INVESTIGATION OF MIG WELDING TO THE CORROSION BEHAVIOUR OF

HEAT TREATED CARBON STEEL

NG SHING SHIAN

Report submitted in partial fulfilment of the requirements for the award of the degree of

Bachelor of Mechanical Engineering with Manufacturing Engineering

Faculty of Mechanical Engineering

UNIVERSITI MALAYSIA PAHANG

JUNE 2012

vii

ABSTRACT

The corrosion behaviour of metal inert gas (MIG) welded heat treated carbon steel was

investigated. Filler material of ER308L and ER70S were used to complete the butt joint

welding on non-heat treated and heat treated low carbon steel (AISI 1010). The welding

heat input was manipulated by varying the welding voltage for MIG welding process.

The microstructures for welded specimens were analysed with the optical microscope.

During the electrochemical test, the corrosion rate on the carbon steel was determined.

Microstructural analysis for specimen without cleaning and after cleaning was analysed

by using SEM. Results showed that the increase in welding heat input was highly

significant to the corrosion rate of carbon steel. The welding heat input was able to

change the ferrite content on the microstructure of specimen which affected the

corrosion rate on the carbon steel.

viii

ABSTRAK

Kajian tentang karat terhadap keluli karbon yang diproses dengan rawatan haba dan

dikimpal dengan mesin MIG (metal inert gas) telah dijalankan. Rod kimpal yang

digunakan semasa proses pengimpalan termasuk ER70S dan ER308L untuk

mengabungkan keluli karbon (AISI 1010). Voltan pengimpalan semasa proses

pengimpalan dimanipulasikan. Imej mikrostruktur terhadap keluli karbon yang dikimpal

telah dianalisiskan dan dibincang. Kadar pengaratan keluli dikaji dengan menggunakan

ujian kakisan. Kajian mikrostruktur terhadap keluli sebelum dan selepas dibersihkankan

telah dianalisis denagan menggunakan SEM. Keputusan menunjukkan bahawa voltan

pengimpalan mempengaruhi kadar pengaratan keluli disebabkan mikrostruktur yang

telah diubah setelah proses pengimpalan.

