Curso CwiLesson 1_1 Total

The Basics of ArcWelding

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Common Electric ArcWelding Processes

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Covered Electrodes forWelding Mild Steels

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HardsurfacingElectrodes

Carbon & Low AlloySteel Filler Metals for

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Welding Filler Metalsfor Stainless Steels

Covered Electrodes forWelding Low Alloy

Steels

Flux Cored Arc WeldingElectrodes for Carbon &

Low Alloy Steels

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Estimating andComparing Weld Metal

Costs

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Reliability of WeldingFiller Metals

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Current ChapterTable ofContents

BASICWELDING FILLER METAL

TECHNOLOGY

A Correspondence Course

LESSON ITHE BASICS OF ARC WELDING

©COPYRIGHT 2000 THE ESAB GROUP, INC.

ESAB ESAB Welding &Cutting Products

! An Introduction to Metals! Electricity for Welding

-i-© COPYRIGHT 1998 THE ESAB GROUP, INC.


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TABLE OF CONTENTSLESSON I

THE BASICS OF ARC WELDING

PART A. AN INTRODUCTION TO METALS

Section Nr. Section Title Page

1.1 Source and Manufacturing ............................................................. 1

1.1.1 Rimmed Steel ................................................................................... 2

1.1.2 Capped Steel .................................................................................... 2

1.1.3 Killed Steel ........................................................................................ 3

1.1.4 Semi-Killed Steel............................................................................... 3

1.1.5 Vacuum Deoxidized Steel ................................................................. 3

1.2 Classification of Steels ................................................................... 3

1.2.1 Carbon Steel ..................................................................................... 3

1.2.2 Low Alloy Steel .................................................................................. 3

1.2.3 High Alloy Steel ................................................................................. 4

1.3 Specifications ................................................................................. 5

1.4 Crystalline Structure of Metals ...................................................... 6

1.4.1 Grains and Grain Boundaries ........................................................... 7

1.5 Heat Treatment ................................................................................ 8

1.5.1 Preheat ............................................................................................. 8

1.5.2 Stress Relieving ................................................................................ 9

1.5.3 Hardening ......................................................................................... 9

1.5.4 Tempering ......................................................................................... 9

1.5.5 Annealing .......................................................................................... 9

1.5.6 Normalizing ....................................................................................... 10

1.5.7 Heat Treatment Trade-Off ................................................................. 10

1.6 Properties of Metals ........................................................................ 10

1.6.1 Tensile Strength ................................................................................ 10

1.6.2 Yield Strength.................................................................................... 11

1.6.3 Ultimate Tensile Strength .................................................................. 11

1.6.4 Percentage of Elongation ................................................................. 11

-ii-© COPYRIGHT 1998 THE ESAB GROUP, INC.


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1.6.5 Reduction of Area ............................................................................. 11

1.6.6 Charpy Impacts ................................................................................. 11

1.6.7 Fatigue Strength ............................................................................... 12

1.6.8 Creep Strength.................................................................................. 13

1.6.9 Oxidation Resistance ........................................................................ 13

1.6.10 Hardness Test ................................................................................... 13

1.6.11 Coefficient of Expansion ................................................................... 14

1.6.12 Thermal Conductivity ........................................................................ 14

1.7 Effects of Alloying Elements .......................................................... 14

1.7.1 Carbon .............................................................................................. 14

1.7.2 Sulphur ............................................................................................. 14

1.7.3 Manganese ....................................................................................... 15

1.7.4 Chromium ......................................................................................... 15

1.7.5 Nickel ................................................................................................ 15

1.7.6 Molybdenum ..................................................................................... 15

1.7.7 Silicon ............................................................................................... 15

1.7.8 Phosphorus....................................................................................... 15

1.7.9 Aluminum .......................................................................................... 15

1.7.10 Copper .............................................................................................. 15

1.7.11 Columbium........................................................................................ 16

1.7.12 Tungsten ........................................................................................... 16

1.7.13 Vanadium .......................................................................................... 16

1.7.14 Nitrogen ............................................................................................ 16

1.7.15 Alloying Elements summary ............................................................. 16

PART B. ELECTRICITY FOR WELDING


1.8 Electricity for Welding ....................................................................... 17

1.8.1 Principles of Electricity ...................................................................... 17

1.8.2 Ohm’s Law ........................................................................................ 18

1.8.3 Electrical Power ................................................................................ 19

1.8.4 Power Generation ............................................................................. 20

-iii-© COPYRIGHT 1998 THE ESAB GROUP, INC.


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1.8.5 Transformers .................................................................................... 22

1.8.6 Power Requirements ........................................................................ 24

1.8.7 Rectifying AC to DC .......................................................................... 25

1.9 Constant Current or Constant Voltage .............................................. 26

1.9.1 Constant Current Characteristics ...................................................... 26

1.9.2 Constant Voltage Characteristics ...................................................... 26

1.9.3 Types of Welding Power Sources ..................................................... 27

1.9.4 Power Source Controls ..................................................................... 28

Appendix A Glossary of Terms ............................................................................. 29

-1-© COPYRIGHT 1999 THE ESAB GROUP, INC.

LESSON I, PART AThe Basics of Arc

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Current ChapterTable ofContentsAN INTRODUCTION TO METALS

1.1 SOURCE AND MANUFACTURING

Metals come from natural deposits of ore in the earth’s crust. Most ores are contaminated

with impurities that must be removed by mechanical and chemical means. Metal extracted

from the purified ore is known as primary or virgin metal, and metal that comes from scrap

is called secondary metal. Most mining of metal bearing ores is done by either open pit or

underground methods. The two methods of mining employed are known as “selective” in

which small veins or beds of high grade ore are worked, and “bulk” in which large quantities

of low grade ore are mined to extract a high grade portion.

1.1.0.1 There are two types of ores, ferrous and nonferrous. The term ferrous comes

from the Latin word “ferrum” meaning iron, and a ferrous metal is one that has a high iron

content. Nonferrous metals, such as copper and aluminum, are those that contain little or

no iron. There is approximately 20 times the tonnage of iron in the earth’s crust compared

to all other nonferrous products combined; therefore, it is the most important and widely

used metal.

1.1.0.2 Aluminum, because of its attractive appearance, light weight and strength, is the

next most widely used metal. Commercial aluminum ore, known as bauxite, is a residual

deposit formed at or near the earth’s surface.

1.1.0.3 Some of the chemical processes that occur during steel making are repeated

during the welding operation and an understanding of welding metallurgy can be gained by

imagining the welding arc as a miniature steel mill.

1.1.0.4 The largest percentage of commercially produced iron comes from the blast

furnace process. A typical blast furnace is a circular shaft approximately 90 to 100 feet in

height with an internal diameter of approximately 28 feet. The steel shell of the furnace is

lined with a refractory material, usually a hard, dense clay firebrick.

1.1.0.5 The iron blast furnace utilizes the chemical reaction between a solid fuel charge

and the resulting rising column of gas in the furnace. The three different materials used for

the charge are ore, flux and coke. The ore consists of iron oxide about four inches in

diameter. The flux is limestone that decomposes into calcium oxide and carbon dioxide.

The lime reacts with impurities in the ore and floats them to the surface in the form of a

slag. Coke, which is primarily carbon, is the ideal fuel for blast furnaces because it

produces carbon monoxide gas, the main agent for reducing iron ore into iron metal.



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Current ChapterTable ofContents1.1.0.6 The basic operation of the blast furnace is to reduce iron oxide to iron metal and

to remove impurities from the metal. Reduced elements pass into the iron and oxidized

elements dissolve into the slag. The metal that comes from the blast furnace is called pig

iron and is used as a starting material for further purification processes.

1.1.0.7 Pig iron contains excessive amounts of elements that must be reduced before

steel can be produced. Different types of furnaces, most notably the open hearth, electric

and basic oxygen, are used to continue this refining process. Each furnace performs the

task of removing or reducing elements such as carbon, silicon, phosphorus, sulfur and

nitrogen by saturating the molten metal with oxygen and slag forming ingredients. The

oxygen reduces elements by forming gases that are blown away and the slag attracts

impurities as it separates from the molten metal.

