study on thickness effects of low carbon steel in … · 2.4 the percentage of phases after heat...

STUDY ON THICKNESS EFFECTS OF LOW CARBON STEEL IN RAPID

COOLING PROCESS

NUR ADJLA BINTI MOHAMAD HAFIZIN

This thesis is submitted as a partial fulfillment of the requirements for the award of the

Bachelor of Mechanical Engineering with Manufacturing Engineering

Faculty of Mechanical Engineering

UNIVERSITI MALAYSIA PAHANG

JUNE 2012

vii

ABSTRACT

Hot stamping is among the latest technology applied in sheet metal manufacturing. It is

a combination of a forming and rapid cooling to produce stronger products with the

creation of martensite inside the sheet metal components. To cope with a huge range of

sheet metal that suitably used for hot forming process, this research investigates

microstructure changes during the rapid cooling process of three different sheet metal

thicknesses. The Low Carbon Steel specimens of 2, 4 and 6mm thick were heated in a

furnace to austenite region before immediately quenched in water and normalized in air.

During the process, cooling rates were measured with acquisition system. The quenched

specimens were observed using metallurgical microscope. The results show that water

quench produced martensite structures while air quench produced pearlite and ferrite

structures. The 2mm specimen of water quench formed a higher percentage of

martensite and compactly bonded structure compared to 4mm and 6 mm specimens’

structure. For the air quench, 2mm specimen formed finely dispersed pearlite and

ferrite. However, the microstructure of 4mm and 6mm specimens’ shows coarse pearlite

and ferrite with lower hardness surface.

viii

ABSTRAK

Proses pembuatan bersuhu panas ke atas logam adalah antara teknologi terkini yang

digunakan dalam industri pembuatan. Ia adalah gabungan pembentukan dan proses

penyejukan pantas untuk menghasilkan produk lebih kukuh dengan penciptaan

martensit di dalam komponen yang dihasilkan dari lembaran logam. Untuk berhadapan

dengan pelbagai besar kepingan logam yang sesuai digunakan untuk proses

pembentukan panas, kajian ini mengkaji perubahan mikrostruktur semasa proses

penyejukan pada tiga kepingan logam yang berlainan ketebalan. Spesimen keluli carbon

rendah daripada ketebalan 2, 4 dan 6mm telah dipanaskan dalam relau sehingga

mencapai fasa austenit sebelum segera disejukkan dalam media penyejukan iaitu air dan

udara. Semasa proses penyejukan, kadar penyejukkan setiap specimen telah diukur

dengan sistem pemerolehan data. Mikrostruktur dalam spesimen telah di analisis

menggunakan mikroskop metalurgi. Keputusan menunjukkan bahawa air lindap kejut

menghasilkan struktur martensit manakala udara lindap kejut menghasilkan struktur

pearlit dan ferit. Spesimen berketebalan 2mm pada proses air lindap kejut membentuk

peratusan martensit yang lebih tinggi dan struktur yang kemas terikat dengan kekerasan

permukaan yang tinggi berbanding mikrostuktur yang dihasilkan oleh specimen 4mm

dan 6mm. Bagi lindap kejut udara, spesimen 2mm membentuk taburan halus pearlit dan

ferit. Walau bagaimanapun, mikrostruktur 4mm dan 6mm spesimen menghasilkan

pearlit dan ferit yang kasar dengan kekerasan permukaan yang lebih rendah.