ix

TABLE OF CONTENTS

Page

EXAMINER’S DECLARATION ii

SUPERVISOR’S DECLARATION iii

STUDENT’S DECLARATION iv

DEDICATION v

ACKNOWLEDGEMENTS vi

ABSTRACT vii

ABSTRAK viii

TABLE OF CONTENTS ix

LIST OF TABLES xiii

LIST OF FIGURES xiv

LIST OF SYMBOLS xvii

LIST OF ABBREVIATIONS xix

CHAPTER 1 INTRODUCTION

1.1 Background of Study 1

1.2 Problem Statement 2

1.3 Objectives 2

1.4 Scope of Work 3

CHAPTER 2 LITERATURE REVIEW

2.1 Heat Treatment 4

2.1.1 Annealing 4

2.1.2 Normalizing 5

2.1.2 Hardening 6

2.1.2 Tempering 6

2.2 Welding 8

2.2.1 MIG Welding 9

x

2.3 Corrosion 13

2.3.1 Uniform Attack 14

2.3.2 Crevice Corrosion 14

2.3.3 Pitting Corrosion 15

2.3.4 Intergranular Corrosion 16

2.4 Materials and Microstructures 17

CHAPTER 3 METHODOLOGY

3.1 Introduction 20

3.2 Sample Preparation 22

3.2.1 Heat Treatment 25

3.2.2 Welding 27

3.3 Microstructure 29

3.3.1 Cross Section Cutting 30

3.3.2 Cold Mounting 30

3.3.3 Grinding and Polishing 32

3.3.4 Etching and Microstructure 34

3.4 Electrochemical Testing 35

CHAPTER 4 RESULTS AND DISCUSSIONS

4.1 Introduction 38

4.2 Chemical Compositional Analysis 39

4.3 Microstructure Comparison After Heat Treatment Process 40

4.4 Microstructure of Welded Low Carbon Steel 42

4.5 Polarization Diagram Analysis 46

4.6 Effect of Welding Voltage on the Corrosion Rate 49

4.7 Scanning Electron Microscopy Result 57

CHAPTER 5 CONCLUSION AND RECOMMENDATIONS

5.1 Conclusion 59

5.2 Recommendations For Future Work 60

xi

REFERENCE 61

APPENDICES

A1 AISI-SAE Standard Carbon Steels 64

A2 Complete Compositional Analysis of Sample Material 65

B Sample Calculation of Corrosion Rate for Electrochemical Test 66

C1 Microstructure Of Welded Non-Heat Treated Specimen With

ER70S Filler At 19V

67

C2 Microstructure Of Welded Non-Heat Treated Specimen With

ER70S Filler At 20V

68

C3 Microstructure Of Welded Non-Heat Treated Specimen With

ER70S Filler At 21V

69

C4 Microstructure Of Welded Heat Treated Specimen With ER70S

Filler At 19V

70

C5 Microstructure Of Welded Heat Treated Specimen With ER70S

Filler At 20V

71

C6 Microstructure Of Welded Heat Treated Specimen With ER70S

Filler At 21V

72

C7 Microstructure Of Welded Non-Heat Treated Specimen With

ER308L Filler At 19V

73

C8 Microstructure Of Welded Non-Heat Treated Specimen With

ER308L Filler At 20V 74

C9 Microstructure Of Welded Non-Heat Treated Specimen With

ER308L Filler At 21V

75

C10 Microstructure Of Welded Heat Treated Specimen With ER308L

Filler At 19V

76

C11 Microstructure Of Welded Heat Treated Specimen With ER308L

Filler At 20V

77

C12 Microstructure Of Welded Heat Treated Specimen With ER308L

Filler At 21V

78

xii

D1 Gantt Chart PSM 1 79

D2 Gantt Chart PSM 2 80

xiii

LIST OF TABLES

Table No. Page

1.1 Cost related to corrosion 1

2.1 Effect of pitting resistance at different elements 15

3.1 Comparison of chemical composition with the ASTM standard 25

3.2 Chemical composition of the welding fillers used 27

3.3 Parameter set up values for the electrochemical test 36

4.1 Chemical composition of as-received carbon steel and the AISI

1010 carbon steel

39

4.2 Corrosion rate of non-heat treated low carbon steel welded with

ER70S mild steel filler

49

4.3 Corrosion rate of full annealing heat treated low carbon steel

welded with ER70S mild steel filler

50

4.4 Corrosion rate of non-heat treated low carbon steel welded with

ER308L stainless steel filler

51

4.5 Corrosion rate of full annealing heat treated low carbon steel

welded with ER308L stainless steel filler

52

4.6 Corrosion rate of specimens at different welding parameters and

materials’ treatment

53

xiv

LIST OF FIGURES

Figure No. Page

2.1 Photographs showing the (a) initial and final specimens in (b) air

and (c) water 11

2.2 Photographs showing the SEM observation of 1000x

magnification for (a) base metal, (b) CG-HAZ and (c) ICCG-

HAZ

12

2.3 Effect of welding heat input on the corrosion rate 13

3.1 Photograph of as-received low carbon steel 20

3.2 A flow chart showing a summary of research methodology 21

3.3 The MVS-C 6/31 shear machine used during cutting process 22

3.4 Drawing of specimen in dimension of mm 23

3.5 Dimension of specimen 24

3.6 Thermconcept model KM 225/13 26

3.7 Heat treatment phase diagram for carbon steel 26

3.8 Flow charts that showing the welding parameters 28

3.9 Miller welding in FKM lab UMP 29

3.10 LECO MSX200M sectioning cut-off machine 30

3.11 LECOSET 7007 cold curing resin and acrylic 31

3.12 LECOMAT pressure vessel 31

3.13 HandiMet 2 Roll Grinder 32

3.14 Metkon Forcipol 2V modular grinding and polishing machine 33

3.15 Diamond liquids used for polishing 33

3.16 Microscope used in the FKM lab 34

3.17 Electrochemical test by using potentiostat 35

3.18 Hydrochloride acid used for the cleaning purpose 37

xv

4.1 Microstructure of base metal for the non-heat treated low carbon

steel at magnification of 500x

41

4.2 Microstructure of base metal for the heat treated low carbon steel

at magnification of 500x

41

4.3 Microstructure of the heat affected zone (HAZ) of non-heat treated

low carbon steel at welding voltage of 20 V with ER70S welding

filler

43

4.4 Microstructure of the heat affected zone (HAZ) of non-heat treated

low carbon steel at welding voltage of 21 V with ER70S welding

filler

43

4.5 Microstructure of weldment for specimen of welding voltage 20V

with ER70S mild steel filler in magnification of 500x (non-heat

treated)