1.1.0.8 Depending upon the type of slag that is used, refining furnaces are classed as

either acid or basic. Large amounts of lime are contained in basic slags and high quantities

of silica are present in acid slags. This differential between acid and basic slags is also

present in welding electrodes for much of the same refining process occurs in the welding

operation.

1.1.0.9 After passing through the refining furnace, the metal is poured into cast iron ingot

molds. The ingot produced is a rather large square column of steel. At this point, the metal

is saturated with oxygen. To avoid the formation of large gas pockets in the cast metal, a

substantial portion of the oxygen must be removed. This process is known as deoxidation,

and it is accomplished through additives that tie up the oxygen either through gases or in

slag. There are various degrees of oxidation, and the common ingots resulting from each

are as follows:

1.1.1 Rimmed Steel - The making of rimmed steels involves the least deoxidation. As

the ingots solidify, a layer of nearly pure iron is formed on the walls and bottom of the mold,

and practically all the carbon, phosphorus, and sulfur segregate to the central core. The

oxygen forms carbon monoxide gas and it is trapped in the solidifying metal as blow holes

that disappear in the hot rolling process. The chief advantage of rimmed steel is the excel-

lent defect-free surface that can be produced with the aide of the pure iron skin. Most

rimmed steels are low carbon steels containing less than .1% carbon.

1.1.2 Capped Steel - Capped steel regulates the amount of oxygen in the molten

metal through the use of a heavy cap that is locked on top of the mold after the metal is

allowed to reach a slight level of rimming. Capped steels contain a more uniform core

composition than the rimmed steels. Capped steels are, therefore, used in applications



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Current ChapterTable ofContentsthat require excellent surfaces, a more homogenous composition, and better mechanical

properties than rimmed steel.

1.1.3 Killed Steel - Unlike rimmed or capped steel, killed steel is made by completely

removing or tying up the oxygen before the ingot solidifies to prevent the rimming action.

This removal is accomplished by adding a ferro-silicon alloy that combines with oxygen to

form a slag, leaving a dense and homogenous metal.

1.1.4 Semi-killed Steel - Semi-killed steel is a compromise between rimmed and killed

steel. A small amount of deoxidizing agent, generally ferro-silicon or aluminum, is added.

The amount is just sufficient to kill any rimming action, leaving some dissolved oxygen.

1.1.5 Vacuum Deoxidized Steel - The object of vacuum deoxidation is to remove

oxygen from the molten steel without adding an element that forms nonmetallic inclusions.

This is done by increasing the carbon content of the steel and then subjecting the molten

metal to vacuum pouring or steam degassing. The carbon reacts with the oxygen to form

carbon monoxide, and as a result, the carbon and oxygen levels fall within specified limits.

Because no deoxidizing elements that form solid oxides are used, the steel produced by

this process is quite clean.

1.2 CLASSIFICATIONS OF STEEL

The three commonly used classifications for steel are: carbon, low alloy, and high alloy.

These are referred to as the “type” of steel.

1.2.1 Carbon Steel - Steel is basically an alloy of iron and carbon, and it attains its

strength and hardness levels primarily through the addition of carbon. Carbon steels are

classed into four groups, depending on their carbon levels.

Low Carbon Up to 0.15% carbon

Mild Carbon Steels .15% to 0.29% carbon

Medium Carbon Steels .30% to 0.59% carbon

High Carbon Steels .60% to 1.70% carbon

1.2.1.1 The largest tonnage of steel produced falls into the low and mild carbon steel

groups. They are popular because of their relative strength and ease with which they can

be welded.

1.2.2 Low Alloy Steel - Low alloy steel, as the name implies, contains small amounts

of alloying elements that produce remarkable improvements in their properties. Alloying



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Current ChapterTable ofContentselements are added to improve strength and toughness, to decrease or increase the

response to heat treatment, and to retard rusting and corrosion. Low alloy steel is gener-

ally defined as having a 1.5% to 5% total alloy content. Common alloying elements are

manganese, silicon, chromium, nickel, molybdenum, and vanadium. Low alloy steels may

contain as many as four or five of these alloys in varying amounts.

1.2.2.1 Low alloy steels have higher tensile and yield strengths than mild steel or carbon

structural steel. Since they have high strength-to-weight ratios, they reduce dead weight in

railroad cars, truck frames, heavy equipment, etc.

1.2.2.2 Ordinary carbon steels, that exhibit brittleness at low temperatures, are unreliable

in critical applications. Therefore, low alloy steels with nickel additions are often used for

low temperature situations.

1.2.2.3 Steels lose much of their strength at high temperatures. To provide for this loss

of strength at elevated temperatures, small amounts of chromium or molybdenum are

added.

1.2.3 High Alloy Steel - This group of expensive and specialized steels contain alloy

levels in excess of 10%, giving them outstanding properties.

1.2.3.1 Austenitic manganese steel contains high carbon and manganese levels, that

give it two exceptional qualities, the ability to harden while undergoing cold work and great

toughness. The term austenitic refers to the crystalline structure of these steels.

1.2.3.2 Stainless steels are high alloy steels that have the ability to resist corrosion. This

characteristic is mainly due to the high chromium content, i.e., 10% or greater. Nickel is

also used in substantial quantities in some stainless steels.

1.2.3.3 Tool steels are used for cutting and forming operations. They are high quality

steels used in making tools, punches, forming dies, extruding dies, forgings and so forth.

Depending upon their properties and usage, they are sometimes referred to as water

hardening, shock resisting, oil hardening, air hardening, and hot work tool steel.

1.2.3.4 Because of the high levels of alloying elements, special care and practices are

required when welding high alloy steels.



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Current ChapterTable ofContents1.3 SPECIFICATIONS

Many steel producers have developed steels that they market under a trade name such as

Cor-Ten, HY-80, T-1, NA-XTRA, or SS-100, but usually a type of steel is referred to by its

specification. A variety of technical, governmental and industrial associations issue

specifications for the purpose of classifying materials by their chemical composition,

properties or usage. The specification agencies most closely related to the steel industry

are the American Iron and Steel Institute (AISI), Society of Automotive Engineers (SAE),

American Society for Testing and Materials (ASTM), and the American Society of

Mechanical Engineers (ASME).

1.3.0.1 The American Iron and Steel Institute (AISI) and the Society of Automobile

Engineers (SAE) have collaborated in providing identical numerical designations for their

specifications. The first two digits of a four digit index number refer to a series of steels

classified by their composition or alloy combination. While the last two digits, which can

change within the same series, give an approximate average of the carbon range. For

example, the first two digits of a type 1010 or 1020 steel indicate a “10” series that has

carbon as its main alloy. The last two digits indicate an approximate average content of

.10% or .20% carbon, respectively. Likewise, the “41” of a 4130 type steel refers to a group

that has chromium and molybdenum as their main alloy combination with approximately

.30% carbon content.

1.3.0.2 The AISI classifications for certain alloys, such as stainless steel, are somewhat

different. They follow a three digit classification with the first digit designating the main

alloy composition or series. The last two digits will change within a series, but are of an

arbitrary nature being agreed upon by industry as a designation for certain compositions

within the series. For example, the “3” in a 300 series of stainless steel indicates chromium

and nickel as the main alloys, but a 308 stainless has a different overall composition than a

347 type. The “4” of a 400 series indicates the main alloy as chromium, but there are

different types such as 410, 420, 430, and so forth within the series.

1.3.0.3 The American Society for Testing and Materials (ASTM) is the largest

organization of its kind in the world. It has compiled some 48 volumes of standards for

materials, specifications, testing methods and recommended practices for a variety of

materials ranging from textiles and plastics to concrete and metals.

1.3.0.4 Two ASTM designated steels commonly specified for construction are A36-77

and A242-79. The prefix letter indicates the class of a material. In this case, the letter “A”

indicates a ferrous metal, the class of widest interest in welding. The numbers 36 and 242



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are index numbers. The 77 and 79 refer to the year that the standards for these steels

were originally adopted or the date of their latest revision.

