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i
EFFECTS OF HEAT TREATMENT AND
ALLOYING ELEMENTS ON CHARACTERISTICS
OF AUSTEMPERED DUCTILE IRON
Submitted By:
ii
MUHAMMAD ASHRAF SHEIKH
2001-PhD-Met-02
Department of Metallurgical and Materials Engineering
UNIVERSITY OF ENGINEERING AND TECHNOLOGY
LAHORE – PAKISTAN
2008
iii
EFFECTS OF HEAT TREATMENT AND
ALLOYING ELEMENTS ON CHARACTERISTICS
OF AUSTEMPERED DUCTILE IRON
A thesis submitted to the University of Engineering and Technology,
Lahore as a partial fulfillment for the degree of Doctor of Philosophy in
Metallurgical and Materials Engineering
Approved on ___12-01-2008____
Internal Examiner: Signature: _______________________
(Supervisor) Name: Professor Dr Javed Iqbal
External Examiner: Signature:________________________
Name: Professor Dr M. Saleem Shuja
Rector, University of Lahore.
Chairman of the Department: Signature: ________________________
Name: Prof. Qasim Hassan Zaidi
Dean - Faculty of Chemical Signature: ________________________
Min. & Met. Engg. Name: Prof. Dr. Faiz ul Hasan
Department of Metallurgical and Materials Engineering
UNIVERSITY OF ENGINEERING AND TECHNOLOGY
iv
LAHORE-PAKISTAN
2008
v
This thesis was evaluated by the following Examiners:
External Examiners:
From Abroad:
Dr. Ramin Raiszadeh,
Metallurgy Department, Engineering School,
Shahid Bahonar University Of Kerman
Kerman, Iran.
Dr. Derya Dispinar,
Metallurgy and Materials Engineering,
University of Istanbul,
Turkey
From Pakistan:
Professor Dr. M. Saleem Shuja
Rector,
University of Lahore.
Internal Examiner:
Professor Dr Javed Iqbal
Department Metallurgical and Materials Engineering.
University of Engineering and Technology,
Lahore.
vi
“Glory to You: of knowledge we have none, save what You have taught us;
in truth it is You Who are perfect in knowledge and wisdom.”
(Al-Quran 2:32)
vii
Dedication
to
my mother,
my wife and children
viii
ACKNOWLEDGEMENTS
I am highly grateful to my supervisor, Professor Dr. Javed Iqbal for his
guidance, encouragement and supervision given throughout this work.
I feel great obligation to Professor Dr. John Campbell, Head IRC Department
and Dr. T. U. Din, Research Associate University of Birmingham, UK for giving me
permission to do my experimental work in the department, for their co-operation and
valuable guidance.
My special thanks to Lt. General (R) Muhammad Akram Khan, the Vice
Chancellor, University of Engineering and Technology, Lahore, for his continued support
and encouragement throughout this research work. I also wish to acknowledge the
financial support of the Higher Education Commission, Islamabad.
I am also grateful to the Director General Research, Dr. K. E. Durrani, Dean,
Professor Dr. Faiz ul Hasan, Chairman, Prof. Qasim Hassan Zaidi, Director Post
Graduate Studies, Prof. Dr. M. Ajmal Chishti for their help and cooperation during my
research work.
I would like to acknowledge the support of Mr. Munir Ahmad, M.D. and Mr.
Izhar Ahmad of Pakistan Standards and Quality Control Authority, Lahore, Dr. Shahzad
Alam and Mr. Junaid of PCSIR Laboratories, Lahore and Mr. M. Sadiq Qureshi of
Flames International for helping me in testing of the samples.
My sincere thanks and appreciation are also due to M/S ADI Treatments, U.K.
for the heat treatment of some of the samples. I would like to thank Mr. Adrian and Mr.
Michael of Casting Research Group, University of Birmingham , UK and Mr. Rashid
Ahmad of Star Agro Engineering & Foundry, Lahore; for making the melts and Mr.
Furqan Ahmad & Mr. Asif Rafiq of University of Engineering & Technology, Lahore for
their help in metallography.
I would like to acknowledge the assistance of Mr. Muhammad Saeed, Mr.
Manzoor Ahmad and Mrs Azra Haroon of University of Engineering & Technology,
Lahore.
I wish to thank all my colleagues and staff of various laboratories for their help
specially Mr. Shahzad Ali and Mr. Abdul Qayyum for their assistance.
Finally I take this opportunity to express my gratitude to my family, specially
my wife for her encouragement and support.
M. ASHRAF SHEIKH
ix
ABSTRACT
The effect of three variables on ductile iron has been investigated in this study.
The first variable was the effect of austempering time on ductile iron. The second
variable was the effect of austenitizing temperature and the third major variable was the
effect of alloying additions on ductile iron. The alloying elements selected for this
purpose were copper, nickel, a combination of copper and nickel and lanthanum.
The initial study was conducted on unalloyed ductile iron castings. The effect of
austempering time was examined by varying austempering time in the range of 30
minutes to 90 minutes, while keeping austenitization temperature and austempering
temperature constant. It was found that with the increase of austempering time, the tensile
strength increased significantly. However, at 90 minutes the tensile strength decreased.
The optimum temperature was found to be 60 minutes. The second variable was the
effect of austenitization temperature on ductile iron. Based on the result of the first
experiment, the austempering was carried out for 90 minutes. The austempering
temperatures were kept at 270 oC and 370
oC. The austenitization temperature was varied
from 850 o
C to 925 oC. The study revealed that tensile strength increased at 900
oC but it
decreased at 925oC. The third major variable involving the effect of alloying additions on
ductile iron, was studied by adding copper with three different values i.e. 0.5 wt. %, 1.0
wt. % and 1.5 wt. %. The fourth melt was without the addition of copper. It was found
that with the increase of copper the tensile strength continued to increase up to 1.5 wt. %.
The second alloying addition was nickel. One melt was made without nickel while the
remaining three melts were made with the addition of 1.0 wt. %, 2.0 wt. % and 3.0%
x
nickel. The tensile strength increased correspondingly with the increase in the addition of
nickel to 3.0 wt. %. The effect of a combination of copper and nickel on ductile iron was
also examined. The effect of the last alloying element which was studied was lanthanum.
Four melts were made for this study. The first melt was without the addition of
lanthanum while the remaining three had 0.006 wt.%, 0.02 wt.% and 0.03 wt.%
lanthanum. The results indicated that the tensile strength increased with the increase of
lanthanum content with and without austempering. Furthermore, the highest nodule count
was obtained with 0.03 wt.% lanthanum while the nodularity remained almost
unchanged.
Thus, it was observed that the addition of alloying elements results in an
increase of tensile strength. The optimum austempering time was 90 minutes and the
optimum austenitizing temperature was found to be 900 oC.