45

4.6 Microstructure of weldment for specimen of welding voltage 21V

with ER70S mild steel filler in magnification of 500x (non-heat

treated)

45

4.7 Polarization diagram for non-heat treated carbon steel welded with

ER70S filler

47

4.8 Polarization diagram for heat treated carbon steel welded with

ER70S filler

48

4.9 Polarization diagram for non-heat treated carbon steel welded with

ER308L filler

48

4.10 Polarization diagram for heat treated carbon steel welded with

ER308L filler

49

4.11 Graph of corrosion rate versus welding voltage for non-heat

treated specimens welded with ER70S mild steel filler

50

4.12 Graph of corrosion rate versus welding voltage for full annealing

heat treated specimens welded with ER70S mild steel filler

51

4.13 Graph of corrosion rate versus welding voltage for non-heat

treated specimens welded with ER308L stainless steel filler

52

4.14 Graph of corrosion rate versus welding voltage for full annealing

heat treated specimens welded with ER308L stainless steel filler

53

4.15 Graph of corrosion rate versus the welding voltage for welded low

carbon steel

54

xvi

4.16 SEM micrograph at magnification of 2000x for the specimen

without cleaning process

58

4.17 SEM micrograph at magnification of 1500x for the specimen after

cleaning process

58

xvii

LIST OF SYMBOLS

A Exposed area

Ar Argon

C Carbon

Cr Chromium

Cu Copper

d Density

Ecorr Corrosion potential

Fe Iron

Fe2O3·nH2O Iron (III) oxides

FeO(OH) Iron (III) oxide-hydroxide

Fe(OH)3 Iron (III) oxide-hydroxide

HCL Acid hydrochloric

I Welding voltage

Icorr Corrosion current

K Constant value

Mn Manganese

Mo Molybdenum

NaCl Sodium chloride

P Phosphorus

Q Heat input

S Welding speed

S Sulphur

Si Silicon

V Vanadium

xviii

Wt% Weight percentage

V Welding voltage

% Percentage

xix

LIST OF ABBREVIATIONS

AISI American Iron and Steel Institute

ASTM American Society for Testing and Materials

CR Corrosion rate

EW Equivalent weight

GMAW Gas Metal Arc Welding

HAZ Heat affected zone

MIG Metal inert gas

SAE Society of Automotive Engineers

SEM Scanning electron microscopy

SCC Stress corrosion cracking

UMP University Malaysia Pahang

CHAPTER 1

INTRODUCTION

1.1 BACKGROUND OF STUDY

Eventually, millions of dollars are lost every year in industrials because of the

existence of corrosion. Maintenances and replacements are spent to prevent losses and

improve the productivity of output. Much of the losses are due to the corrosion of iron

and steel, as well as the other metals. Corrosion of bridges is a major problem as they

age and require replacement, which may cost billions at least. In chemical industry,

corrosion maintenance always the priority in its acid plants even though the corrosion

conditions are not considered to be particularly severe. Bunch of costing are needed on

painting steel to prevent rusting by a marine atmosphere. The petroleum industry spends

a million dollars per day to protect underground pipelines. In prediction, costs of

corrosion will increase substantially because of shortages of construction materials,

higher energy cost, and aggressive corrosion environments in coal conversion process.

Welding is an efficient metal joining process which is commonly used in the

industry. This type of welding will need to use a welding gas to shield the welding

puddle and arc during the welding process. MIG welding is considered as the most

common welding process in the light or even the heavy industries nowadays. However,

during the welding process, due to the different quantity of heat input as well as the

quality of the weldments, many problems arise from the process especially in corrosion.

Thus, joint is always considered as the weakest part in the components.

2

Table 1.1: Cost related to corrosion

Region / Industry Cost of Corrosion Reference

Aircraft (lost revenue when

grounded for corrosion

maintenance/repairs)

$100,000 per day IAR Flyer, Spring 2000

Edition, NRC Institute for

Aerospace Research

(Canada).

Bridge (Old Severn Bridge)

in England

£20 million to mitigate

corrosion (projected), with

£3 million previously spent

on corrosion assessment of

suspension cables

BBC news (on-line

edition), March 7, 2007.

Coast Guard, United States,

Aircraft

$20 million per year Lee Ann Tegtmeier: "U.S.

Coast Guard Treads in

Deep Water", Overhaul &

Maintenance Magazine

(May 1, 2002).