1.3.0.5 The ASTM designation may be further subdivided into Grades or Classes. Since

many standards for ferrous metals are written to cover forms of steel (i.e., sheet, bar, plate,

etc.) or particular products fabricated from steel (i.e., steel rail, pipe, chain, etc.), the user

may select from a number of different types of steel under the same classification. The

different types are than placed under grades or classes as a way of indicating the

differences in such things as chemistries, properties, heat treatment, etc. An example of a

full designation is A285-78 Grade A or A485-79 Class 70.

1.3.0.6 The American Society of Mechanical Engineers (ASME) maintains a widely used

ASME Boiler and Pressure Vessel Code. The material specification as adopted by the

ASME is identified with a prefix letter “S”, while the remainder is identical with ASTM with

the exception that the date of adoption or revision by ASTM is not shown. Therefore, a

common example of an ASME classification is SA 387 Grade 11, Class 1.

1.4 CRYSTALLINE STRUCTURE OF METALS

When a liquid metal is cooled, its atoms will assemble into a regular crystal pattern and we

say the liquid has solidified or crystallized. All metals solidify as a crystalline material. In a

crystal the atoms or molecules are held in a fixed position and are not free to move about

as are the molecules of a liquid or gas. This fixed position is called a crystal lattice. As the

temperature of a crystal is raised, more thermal energy is absorbed by the atoms or

molecules and their movement increases. As the distance

between the atoms increases, the lattice breaks down and

the crystal melts. If a lattice contains only one type of atom,

as in pure iron, the conditions are the same at all points

throughout the lattice, and the crystal melts at a single

temperature (see Figure 1).

FIGURE 1

TE

MP

ER

AT

UR

E °

F

4000

3000

2000

1000

TIMESOLID-LIQUID TRANSFORMATION, PURE IRON

LIQUID

2795°F

SOLID



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Current ChapterTable ofContents1.4.0.1 However, if the lattice contains two

or more types of atoms, as in any alloy-steel,

it may start to melt at one temperature but not

be completely molten until it has been heated

to a higher temperature (See Figure 2). This

creates a situation where there is a

combination of liquids and solids within a

range of temperatures.

1.4.0.2 Each metal has a characteristic

crystal structure that forms during

solidification and often remains the permanent

form of the material as long as it remains at

room temperature. However, some metals

may undergo an alteration in the crystalline

form as the temperature is changed. This is known as phase transformation. For example,

pure iron solidifies at 2795°F, the delta structure transforms into a structure referred to as

gamma iron. Gamma iron is commonly known as austenite and is a nonmagnetic

structure. At a temperature of 1670°F., the pure iron structure transforms back to the delta

iron form, but at this temperature, the metal is known as alpha iron. These two phases are

given different names to differentiate between the high temperature phase (delta) and the

low temperature phase (alpha). The capability of the atoms of a material to transform into

two or more crystalline structures at different temperatures is defined as allotropic. Steels

and iron are allotropic metals.

1.4.1 Grains and Grain Boundaries - As the metal is cooled to its freezing point, a

small group of atoms begin to assemble into crystalline form (refer to Figure 3). These

small crystals scattered throughout the body of the liquid are oriented in all directions and

as solidification continues, more crystals are formed from the surrounding liquid. Often,

they take the form of dendrites, or a treelike structure. As crystallization continues, the

crystals begin to touch one another, their free growth hampered, and the remaining liquid

freezes to the adjacent crystals until solidification is complete. The solid is now composed

of individual crystals that usually meet at different orientations. Where these crystals meet

is called a grain boundary.

1.4.1.2 A number of conditions influence the initial grain size. It is important to know that

cooling rate and temperature has an important influence on the newly solidified grain

structure and grain size. To illustrate differences in grain formation, let's look at the cooling

phases in a weld.

FIGURE 2

TIMESOLID-LIQUID TRANSFORMATION, ALLOY METAL

TE

MP

ER

AT

UR

E °

F

4000

3000

2000

1000

LIQUID

SOLID &LIQUID

SOLID



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1.4.1.3 Initial crystal formation begins at the coolest spot in the weld. That spot is at the

point where the molten metal and the unmelted base metal meet. As the metal continues

to solidify, you will note that the grains in the center are smaller and finer in texture than the

grains at the outer boundaries of the weld deposit. This is explained by the fact that as the

weld metal cools, the heat from the center of the weld deposit will dissipate into the base

metal through the outer grains that solidified first. Consequently, the grains that solidified

first were at high temperatures for a longer time while in the solid state and this is a

situation that encourages grain growth. Grain size can have an effect on the soundness of

the weld. The smaller grains are stronger and more ductile than the larger grains. If a

crack occurs, the tendency is for it to start in the area where the grains are largest.

1.4.1.4 To summarize this section, it should be understood that all metals are composed

of crystals of grains. The shape and characteristics of crystals are determined by the

arrangement of their atoms. The atomic pattern of a single element can change its

arrangement at different temperatures, and that this atomic pattern or microstructure

determines the properties of the metals.

1.5 HEAT TREATMENT

The temperature that metal is heated, the length of time it is held at that temperature, and

the rate that it is cooled, all have an effect on a metal's crystalline structure. This crystalline

structure, commonly referred to as "microstructure," determines the specific properties of

metals. There are various ways of manipulating the microstructure, either at the steel mill

or in the welding procedure. Some of the more common ways are as follows:

1.5.1 Preheat - Most metals are rather good conductors of heat. As a result, the heat

in the weld area is rapidly dispersed through the whole weldment to all surfaces where it is

radiated to the atmosphere causing comparatively rapid cooling. In some metals, this rapid

cooling may contribute to the formation of microstructures in the weld zone that are detri-

mental. Preheating the weldment before it is welded is a method of slowing the cooling

FIGURE 3

GRAINBOUNDARIES

DENDRITE INITIAL COMPLETEFORMATION CRYSTAL FORMATION SOLIDIFICATION

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rate of the metal. The preheat temperature may vary from 150°F to 1000°F, but more

commonly it is held in the 300°F to 400°F range. The thicker the weld metal, the more

likely will it be necessary to preheat, because the heat will be conducted away from the

weld zone more rapidly as the mass increases.

1.5.2 Stress Relieving - Metals expand when heated and contract when cooled. The

amount of expansion is directly proportional to the amount of heat applied. In a weldment,

the metal closest to the weld is subjected to the highest temperature, and as the distance

from the weld zone increases, the maximum temperature reached decreases. This nonuni-

form heating causes nonuniform expansion and contraction and can cause distortion and

internal stresses within the weldment. Depending on its composition and usage, the metal

may not be able to resist these stresses and cracking or early failure of the part may occur.

One way to minimize these stresses or to relieve them is by uniformly heating the structure

after it has been welded. The metal is heated to temperatures just below the point where a

microstructure change would occur and then it is cooled at a slow rate.

1.5.3 Hardening - The hardness of steel may be increased by heating it to 50°F to

100°F above the temperature that a microstructure change occurs, and then placing the

metal in a liquid solution that rapidly cools it. This rapid cooling, known as "quenching,"

locks in place microstructures known as "martensite" that contribute to a metal's hardness

characteristic. The quenching solutions used in this process are rated according to the

speed that they cool the metal, i.e., Oil (fast), Water (faster), Salt Brine (fastest).

1.5.4 Tempering - After a metal is quenches, it is then usually tempered. Tempering is

a process where the metal is reheated to somewhere below 1335°F, held at that tempera-

ture for a length of time, and then cooled to room temperature. Tempering reduces the

brittleness that is characteristic in hardened steels, thereby producing a good balance

between high strength and toughness. The term toughness, as it applies to metals, usually

refers to resistance to brittle fracture or notch toughness under certain environmental

conditions. More information on these properties will be covered later in this lesson and in

subsequent lessons. Steels that respond to this type of treatment are known as "quenched

and tempered steels."

1.5.5 Annealing - A metal that is annealed is heated to a temperature 50° to 100°

above where a microstructure change occurs, held at that temperature for a sufficient time

for a uniform change to take place, and then cooled at a very slow rate, usually in a fur-

nace. The principal reason for annealing is to soften steel and create a uniform fine grain

structure. Welded parts are seldom annealed for the high temperatures would cause

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1.5.6 Normalizing - The main difference between normalizing and annealing is the

method of cooling. Normalized steel is heated to a temperature approximately 100° above

where the microstructure transforms and then cooled in still air rather than in a furnace.

