridging gaps

poor fit up situations.

THE MIG WELDING CIRCUIT

Welding is done by using

constant voltage weldi

machine to supply the pow

a wire feed unit with

attached gun to feed the fi

wire to the arc, and a g

supply system to shield t

weld area.

MAKING THE WELD

The voltage, wire speed a

gas flow are set by the weld

according to recommend

ranges for the applicati

before welding.

After positioning the gun, t

welder pulls and holds t

trigger to start the gas flo

and the arc. The welder th

controls the nozzle distan

from the work, the angle

the gun, and rate of tra

speed across the joint. At t

end of the joint the trigge

released to stop the wire fee

gas flow, and break the arc


16/20


MAJOR EQUIPMENT FOR MIG

WELDING

In addition to the safety

clothing and hand tools

generally used in the welding

trade the major parts of the

Mig welding process are:

1. THE WELDING MACHINE

(POWER SOURCE)

2. THE WIRE FEED UNIT

3. WELDING GUN

4. SHIELDING GAS SUPPLY

Shielded Metal Arc Welding

(SMAW)

Shielded Metal Arc Welding

(SMAW) is defined as "an arc

welding process in which

coalescence of metals is

produced by heat from an

electric arc that is maintained

between the tip of a flux

covered electrode and thesurface of the base metal in

the joint being welded." This

process is commonly referred

to as stick welding.

The electrode consists of a

solid metal core, which is

covered by a metallic

coating. The coatings

composition is dependent on

the type of electrode and

welding polarity. It serves

various functions during the

welding process. These

include; provide a shielding

agent from the atmosphere

which protects the molten

pool; act as fluxing agents to

cleanse the weld metal

deposit, establish electrical

characteristics of the

electrode, provide a slag

covering during cooling which

can improve weld properties,

enhance the ability to weld

out of position, improve beadprofile and appearance, and

can add alloying elements to

the weld to affect

mechanical properties.

SMAW is the most widely used

welding process in the US and

the world. The equipment cost

is low and can be portable, it

can be done in areas oflimited access, it can be done

in all positions and it is a viable

process for joining most metals

and alloys. The low hydrogen

(LH) mild steel electrodes are

the most commonly used and

represent up to 90% of the

total market. All stick

electrode readily absorb

moisture which will

detrimentally effect weld

quality. The LH being the most

affected and should be stored

in an electrode oven before

use.

SMAW is typically done with

DC current, either electrode

positive or negative, but can

also be used with AC curre

Electrode positive produc

higher penetration patte

and typically operate bett

but electrode negative resu

in the highest melting ra

Electrodes with high magne

properties can experience a

blow with DC current a

may be welded with A

setting. These include iron a

nickel alloy rods. Also the A

power sources are l

expensive. The proc

limitations include relativ

low deposition rates and du

cycles, low efficiency, hi

spatter and fume generatiand slag removal is oft

required upon we

completion.

http://www.praxair.com/pra

air.nsf/7a1106cc7ce1c54e85

56a9c005accd7/166df4d340

8e56485256570007de7bc?o

ndocument

OXY-ACETYLENE WELDINAND CUTTING

As mentioned in the previo

chapter, gas welding, usi

oxygen and hydrogen, da

back to the 1850s.

However, the oxy-hydrog

flame is virtually useless

welding steel. Gas welding,

be broadly useful, had to

await the discovery of tremarkable properties of t

oxy-acetylene flame, and o

way to make acetylene at

reasonable cost. These eve

took place in the 1890s.

Acetylene gas is unknown

nature. Edmund Davy,

famous British chemist,


17/20


generally considered the first

man

to make acetylene. In 1836,

attempting to produce

potassium metal, he came up

with a black compound

(potassium carbide) which

reacted with water to

produce a gas which burned

with great brilliancy. He

thought it

would make an excellent

illuminating gas if it could be

produced at moderate cost.

That was not possible, using

potassium carbide as the

starting point. Calcium

carbide (which, likeacetylene, does not exist in

nature) was not

made and identified until

1862. Like potassium carbide,

it reacted with water to form

acetylene. Again, the process

by which it was first made did

not offer economic

possibilities.

Although calcium carbidewas undoubtedly accidentally

produced in electric furnace

operations before 1892, not

until that year was it produced

and identified both in France

and the U.S. In both cases, the

experimenters were

trying to make something else.

The Frenchman did not

immediately recognize the

potential commercial value of

what he had created. The

Americans did.