Gas Pipeline Industry

(North America)

$80 million per year

purchased in coatings to

coat new pipelines and

recoat existing pipelines

(1993 reference)

Cavassi and Cornago: "The

Cost of Corrosion in the

Oil and Gas Industry",

JPCL, May 1999, pp30-40.

1.2 PROBLEM STATEMENT

The modernization of this era expands the demands and applications of carbon

steel especially in construction industry and the naval structure builder. A lot of mega

steel structure platforms have been designed and constructed to fulfill the exploitation of

civilization. Most of these structures are fabricated by technique of welding. However,

these weldments usually are more vulnerable to steel corrosion crack than the

corresponding base plates, the welded zones represent potential weak links which may

limit or impair performance. Moreover, the corrosivity of the seawater is reflected by

the fact that most of the common structural metals and alloys are attacked by this liquid

or its surrounding environments. Thus, improvements in weldment properties are

critical to increase the reliability of high-performance structures utilizing welded carbon

materials.

3

1.3 OBJECTIVE

The objective of this study is to investigate the corrosion behavior of metal inert

gas (MIG) welded heat treated low carbon steel.

1.4 SCOPE OF WORK

The scope of works involve in this study:

i. Material used was low carbon steel (AISI 1010).

ii. Full annealing heat treatment process on low carbon steel (AISI 1010).

iii. The two metal inert gas welding fillers used were ER70S and ER308L at

welding voltage of 19 V, 20 V and 21 V.

iv. Microstructures analysis on welded specimen by using optical

microscope.

v. Electrochemical test with solution of 3.5 wt % NaCl by using

potentiostat.

vi. Scanning Electron Microscope (SEM) was used to identify the corrosion

product.

CHAPTER 2

LITERATURE REVIEW

2.1 HEAT TREATMENT

Heat treatment is the metal working process that controls the heating and the

cooling of metals to modify the physical and the chemical properties of the metals

without changing the product shape. Heat treatment is sometimes done unwittingly due

to the manufacturing processes involved such as welding or forming processes. Heat

treatment process usually associated with increasing and altering certain

manufacturability of metals such as to improve the machining quality and restore the

ductility of the target materials. There are four basics types of heat treatment processes

nowadays which including the annealing, normalizing, hardening, and tempering.

2.1.1 Annealing

In general, annealing process relieves the stresses set up in the metal, soften the

metal and make the metal become more ductile by refining the grain structures. As

annealing process, the metal is heated to specific temperature and holds it at that

temperature for a set length of time. The cooling temperature for the annealing process

will be at the room temperature. Somehow, the cooling method usually depends on the

metal and the properties required. Some metals are cooled in furnace, by means that the

metal will has the slow cooling rate as some to the furnace. Others cooling method

found to bury the metal in ashes, lime or other insulating materials.

To enhance the maximum softness in steel, the metal is heated to proper

temperature and cooled in a very slow cooling rate. The cooling can be done by burying

the hot part in an insulating material or by shutting off the furnace and allowing the

5

furnace and the part to cool together. The soaking period is depending on both the mass

of the part and the type of metal.

The nonferrous metal such as copper is hard and brittle when undergoes the

mechanical work. However, it can be soften by this annealing process. The annealing

temperature for copper is between 700 °F to 900 °F. Copper maybe cooled rapidly or

slowly since the cooling rate has no effect on the heat treatment. The one drawback

experienced in annealing copper is the phenomenon called “hot shortness.” At about

900°F, copper loses its tensile strength, and if not properly supported, it could fracture.

For aluminum, the similar reaction as the copper when undergo heat treating.

There is a number of aluminum alloys exist and each of these alloys are requiring

special heat treatment to gain the best mechanical properties (Sanjib, 2008).

2.1.2 Normalizing

Normalizing heat treatment process is only applicable to the ferrous metal only.

It is differing from the annealing process in that the metal is heated to a higher

temperature and will be removed from the furnace for air cooling purpose.

The main objective of normalizing is to remove the internal stresses induced by

heat treating, welding, machining, forming and other machinability works. Stress is very

important where if there’s no proper control, it may leads to metal failure. Hence, before

any hardening process, the materials should undergo normalizing process first to ensure

the maximum desired results. Normally, low carbon steel does not need this pre

normalizing process. However, there is no harmful reason if this steel is normalized.

Normalized steels are harder and stronger than annealed steels. In the normalized

condition, steel is much tougher than in any other structural condition. Parts subjected to

impact and those that require maximum toughness with resistance to external stress are

usually normalized (Dosset and Boyer, 2002).