1.5.7 Heat Treatment Trade-Off - It must be noted that these various ways of control-

ling the heating and cooling of metals can produce a desired property, but sometimes at the

expense of another desirable property. An example of this trade-off is evident in the fact

that certain heat treatments can increase the strength or hardness of metal, but the same

treatments will also make the metal less ductile or more brittle, and therefore, susceptible

to welding problems.

1.6 PROPERTIES OF METALS

The usefulness of a particular metal is determined by the climate and conditions in which it

will be used. A metal that is stamped into an automobile fender must be softer and more

pliable than armor plate that must withstand an explosive force, or the material used for an

oil rig on the Alaska North Slope must perform in a quite different climate than a steam

boiler. It becomes obvious that before a material is recommended for a specific use, the

physical and mechanical properties of that metal and the weld metal designed to join it

must be evaluated. Some of the more important properties of metals and the means of

evaluation are as follows:

1.6.1 Tensile Strength - Tensile strength is one of the most important determining

factors in selecting a metal, especially if it is to be a structural member, part of a machine,

or part of a pressure vessel.

1.6.1.1 The tensile test is performed as shown in Figure 4. The test specimen is

machined to exact standard dimensions and clamped into the testing apparatus at both

ends. The specimen is then

pulled to the point of fracture

and the data recorded.

1.6.1.2 The tensile strength

test gives us 4 primary pieces

of information: (1) Yield

Strength, (2) Ultimate Tensile

Strength, (3) Elongation, and (4)

Reduction in Area.

FIGURE 4

RECORDINGDIAL

TESTSPECIMEN

FORCE

TENSILE TESTING APPARATUS



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Current ChapterTable ofContents1.6.2 Yield Strength - When a metal is placed in tension, it acts somewhat like a

rubberband. When a load of limited magnitude is applied, the metal stretches, and when

the load is released, the metal returns to its original shape. This is the elastic characteristic

of metal and is represented by letter A in Figure 5. As a greater load is applied, the metal

will reach a point where it will no longer return to its original shape but will continue to

stretch. Yield strength is the point where the metal reaches the limit of its elastic character-

istic and will no longer return to its original shape.

1.6.3 Ultimate Tensile Strength - Once a metal has exceeded its yield point, it will

continue to stretch or deform, and if the load is suddenly released, the metal will not return

to its original shape, but will remain in its elongated form. This is called the plastic region of

the metal and is represented by the letter B in Figure 5. As this plastic deformation in-

creases, the metal strains

against further elongation, and

an increased load must be

applied to stretch the metal. As

the load is increased, the metal

will finally reach a point where it

no longer resists and any fur-

ther load applied will rapidly

cause the metal to break. That

point at which the metal has

withstood or resisted the maximum applied load is its ultimate tensile strength. This infor-

mation is usually recorded in pounds per square inch (psi).

1.6.4 Percentage of Elongation - Before a tensile specimen is placed in the tensile

tester, two marks at a measured distance are placed on the opposing ends of the circular

shaft. After the specimen is fractured, the distance between the marks is measured and

recorded as a percentage of the original distance. See Figure 5. This is the percentage of

elongation and it gives an indication of the ductility of the metal at room temperature.

1.6.5 Reduction of Area - A tensile specimen is machined to exact dimensions. The

area of its midpoint cross-section is a known figure. As the specimen is loaded to the point

of fracture, the area where it breaks is reduced in size. See Figure 5. This reduced area is

calculated and recorded as a percentage of the original cross-sectional area. This informa-

tion reflects the relative ductility or brittleness of the metal.

1.6.6 Charpy Impacts - Metal that is normally strong and ductile at room temperature

may become very brittle at much lower temperatures, and thus, is susceptible to fracture if

FIGURE 5

REDUCTIONOF AREA

ELONG-ATION

ULTIMATE STRENGTH

YIELD STRENGTH FRACTURE

A

B

C

STRAIN - INCHESA B C

NOMINAL STRESS - STRAIN CURVE

ST

RE

SS

- lb

s



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Current ChapterTable ofContentsa sharp abrupt load is applied to it. An impact tester measures the degree of susceptibility

to what is called brittle fracture.

1.6.6.1 The impact specimen is machined to exact dimensions (Figure 6) and then

notched on one side. Quite often, the notch is in the form of a "V" and the test in this case

is referred to as a Charpy V-Notch Impact Test. The specimen is cooled to a

predetermined temperature and then placed in a stationary clamp at the base of the testing

machine. The specimen is in the direct path of a weighted hammer attached to a

pendulum (Figure 6).

1.6.6.2 The hammer is released from a fixed height and the energy required to fracture

the specimen is recorded in ft-lbs. A specimen that is cooled to -60°F and absorbs 40 ft-lbs

of energy is more ductile, and therefore, more suitable for low temperature service than a

specimen that withstands only 10 ft-lbs at the same temperature. The specimen that

withstood 40 ft-lbs energy is said to have better toughness or notch toughness.

1.6.7 Fatigue Strength - A metal will withstand a load less than its ultimate tensile

strength but may break if that load is removed and then reapplied several times. For ex-

ample, if a thin wire is bent once, but if it is bent back and forth repeatedly, it will eventually

fracture and it is said to have exceeded its fatigue strength. A common test for this

strength is to place a specimen in a machine that repeatedly applies the same load first in

tension and then in compression. The fatigue strength is calculated from the number of

cycles the metal withstands before the point of failure is reached.

FIGURE 6

FRACTURES CRACKS DEFORMS

CHARPY V-NOTCHSPECIMEN

ENERGYIN FT/LBS

CHARPY IMPACT TEST MACHINE

CHARPY V-NOTCH IMPACT TEST

.312"

2.16

5"

.394"

.394"

L

L 2



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Current ChapterTable ofContents1.6.8 Creep Strength - If a load below a metal's tensile strength is applied at room

temperature (72°F), it will cause some initial elongation, but there will be no further measur-

able elongation if the load is kept at a constant level. If that same load were applied to a

metal heated to a high temperature, the situation would change. Although the load is held

at a constant level, the metal will gradually continue to elongate. This characteristic is

called creep. Eventually, the material may rupture depending on the temperature of the

metal, the degree of load applied and the length of time that it is applied. All three of these

factors determine a metal's ability to resist creep, and therefore, its creep strength.

1.6.9 Oxidation Resistance - The atoms of metal have a tendency to unite with oxy-

gen in the air to form oxide compounds, the most visible being rust and scale. In some

metals, these oxides will adhere very tightly to the skin of the metal and effectively seal it

from further oxidation as is evident in stainless steel. These materials have high oxidation

resistance. In other metals, the bond is very loose, creating a situation where the oxides

will flake off, and the metal gradually deteriorates as the time of exposure is extended.

1.6.10 Hardness Test - The resistance of a metal to indentation is a measure of its

hardness and an indication of the materials's strength. To test for hardness, a fixed load

forces an indenter into the test material (Figure 7). The depth of the penetration or the size

of the impression is measured. The measurement is converted into a hardness number

through the use of a variety of established tables. The most common tables are the Brinell,

Vickers, Knoop and Rockwell. The Rockwell is further divided into different scales, and

FIGURE 7

HARDNESS TEST SHAPE OF INDENTER INDENTER DESCRIPTION

ROCKWELLA DiamondC ConeDB 1/16 in. DiameterF Steel SphereG

1/8 in. DiameterE Steel Sphere

10 mm Sphere of SteelBRINNELL or Tungsten Carbide

VICKERS Diamond Pyramid

KNOOP Diamond Pyramid

}}

Types of Indenters - Hardness Tests



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Current ChapterTable ofContentsdepending on the material being tested, the shape of the indenter and the load applied, the

conversion tables may differ. For example, a material listed as having a hardness of Rb or

Rc means its hardness has been determined from the Rockwell "B" scale or the Rockwell

"C" scale.