xi
TABLE OF CONTENTS
Description Page
Acknowledgement
Abstract
Table of Contents
List of tables
List of figures
Chapter–1 INTRODUCTION 1
Chapter–2 LITERATURE REVIEW 2
2.1 Ductile Iron 4
2.1.1 History of Ductile Iron 4
2.1.2 Production of Ductile Iron 5
2.1.2.1 Raw Materials 5
2.1.2.2 Control of the Composition of Ductile Iron 5
2.1.2.3 Charge Materials 8
2.1.2.4 Desulphurization 9
2.1.2.5 Spheroidizing Treatment Alloys 10
2.1.2.6 Melting Techniques for the Production of
Ductile Iron
10
2.1.2.7 Spheroidizing Treatment 11
2.1.2.8 Amount of Magnesium Required 13
2.1.2.9 Inoculation 15
2.1.3 Formation of Carbides in Ductile Iron 17
2.1.4 Pouring 18
2.1.5 Importance of Ductile Iron 18
2.2 Austempered Ductile Iron (ADI) 20
2.2.1 Austempering 20
2.2.2 Introduction to Austempered Ductile Iron 20
xii
2.2.3 Production of Austempered Ductile Iron 21
2.2.3.1 Composition of ADI 22
2.2.3.2 Effects of Alloying Elements 22
2.2.3.3 Production of Austempered Ductile Iron 27
2.2.3.4 Heat Treatment Considerations 29
2.2.4 Specifications of Austempered Ductile Iron 30
2.2.5 Cost Benefits of Austempered Ductile Iron 31
2.2.6 Properties Of Austempered Ductile Iron 32
2.2.7 Disadvantages of Austempered Ductile Iron 33
2.2.8 Application of Austempered Ductile Iron 33
Chapter–3 EXPERIMENTAL WORK 36
Research Methodology 36
3.1 Production of Ductile Iron 39
3.1.1 Ductile Iron without and with Copper, Nickel and
Copper-Nickel Together
39
3.1.2 Ductile Iron Prepared without Lanthanum 42
3.1.3 Ductile Iron Produced with Lanthanum 42
3.1.4 Moulding Method 44
3.1.5 Melting Technique 45
3.1.6 Spheroidizing Treatment 45
3.1.7 Inoculant 45
3.1.8 Chemical Analysis 46
3.1.9 Filtration of Ductile Iron 47
3.2 Microstructure 47
3.3 Salts Used 47
3.4 Equipments Used 48
3.4.1 Melting Furnaces 48
3.4.2 Heat Treatment Furnaces 48
3.4.3 Microscopes Used 49
3.4.4 Tensile Testing Machines 50
xiii
Chapter – 4 RESULTS AND DISCUSSION 51
4.1 Effect of Austempering Time on Ductile Iron 51
4.2 Effect of Austenitizing Temperature on Ductile Iron 55
4.3 Effect of Alloying Elements on Ductile Iron 58
4.3.1 Effect of Copper on the Tensile Strength of Ductile Iron 59
4.3.2 Effect of Nickel on Tensile Strength of Ductile Iron 63
4.3.3 Effect of a combination of Copper and Nickel on Ductile
Iron
67
4.3.4 Effect of Lanthanum on Ductile Iron 71
4.3.4.1 Effect of Lanthanum on Nodule Count and
Nodularity of Ductile Iron
72
4.3.4.2 Effect of Heat treatment with Lanthanum on
Tensile Strength
78
4.3.4.3 Effect of Heat treatment on Microstructure of
Ductile Iron
81
Chapter-5 CONCLUSIONS 88
FUTURE WORK 90
REFERENCES 91
Appendix 1 The bismuth-cerium phase diagram 98
Appendix 2 The lanthanum-bismuth phase diagram 99
xiv
LIST OF TABLES
Table No. Description Page
2.1 Composition of Grey Iron for Low Grade 3
2.2 Composition of Grey Iron for H
igh Grade
3
2.3 Composition of Ferro-Silicon-Magnesium Alloy 10
2.4 Chemical Composition of Ni-base alloy Containing Magnesium
in wt.%
12
2.5 Chemical Composition of Inoculants 16
2.6 Typical Composition of Ductile Iron for Austempered Ductile
Iron
22
2.7 Composition of a Typical Alloy of Cerium 27
2.8 Austempered Ductile Iron ASTM A897-90 (ASTM 897 M-90) 30
2.9 British Standards Specification for ADI EN 1564:1997 31
3.1 Chemical Composition of Pig Iron in wt % 39
3.2 Chemical Composition of Mild Steel in wt % 39
3.3 Chemical Composition of Ferro-Silicon-Magnesium in wt % 40
3.4 Chemical Composition of Ductile Iron Produced with Copper
in wt %
40
3.5 Chemical Composition of Ductile Iron Produced with nickel
in wt %
41
3.6 Chemical Composition of Ductile Iron Produced with Copper &
Nickel together in wt %.
42
3.7 Chemical Composition of Ductile Iron without Lanthanum
in wt. %
42
3.8 Chemical Composition of Sorel Metal in wt. % 43
3.9 Chemical Composition of Swedish Iron in wt. % 43
xv
3.10 Chemical Analysis of Ductile Iron Alloyed with Lanthanum 46
4.1 Effect of Time on the Tensile Strength of Ductile Iron 52
4.2 Effect of Austenitizing Temperature on Tensile Strength of
Ductile Iron
55
4.3 Effect of Copper on Tensile Strength of Ductile Iron 59
4.4 Effect of Nickel on Tensile Strength of Ductile Iron 63
4.5 Effect of Copper and Nickel together on Tensile Strength of
Ductile Iron
68
4.6 Effect of Lanthanum on Nodule count and Nodularity on Ductile
Iron
73
4.7 Dimensions of the Tensile Specimen (mm) 78
4.8 Effect of Lanthanum on the Tensile Strength of Ductile Iron 79
xvi
LIST OF FIGURES
Fig. No. Descriptions Page
2.1 Schematic diagram of a typical austempering heat treatment cycle 27
2.2 Schematic arrangement of the austempering process 29
3.1 Test bar mould (Dimension in mm) 44
4.1 Effect of time on tensile strength of ductile iron austenitized at 900oC
and austempered at 270oC.
52
4.2 Effect of time on tensile strength of ductile iron austenitized at 900oC
and austempered at 370oC.
53
4.3 Effect of austenitizing temperature on the tensile strength of ductile
iron austempered at 270oC.
56
4.4 Effect of austenitizing temperature on the tensile strength of ductile
iron austempered at 370oC.
57
4.5 Effect of copper on tensile strength without any heat treatment 61
4.6 Effect of copper on tensile strength when austenitized at 900oC and
austempered at 270 oC.
61
4.7 Effect of copper on tensile strength when austenitized at 900 oC and
austempered at 370 oC.
62
4.8 Effect of nickel on tensile strength without any heat treatment 65
4.9 Effect of nickel on tensile strength when austenitized at 900 o
C and
austempered at 270 oC
66
4.10 Effect of nickel on tensile strength when austenitized at 900 oC and
austempered at 370 oC
66
4.11 Effect of copper and nickel without heat treatment. 70
4.12 Effect of copper and nickel together on tensile strength of ductile Iron
when austenitized at 900 oC and austempered at 270
oC.
70
4.13 Effect of copper and nickel on tensile strength when austenitized at
900oC and austempered at 370
oC
71
xvii
4.14 Effect of lanthanum on the nodule count of ductile iron 74
4.15 Micrographs of ductile iron with 0.00%, 0.006 %, 0.02 and 0.03 %
Lanthanum
75
4.16 Effect of lanthanum on nodule count of ductile iron 76
4.17 Effect of lanthanum on nodularity of ductile iron 77
4.18 Schematic diagram of tensile test sample 78
4.19 Micrographs of ductile iron austenitized at 900oC and austempered at
370oC for one hour with (a) 0.0 % La (b) 0.006 % La (c) 0.02 % La
(d) 0.03 % La.
82
4.20 Micrographs of ductile iron austenitized at 900oC and austempered at
270oC for one hour with (a) 0.0 % La (b) 0.006 % La (c) 0.02 % La
(d) 0.03 % La.
83
4.21 SEM photograph of ductile iron austenitized at 900oC and
austempered at 370oC
84
4.22 SEM photograph of ductile iron austenitized at 900oC and
austempered at 370oC
84
4.23 SEM photograph of ductile iron austenitized at 900oC and
austempered at 270oC
85
xviii
Chapter - 1
INTRODUCTION
The increasing interest in energy saving has led to the development of
lightweight materials to reduce the weight of existing materials without compromising
their properties. In the automotive industries, attempts have been made to replace cast
iron and steel components with aluminum and austempered ductile iron.
Austempered ductile iron (ADI) is a ductile iron that has undergone a special
isothermal heat treatment called austempering. Unlike conventional “as-cast” irons, its
properties are achieved by specific heat treatment. Therefore, the only prerequisite for
good ADI is a good quality ductile iron.
ADI offers superior combination of properties because it can be cast, like any
other member of the ductile iron family. It offers all production advantages of
conventional ductile iron castings. Subsequently it is subjected to the austempering
process to produce mechanical properties that are superior to conventional ductile iron,
many cast and forged steels.