ix

TABLE OF CONTENTS

TITLE PAGE Page

SUPERVISOR’S DECLARATION i

STUDENT’S DECLARATION iii

DEDICATION iv

ACKNOWLEDGEMENTS v

ABSTRACT vi

ABSTRAK viii

TABLE OF CONTENTS ix

LIST OF FIGURES xii

LIST OF TABLES xv

LIST OF SYMBOLS xvi

LIST OF ABBREVIATIONS xviii

CHAPTER 1 INTRODUCTION

1.1 Introduction 1

1.2 Problem Statement 3

1.3 Objectives 3

1.4 Background of Project 3

1.5 Scope of Project 4

CHAPTER 2 LITERATURE REVIEW

2.1 Introduction 5

2.2 Material 5

2.3 Heat Treatment 7

2.4 Internal Structure After Heat treatment 7

2.4.1 Austenite 7

2.4.2 Ferrite 8

2.4.3 Carbon 8

x

2.4.4 Cementite 8

2.4.5 Pearlite 8

2.4.6 Martensite 8

2.5 Normalizing process 9

2.6 Quenching Process 10

2.7 Slow cooling of Hypotectoid Steel 11

2.8 Continuous-Cooling Transformation Diagram For

Carbon Steel

14

2.9 Time-Temperature Transformation Diagram 17

2.9.1 Austenite to Pearlite Transformation 18

2.9.2 Austenite to Bainite Transformation 19

2.9.3 Austenite to martensite transformation 19

2.10 Microstructure Theory 20

2.11 Testing On Mechanical Properties 20

2.11.1 Hardness Test 20

2.11.2 Rockwell Hardness Tests 21

2.12 Heat Treatment Effects of Low Carbon Steel 23

2.13 Effect Of Cooling Rate On Hardness And

Microstructure

25

2.14 Heat Treatment In Industry 27

2.14.1 Thermocouple 27

CHAPTER 3 METHODOLOGY

3.1 Introduction 29

3.2 Specimen Preparation 31

3.2.1 Design of specimen 31

3.2.2 Milling 32

3.2.3 EDM Wire cut 33

3.2.4 Material composition test for specimens 34


3.3.1 Thermocouple 35

3.3.2 Data Logger 35

3.3.3 National Instrument, NI 9213 36

3.3.4 Heat treatment Setup 37

3.3.5 Air Cooling Process 38

3.3.6 Water Quenching Process 39

xi

3.4 Microstructure Analysis 39

3.4.1 Sectional cut 39

3.4.2 Mounting 41

3.4.3 Grinding 42

3.4.4 Final Polishing 43

3.4.5 Etching 44

3.4.6 Microstructure and composition analysis 45

3.5 Hardness Test 46

CHAPTER 4 RESULT AND DISCUSSION

4.1 Introduction 48

4.2 Material Composition 48


4.4 Microstructure Analysis 54

4.4.1 AISI 1023 57

4.4.2 Air cooling process 58

4.4.3 Water Quenching 65

4.4 Hardness Test 70

4.4.1 Surface Hardness for Air Cooled

Specimens

70

4.4.2 Surface Hardness for Water Quenched

Specimens

72

4.5 Comparison Between Normalizing And Water

Quenching With Different Thickness

74

4.6 Conclusion 75

CHAPTER 5 CONCLUSION AND RECOMMENDATION

5.1 Introduction

5.2 Conclusion 76

5.3 Recommendation 78

REFERENCES 77

xii

APPENDICES 80

A1 Digimizer Software results of Pearlite area

measurement for AISI 1023 untreated specimen

81


measurement for AISI 1023 6mm specimen after Air

Cooling process

82



Cooling process

85



Cooling process

87

B Chemical composition results of Low carbon Steel 89

xiii

LIST OF FIGURES

Figure

No. Title Page

2.1 Carbon Steel Classification 6

2.2 The microstructure of 0.2% carbon steel after air cooled. Fine

dispersed Pearlite (black) and Ferrite (white)

10

2.3 Transformation of 0.4% carbon Hypoeuctectoid steel 12

2.4 Microstructure of a 0.38 wt% C steel having a microstructure

consisting of pearlite (dark region) and proeutectoid ferrite

(white region)