1.6.11 Coefficient of Expansion - All metals expand when heated and contract when

cooled. This dimensional change is related to the crystalline structure and will vary with

different materials. The different expansion and contraction rates are expressed numeri-

cally by a coefficient of thermal expansion. When two different metals are heated to the

same temperature and cooled at the same rate, the one with the higher numerical coeffi-

cient will expand and contract more than the one with the lesser coefficient.

1.6.12 Thermal Conductivity - Some metals will absorb and transmit heat more readily

than others. They are categorized as having high thermal conductivity. This characteristic

contributes to the fact that some metals will melt or undergo transformations at much lower

temperatures than others.

1.7 EFFECTS OF THE ALLOYING ELEMENTS

Alloying is the process of adding a metal or a nonmetal to pure metals such as copper,

aluminum or iron. From the time it was discovered that the properties of pure metals could

be improved by adding other elements, alloy steel has increased by popularity. In fact,

metals that are welded are rarely in their pure state. The major properties that can be

improved by adding small amounts of alloying elements are hardness, tensile strength,

ductility and corrosion resistance. Common alloying elements and their effect on the

properties of metals are as follows:

1.7.1 Carbon - Carbon is the most effective, most widely used and lowest in cost

alloying element available for increasing the hardness and strength of metal. An alloy

containing up to 1.7% carbon in combination with iron is known as steel, whereas the

combination above 1.7% carbon is known as cast iron. Although carbon is a desirable

alloying element, high levels of it can cause problems; therefore, special care is required

when welding high carbon steels and cast iron.

1.7.2 Sulphur - Sulphur is normally an undesirable element in steel because it causes

brittleness. It may be deliberately added to improve the machinability of the steel. The

sulphur causes the

machine chips to break rather than form long curls and clog the machine. Normally, every

effort is made to reduce the sulphur content to the lowest possible level because it can



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Current ChapterTable ofContentscreate welding difficulties.

1.7.3 Manganese - Manganese in contents up to 1% is usually present in all low alloy

steels as a deoxidizer and desulphurizer. That is to say, it readily combines with oxygen

and sulphur to help negate the undesirable effect these elements have when in their natu-

ral state. Manganese also increases the tensile strength and hardenability of steel.

1.7.4 Chromium - Chromium, in combination with carbon, is a powerful hardening

alloying element. In addition to its hardening properties, chromium increases corrosion

resistance and the strength of steel at high temperatures. Chromium is the primary alloying

element in stainless steel.

1.7.5 Nickel - The greatest single property of steel that is improved by the presence of

nickel is its ductility or notch toughness. In this respect, it is the most effective of all alloy-

ing elements in improving a steel's resistance to impact at low temperatures. Electrodes

with high nickel content are used to weld cast iron materials. Nickel is also used in combi-

nation with chromium to form a group known as austenitic stainless steel.

1.7.6 Molybdenum - Molybdenum strongly increases the depth of the hardening

characteristic of steel. It is quite often used in combination with chromium to improve the

strength of the steel at high temperatures. This group of steels is usually referred to as

chrome-moly steels.

1.7.7 Silicon - Silicon is usually contained in steel as a deoxidizer. Silicon will add

strength to steel but excessive amounts can reduce the ductility. Additional amounts of

silicon are sometimes added to welding electrodes to increase the fluid flow of weld metal.

1.7.8 Phosphorus - Phosphorus is considered a harmful residual element in steel

because it greatly reduces ductility and toughness. Efforts are made to reduce it to its very

lowest levels; however, phosphorus is added in very small amounts to some steels to

increase strength.

1.7.9 Aluminum - Aluminum is primarily used as a deoxidizer in steel. It may also be

used in very small amounts to control the size of the grains.

1.7.10 Copper - Copper contributes greatly to the corrosion resistance of carbon steel

by retarding the rate of rusting at room temperature, but high levels of copper can cause

welding difficulties.



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Current ChapterTable ofContents1.7.11 Columbium - Columbium is used in austenitic stainless steel to act as a stabi-

lizer. Since the carbon in the stainless steel decreases the corrosion resistance, a means

of making carbon ineffective must be found. Columbium has a greater affinity for carbon

than chromium, leaving the chromium free for corrosion protection.

1.7.12 Tungsten - Tungsten is used in steel to given strength at high temperatures.

Tungsten also joins with carbon to form carbides that are exceptionally hard, and therefore

have exceptional resistance to wear.

1.7.13 Vanadium - Vanadium helps keep steel in the desirable fine grain condition after

heat treatment. It also helps increase the depth of hardening and resists softening of the

steel during tempering treatments.

1.7.14 Nitrogen - Usually, efforts are made to eliminate hydrogen, oxygen and nitrogen

from steel because their presence can cause brittleness. Nitrogen has the ability to form

austenitic structures; therefore, it is sometimes added to austenitic stainless steel to reduce

the amount of nickel needed, and therefore, the production costs of that steel.

1.7.15 Alloying Elements Summary - It should be understood that the addition of

elements to a pure metal may influence the crystalline form of the resultant alloy. If a pure

metal has allotropic characteristics (the ability of a metal to change its crystal structure) at a

specific temperature, then that characteristic will occur over a range of temperatures with

the alloyed metal. The range in which the change takes place may be wide or narrow,

depending on the alloys and the quantities in which they are added. The alloying element

may also effect the crystalline changes by either suppressing the appearance of certain

crystalline forms or even by creating entirely new forms. All these transformations induced

by alloying elements are dependent on heat input and cooling rates. These factors are

closely controlled at the steel mill, but since the welding operation involves a nonuniform

heating and cooling of metal, special care is often needed in the welding of low and high

alloy steel.


LESSON I, PART BThe Basics of Arc

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Current ChapterTable ofContents1.8 ELECTRICITY FOR WELDING

1.8.1 Principles of Electricity - Arc welding is a method of joining metals accom-

plished by applying sufficient electrical pressure to an electrode to maintain a current path

(arc) between the electrode and the work piece. In this process, electrical energy is

changed into heat energy, bringing the metals to a molten state; whereby they are joined.

The electrode (conductor) is either melted and added to the base metal or remains in its

solid state. All arc welding utilizes the transfer of electrical energy to heat energy, and to

understand this principle, a basic knowledge of electricity and welding power sources is

necessary.

1.8.1.1 The three basis principles of static electricity are as follows:

1. There are two kinds of electrical charges in existence - negative and positive.

2. Unlike charges attract and like charges repel.

3. Charges can be transferred from one place to another.

1.8.1.2 Science has established that all matter is made up of atoms and each atom

contains fundamental particles. One of these particles is the electron, which has the ability

to move from one place to another. The electron is classified as a negative electrical

charge. Another particle, about 1800 times as heavy as the electron, is the proton and

under normal conditions the proton will remain stationary.

1.8.1.3 Material is said to be in an electrically uncharged state when its atoms contain an

equal number of positive charges (protons) and negative charges (electrons). This balance

is upset when pressure forces the electrons to move from atom to atom. This pressure,

sometimes referred to as electromotive force, is commonly known as voltage. It should be

noted that voltage that does not move through a conductor, but without voltage, there would

be no current flow. For our purposes, it is easiest to think of voltage as the electrical

pressure that forces the electrons to move.

1.8.1.4 Since we know that like charges repel and unlike charges attract, the tendency is

for the electrons to move from a position of over-supply (negative charge) to an atom that

lacks electrons (positive charge). This tendency becomes reality when a suitable path is

provided for the movement of the electrons. The transfer of electrons from a negative to a

positive charge throughout the length of a conductor constitutes an electrical current. The

rate that current flows through a conductor is measured in amperes and the word ampere

is often used synonymously with the term current. To give an idea of the quantities of

electrons that flow through a circuit, it has been theoretically established that one ampere

equals 6.3 quintillion (6,300,000,000,000,000,000) electrons flowing past a fixed point in a

conductor every second.



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Current ChapterTable ofContents1.8.1.5 Different materials vary in their ability to transfer electrons. Substances, such as

wood and rubber, have what is called a tight electron bond and their atoms greatly resist

the free movement of electrons. Such materials are considered poor electrical conductors.