The mechanical properties of ductile iron and austempered ductile iron (ADI)
are determined by the metal matrix. In conventional ductile iron it is controlled by the
mixture of pearlite and ferrite. However the properties of ADI are due to its unique
matrix of acicular ferrite and carbon stabilized austenite called ausferrite.
xix
It is a well known that an appropriate amount of rare earth is often used in
ductile iron production in order to counteract the deleterious effects of subversive
elements, e.g. titanium, bismuth and others. It is believed that the rare earths combine
chemically with the subversive elements to effectively remove them from the system
although reactions between titanium and rare earths have not, as yet, been identified [1].
However, an excessive amount of rare earth elements is known to promote the formation
of chunky graphite [2-4].
While doing a preliminary survey on the production of ductile iron in Pakistan,
it was noticed that only a few foundries were producing ductile iron castings of a
reasonably good quality. Austempered ductile iron, however, was not being produced at
all in any foundry in Pakistan. Research in this area was also found to be limited to a
couple of research papers on ADI.
Realizing the importance of ADI and its use in automobile and in other sectors
in western countries, this researcher thought it necessary to explore the production of
ADI locally. ADI was therefore produced at laboratory scale in Pakistan using raw
materials available locally.
In the present work, the effect of alloying elements (copper, nickel, a
combination of copper and nickel and lanthanum) as well as the effect of changing
different parameters of heat treatment i.e. time and temperature on ductile iron were
studied.
xx
CHAPTER - 2
LITERATURE REVIEW
The term, cast iron, identifies a large family of ferrous alloys. Cast irons are
primarily alloys of iron that contain more than 2.0 wt. % carbon. It also contains 1.0 to
3.0 wt. % silicon. The different properties of castings can be achieved by changing
carbon content, silicon content, by alloying with various elements, and by varying
melting, casting and heat treatment practice. Cast irons, as the name implies, are indeed
to be cast to shape rather than formed in solid state. Cast irons have low melting
temperatures and are very fluid when molten and have undergone slight to moderate
shrinkage during solidification. However, cast irons have relatively low impact resistance
and ductility, which limits their use. [5]. This must be taken into account when designing
castings to withstand service stresses. Irons of the composition given below in table 2.1
and table 2.2 satisfy a low and high grade specification of grey cast iron in a medium
size, uniform sections sand castings [6].
Table 2.1 (G 150) Composition of Grey Iron for Low Grade
C % Si % Mn % S % P %
3.1-3.4 2.5-2.8 0.5-0.7 0.15 0.9
Table 2.2 (G 350) Composition of Grey Iron for High Grade
C % Si % Mn % S %
3.1 max 1.4-1.6 0.6-0.75 0.12
xxi
The properties of flake iron depend on size, amount, distribution of graphite
flakes and matrix structure.
2.1 DUCTILE IRON
Ductile iron derives its name from the fact that, in the as-cast form, it exhibits
measurable ductility. By contrast, neither white iron nor grey iron exhibits significant
ductility in a standard tensile specimen [5].
Ductile iron is defined as a high carbon containing, iron-base alloy in which
graphite is present in a compact, spheroidal shape [7]. Ductile iron is also known as
nodular iron or spheroidal graphite iron. Unlike grey iron that contains graphite flakes;
the ductile iron has as- cast structure containing graphite particles in the form of small
rounded spheroidal nodules in the matrix. Therefore, ductile iron has much higher
strength than grey iron and a considerable degree of ductility.
2.1.1 History of Ductile Iron
Foundry men continued to search for an ideal cast iron an as cast “grey iron”
with mechanical properties equal or superior to malleable iron. In 1943, Keith Dwight
Mills made a ladle addition of Magnesium (as copper-magnesium alloy) to cast iron in
the International Nickel Company Research Laboratory. The solidified castings contained
no flakes but nearly perfect spheres of graphite [8].
Five years later, at 1948 AFS Convention, Henton Morrogh of British Cast Iron
Research Association announced the successful production of spheroidal graphite in
hyper eutectic grey iron by addition of small amount of cerium [8].
xxii
At the same time Morrogh from the International Nickel Company, presented a
paper which revealed the development of magnesium as graphite spheroidizer. On
October 25, 1949, patent 2,486,760 was granted to the International Nickel Company,
assigned to Keith D. Mills [8], Albert P. Gegnebin and Norman B. Pilling. This was the
official birth of ductile iron.
2.1.2 Production of Ductile Iron
Ductile iron can be produced by treating low sulphur liquid cast iron with an
additive usually containing magnesium and then inoculated just before or during casting
with a silicon-containing alloy.
2.1.2.1 Raw Materials
To produce ductile iron with the best combination of strength and toughness,
raw materials must be chosen which have lower than 0.02 wt.% sulphur and are low in
trace elements. Low manganese content is also needed to achieve as-cast ductility. Higher
strength grades of ductile iron can also be made with common grades of constructional
steel scrap, pig iron and foundry returns, but certain trace elements e.g. lead, antimony
and titanium are usually kept as low as possible to achieve good graphite structure.
2.1.2.2 Control of the Composition of Ductile Iron
Composition of Ductile Iron
The composition of unalloyed ductile iron is similar to that of grey iron with
respect to carbon and silicon contents. Carbon contents of unalloyed ductile iron ranges
from 3.0 wt.% to 4.0 wt.% and silicon content from 1.6 wt.% to 2.8 wt.%. The sulphur
and phosphorus levels of high quality ductile iron, however, must be kept very low at
xxiii
0.03 wt.% S maximum and 0.1 wt.% P maximum, which are ten times lower than the
maximum levels for grey cast iron. Other impurities must also be kept low because they
interfere with formation of graphite nodules in ductile iron [9].
All the elements in the composition of ductile iron should be controlled. The
following are the important elements in the production of ductile iron [10].
Total Carbon
The optimum range of carbon is usually 3.4 to 3.8 wt.% depending on the silicon
content. Above this range there is a danger of graphite floatation, especially in heavy
sections.
Silicon
Silicon enters ductile iron from raw materials, including cast iron scrap, pig iron,
ferro-alloys and to small extent from silicon-containing alloys during inoculation. The
preferred range is about 2.0 to 2.8 wt.%. Lower silicon levels lead to high ductility in
heat-treated iron but there is danger of carbides in thin section. High silicon helps to
avoid carbides in thin sections. It also increases hardness and tensile strength.
Carbon Equivalent
The carbon, silicon and phosphorus contents can be considered together as a
Carbon Equivalent Value. This is a useful guide to foundry behavior. There are several
Carbon Equivalent formulas and they are useful in assessing the casting properties. The
formula that is commonly used is as follows:
CE = C% + 1/3(%Si + %P) [10]
xxiv
If the value CE is equal to 4.3wt.%, the iron will be wholly of eutectic
composition. When CE is lower than 4.3, there will be a proportion of dendrites; if CE is
higher than 4.3 wt.% there will be primary graphite nodules in the structure.
Manganese
The main source of manganese is steel scrap used in the charge. Manganese
should be limited in order to obtain maximum ductility. In as- cast ferrite iron, it should
be 0.2 wt. % or less. Manganese produces to undesirable micro segregation especially in
heavy section. It encourages the formation of grain- boundary carbides which promote
low ductility, low toughness and persistent pearlite.
Magnesium
The magnesium content which is required to produce spheroidal graphite usually
ranges from 0.04 to 0.06 wt.%. If the initial sulphur content is below 0.015 wt.%, lower
magnesium content in the range of 0.035 to 0.04 wt.% may be satisfactory. Compacted
graphite structure with inferior properties may be produced if magnesium is low, while
too high magnesium content may promote dross defects and carbide formation.
Minor Elements Promoting Non-spheroidal Graphite
Lead, antimony, bismuth and titanium are undesirable elements that may be
introduced in trace amount with raw materials in the charge. Their effects can be
neutralized by cerium addition as reported by I. C. Hughes [10].
Aluminium
The presence of even trace amount of aluminium in ductile iron may promote
surface pinhole- porosity and dross formation. The common source of aluminium is
xxv
contaminants in steel and cast-iron scrap. Another source is aluminium containing
inoculants so use of inoculants of low aluminium is advisable. Aluminium as low as 0.01
wt.% may cause pinholes in ductile iron.
Phosphorus
Phosphorus is normally kept below 0.05% because it promotes unsoundness and
lowers ductility.