2.1.3 Hardening

The hardening treatment for most steels is that the metals are heating to a set

temperature and cooled rapidly by quenching into water, oil or brine. Most of the steels

need this rapid cooling for hardening but few can be air cooled to have the same result.

6

By undergoing the hardening process, the hardness and the strength of the steel are

increasing but less ductile. Generally, the harder the steel, the more brittle it becomes.

To remove the brittleness, metal should be tempered after the hardening process. Many

nonferrous metals can be hardened and their strength increased by controlled heating

and rapid cooling. In this case, the process is called heat treatment, rather than

hardening.

Case hardening always produce hard, wear resistant surface over a strong and

tough core. The principal forms of case hardening are carburizing, cyaniding and

nitriding. However, only ferrous metals are case hardened. Case hardening is ideal for

the parts that require wear resistant on the surface with tough enough to withstand the

heavy loading during work. In case hardening, the surface of the metal is changed

chemically by introducing a high carbide or nitride content. However, the core remains

chemically unaffected. When heat treated, the high carbon surface responds to

hardening and the core toughness.

Flame hardening is another procedure that is used to harden the surface of metal

parts. With the existence of oxyacetylene flame, thin layer at the surface of the part is

rapidly heated to its critical temperature and then immediately quenched by a

combination of a water spray and the cold base metal. This process produces a thin,

hardened surface. At the same time, the internal parts retain their original properties

because the heating process only affects the surface of the product. Whether the process

is manual or mechanical, a close watch must be maintained, since the torches heat the

metal rapidly and the temperatures are usually determined visually (Rajan and Sharma,

1992).

2.1.4 Tempering

After applying the hardening treatment, steel is often harder than desired. This

situation creates difficulty for most practical uses because it is too brittle. Besides,

severe of the internal stresses are set up during the fast cooling rate from the high

temperature during the hardening process. Hence, tempering process is carried out to

relieve the internal stresses and reduce the brittleness. Tempering consists of heating the

steel to a specific temperature (below its hardening temperature) and hold the steel at

the high temperature for certain of period before cooling it at still air condition. The

7

resultant of mechanical properties of the steel is based on the working temperature

during the tempering process.

Tempering is always conducted at temperatures be-low the low-critical point of

the steel. In this respect, tempering differs from annealing, normalizing, and hardening

in which the temperatures are above the upper critical point. Normally, the rate of

cooling from the tempering temperature has no effect on the steel. Steel parts are

usually cooled in still air after being removed from the tempering furnace; however,

there are a few types of steel that must be quenched from the tempering temperature to

prevent brittleness (Pierre et al., 2006).

Seawater is one of the most corroded and most abundant naturally occurring

electrolytes. The corrosivity of the seawater is reflected by the fact that most of the

common structural metals and alloys are attacked by this liquid or its surrounding

environments (Saleh Fozan and Anees Malik, 2005).

The corrosion behavior of metals and alloys differ from one zone to another. In

splash zone the stainless steels have usually satisfactory performance while, the carbon

and low alloy steels do not. Anderson and Ross had found that the austenitic grades

performed much better than martensitic and ferritic grades (Anderson and Ross, 1976).

The Ni, Cu and P alloyed steels were found to be much more resistant than carbon steel

in splash zone (Larrablee, 1958). Also, it was found that Mn, P and Al had measurable

influence on corrosion rates of low carbon steels under tidal exposure. After 5 years

exposure test it was found that the rate of attack in splash zone was much higher than

the atmosphere and deep submerged zones (Humbles, 1949).

Welded pipelines are widely used and for these the most severe pitting corrosion

has been observed to occur primarily at and near the welds. Such pitting tends to occur

at very different rates in the parent metal, the heat affected zone (HAZ) and in the weld

metal (Vetters, 1978; Hunkler, 1987). The HAZ is present on either side of a weld seam

as a result of the thermal gradient set up between the weld metal and the parent metal

(Traverso and Ventura, 1974). This region of parent metal has been heated briefly to

temperatures approaching the melting temperature of the steel and then cooled rapidly

(Vetters, 1978). In contrast, general corrosion loss, important for strength considerations,

tends to occur at a more moderate rate (Bai, 2005).