14

2.5 Continuous-cooling diagram for a plain-carbon eutectoid steel 15

2.6 Variation in the microstructure of a eutectoid plain-carbon steel

by continuously cooling at different rates

16

2.7 Time-temperature diagram for carbon steel 17

2.8 Summary of basic thermocouple properties 28

3.1 Flow of methodology process 30

3.2 Low carbon steel blocks 31

3.3 Microstructure and hardness test specimen 31

3.4 Milling machine Partner VMM3917 32

3.5 Face milling process 33

3.6 EDM Wire Cut Machine 33

3.7 Cutting wire 33

3.8 Arc Spark Spectrometer for testing the metal composition 34

3.9 Cutting wire 34

3.10 Thermocouple wire type K 35

xiv

3.11 Tested specimen with 4 testing sparks 36

3.12 Furnace 37

3.13 National instrument NI 9123 37

3.14 Data logger set up 38

3.15 Sectional Cut off Machine MSX200M 40

3.16 Cutting section of specimen 40

3.17 Mounted specimens 42

3.18 HandiMet 2Roll Grinder frames 42

3.19 Grinding process using HandiMet 2Roll Grinder 43

3.20 Metken Forcipol 2V Grinding/Polishing M/C 43

3.21 Polycrystalline solution 6 µm, 1 µm and 0.05 µm 44

3.22 Fume hood 44

3.23 Metallurgical Microscope 45

3.24 Metallurgical analysis process 45

3.25 Matsuzawa Digital Rockwell Hardness Tester 46

3.26 Scale table of Rockwell testing test 47

4.1 Tested specimen with 4 tested sparks 49

4.2 Experiment equipment and setup 50

4.3 Heat treatment experiment setup 51

4.4 Cooling curve of water quenching 52

4.5 Cooling curve for air cooling specimen 53

4.6 Specific part of specimen for microstructure analysis 54

4.7 Microstructure analysis using Digimizer software 55

4.8

(a) The microstructure of AISI 1023 steel without heat

treatment

56

(b) Inverted image of microstructure of AISI 1023 untreated 56

xv

specimen using Digimizer software

4.9 (a) Microstructure of 2mm specimen after normalizing

process

59

(b) Inverted image of microstructure 2mm specimen of

normalizing using Digimizer software

60


process

61



61


process

61



61

4.12 Influence of thickness on Pearlite structure area 64

4.13 (a) Microstructure of 2mm specimen after water quenching

process

65

(b) The inverted image of 2mm specimen by Digimizer

software

66


process

67


software

67


process

68


software

69

4.16 Graph effect of thickness of Air cooled specimen 71

4.17 Graph of Rockwell Hardness of Water quenched specimens 73

4.18 Cooling rate and thickness effect on hardness 74

xvi

LIST OF TABLES

Table

No.

Title Page

2.1 Application of Low Carbon Steel based on carbon contain 7

2.2 Hardness Testing Techniques 22

2.3 Rockwell Hardness Scale 23

2.4 The percentage of phases after heat treatment of AISI 1020 steel 26

3.1 Geometry dimension for microstructure and hardness test

specimen

32

4.1 Average composition of material for the specimen 49

4.2 Classification of specimen 51

4.3 Microstructures of Air Cooled and Water quenched Specimens 55

4.4 The total value of pearlite measurement of untreated specimen 58

4.5 Total measurement of pearlitic phase of 2mm air cooled

specimen

60


specimen

62


specimen

63

4.8 Summarization of Resulted Pearlitic area of 2mm, 4mm and

6mm Air Cooled Specimen

64

4.9 Rockwell hardness (60kgf) of AISI 1023 at the surface of

specimens with Air Cooling Process

70

4.10 Rockwell hardness (60kgf) of AISI 1023 at the surface of

specimens with Water Quenching Process

72

xvii

LIST OF SYMBOLS

𝛾 Austenite Phase

𝛼 Ferrite Phase

Te Eutectoid Temperature

Ms Martensite Start

Mf Martensite Finish

C Carbon

Fe Ferrous

Si Silicon

Mn Manganese

P Phosphorus

S Sulphur

Cr Chromium

Mo Molybdenum

Ni Nickel

Al Aluminum

Co Cobalt

Cu Copper

Nb Niobium

Ti Titanium

V Vanadium

Pb Plumbum

Sn Stannum

xviii

B Boron

Ca Calcium

Zr Zirconium

As Arsenic

Bi Bismuth

Fe3C Cementite

xix

LIST OF ABBREVIATIONS

AISI American Iron and Steel Institute

BCC Body-centered Cubic

BCT Body-centered Cubic

CCT Continuous-cooling Time

EDM Electric-discharged Machine

FCC Face-centered Cubic

ISA Instrument Society of America

NI National Instrument

CHAPTER 1

INTRODUCTION

1.1 INTRODUCTION

Process of manufacturing activities can be improved by the understanding of

microstructure characteristic of the used material. In this case, the most useful material

is metals where their material properties are usually determined by tests. As a

mechanical engineer, it is necessary to engage in testing work to have a general

understanding of the common methods of properties test of metals. From an engineering

point of view, the mechanical properties are the most important requirement in selecting

material for design purpose. Mechanical properties and microstructure of metals

describes their behavior under mechanical and physical usage.