Minor Elements Promoting Carbides
Chromium, vanadium and boron are carbide promoters. They are controlled by a
careful selection of metallic raw materials for melting
2.1.2.3 Charge Materials
The metallic charge for ductile iron base consists mainly of:
Pig iron, steel scrap, return ductile iron scrap and ferroalloys [7]
Pig Iron
The ideal pig iron for ductile iron charge is pure iron- carbon alloy, which is not
available. It is believed that sorel metal is the best charge. In sorel metal the manganese
content is very low i.e. 0.009 wt.% and its content of elements which either promote
carbides or interfere with spheroidization of graphite is low.
Steel Scrap
Steel scrap is an important component of ductile iron charge. Chemical
composition and physical shape are to be considered. The physical shape includes
dimensions and specific surface. All melting equipment has its limitations as to
maximum size. The cupola furnace also has a minimum size limitation.
xxvi
Even though very small pieces may be charged into electric induction or arc
furnaces (such as thin plate chippings) these have very large specific surface areas which
rust rapidly. Even though rust is not believed to cause metallurgical deterioration, it
certainly increases slag quantity, acidity and corrosiveness. Whenever possible, such
scrap should be used in a balanced condition.
Despite these difficulties, steel scrap will remain in use because it is normally
less expensive than pig iron and also available in plentiful supply [7].
Ductile Iron Scrap
Only scrap of ductile iron of known quality should be used.
Ferro Alloys
When Ferro alloys are needed in the charge, the chemical composition of the
alloys should be known.
2.1.2.4 Desulphurization
A variety of compounds are capable of removing sulphur from molten iron.
Even manganese desulphurizes but it is an expensive material [7].
More practical desulphurizing agents are [7]:
Caustic Soda NaOH
Soda Ash Na2CO3
Burnt Lime CaCO3
Calcium Carbide CaC2
Calcium Cyanide CaCN2
xxvii
Of these, caustic soda is rarely used because of the health hazard. Lime stone is
first reduced to CaO before use. CaCN2 should be ruled out because it increases base iron
nitrogen content with the result of a danger of nitrogen gas defects in the castings.
In the tradition of ferrous metallurgy, CaO is the most established of
desulphurizing compounds. In ductile iron practice, it is used in basic cupola and electric
arc furnace. Limestone (CaCO3) is injected into large ladles resulting in both economical
and excellent desulphurization.
2.1.2.5 Spherodizing Treatment Alloys
There are two main alloys in use, nickel magnesium (NiMg) and ferro-silicon-
magnesium (FSM). Ferro-silicon-magnesium alloy is commonly used. It should have the
composition shown in table 2.3.
Table 2.3 Composition of Ferro-silicon-magnesium Alloy
Mg % Si % Ca % Ce % Fe %
4-6 45-50 1 max 0.5 balance
2.1.2.6 Melting Techniques for the Production of Ductile Iron
Any furnace which is used for melting of ductile iron must be capable of
producing an iron of correct composition at correct temperature. The need to maintain
these factors consistently is most important.
xxviii
Types of Furnaces
Various furnaces are available for the production of ductile iron e.g. fuel fired
furnace, electric arc furnace, induction furnace and cupola.
Optimum economy and quality is achieved through cupola-induction furnace
duplexing, but this optimum is obtainable for large volume production only. Externally
water-cooled cupola furnaces are being used for large scale operations. Water-cooling
causes too much heat losses in small cupolas. The main reason for their popularity is the
fact that these can operate continuously for several days [7].
Electric Melting
Electric melting is simple, clean and reliable. It also offers the greatest flexibility
for melting irons of different grades. Electric arc furnaces are far less popular than
induction furnaces. An additional disadvantage of electric arc melting is its noise
pollution.
Electric induction furnaces are most common. High frequencies units are usually
used for laboratory scale production. In commercial production either 50 or 60 HZ
frequencies are being used; the lower the frequency, the better the stirring action and
thus, homogenization.
2.1.2.7 Spheroidizing Treatment
One of greatest practical difficulties is the required amount of magnesium into
the melt with the necessary degree of consistency. Magnesium boils at 1120oC and when
plunged into cast iron at 1400oC, Magnesium metal melts and vaporizes instantaneously,
escaping with violence and carrying some of the cast iron with it. Different alloys are
xxix
used to overcome this difficulty. An alloy of nickel and magnesium (5 or 15 % Mg) is
efficient as it sinks in molten iron and the reaction is relatively quiet , especially with 5
wt.% alloy. However it is expensive and simultaneously addition of nickel is not always
welcomed [11]. Following are four most generally used nickel base magnesium alloys
shown in the table 2.4 [7].
Table 2.4: Chemical composition of Ni-base alloy containing magnesium in wt.%
Mg % Ni % Si % C % Fe %
Ni-Mg 1 15 83 --- 2.0 ---
Ni-Mg 2 15 50 30 --- Bal
Ni-Mg 3 4.5 93 --- 1.5 ---
Ni-Mg 4 4.5 60 --- 2.5 Bal
A range of magnesium-ferro-silicon alloys are available containing 3 to 15 wt.%
magnesium with approximately 45 % silicon [11].
Although various methods are employed for introducing magnesium into molten
metal, the universally accepted procedure is the sandwich method.
Because of the relatively low density, ferrosilicon- magnesium alloys tend to
float on the surface of the liquid iron and react inefficiently. Accordingly, the alloy is
placed in the bottom of the treatment ladle, preferably in a „pocket‟ moulded in the
bottom of the ladle, and covered with steel plate. Before use, the ladle should be heated
to a temperature of red heat.
xxx
After placing the ferrosilicon-magnesium alloy in the pocket and covering the
alloy with the plate, the ladle is positioned so that liquid metal stream does not impinge
directly on the magnesium alloy/sandwich. This allows the metal to flow back over the
sandwich, which due to presence of cover, delays the reaction of magnesium alloy until a
sufficient depth of alloy is built up in the ladle, and it also prevents the alloy floating to
the surface of the liquid iron. The ladle should be filled as quickly as possible. This
improves the magnesium recovery. The magnesium recovery depends on metal
temperature, the quantity of metal treated and the design of the ladle.
2.1.2.8 Amount of Magnesium Required
In practice it is normal to allow for minimum residual magnesium content of
0.035 to 0.04 wt.%, plus the amount of magnesium required to neutralize the sulfur in the
iron. The amount of magnesium alloy required depends on two factors:
a) The temperature of metal, the higher the temperature, the lower the recovery of
magnesium.
b) Sulphur content of the base iron to be treated; the higher the sulphur content, the
greater is the amount of magnesium to be added.
Calculation of Magnesium:
Different formulas are used to calculate the amount of magnesium required. The
commonly used formula is [7]:
Mg to add (%) = %SBase01.0%erycovreMg
%requiredcontentMg
xxxi
Fading of Magnesium
There is a gradual decrease in nodularity and an increase in carbide formation,
as treated iron is held for some time. The results from different research centers indicated
that fading is rather complicated phenomenon. The simplest component is loss of
magnesium content through oxidation or combining with sulphur. Stephen [7] described
the following corresponding reactions:
Mg + O = MgO or
Mg + S = MgS
Considering the relative stabilities of the above two compounds, a more likely
reaction is:
Mg + S + O = MgO + S
If the source of oxygen is an oxide or silica as an oxide, the corresponding
reactions are:
2Mg + SiO2 = Si + 2MgO and
2MgS + SiO2 = Si + 2MgO +2S
It is well established that fading rate is influenced by:
a) Initial Magnesium content; the higher the magnesium content the faster the
fading
b) Temperature; the higher the temperature the faster the fading
c) Slag handling; the faster the slag is removed, the better for magnesium recovery.
xxxii
d) Furnace lining; the worst is silica, the best is magnesia.
2.1.2.9 Inoculation
The metallurgical meaning of the word “inoculation” is to provide the melt with
seeds or “nuclei” on to which the solid phases grow during freezing. In some cases these
nuclei result from adding fines of the same phase which is freezing. If the fines do not
completely dissolve before solidification starts, they provide convenient sites for crystal
growth. In other cases particles of material other than the one to freeze can perform the
same act i.e. heterogeneous nucleation.