8

Naval structures are especially vulnerable because they are constantly exposed

to a particularly aggressive environment which provides conditions favorable to

corrosion cracking damage. Many of these structures are fabricated by welding, and

because weldments frequently are more vulnerable to corrosion crack than the

corresponding base plates, the welded zones represent potential weak links which may

limit or impair performance (Gooch, 1974). Thus, improvements in weldment properties

are critical for increasing the reliability of high-performance structures utilizing welded

high-strength materials.

2.2 WELDING

Welding always refer as the fabrication process that joins materials causing by

coalescence. These joining materials usually include metals or thermoplastics. This

joining process is often done by the melting between the two base materials with the

adding of a filler material to form a pool of molten material which also known as weld

pool. As the weld pool is cold, the joint between the two base materials become strong,

with pressure sometimes used in conjunction with heat to produce the weld. The

welding process can be contrasted as the soldering and the brazing in which these

processes involve the melting on a lower melting point material between the workpieces

to form a bond between them.

There are several energy sources can be used for welding including a gas flame,

an electric arc, a laser, an electron beam and etc. In industrial process, welding can be

performed in different environments such as open air, under water and in outer space.

While performing the process of welding, hazardous undertaking and precautions must

be taking care to avoid burns, electric shock, vision damages and inhalation of

poisonous gases and fumes (Gooch, 1974).

To a large extent, the welding process determines the metallurgical and

microstructural characteristics of deposited weld metals. The thermal cycles associated

with multi-pass welding techniques invariably produce a spectrum of tempered material

and residual stresses which impact on the mechanical and SCC behavior of weldments.

Additionally, based on the results of several studies on the embrittlement of high-

strength steels, the effect of impurities may also be a very significant factor which

9

influences the mechanical and stress corrosion crack properties of steel weldments

(Kenichiro and McMahon, 1974; Briant and Banerji, 1979; Robert, 1977; Viswanathan

and Hudak, 1977).

2.2.1 MIG welding

The MIG, metal inert gas welding process is actually also known as Gas Metal

Arc Welding (GMAW). This type of welding will need to use a welding gas to shield

the welding puddle and arc during the welding process. MIG welding is considered as

the most common welding process in the light or even the heavy industries nowadays.

MIG welding is so popular due to the easy handling during the welding process. It is

faster and quicker to perform the joining process for the common metals in industries.

MIG welding usually used with the robots and automation purposes. The main reason

why MIG welding is popular because it is able to weld joins variety of metals at

different thickness. For MIG welding, it uses a consumable wire electrode that is fed

from a spool.

The most common weld joins include the lap joint, butt joint, T-joint and the

edge joint. MIG is used to weld many materials with different gases to form the arc

which is depending on the materials to be welded together. Argon CO2 blend is usually

used to join the mild steel, aluminum, titanium and alloy metals as well. Helium is

sometimes used to weld the carbon steel and titanium in high speed process. However,

carbon dioxide is the most often used to weld the carbon steel and low alloy steel for the

MIG process.

The very advantage of MIG is the fact that it is able to weld at different positions.

Welder can weld overhead, horizontal, vertical up and down with the one welding

machine. Welder does not need to stop all the time to change the electrode compare to

the stick welder. There are fewer spatters if using the solid MIG welding wire as

comparing to the gasless welding wire where spatter is around about the same as that of

manual metal arc welding. By performing the MIG welding, the heat affected zone

(HAZ) will be smaller because of the fast travel rates that are attainable with this

welding process. The fast travel rates have the less heat input and energy going into the

same weld joint.

10

However, the use of inert gas causes this MIG welding less portable than the arc

welding where the arc welding requires no external source of shielding gas. The other

disadvantage of MIG welding is that it is able to produce sloppier and less controlled

weld as compared to the tungsten inert gas welding (Gooch, 1974).

The thermal diffusity of the base metal is important where if the diffusivity is

higher, the material cooling rate is high and hence the heat affected zone is relatively

small. The amount of heat inputted at the welding process is also important where high

heat input will increase the size of the HAZ. Inversely, limited amount of heat at the

welding process will resulting in a small heat affected zone. The formula of the welding

heat input is the significant and showed as followed:

(2.1)

Where Q = heat input, kJ/min, V = voltage, V, I = base current, A and S = welding

speed, mm/min

From the research of Dursun O zyurek, the increase in heat input related with

current will caused the coarsening of the microstructure of weld nugget and also of heat

affected zone, HAZ. From the experiment conducted, the tensile shear load bearing

capacity of welded materials increases with the increasing of heat input related with

weld current due to the enlargement of nugget size which shown in Figure 2.1 (Dursun,

2007).