Metals, on the other hand, have large amounts of electrons that transfer freely. Their

comparatively low electrical resistance classifies them as good electrical conductors.

1.8.1.6 Electrical resistance is primarily due to the reluctance of atoms to give up their

electron particles. It may also be thought of as the resistance to current flow.

1.8.1.7 To better understand the electrical terms discussed above, we might compare

the closed water system with the electrical diagram shown in Figure 8. You can see that as

the pump is running, the water will move in the direction of the arrows. It moves because

pressure has been produced and that pressure can be likened to voltage in an electrical

circuit. The pump can be compared to a battery or a DC generator. The water flows

through the system at a certain rate. This flow rate in an electrical circuit is a unit of

measure known as the ampere. The small pipe in the fluid circuit restricts the flow rate and

can be likened to a resistor. This unit resistance is known as the ohm. If we close the

valve in the fluid circuit, we stop the flow, and this can be compared to opening a switch in

an electrical circuit.

1.8.2 Ohm's Law - Resistance is basic to electrical theory and to understand this

principle, we must know the Ohm's Law, which is stated as follows: In any electrical circuit,

the current flow in amperes is directly proportional to the circuit voltage applied and in-

versely proportional to the circuit resistance. Directly proportional means that even though

the voltage and amperage may change, the ratio of their relationship will not. For example,

if we have a circuit of one volt and three amps, we say the ratio is one to three. Now if we

increase the volts to three, our amperage will increase proportionately to nine amps. As

can be seen, even though the voltage and amperage changed in numerical value, their

ratio did not. The term "inversely proportional" simply means that if the resistance is

FIGURE 8

VALVE

SWITCH

RESISTOR10 OHM

BATTERY12 VOLT

ELECTRICAL DIAGRAM

SMALL PIPEPUMP

CLOSED WATER SYSTEM

LARGEPIPE



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Current ChapterTable ofContentsdoubled, the current will be reduced to one-half. Ohm's Law can be stated mathematically

with this equation:

I = E ÷ R or E = I × R or R = E ÷ I

(E = Volts, I = Amperes, R = Resistance (Ohms))

1.8.2.1 The equation is easy to use as seen in the following problems:

1) A 12 volt battery has a built-in resistance of 10 ohms. What is the amperage?

12 ÷ 10 = 1.2 amps

2) What voltage is required to pass 15 amps through a resistor of 5 ohms?

15 × 5 = 75 volts

3) When the voltage is 80 and the circuit is limited to 250 amps, what is the valueof the resistor?

80 ÷ 250 = .32 ohms

1.8.2.2 The theory of electrical resistance is of great importance in the arc welding

process for it is this resistance in the air space between the electrode and the base metal

that contributes to the transfer of electrical energy to heat energy. As voltage forces the

electrons to move faster, the energy they generate is partially used to overcome the

resistance created by the arc gap. This energy becomes evident as heat. In the welding

process, the temperature increases to the point where it brings metals to a molten state.

1.8.3 Electrical Power - The word "watt" is another term frequently encountered in

electrical terminology. When we pay our electrical bills, we are actually paying for the

power to run our electrical appliances, and the watt is a unit of power. It is defined as the

amount of power required to maintain a current of one ampere at a pressure of one volt.

The circuit voltage that comes into your home is a constant factor, but the amperage drawn

from the utility company depends on the number of watts required to run the electrical

appliance. The watt is figured as a product of volts times amperes and is stated math-

ematically with the following equation:

W =E × I E = W ÷ I I = W ÷ E

(W = Watts, E = Volts, I = Amperes)

1.8.3.1 The amperage used by an electrical device can be calculated by dividing the

watts rating of the device by the primary voltage for which it is designed.



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Current ChapterTable ofContents1.8.3.2 For example, if an appliance is designed for the common household primary

voltage of 115 and the wattage stamped on the appliance faceplate is 5, then the

amperage drawn by the appliance when in operation is determined as shown:

5 ÷ 115 = .04 amperes

1.8.3.3 Kilowatt is another term common in electrical usage. The preface "kilo" is a

metric designation that means 1,000 units of something; therefore, one kilowatt is 1,000

watts of power.

1.8.4 Power Generation - Electrical energy is supplied either as direct current (DC) or

alternating current (AC). With direct current, the electron movement within the conductor is

in one direction only. With alternating current, the electron flow reverses periodically. Al-

though some types of electrical generators will produce current directly (such as batteries,

dry cells, or DC generators), most direct current is developed from alternating current.

1.8.4.1 Through experimentation, it was discovered that when a wire is moved through a

magnetic field, an electrical current is induced into the wire, and the current is at its

maximum when the motion of the conductor is at right

angles to the magnetic lines of force. The sketch

in Figure 9 will help to illustrate this principle.

1.8.4.2 If the conductor is moved upwards in

the magnetic field between the N and S poles,

the galvanometer needle will deflect plus (+).

Likewise, if the conductor is moved downwards

the needle will deflect minus (-). With this

principle of converting mechanical energy into

electrical energy understood, we can apply it to

the workings of an AC generator.

1.8.4.3 Figure 10 is a simplified sketch of an AC

generator. Starting at 0° rotation, the coil wire is moving

parallel to the magnetic lines of force and cutting none of them. Therefore, no current is

being induced into the winding.

1.8.4.4 From 0° to 90° rotation, the coil wire cuts an increasing number of magnetic lines

of force and reaches the maximum number at 90° rotation. The current increases to the

maximum because the wire is now at right angles to the lines of force.

S

N

FIGURE 9

GALVANOMETER

ELECTRO-MAGNETICINDUCTION



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1.8.4.5 From 90° to 180° rotation, the coil wire cuts a diminishing number of lines of

force and at 180° again reaches zero.

1.8.4.6 From 180° to 270°, the current begins to rise again but in the opposite direction

because now the wire is in closer proximity to the opposite pole.

1.8.4.7 One cycle is completed as the coil wire moves from 270° to 0° and the current

again drops to zero.

1.8.4.8 With the aid of a graph, we can visualize the rate at which the lines of force are

cut throughout the cycle. If we plot the current versus degree of rotation, we get the

familiar sine wave as seen in Figure 11.

1.8.4.9 With this sine wave, we can

see that one complete cycle of

alternating current comprises one

positive and one negative wave

(negative and positive meaning

electron flow in opposing directions).

The frequency of alternating current is

the number of such complete cycles

per second. For most power

applications, 60 cycles per second (60

Hertz) is the standard frequency in

North America.

FIGURE 10

SLIDINGCONTACTS

N N

N N

S S

S

ROTATING COILOR ARMATURE

PERMANENT MAGNETSOR FIELD COILS

S

N

S

270°180°

0° 90°

BASIC AC POWER GENERATION

FIGURE 11

MAXIMUM (+)

CURRENT

MAXIMUM (–)

0° 90° 180° 270° 360°START 1/4 TURN 1/2 TURN 3/4 TURN FULL TURN

(+)

(–)

000

ONE CYCLE - ALTERNATING CURRENT



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Current ChapterTable ofContents1.8.4.10 Some welders use a three-phase AC supply. Three-phase is simply three

sources of AC power as identical voltages brought in by three wires, the three voltages or

phases being separated by 120 electrical degrees. If

the sine wave for the three phases are plotted on one

line, they will appear as shown in Figure 12.

1.8.4.11 This illustrates that three-phase power is

smoother than single-phase because the overlapping

three phases prevent the current and voltage from

falling to zero 120 times a second, thereby producing a

smoother welding arc.

1.8.4.12 Since all shops do not have three-phase power, welding machines for both

single-phase and three-phase power are available.

1.8.5 Transformers - The function of a transformer is to increase or decrease voltage

to a safe value as the conditions demand. Common household voltage is usually 115 or

230 volts, whereas industrial power requirements may be 208, 230, 380, or 460 volts.