The inoculation of ductile irons produces heterogeneous nuclei for the graphite
spheroids. Neither their material nature nor mechanism of their action is factually
known [7].
The Effects of Inoculations
The principle effects of ductile iron inoculation can be described as follows [12].
The inoculation process:
Promotes the formation of small and uniformly dispersed graphite in grey iron
and increase the nodule count in ductile iron.
Minimizes the formation of primary iron carbides. These carbides create hard
edges on iron castings that make machining difficult, which is a contributing
factor to tool breaking.
xxxiii
Reduces the non-uniform properties within a casting of varying section sizes.
Thinner sections solidify at a faster rate than thicker sections. As a result, the
properties such as tensile strength of these sections will be different. Inoculation
provides more uniform properties within the casting by reducing the
solidification rate in thinner sections.
Improves the tensile strength, impact strength, toughness, wear resistance and
machinability of the casting.
Inoculants
Almost every material inoculates to some degree. For effective and well
controlled inoculation, ferro-silicon of controlled chemical composition are usually used.
Active inoculating elements are: Ca, Al, Ba, Sr, and some others. The chemical
composition of commonly used inoculants is given in table 2.5 [7].
Table: 2.5 Chemical Composition of Inoculants
Si % Ca % Al % Ba % Fe %
75 1.5 1.0 --- Bal
63 2.0 1.0 5 Bal
The inoculants contain relatively little aluminium because aluminium promotes
hydrogen pinholes defects, particularly in thin sections [7].
The sizing of the inoculant is usually ½ inches (13 mm) maximum. Since fines
do not inoculate effectively, a minimum size limit of 1/6 inches (1.5 mm) is advisable [7].
xxxiv
The inoculant should be stored in closed containers. Its effectiveness
deteriorates with time when exposed to open air.
Methods of Inoculation
Cast iron may be inoculated by several methods [12].
Ladle Inoculation
Iron is inoculated by adding inoculant to the metal as it is transferred from the
furnace to the pouring ladle. The turbulence quickly dissolves the inoculant and evenly
disperses it throughout the molten bath.
In-stream Inoculation
In many automatic pouring operations, inoculation is done in-the-stream
In-mould Inoculation
Inoculants may also be added as a preformed insert placed in the pouring basin
of a mould or as granulated inoculant placed in the gating system.
In-stream and in-the-mould inoculation techniques offer little inoculation fade,
and generally require less inoculant material to provide the desired results.
2.1.3 Formation of Carbides in Ductile Iron:
The attainment of mechanical properties in cast ductile iron depends primarily
upon the microstructure developed during solidification and solid-state transformation. A
recent trend in vehicle component has been towards higher strength and lighter weight to
save both materials and energy. Reducing the weight of ductile iron castings by
producing thin-wall parts is an important method for saving energy and material. In thin
xxxv
castings the cooling rate is fast so carbides are formed if special practice and procedure
are not adopted. The general reasons for the carbide formation are as under [13].
1. High solidification cooling rate
2. Carbide-formation elements in the charge
3. Low CE and / or Si content
4. Excessive Mg content
5. Inadequate and poor inoculation (low nodule count)
6. High superheat
2.1.4 Pouring
Pouring of ductile iron should be done quickly while keeping the pouring basin
full through the pour. A majority of ductile iron castings are hand-poured even in highly
mechanized foundries. It must be recognized that hand-pouring is a demanding job and
the pourer is exposed to some hazard. For this reason and, for potential economic and
quality benefits, much effort is being invested in designing pouring machines [7].
2.1.5 Importance of Ductile Iron
Ductile iron is a very useful invention. It offers the design engineers the option
of choosing high ductility, more than 18% elongation, or high strength exceeding 825
MPa. Ductile iron, when compared to steel and malleable iron castings offers cost
savings [14].
Like most commercial cast metals, steel and malleable irons decrease in volume
during solidification, and as a result, require attached reservoirs (feeders or risers) of
liquid metal to compensate shrinkage. The formation of graphite during solidification
xxxvi
causes an internal expansion of ductile iron as it solidifies. As a result, it may be cast free
of significant shrinkage defects with feeders that are much smaller than those used in
malleable iron and steel. This reduced requirement for feed metal increases productivity
of ductile iron and reduces its material and energy requirements resulting in substantial
cost savings. The use of most common grades of ductile iron “as-cast” eliminates heat
treatment cost, offering a further advantage.
Ductile iron castings are used for many structural applications, particularly those
requiring strength and toughness combined with good machinability and low cost. A
ductile iron casting can be poured and shipped the same day. As-cast ductile iron castings
are consistent in dimensions and weight because there is no distortion or growth due to
heat treatment.
Ductile iron is finding increasing applications in automobile parts e.g.
crankshafts, piston rings and cylinder liners. The use of ductile iron in these applications
provides increased strength and permits weight savings.
In agricultural and earth-moving application, brackets, sprockets wheels and
track components of improved strength are made of ductile iron.
General engineering applications include hydraulic cylinders, mandrels,
machine frames, switch gears, rolling mill rolls, tunnel segments, bar stock, street
furniture and railway rail-clip supports
xxxvii
2.2 AUSTEMPERED DUCTILE IRON (ADI)
2.2.1 Austempering
To achieve the full potential of ductile iron, austempering heat treatment is
adopted. It is possible to achieve much higher ranges of tensile strength and elongation
by adopting austempering treatment for ductile iron. For austempering treatment, a defect
free casting should be chosen. Any lapse in quality control of starting material will result
in inferior end product. The process is simple. The first stage consists of soaking the
castings at austenitizing temperature of 850-950oC. The austenitized castings are then
quickly transferred to a liquid bath (salt bath) maintained at temperature range of 235-
425oC. The transformation is allowed to proceed for a period of up to four hours when
austenite transforms to bainite. The castings are finally cooled to room temperature after
transformation. By adopting austempering heat treatment process instead of conventional
hardening and tempering treatment for ductile iron, the chances of cracking and distortion
are reduced. Thus, it becomes possible to carry out rough and final machining before heat
treatment. It is possible to achieve various combinations of high strength, high hardness,
limited ductility or lower strength, lower hardness, high ductility by varying the
temperature of austempering.
2.2.2 Introduction to Austempered Ductile Iron:
Austempered ductile iron (ADI) is a ductile iron that has undergone a special,
isothermal heat treatment called austempering. Unlike conventional “as-cast” irons, its
properties are achieved by heat treatment, not by specific addition. Therefore the only
prerequisite for a good ADI is a quality ductile iron [15]
xxxviii
ADI offers superior combination of properties because it can be cast like any
other member of the ductile iron family. It offers all production advantages of
conventional ductile iron castings. Subsequently, it is subjected to the austempering
process to produce mechanical properties that are superior to conventional ductile iron,
cast and forged aluminium and many cast and forged steels.
The metal matrix determines the mechanical properties of ductile iron and ADI.
The matrix in conventional ductile iron is controlled by a mixture of pearlite and ferrite.
The properties of ADI are due to its unique matrix of acicular ferrite and carbon-
stabilized austenite, called ausferrite. Austempering was commercially applied to
austempered ductile iron in 1972. A small hallow crankshaft cast by Wagner Casting
Company of Decatur, Illinois was machined and installed in a Tecumser products type
AE compressor. Meanwhile General Motors successfully implemented ADI rings and
pinion gears and constant velocity joints on its production trucks and automobiles [15].
From its infancy in 1972 until today, the application of ADI has grown
worldwide. Its annual growth is estimated at 15%. Its combination of high strength- to-
weight ratio, wear resistance and low cost have made it a “high-tech” material.
Researchers are continuously studying its new parameters [15].
2.2.3 Production of Austempered Ductile Iron
The mechanical properties offered by ADI make it an attractive material for
demanding applications. Austempered ductile iron castings must be produced free from
surface defects, free from carbides, porosity, inclusions and having a consistent chemical
composition .
xxxix
2.2.3.1 Composition of ADI
In many cases, the composition of an ADI casting differs a little from that of a
conventional ductile iron casting. When selecting the composition, consideration should
be given to the elements that adversely affect casting quality e.g. formation of carbides
and inclusions. A typical composition of ductile iron casting used for making
austempered ductile iron is given in the table 2.6 [16].