Major J. Turner Morehead and

Thomas L. Wilson, using an

electric furnace they had set

up in Spray, N.C., were

attempting to make calcium

metal from a mixture of

quicklime and coal tar. If

successful, they hoped to use

the

calcium to reduce aluminum

oxide and come up with

aluminum metal. However,

the product of their electric

furnace run was a dark

crystalline mass which reacted

violently with water. They had

found a way to make

acetylene economically, and

they were not slow to

recognize the value of their

discovery. Because they sent

a

sample of the calcium

carbide to Lord Kelvin inEngland, together with details

on the method by which they

had

produced it, by 1895 calcium

carbide plants were operating

in both England and France,

as well as in the U.S.

Plants in Norway and

Switzerland followed close

behind.

OXYGEN AND ACETYLENEThis

chapter will deal with the two

gases which, burned together,

produce the oxy-acetylene

flame. It will cover

theirproperties, their

production, their commercial

distribution, the containers in

which they are stored and

distributed,and the

precautions which should be

observed when using the

gases or handling and storing

the

containers.OxygenOxygen,

which makes up about

percent of the air we norma

breathe, as well as about

percent by weight ofall t

water on earth, may

considered the m

important element in t

universe. Without it, the

would be nolife as we know

Every living animal bur

oxygen with carbon a

hydrogen to produce t

energy that it needsin order

live, grow, and mov

Fortunately for the anim

kingdom, all green pla

produce more oxyg

thanthey consume, so that t

reservoir of oxygen in o

atmosphere remains at

constant level from cent

tocentury.Oxygen not o

combines with carbon a

hydrogen to produce ener

(heat), but combines w

most of theother eleme

found in the univer

including all meta

Fortunately, its reaction w

most eleme

andcompounds takes pla

very slowly or not at all

normal temperatur

However, almost everythi

made uppredominantly

carbon and hydrogen (co

wood, petroleum produc

has a kindling temperatur

Once thattemperature

reached, oxidatio

suddenly becomes burning


18/20


which then proceeds to

produce enough heat

tomaintain the reaction until

the supply of oxygen or fuel

runs out, or until other

influences produce enough

coolingeffect to quench the

fire.Its perhaps fortunate we

have only 21 percent oxygen

in our atmosphere, and that

78 percent is made up

ofnitrogen, which wont

combine with oxygen at any

temperature normally

reached by the burning of

other materials.We dont often

think of it in that way, but the

nitrogen acts as a cooling

agent. A good part of the

energy producedby the

burning of carbon and

hydrogen in air is used up in

heating the nitrogen. In an

atmosphere of 100

percentoxygen, burning takes

place at a greatly

accelerated rate. Given such

an atmosphere, a wooden

house that caught fire would

probably burn flat in a matter

of minutes, rather than hours. If

theres one thing you must

rememberabout oxygen, its

that things burnmuch faster in

pure oxygen (or even in a

mixture of half oxygen, half

nitrogen)than they do in air.

Thats why passing a lighted

cigarette to a person in an

oxygen tent is almost

equivalent tosigning his death

warrant. The other thing you

must remember is this: that

when surrounded by pure

oxygen, someoils and greases

oxidize rapidly, fast enough to

reachkindling temperature in

a short time. Thats why you

mustalways keep oxygen

away from oils and grease,

and keep oil and grease from

getting into an oxygen

regulator orhose. The only

lubricants which can be used

with oxy-acetylene apparatus

and then only on threads

and O-rings are special

products approved for such

use.AcetyleneAcetylene is a

hydrocarbon, just as are

propane, methane, and

virtually all the components

which make upgasoline and

fuel oils. However, it differs

from those hydrocarbons in

this respect: in the acetylene

molecule, madeup of two

carbon atoms and two

hydrogen atoms, the carbon

atoms are joined by what

chemists call a triplebond.

When acetylene reaches its

kindling temperature (and

under some other conditions

as well, which wellcover

shortly) the bond breaks

andreleases energy. In other

hydrocarbons, the breaking of

the bonds between

thecarbon atomsabsorbs

energy. The triple bond is the

reason why the oxy-acetylene

flame is hotter than t

flameproduced by burni

any other hydrocarbon g

with oxygen.Acetylene

almost unknown in the natu

world. There are ways

produce acetylene fro

natural gas, but theya

economical only on a lar

scale. Virtually all t

acetylene distributed

welding and cutting use

created byallowing calciu

carbide, an electric furna

product, to react with wat

As mentioned in Chapter

the discoveryof the elect

furnace method of produci

calcium carbide w

accidental. It turned out to

a lucky accident.The ni

thing about the calciu

carbide method of produci

acetylene is that it can

done on almost a

scaledesired. In tightly-seal

cans, calcium carbide kee

indefinitely. For years, mine

lamps produced acetyle

byadding water, a drop at

time, to lumps of carbid

Before acetylene in cylind

became available in alm

everycommunity

appreciable size, as it is toda

many users of acetyle

produced their own gas fro

calciumcarbide, usi

acetylene generators whi

ranged in output from as lit


19/20


as 20 to as much as 1000

cubic feet perhour (cfh).