Transmitting such relatively low voltages over long distances would require a conductor of

enormous and impractical size. Therefore, power transmitted from a power plant must be

stepped up for long distance transmission and then stepped down for final use

1.8.5.1 As can be seen in Figure 13, the voltage is generated at the power plant at

13,800 volts. It is increased, transmitted over long distances, and then reduced in steps for

the end user. If power supplied to a transformer circuit is held steady, then secondary

current (amperes) decreases as the primary voltage increases, and conversely, secondary

current increases as primary voltage decreases. Since the current flow (amperes)

determines the wire or conductor size, the high voltage line may be of a relatively small

diameter.

FIGURE 12

120°

1 CYCLE

THREE PHASE AC

240°

0°

FIGURE 13

POWER TRANSMISSION

13,800 V

POWERPLANT STEP

UP

287,000V

HIGH VOLTAGE

300 MILES

STEPDOWN

132,000 V

34,000 V

4,600V

208V230V460VFINALUSE



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Current ChapterTable ofContents1.8.5.2 The transformer in a welding machine performs much the same as a large power

plant transformer. The primary voltage coming into the machine is too high for safe

welding. Therefore, it is stepped down to a useable voltage. This is best illustrated with an

explanation of how a single transformer works.

1.8.5.3 In the preceding paragraphs, we have found than an electrical current can be

induced into a conductor when that conductor is moved through a magnetic field to

produce alternating current. If this alternating current is passed through a conductor, a

pulsating magnetic field will surround the exterior of that conductor, that is the magnetic

field will build in intensity through the first 90 electrical degrees, or the first cycle. From that

point, the magnetic field will decay during the next quarter cycle until the voltage or current

reaches zero at 180 electrical degrees. Immediately, the current direction reverses and the

magnetic field will begin to build again until it reaches a maximum at 270 electrical degrees

in the cycle. From that point the current and the magnetic field again begin to decay until

they reach zero at 360 electrical degrees, where the cycle begins again.

1.8.5.4 If that conductor is wound around a material with high magnetic permeability

(magnetic permeability is the ability to accept large amounts of magnetic lines of force)

such as steel, the magnetic field permeates that core. See

Figure 14. This conductor is called the primary coil, and if

voltage is applied to one of its terminals and the circuit is

completed, current will flow. When a second coil is wound

around that same steel core, the energy that is stored in

this fluctuating magnetic field in the core is induced into

this secondary coil.

1.8.5.5 It is the build-up and collapse of this magnetic

field that excite the electrons in the secondary coil of the

transformer. This causes an electrical current of the same frequency as the primary coil to

flow when the secondary circuit is completed by striking the welding arc. Remember that

all transformers operate only on alternating current.

1.8.5.6 A simplified version of a welding transformer is schematically shown in Figure 15.

This welder would operate on 230 volts input power and the primary winding has 230 turns

of wire on the core. We need 80 volts for initiating the arc in the secondary or welding

circuit, thus we have 80 turns of wire in the secondary winding of the core. Before the arc

is struck, the voltage between the electrode and the work piece is 80 volts. Remember that

no current (amperage) flows until the welding circuit is completed by striking the arc.

FIGURE 14

STEEL CORE

PRIMARYCOIL

SECONDARYCOIL

80 V

80TURNS

460 V

460TURNS

BASIC TRANSFORMER



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Current ChapterTable ofContents1.8.5.7 Since the 80 volts

necessary for initiating the arc

is too high for practical

welding, some means must be

used to lower this voltage to a

suitable level. Theoretically, a

variable resistor of the proper

value could be used as an

output control since voltage is

inversely proportional to

resistance as we saw when studying Ohm's Law. Ohm's Law also stated that the

amperage is directly proportional to the voltage. This being so, you can see that adjusting

the output control will also adjust the amperage or welding current.

1.8.5.8 After the arc is initiated and current begins to flow through the secondary or

welding circuit, the voltage in that circuit will be 32 volts because it is then being controlled

by the output control.

1.8.6 Power Requirements - We can make another calculation by looking back at

Figure 15, and that is power consumption. Earlier, we explained that the watt was the unit

of electrical power and can be calculated by the formula:

Watts = Volts × Amperes

1.8.6.1 From Figure 15, we can see that the instantaneous power in the secondary

circuit is:

Watts = 32 × 300

Watts = 9600 Watts

1.8.6.2 The primary side of our transformer must be capable of supplying 9600 watts

also (disregarding losses due to heating, power factor, etc.), so by rearranging the formula,

we can calculate the required supply line current or amperage:

Amperage = Watts ÷ Volts

A = 9600 ÷ 230 = 41.74 Amps

1.8.6.3 This information establishes the approximate power requirements for the welder,

and helps to determine the input cable and fuse size necessary.

FIGURE 15

9600 WATTS 9600 WATTS

230 TURNS 80 TURNS

80OCV

OUTPUTCONTROL

230VOLTS PRIMARY SECONDARY

41.74AMPS

SIMPLIFIED WELDING TRANSFORMER

32 VOLTS300 AMPS



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Current ChapterTable ofContents1.8.7 Rectifying AC to DC - Although much welding is accomplished with AC welding

power sources, the majority of industrial welding is done with machines that produce a

direct current arc. The commercially produced AC

power that operates the welding machine

must then be changed (rectified) to direct

current for the DC arc. This is accom-

plished with a device called a rectifier.

Two types of rectifiers have been used

extensively in welding machines, the

old selenium rectifiers and the more

modern silicon rectifiers, often referred

to as diodes. See Figure 16.

1.8.7.1 The function of a rectifier in the

circuit can best be shown by the use of the

AC sine wave. With one diode in the circuit,

half-wave rectification takes place as shown

in Figure 17.

1.8.7.2 The negative half-wave is simply cut off and a pulsating DC is produced. During

the positive half-cycle, current is allowed to flow through the rectifier. During the negative

half-cycle, the current is blocked. This produces a DC composed of 60 positive pulses per

second.

1.8.7.3 By using four rectifiers connected in a

certain manner, a bridge rectifier is created, producing

full wave rectification. The bridge rectifier results in

120 positive half-cycles per second, producing a

considerably smoother direct current than half-wave

rectification. See Figure 18.

1.8.7.4 Three-phase AC can be rectified to

produce an even smoother DC than single-phase

AC. Since three-phase AC power produces three

times as many half-cycles per second as single-

phase power, a relatively smooth DC voltage

results as shown in Figure 19.

SINGLE PHASE HALF WAVE RECTIFICATION

FIGURE 17

FIGURE 16

SILICON RECTIFIERSELENIUM RECTIFIER

SINGLE PHASE FULL WAVE RECTIFICATION

FIGURE 18

1 CYCLE

3 PHASE FULL WAVE RECTIFICATION

FIGURE 19



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Current ChapterTable ofContents1.9 CONSTANT CURRENT OR CONSTANT VOLTAGE

Welding power sources are designed in many sizes and shapes. They may supply either

AC or DC, or both, and they may have various means of controlling their voltage and

amperage output. The reasons for this is that the power source must be capable of

producing the proper arc characteristics for the welding process being used. A power

source that produces a satisfactory arc when welding with coated electrodes will be less

than satisfactory for welding with solid and flux cored wires.

1.9.1 Constant Current Characteristics - Constant current power sources are used

primarily with coated electrodes. This type of power source has a relatively small change in

amperage and arc power for a corresponding relatively large change in arc voltage or arc

length, thus the name constant current. The characteristics of this power source are best

illustrated by observing a graph that plots the volt-

ampere curve. As can be seen in Figure 20, the

curve of a constant current machine drops down-

ward rather sharply and for this reason, this type of

machine is often called a "drooper."

1.9.1.1 In welding with coated electrodes, the

output current or amperage is set by the operator

while the voltage is designed into the unit. The

operator can vary the arc voltage somewhat by

increasing or decreasing the arc length. A slight

increase in arc length will cause an increase in arc

voltage and a slight decrease in amperage. A slight

decrease in arc length will cause a decrease in arc

voltage and a slight increase in amperage.

1.9.2 Constant Voltage Characteristics - Constant voltage power sources, also

known as constant potential, are used in welding with solid and flux cored electrodes, and

as the name implies, the voltage output remains relatively constant. On this type of power

source, the voltage is set at the machine and amperage is determined by the speed that

the wire is fed to the welding gun. Increasing the wire feed speed increases the amperage.