Table 2.6 Typical Composition of Ductile Iron for Austempered Ductile Iron
C % Si % Mn % Cu % Ni % Mo %
3.5-3.7 2.5-2.7 0.25-0.31 0.05-0.8 0.01-0.8 If required
0.25 max
There are three important points to consider when selecting the chemical
composition of ADI [17].
1) The iron should be sufficiently alloyed to avoid transformation of pearlite but
not over alloyed.
2) The micro structure should be free from intercellular carbides and phosphides.
3) The tendency for chemical segregation should be minimized for the sake of
uniformity in the cast component.
2.2.3.2 Effects of Alloying Elements
Alloying elements are generally used in ductile iron to increase its hardenability.
Only the minimum amount of alloys required should be used. Excessive alloying only
xl
increases the cost and difficulty producing quality ductile iron necessary for ADI. The
following are major alloying elements which are used for austempered ductile iron [16].
Carbon
Increasing carbon in the range of 3 to 4 wt. % increases the tensile strength but
has negligible effect on elongation and hardness. Carbon should be controlled within the
range of 3.6 to 3.8 wt.%.
Silicon
Silicon is one of the most important elements in austempered ductile iron (ADI)
as it promotes graphite formation, decreases the solubility of carbon in austenite and
inhibits the formation of bainite carbide. Increasing the silicon content increases the
impact strength of ADI. Silicon should be controlled closely within the range of 2.4 to 2.8
wt. %
Manganese
Manganese can be both beneficial and harmful as an alloying element. It
strongly increases hardenability, but during solidification, it segregates to cell boundaries
where it forms carbides and retards austempering reaction. It is advisable to restrict the
manganese level to less than 0.3 wt.%
Copper
It increases hardenability and ductility at austempering temperature below 350
oC. It may be added up to 0.8 wt.%
xli
Nickel
Nickel increases hardenability of ductile iron .It increases ductility and fracture
toughness at austempering temperature below than 350 oC. It may be added up to 2.0
wt.%.
Molybdenum
It may be added in heavy section castings to prevent the formation of pearlite.
Tensile strength and ductility decreases as the molybdenum is increased beyond that
amount which is required for hardenability. This is because of segregation of
molybdenum to cell boundaries and formation of carbides. It should not be added more
than 0.2 wt. %
Phosphorus
Phosphorus forms the very brittle structure known as steadite in ductile iron as
well as in grey cast iron since phosphorus adversely affects toughness and ductility, a
maximum of 0.05 per cent is usually specified [18]
Sulphur
The most important effect of sulphur in ductile iron is to increase the amount of
magnesium required to achieve spheroidal graphite. The level of sulphur in the iron prior
to magnesium treatment is a function of the melting practice used. Sulphur content after
treatment is usually 0.015 per cent [18].
Very little work has been carried out to study the effect of sulphur. Generally
sulphur is considered to be an impurity. Patty Sim, [19] a foundryman was using
autopour at his foundry. His target was 0.013 per cent sulphur in the furnace bath.
According to him lower sulphur target could cause carbides in the finished castings.
xlii
The influence of sulpher on the machinability of grey cast was studied by
Adriana et al. [20]. They found that sulpher addition in grey iron from 0.065 to 0.18 wt.%
did not produce significant alternation on mechanical properties or on microstructure.
From their study viable use of a higher sulpher percentage on grey cast iron production,
without the detrimental effects of mechanical properties, microstructure and
machinability were obtained.
Rare Earth Elements
The addition of rare earth elements has significant effects on the properties of
ductile iron [15]. Following are the most common rare earth elements which are used in
ductile iron.
1. Cerium
Cerium is a powerful desulphuriser. When sulphur content of a cast iron exceeds
about 0.02 wt. % the cerium reduces sulphur content. Cerium combines with sulphur
even in the presence of manganese. Cerium sulphide is formed and it rises to the surface
of the molten metal. The higher the sulphur content of the molten metal the greater will
be the amount of cerium required. As cerium is a relatively expensive material, so
sulphur content of iron to be treated should at the lowest level. Cerium may be added to
cast iron in a variety of alloys i.e. pure cerium, iron-cerium, nickel-cerium, copper-
cerium, silicon-cerium, manganese-cerium, and aluminium-cerium. All dissolve quite
easily in molten cast iron. On account of its ready commercial availability and relatively
low cost, a cerium alloy known as mischmetal is most frequently employed. Mischmetal
contains approximately 50% of cerium. Chemical analysis of a typical cerium alloy [11]
is given the table 2.7.
xliii
Table 2.7 Composition of a Typical alloy of Cerium
Ce % La % Nd % Other rare earths Fe %
45-53 22-25 15-17 8-10 5 max
An important point to be observed is that the mischmetal should not be finally
divided in the form of powder, as loss by oxidation may occur.
Morrogh [21] and Wallance et al. [22] have reported that a very small addition
of cerium as mischmetal has a controlling effect on the deleterious elements such as lead,
arsenic, antimony, titanium, and tin. McCluhan [23] has shown that an optimum addition
of cerium as mischmetal to laboratory magnesium-ferro-silicon (MgFeSi) results in
ductile cast irons with high graphite nodule counts and low levels of carbide formation.
2. Lanthanum
Very little work has been done on the effect of lanthanum on the properties of
the ductile iron. When lanthanum is added to ductile iron as a lone rare earth element in
the nodulizing alloy, mixed results have been reported. Horie et al [24] claimed that
nodule count increases and carbides are reduced when the La: S ratio is between 2.5 and
6.0.
However, Stefanescu et al. [25] found that nodule count steadily decreases as the
lanthanum content in MgFeSi increases. Very little work has been done on the effect of
rare earth elements on the microstructure and the properties of ductile iron. It would be of
considerable interest to determine whether the addition of lanthanum is significant in
affecting nodule count and nodularity
xliv
2.2.3.3 Production of Austempered Ductile Iron:
Austempered ductile iron is produced by an isothermal heat treatment known as
austempering. It consists of the following steps and it can be represented schematically as
shown in Figure 2.1 [17].
1) Heating the casting to the austenitizing temperature in the range of 850 o
C to
950 oC
2) Holding the part at austenitizing temperature for a time sufficient to get the
entire part to the required temperature and to saturate the austenite with carbon.
3) Quenching the part rapidly enough to avoid formation of pearlite to
austempering temperature in the range 235 oC to 400
oC
4) Austempering the casting at the desired temperature for a time sufficient to
produce matrix of ausferrite.
5) Finally cooling the casting to room temperature.
Figure 2.1 Schematic diagram of a typical austempering heat treatment cycle [17]
xlv
Austenitizing
The austenitizing temperature controls the carbon content of austenite that
affects the structure and properties of austempered castings. High austenitizing
temperature increases the carbon content of austenite, which effect the hardenability. It
makes the transformation problematic and reduces the mechanical properties after
austempering. The higher carbon of austenite requires a longer time to transform.
Austenitizing temperature should be minimum required to heat the entire part to the
desired austenitizing temperature and to saturate the austenite with equilibrium level of
carbon. Austenitizing time is affected by chemical composition, austenitizing
temperature, casting section size and type [16].
Austempering
Cooling from austenitizing temperature must be completed rapidly to avoid the
formation of pearlite. If pearlite is formed, the strength, elongation and toughness will be
reduced.
Austempering temperature is one of the major determinants of mechanical
properties of ADI castings. Higher austempering temperature (350 oC to 400
oC
produces ADI with lower strength and hardness but high elongation and fracture
toughness. Higher austempering temperature produces coarse ausferrite matrix.
For production of ADI with higher strength and greater wear resistance but
lower fracture toughness and lower elongation, austempering temperature below 350 oC
should be employed.
xlvi
When austempering temperature is selected, the austempering time should be
chosen to give a stable structure of ausferrite. Shorter austempering time will give rise to
insufficient diffusion of carbon to austenite to stabilize it and martensite may form when
cooling to room temperature. This type of structure would give higher hardness and lower
ductility. Excessive austempering time can result in decomposition of ausferrite into
ferrite and carbides, so austempering time selection should be appropriate [16].