Generally, the applications of metals have been studied a long time ago. Since

the Iron Age, metal have been practically used in daily life and become very important

in the development of industry. The ability to withstand service for higher loadings

without permanently deforms and ruptures. In the other hand, the ability of metal

undergoes large permanent deformation, thus permitting the formation of different

shapes under the application of proper forces [Izabel, 2005]. Plenty of studies have been

done on metal, so the microstructures of most metals are well identified as well as the

effects of heat treatment in altering their mechanical properties.

The most popular metal is known as carbon steel. Carbon steel is being divided

as low carbon steel, medium carbon steel and high carbon steel which based on the

carbon content. Normally carbon steel is not enough hard to use in certain

manufacturing. Therefore heat treatment is important process to improve the mechanical

2

properties of the carbon steel. The common forms of heat treatment for steels are

hardening, tempering, annealing, normalizing and case hardening [Krauss, 1989].

The different mechanical properties of carbon steels will be resulted as different

microstructures formed during cooling. Furthermore, the diffusionless transformations

obtain the martensite formation which is the highest hardness in iron-carbon system and

the lowest hardness is obtained due to a diffusion transformation, which cause the

ferrite and/or pearlite formation by a eutectoid reaction. Martensite is obtained during

rapid cooling while ferrite and pearlite obtained from austenite during slow cooling near

the equilibrium. Therefore, both steel microstructure and mechanical properties are

related to steel thermal history [W. Wen, 2005]. Pressure also has an influence on

stability of phase of equilibrium. However, this parameter is not usually taken in

account, mainly in solid state reaction. It is because the influence of pressure is limited

[Jokhio et al., 2000].

Low carbon steel as one of engineering material that has been selected in this

project is to investigate the comparison of the effects on thickness of material of rapid

cooling process and develop a mechanical data of low carbon steel after the heat

treatment. This material have been chosen because this type of steel is mostly used for

deep drawing of motor car bodies, motor cycle parts, and other domestic applications

where it is must involved the desired mechanical properties processes.

From the engineering material theory, the required properties of material can be

changed by manufacturing process and heat treatment process method. Therefore the

present work was planned to investigate the relationship between thickness,

microstructure, mechanical properties, and heat treatment for intelligent selection of

manufacturing process, properties, and application for particular purpose [Sim et al,.

2004].

3

1.2 PROBLEM STATEMENT

This project is being undertaken to compare microstructure effects of different

thickness on metal in rapid cooling process. In present day, new approaches of

manufacturing process have been introduced. For example, the application of hot

stamping technique is used to form many types of car component. This process involves

thermo mechanical effects where the sheet metal parts are quenched after forming

process to improve the mechanical properties. It is important to investigate the thickness

effects on microstructure development.

1.3 OBJECTIVES

The aim of this research is to understand the mechanism of investigation

microstructures changes during rapid cooling process of different sheet metal

thicknesses. In order to achieve the objectives, the approach to the research has been

determined as listed below:

1) To conduct experiment of rapid cooling process on low carbon steel

2) To identify the material composition of low carbon steel

3) To investigate the microstructure effect after water quenching and air cooling

process with different thickness

4) To determine the effect of material thickness on surface hardness

1.4 PROJECT BACKGROUND

Quenching is common process to harden the steel. This process are widely used

in industry which to obtain the required properties of any application. From this study,

the mechanical properties of the specimen from in-received after quenching process

with different material thickness can be differentiate. The formation of martensite and

pearlite can be analyzed and the investigation of influences of thickness effecting the

microstructure and hardness of material.

4

1.5 PROJECT SCOPE

1.5.1 Literature Reviews

i. Researching and surveying on the paper work that have been made by the

previous researcher.

ii. Collect the information related to the project’s objectives.

1.5.2 Experiment

i. Planning the methodology flow of experiment.

ii. Specimen preparation of experiment.

iii. Do the experiment to compares the effects of different thickness in two types of

different process which are normalizing and quenching

iv. Apply two types of test on the low carbon steel after heat treatment

(Microstructure test and Rockwell hardness test).