Decreasing the wire feed speed decreases the amperage.

1.9.2.1 Arc length plays an important part in welding with solid and flux cored electrodes,

just as it does in welding with a coated electrode. However, when using a constant voltage

power source and a wire feeder that delivers the wire at a constant speed, arc length

caused by operator error, plate irregularities, and puddle movement are automatically

34V - 290A

32V - 300 A

30V - 308 A

VOLT / AMPERE CURVECONSTANT CURRENT

100 200 300AMPERES

CONSTANT CURRENT VOLT / AMPERE CURVE

FIGURE 20

80

70

60

50

40

30

20

10

VOLTS



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Current ChapterTable ofContentscompensated for by the characteristics of this process. To understand this, keep in mind

that with the proper voltage setting, amperage setting, and arc length, the rate that the wire

melts is dependent upon the amperage. If the amperage decreases, this melt-off rate

decreases and if the amperage increases, so does the melt-off rate.

1.9.2.2 In Figure 21, we see that condition #2 produces the desired arc length, voltage,

and amperage. If the arc length is increased as in #1, the voltage increases slightly; the

amperage decreases considerably, and therefore, the melt-off rate of the wire decreases.

The wire is now feeding faster than it is melting

off. This condition will advance the end of the

wire towards the work piece until the proper arc

length is reached where again, the melt-off rate

equals the feeding rate. If the arc length is

decreased as in #3, the voltage drops off

slightly, the amperage is increased

considerably, and the melt-off rate of the wire

increases. Since the wire is now melting off

faster than it is being fed, it melts back to the

proper arc length where the melt-off rate

equals the feeding rate. This is often referred

to as a self-adjusting arc. These automatic

corrections take place in fractions of a second,

and usually without the operator being aware

of them.

1.9.2.3 There are a variety of different welding machines, each with its own unique

internal design. Our purpose is not to detail the function of each part of the machine, but to

emphasize that their main difference is in the way they control the voltage and amperage

output.

1.9.3 Types of Welding Power Sources - A great variety of welding power sources

are being built today for electric arc welding and we shall mention some of the major types

briefly. Welding power sources can be divided into two main categories: static types and

rotating types.

1.9.3.1 Static Types - Static type power sources are all of those that use commercially

generated electrical power to energize a transformer that, in turn, steps the line voltage

down to useable welding voltages. The two major categories of static power sources are

the transformer type and the rectifier type.

1 2 3

VOLTS

40

30

20

10

100 200 300 400AMPERES

VOLT / AMPERE CURVE - CONSTANT VOLTAGE

FIGURE 21



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1.9.3.1.1 The transformer type produce only alternating current. They are commonly

called "Welding Transformers." All AC types utilize single-phase primary power and are of

the constant current type.

1.9.3.1.2 The rectifier types are commonly called "Welding Rectifiers" and produce DC or,

AC and DC welding current. They may utilize either single phase or three phase input

power. They contain a transformer, but rectify the AC or DC by the use of selenium

rectifiers, silicon diodes or silicon controlled rectifiers. Available in either the constant

current or the constant voltage type, some manufacturers offer units that are a combination

of both and can be used for coated electrode welding, non-consumable electrode welding

and for welding with solid or flux cored wires.

1.9.3.2 Rotating Types - Rotating type power sources may be divided into two classifi-

cations:1. Motor-Generators

2. Engine Driven

1.9.3.2.1 Motor-generator types consist of an electric motor coupled to a generator or

alternator that produces the desired welding power. These machines produced excellent

welds, but due to the moving parts, required considerable maintenance. Few, if any, are

being built today.

1.9.3.2.2 Engine driven types consist of a gasoline or diesel engine coupled to a generator

or alternator that produces the desired welding power. They are used extensively on jobs

beyond commercial power lines and also as mobile repair units. Both rotating types can

deliver either AC or DC welding power, or a combination of both. Both types are available

as constant current or constant voltage models.

1.9.4 Power Source Controls - Welding power sources differ also in the method of

controlling the output current or voltage. Output may be controlled mechanically as in

machines having a tapped reactor, a moveable shunt or diverter, or a moveable coil. Elec-

trical types of controls, such as magnetic amplifiers or saturable reactors, are also utilized

and the most modern types, containing silicon controlled rectifiers, give precise electronic

control.

1.9.4.1 A detailed discussion of the many types of welding power sources on the market

today is much too lengthy a subject for this course, although additional information on the

type of power sources for the various welding processes will be covered in Lesson II.

1.9.4.2 Excellent literature is available from power source manufacturers, however, and

should be consulted for further reference.


LESSON I, GLOSSARYThe Basics of Arc

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Current ChapterTable ofContentsAPPENDIX A

LESSON I - GLOSSARY OF TERMS

AISI — American Iron and Steel Institute

Allotropic — A material in which the atoms are capable of transforming into two

or more crystalline structures at different temperatures.

Alternating — An electrical current which alternately travels in either direction in a

Current conductor. In 60 cycles per second (60 Hz) AC, the frequency

used in the U.S.A., the current direction reverses 120 times every

second.

Ampere — Unit of electrical rate of flow. Amperage is commonly referred to as

the “current” in an electrical circuit.

ASME — American Society of Mechanical Engineers

ASTM — American Society for Testing and Materials

Atom — The smallest particle of an element that posses all of the

characteristics of that element. It consists of protons, neutrons,

and electrons.

Carbon Steel — (Sometimes referred to as mild steel.) An alloy of iron and carbon.

Carbon content is usually below 0.3%.

Conductor — A material which has a relatively large number of loosely bonded

electrons which may move freely when voltage (electrical pressure)

is applied. Metals are good conductors.

Constant Current — (As applied to welding machines.) A welding power source which

will produce a relatively small change in amperage despite

changes in voltage caused by a varying arc length. Used mostly

for welding with coated electrodes.



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Current ChapterTable ofContentsConstant Voltage — (As applied to welding machines.) A welding power source which

will produce a relatively small change in voltage when the

amperage is changed substantially. Used mostly for welding with

solid or flux cored electrodes.

Direct Current — An electrical current which flows in only one direction in a

conductor. Direction of current is dependent upon the electrical

connections to the battery or other DC power source. Terminals on

all DC devices are usually marked (+) or (-). Reversing the leads

will reverse the direction of current flow.

Electron — Negatively charged particles that revolve around the positively

charged nucleus in an atom.

Ferrous — Containing iron. Example: carbon steel, low alloy steels, stainless

steel.

Hertz — Hertz (Hz) is the symbol which has replaced the term “cycles per

second.” Today, rather than saying 60 cycles per second or simply

60 cycles, we say 60 Hertz or 60 Hz.

High Alloy Steels — Steels containing in excess of 10% alloy content. Stainless steel is

considered a high alloy because it contains in excess of 10%

chromium.

Induced Current or

Induction — The phenomena of causing an electrical current to flow through a

conductor when that conductor is subjected to a varying magnetic

field.

Ingot — Casting of steel (weighing up to 200 tons) formed at mill from melt

of ore, scrap limestone, coke, etc.

Insulator — A material which has a tight electron bond, that is, relatively few

electrons which will move when voltage (electrical pressure) is

applied. Wood, glass, ceramics and most plastics are good

insulators.



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Current ChapterTable ofContentsKilowatt — 1,000 watts

Low Alloy Steels — Steels containing small amounts of alloying elements (usually 1½%

to 5% total alloy content) which drastically improves their

properties.

Non-Ferrous — Containing no iron. Example: Aluminum, copper, copper alloys.

Ohm — Unit of electrical resistance to current flow.

Phase

Transformation — The changes in the crystalline structure of metals caused by

temperature and time.

Proton — Positively charged particles which are part of the nucleus of atoms.

Rectifier — An electrical device used to change alternating current to direct

current.

SAE — Society of Automotive Engineers

Transformer — An electrical device used to raise or lower the voltage and inversely

change the amperage.

Volt — Unit of electromotive force, or electrical pressure which causes

current to flow in an electrical circuit.

Watt — A unit of electrical power. Watts = Volts x Amperes

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Curso CwiLesson 1_1 Total