2.2.3.4 Heat Treatment Considerations
It is important that the heat treatment operation is closely controlled to ensure
the production of castings with consistent and satisfactory mechanical properties. A
schematic representation of batch austempering heat treatment process is shown in figure
2.2 [17].
Figure 2.2 Schematic arrangement of the austempering process [17]
xlvii
2.2.4 Specifications of Austempered Ductile Iron
The ASTM specification A 897 is now most commonly accepted for
austempered ductile iron. The five grades specified are detailed in table 2.8 below. They
are readily differentiated by their hardness. Tensile, yield and elongation values are also
specified as shown in table 2.8 [26].
Table 2.8 Austempered Ductile Iron ASTM A897-90 (ASTM 897 M-90)[26]
Grade Min. Tensile Min. Yield Elongation BHN
Psi. N/m2 Psi. N/mm
2 % Range
1 125,000 850 80,000 550 10 269-321
2 150,000 1050 100,000 700 7 302-363
3 175,000 1200 125,000 850 4 341-444
4 200,000 1400 155,000 1200 1 388-477
5 230,000 1600 185,000 1300 - 444-555
The British Standards Specification for austempered ductile iron is also used
mostly in Europe. The four grades are detailed in table 2.9. They are differentiated by
their tensile, proof stress and elongation. The standard EN 1564:1997 is shown in table
2.9 [26].
xlviii
Table 2.9 British Standards Specification for ADI EN 1564:1997 [26]
Material Symbol Number Tensile Strength
N/mm2 (Min.)
0.2 % Proof
Strength N/mm2
(Min.)
Elongation %
EN-GJS-800-8 EN-JS 1100 800 500 8
EN-GJS-1000-5 EN-JS 1110 1000 700 5
EN-GJS-1200-2 EN-JS 1120 1200 850 2
EN-GJS-1400-1 EN-JS 1130 1400 1100 1
2.2.5 Cost Benefits of Austempered Ductile Iron:
The price of austempered ductile iron is lower than per kilogram of steel. ADI
parts can be produced at a cost less than for steel forging. There are many factors which
favour the replacement of steel forging with austempered ductile iron [27].
Excellent castability
It can be cast into complex shapes. Ductile iron has a very high yield.
Low Machining Cost
ADI requires less starting material and less metal removal. Prior to
austempering, ductile iron exhibits better machinability than the steels. Both ductile iron
and ADI produce dense, discontinuous chips that are easily handled.
Heat treatment Savings
Austempering generally costs less than carburizing or induction hardening, and
produces a higher degree of uniformity.
xlix
Low Energy Cost
Producing a typical austempered ductile iron castings consumes 50% less energy
than steel casting and 80 % less energy than steel forging for the producing of the similar
product.
2.2.6 Properties Of Austempered Ductile Iron
Austempered Ductile iron has the following properties [16,27]
Strength
It has strength equal to or greater than steel
Toughness
It has toughness better than ductile iron and equal to or better than cast or forged
steel.
Weight
Austempered ductile iron has 10 % less weight than steel due to the presence of
graphite nodules.
Fatigue Strength
Austempered ductile iron has equal to or better fatigue strength than forged
steel, which increases with machining after heat treatment
Damping
Austempered ductile iron has five times better damping property than steel. The
parts made of this material make less noise.
l
2.2.7 Disadvantages of Austempered Ductile Iron:
There are certain disadvantages of austempered ductile iron. These should be
considered before replacing steel parts with ADI. These are as follows [28].
1) Welding is not recommended for austempered ductile iron.
2) Lower hardness grades can be machined after heat treatment, but higher
hardness grades must be machined before heat treatment.
2.2.8 Application of Austempered Ductile Iron:
The development of austempered ductile iron (ADI) has given the design
engineers a new group of cast ferrous materials. ADI provides an exceptional
combination of mechanical properties equivalent to cast and forged steel. Dr. Richard
Harding [29] overviewed the wide range of application of austempered ductile iron.
Gears
One of the earliest applications of ADI was for the manufacture of gears.
Pioneers in this field were:
General Motors, USA for rear axle hypoid pinion and ring gears for cars
Chinese foundries, who used similar gears for light and medium trucks [30]
Kymi Kymmence, Finland, for various applications including general
engineering gear boxes, rolling mill drives, and large segmented ring gears for
cement mills, rotary kilns and forestry machines [31,32]
li
Crankshafts
One of large potential markets of austempered ductile iron is crankshafts for
high-powered diesel engines [29]. Crankshafts for air-conditioning and refrigerator
application have been produced by companies such as Wagner Casting Co. USA and
Sulzer Brothers, Switzerland [33]
Transmissions
There are following examples for the transmissions [29].
A large number of tripot housings have been used by General Motors, USA, in
front- wheel drive units.
Differential spiders manufactured by Kymi Kymmene, Finland.
Suspensions
The austempered ductile iron suspensions have been used in the industry. The
example includes war shoe restraints made by Advance Cast Products, USA , for use in
suspension units of lorry tractor units [29].
Railway Engineering
A variety of austempered ductile iron components have been used in European
and American railway applications e.g. Axle boxes produced by SKF, Sweden, for
railway vehicles and pick up arms for railway track maintenance machines, produced by
Sulzer Brothers, Switzerland [26,33]
Bracket Trailer
The Australian trucking industry had interesting challenges in terms of hauling
freight over rough and isolated distances that can be exceptionally long. Different
lii
experiments were carried out with different materials. They failed the on-road test.
Ultimately a ductile iron casting was designed and austempered to ASTM Grade 2 ADI.
The bracket was 900mm long and 1200 mm high, with a weight of 105 kg. These ADI
brackets successfully traveled over 322,000 km without any problem [34].
Agricultural Applications
Due to its high strength-to-weight ratio as well as its increased wear resistance,
austempered ductile iron is well suited for agricultural applications from suspension to
ground-engaging components [35]
Defence
The defence industry has been relatively slow to adopt ADI, however some of
the applications include track links, armor and various hardware for trucks and armored
vehicles [16].
liii
CHAPTER - 3
EXPERIMENTAL WORK
RESEARCH METHODOLOGY
During the present research an attempt was made to observe the tensile strength
of ductile iron by the addition of copper, nickel, a combination of copper and nickel and
lanthanum. Different heats with and without copper, nickel and a combination of copper
and nickel were made to find out the effect of these alloying elements on ductile iron.
Samples for this study consisted of tensile test bars having different compositions of
ductile iron with and without the alloying additions. Test bars from one melt without
lanthanum were produced in the Casting Laboratory of the University of Birmingham,
UK. Test bars with varying composition of lanthanum from three melts were produced to
observe the effect of the addition of lanthanum. Different experiments were conducted
for studying the effect of alloying elements and effect of heat treatment on ductile iron.
To find out the optimum austempering time, ductile iron samples were heat
treated at fixed austenitizing temperature at 900 oC and austempering
temperature at 270 oC and 370
oC. The austempering time was varied from half
an hour, one hour and one and a half hour to determine the most suitable
austempering time.
liv
To ascertain the suitable austenitizing temperature, the austempering
temperature at 270 oC and 370
oC and austempering time for one hour was
fixed. Austenitizing temperature of 850 oC, 900
oC and 925
oC was maintained
for one hour to find out the best austenitizing temperature for these samples.
Addition of Copper
Four heats were made to find out the effect of copper on the tensile strength of
ductile iron. Copper content was varied from nil to 1.5 wt. %.
Addition of Nickel
Different heats were produced to examine the effect of nickel on ductile iron.
The melts were made with the addition of 1.0 wt. %, 2.0 wt. % and 3.0 wt. % of nickel.
Addition of a combination of Copper and Nickel
Different melts with a combination of copper and nickel were made to examine
the effect of both of alloying elements together.
Addition of Lanthanum
Four melts were made for this purpose. One melt was made without lanthanum
while three melts with varying compositions of lanthanum were made. Three aspects of
the composition of lanthanum were investigated i.e. nodule count, nodularity and tensile
strength with and without heat treatment.