1.5.3 Analysis of the Experiment

i. Analyze the experiments result and do a comparison related of the experiment.

ii. Summarize the assumption from the research.

1.5.4 Thesis Writing

i. Transfer all the experiment data and results into a form of thesis report including

the conclusion of project.

5

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

Rapid cooling is a process purpose to harden the metals by immersion in oil or

water, of a metal object from the high temperature at which it has been shaped. This

usually is undertaken to maintain mechanical properties associated with a crystalline

structure or phase distribution that would be lost upon slow cooling. The technique is

commonly applied to steel objects, to which it imparts hardness. On the other hand,

copper objects that have become hardened by hammering or other deformation at

ordinary temperatures can be restored to the malleable state by heating and quenching.

2.2 MATERIAL

In this project, low carbon steel has been chosen to serve as experimental

material. Low carbon steel is a type of metal that has an alloying element made up of a

relatively low amount of carbon. Typically, it has a carbon content that ranges between

0.05% and 0.20% and a manganese content that falls between 0.40 and

1.5%. According to Figure 2.1, Low Carbon Steel can be classified into hypoeutectoid

steel based on its carbon content range.

6

Figure 2.1: Carbon Steel Classification

Source: Ginzel (1995)

Low carbon steel is known as low carbon steel is one of the most common types

of steel used for general purposes, in part because it is often less expensive than other

types of steel. While the steel contains properties that work well in manufacturing a

variety of goods, it is most frequently made into flat-rolled sheet or strips of steel.

Steel with low carbon content has the same properties as iron, soft but easily

formed. As carbon content rises the metal becomes harder and stronger but less ductile

and more difficult to weld. Higher carbon content lowers steel's melting point and its

temperature resistance in general. Carbon content influences the yield strength of steel

because they fit into the interstices crystal lattice sites of the body-centered cubic (BCT)

arrangement of the iron molecules [Merbaki et al., 2004].

Steel can be heat-treated which allows parts to be fabricated in an easily

formable soft state. If enough carbon is present, the alloy can be hardened to increase

strength, wear, and impact resistance. Steels are often wrought by cold-working

methods, which is the shaping of metal through deformation at a low equilibrium or met

a stable temperature.

7

Table 2.1: Application of Low Carbon Steel based on carbon contain

Carbon Percentage (%) Applications

0.1-0.2 Chain, stamps, rivets, nails, wire, pipe and where

very soft, plastic steel is needed

0.2 – 0.3 Structural steels, machine parts, soft and tough steels

and use of case hardened machine part and screw

2.3 HEAT TREATMENT

A combination of times heating and cooling operations which applied to the

solid state metal or alloy is called as heat treatment. This method is often used to modify

or alter the microstructure and properties of steels. The various types of heat-treating

processes are similar because they all involve the heating and cooling of metals.

However, they differ in the heating temperatures and the cooling rates used. The final

results of each method are different. The usual methods of heat-treating ferrous metals

(metals with iron) are annealing, normalizing, hardening, and tempering. Most

nonferrous metals can be annealed, but never tempered, normalized, or case-hardened.

According to [Jokio et al., 2005], heat treatment is defined as physical process

which entails the controlled heating and cooling of material, such as metal or alloys to

obtain desired properties. Heat treating is an energy intensive process that is carried out

in different furnaces such as electric and gas. Shortening heat treatment cycles can

provide great environment and financial benefits through energy saving. Many texture

detail furnace equipment and its design but little to no literature can be found in furnace

temperature method [Qiao et al., 2009].

2.4 INTERNAL STRUCTURE AFTER HEAT TREATMENT

2.4.1 Austenite

This phase is only possible in carbon steel at high temperature. It has a Face

Center Cubic (FCC) atomic structure which can contain up to 2% carbon in solution.

8

2.4.2 Ferrite

This phase has a Body Centre Cubic structure (BCC) which can hold very little

carbon. Typically 0.0001% at room temperature and can exist as either alpha or delta

ferrite.

2.4.3 Carbon

A very small interstitial atom that tends to fit into a cluster of iron atoms. It

strengthens steel and gives it the ability to harden by treatment.