Resistance Welding

Resistance Spot Welding

(RSW), Resistance Seam

Welding (RSEW), and

Projection Welding (PW) are

commonly used resistance

welding processes. Resistance

welding uses the application

of electric current and

mechanical pressure to

create a weld between two

pieces of metal. Weld

electrodes conduct the

electric current to the two

pieces of metal as they are

forged together.

The welding cycle must first

develop sufficient heat to

raise a small volume of metal

to the molten state. This metal

then cools while under

pressure until it has adequate

strength to hold the parts

together. The current density

and pressure must be

sufficient to produce a weld

nugget, but not so high as to

expel molten metal from theweld zone.

Resistance Welding Benefits

High speed welding

Easily automated

y Suitable for high rateproduction

y EconomicalResistance Welding Limitations

yInitial equipment costs

y Lower tensile and fatiguestrengths

y Lap joints add weight andmaterial

Common Resistance Welding

Concerns

We can help optimize your

welding process variables.

Evaluate your current welding

parameters and techniques.

Help eliminate common

welding problems and

discontinuities such as those

listed below:

Resistance Welding

Problems and Discontinuities

y Cracksy Electrode deposit on worky Porosity or cavitiesy Pin holesy Deep electrode indentationy Improper weld penetrationy Surface appearancey Weld sizey Irregular shaped welds

A process in which the he

for producing the weld

generated by the resistan

to the flow of current throu

the parts to be joined. T

application of external force

required; however, no flux

filler metals, or external he

sources are necessary. M

metals and their alloys can

successfully joined

resistance welding processe

Several methods are classifi

as resistance weldi

processes: spot, r

spot, seam, projection, upsflash, and percussion.

In resistance spot weldin

coalescence at the fayi

surfaces is produced in o

spot by the heat obtain

from the resistance to elect

current through the work pa

held together under pressu

by electrodes. The size ashape of the individua

formed welds are limit

primarily by the s

and contour of t

electrodes. See also Sp

welding.


20/20


In roll resistance spot welding,

separated resistance spot

welds are made with one or

more rotating circular

electrodes. The rotation of the

electrodes may or may not be

stopped during the making ofa weld.

In resistance seam welding,

coalescence at the faying

surfaces is produced by the

heat obtained from resistance

to electric current through the

work parts held together

under pressure by electrodes.

The resulting weld is a series of

overlapping resistance spot

welds made progressively

along a joint by rotating the

electrodes.

In projection welding,

coalescence is produced by

the heat obtained from

resistance to electric current

through the work parts held

together under pressure byelectrodes. The resulting welds

are localized

at predetermined points by

projections, embossments, or

intersections.

In upset welding,

coalescence is produced

simultaneously over the entire

area of abutting surfaces orprogressively along a joint, by

the heat obtained from

resistance to electric current

through the area of contact

of those surfaces. Pressure is

applied before heating is

started and is maintained

throughout the heating

period.

In flash welding, coalescence

is produced simultaneously

over the entire area of

abutting surfaces by the heatobtained from resistance to

electric current between the

two surfaces and by the

application of pressure after

heating is substantially

completed. Flash and

upsetting are accompanied

by expulsion of the metal from

the joint.See also Flash

welding.

In percussion welding,

coalescence is produced

simultaneously over the entire

abutting surfaces by the heat

obtained from an arc

produced by a rapid

discharge of electrical energy

with pressure percussively

applied during or immediately

following the electricaldischarge.

Most metals and alloys can be

resistance-welded to

themselves and to each other.

The weld properties are

determined by the metal and

by the resultant alloys which

form during the welding

process. Stronger metals andalloys require higher electrode

forces, and poor electrical

conductors require less

current. Copper, silver, and

gold, which are excellent

electrical conductors, are very

difficult to weld because they

require high current densities

to compensate for their lo

resistance. Medium- and hig

carbon steels, whi

are hardened and embrittl

during the normal weldi

process, must be tempered

multiple impulses.

welding, reviewer

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