2.4.4 Cementite

A very hard intermetallic compound which consisting of 6.7% carbon and the

remainder iron. Its chemical symbol is Fe3C. However, when it is mixed with soft ferrite

layers, the average hardness is reduced considerably. Slow cooling gives course

pearlite; soft and easy to machine but poor toughness. Faster cooling gives very fine

layers of ferrite and cementite which is harder and tougher.

2.4.5 Pearlite

Pearlite is a mixture of alternate strips of ferrite and cementite in single grain.

The distance between the plates and their thickness is depending on the cooling rate of

the material. Fast cooling will creates thin plates that are close together and slow

cooling creates a much coarser structure possessing less toughness. A fully pearlitic

structure occurs at 0.8% carbon. Further increases in carbon will create cementite at the

grain boundaries, which will start to weaken the steel.

2.4.6 Martensite

Body centered tetragonal (BCT) structure occur when steel is cooled rapidly

from austenite, the FCC structure rapidly changes to BCC leaving insufficient time for

the carbon to form pearlite. This results in a distorted structure that has the appearance

9

of fine needles. There is no partial transformation associated with martensite, it either

form or it doesn’t. However, only the part of the section that cool fast enough will form

martensite. In the thick section it will only form to a certain depth, and if the shape is

complex it may only form in small pockets. The hardness of martensite is solely

dependent on carbon content; it is normally very high, unless the carbon content is

exceptionally low.

2.5 NORMALIZING PROCESS

This process can be done by heating the steel to austenite phase and cooling it in

the air. In industry, normalizing is applied to eliminate coarse-grained structures

obtained in previous working operations such as rolling and forging. Besides,

normalizing is purpose to modify and improve the cast dendritic structures and reduce

segregation by homogenizing the microstructure. The desired microstructure and

mechanical properties can be obtain and improve the machinability of low carbon steel.

In normalizing steel is uniformly heated to a temperature which causes complete

transformation to austenite. The steel is held at this temperature for sufficient time for

the formation of homogeneous structure throughout its mass. Then, it is allowed to cool

in air. Furthermore, normalizing results in the formation of ferrite, cementite and

lamellar pearlite. Due to the higher cooling rate in this process, the transformation of

austenite is occurring at a lower temperature than in annealing process. Therefore, the

transformation of pearlite is finer where having lower interlamellar spacing between the

two neighboring of cementite plates compared to anneled steel compared to coarse

pearlite [Izabel, 2005].

The amount of pearlite will affect the hardness and strength of steel. The

normalized steels are harder and stronger than annealed steels due to the amount of

pearlite in normalized steel is more despite having the same carbon content [Saquet et

al., 1999]. This differences value of pearlite occurs due to the shifting of eutectoid

composition to a lower value. The microstructure of 0.2% carbon steel in normalized

condition is shown in Figure 2.2 refers to the researched of the Effect of Cooling Rate

on Hardness and Microstructure of AISI 1020 Steels [Adnan C., 2009].

10

In normalizing, the mass of metal has an influence on the cooling rate and on the

resulting structure. Thin pieces cool faster and are harder after normalizing than thick

ones. In annealing (furnace cooling), the hardness of the two are about the same [Holzcr

et al., 1991].

Figure 2.2: The microstructure of 0.2% carbon steel after air cooled. Fine dispersed

pearlite (black) and ferrite (white)

Source: Adnan C. (2009)

Since normalizing is a much more economical process, this type of method is

extensively used in industry as cooling take place in the air. Some typical examples of

normalizing in commercial practice are homogenization of cast and wrought structures,

improvement of machinability, grain size refinement of cast structures and stress

relieving of castings [Monsanto, 1992].

2.6 QUENCHING PROCESS

Quenching is one of the hardening processes of steel that may be defined as

rapid cooling from the austenite phase. The rapid cooling is obtained by immersion of

steel in a liquid bath such as water or oil. The rapid cooling of steel from austenite

phase results in the formation of meta-stable phase called martensite. The main

objective of this process is to increase hardness, strength and wear resistance. Besides,

study on thickness effects of low carbon steel in … · 2.4 the percentage of phases after heat...

Documents