Different properties of ductile iron were studied taking into consideration the
following:
Change of tensile strength with the change of austempering time.
lv
The change of tensile strength with the change of austenitizing temperature.
The change in nodule count and nodularity with the change in the amount of
lanthanum.
The change of tensile strength with the change of austempering temperature in
low and high temperature ranges.
The change of tensile strength with the change of lanthanum, copper and nickel
content
Limitations of Study
Two major constraints were experienced during the experimental design stage.
One was that the ferro-lanthanum alloying element was not available in the local market
and the other was the non availability of relevant technical literature. The difficulties
were overcome by conducting some of the experiments at the University of Birmingham,
UK.
Aim of Study
The aim of the experimental work was to study the effects of copper, nickel, a
combination of copper and nickel and lanthanum and to study the heat treatment
variables (time and temperature) on ductile iron. For this purposes ductile iron castings
were produced with and without copper, nickel, a combination of copper and nickel and
lanthanum. The results were compared to find out the best austempering time,
austenitization temperature and the percentage of alloying addition to get the maximum
tensile strength.
lvi
3.1 PRODUCTION OF DUCTILE IRON
3.1.1 Ductile Iron without and with Copper, Nickel and Copper-
Nickel Together
Ductile iron was made using local materials and local facilities. The melting was
carried out in a commercial electro-induction foundry furnace. The materials used were
pig iron from Pakistan Steel, mild steel from the local market and ductile iron returns of
the foundry. In order to get the required composition, ferro-alloys were added to the melt.
After melting, the metal was poured into a ladle with two pockets at the bottom. In one
pocket ferro-silicon-magnesium alloy and inoculant ( ferro-silicon) were placed while the
other was kept empty. The following raw materials for the production of ductile iron
were used.
Pig Iron
Pig iron from Pakistan Steel was used for making ductile iron. The composition
of the iron is given in table 3.1
Table 3.1 Chemical Composition of Pig iron in wt %
C Si Mn P S Fe
4.1 0.83 0.6 0.025 0.021 Balance
Mild Steel
Mild steel with the following composition mentioned in table 3.2 was used.
Table 3.2 Chemical Composition of Mild Steel in wt %
C Si Mn Fe
0.2 0.3 0.4 Balance
lvii
Ferro-silicon-magnesium
Ferro-silicon-magnesium used for the spheroidization with the composition
shown in table 3.3.
Table 3.3 Chemical Composition of Ferro-silicon-magnesium in wt %
Si Mg Ca Al Fe
42 5.5 1.2 1.0 Balance
The melt was poured from about 1450 oC into a standard Y block sand mould.
Tensile specimens of 15 mm diameter 250 mm long were machined from the castings.
Chemical Composition of Heats Produced
The chemical analysis of ductile iron produced without and with copper addition
is mentioned in the table 3.4. Four heats were made to find out the effect of
copper on ductile iron. The details of heats are as follows:
Heat No. C0 without any copper Heat No. C10 with 1.0 % copper
Heat No. C5 with 0.5 % copper Heat No. C15 with 1.5 % copper
Table 3.4 Chemical Composition of Ductile Iron Produced with copper in wt %
Elements Heat No C0 Heat No C5 Heat No C10 Heat No C15
C 3.6 3.5 3.7 3.9
Si 2.7 2.9 2.6 2.7
Mn 0.1 0.2 0.1 0.2
Cu 0.0 0.5 1.0 1.5
S 0.07 0.09 0.08 0.09
P 0.02 0.03 0.02 0.02
lviii
The chemical analysis of ductile iron produced without nickel and with nickel
addition is mentioned in the table 3.5. The details of heats prepared to find out
the effect of nickel are mentioned below:
Heat No. N0 without any nickel
Heat No. N1 with 1.0 % nickel
Heat No. N2 with 2.0 % nickel
Heat No. N3 with 3.0 % nickel
Table 3.5 Chemical Composition of Ductile Iron Produced with Nickel in wt %
Elements Heat No N0 Heat No N1 Heat No N2 Heat No N3
C 3.6 3.7 3.7 3.8
Si 2.7 2.8 2.7 2.7
Mn 0.1 0.2 0.2 0.2
Nickel 0.0 1.0 2.0 3.0
S 0.08 0.09 0.08 0.08
P 0.02 0.02 0.02 0.02
The chemical analysis of ductile iron produced without and with a combination
of copper and nickel contents is mentioned in the table 3.6. The details of heats
prepared to find out the effect of copper and nickel together are mentioned
below:
Heat No. CN0 without any copper and nickel
Heat No. CN1 with 0.5 wt % copper and 1.0 wt.% nickel
Heat No. CN2 with 1.0 wt. % copper and 2.0 wt.% nickel
Heat No. CN3 with 1.5 wt. % copper and 3.0 wt. % nickel
lix
Table 3.6 Chemical Composition of Ductile Iron Produced with Copper & Nickel
Together in wt %.
Elements Heat No CN0 Heat No CN1 Heat No CN2 Heat No CN3
C 3.8 3.7 3.6 3.8
Si 2.9 2.7 2.8 2.9
Mn 0.2 0.1 0.1 0.2
Nickel 0.0 1.0 2.0 3.0
Copper 0.0 0.5 1.0 1.5
S 0.07 0.09 0.08 0.08
P 0.03 0.02 0.02 0.02
3.1.2 Ductile Iron Prepared without Lanthanum
Ductile iron samples were prepared at the University of Birmingham, U K. The
furnace used was medium frequency induction furnace of capacity 28 kg. The material
used was sorel metal, mild steel and ferroalloys. The investigation for optimum
austempering time and austenitizing temperature was carried out on the samples of this
melt. The composition of ductile iron produced is given in the table 3.7.
Table 3.7 Chemical Composition of Ductile Iron in wt. %
C Si Ni S P Mg
3.5% 2.5% 0.019% 0.05% 0.005% 0.05%
3.1.3 Ductile Iron Produced with Lanthanum
Four melts were made with and without the addition of lanthanum i.e. 0.00
wt.%, 0.006 wt.%, 0.02 wt.% and 0.03 wt.% at the University of Birmingham, UK. For
lx
this purpose, the following charge materials were used. A good quality of charge was
selected for the melting. The composition of the charge was as follows:
Sorel Metal
Sorel Metal of Grade RTF 10 was used. The composition is given the table 3.8.
Table 3.8 Chemical Composition of Sorel Metal in wt. %
C Si Mn S P
4.33 0.134 0.014 0.005% 0.017
Ferro-silicon 75(Base)
Ferro-silicon with 75 wt. % of silicon was used.
Ferro-silicon-magnesium
Ferro-silicon-magnesium with Si = 45.45 wt.% and Mg =4.72wt.% was used for
graphitization
Swedish Iron
Swedish iron was used for the production of ductile iron . The composition of
the iron is given the table 3.9
Table 3.9 Chemical Composition of Swedish Iron in wt. %
C Si Mn S P
0.01 0.03 0.18 0.004 0.011
lxi
3.1.4 Moulding Method
A vertically-parted sand mould was used. Fig.3.1 [36] shows the dimension of
the mould. The mould constituted three parts, a pouring basin, a runner and a series of ten
test bar cavities. Ten cavities were used for the castings. The sand moulds were made
from local silica sand from Kings Lynn AFS grade 60 bonded with Ashland pepset resin.
For making the mould, silica sand with pepset 1505 with catalyst pepset 2590
were used. The pattern was sprayed with silicon-free release agent, from Blayson
company, for easy removal of mould.
Fig.3.1 16mm diameter test bar mould (Dimension in mm)
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3.1.5 Melting Technique
The charge materials were melted in INDUCTOTHERM medium frequency
Induction furnace of 28 kg capacity for making melts with and without lanthanum. To
achieve a good and reliable result, care was taken to maintain a good melting practice
throughout the experimental work. In each experiment 24 kg melt was used. Ductile iron
samples alloyed with copper, nickel and copper & nickel together were produced from a
100 kg high frequency induction furnace installed at a commercial foundry.
3.1.6 Spheroidizing Treatment
The Sandwich method was employed for spheroidizing. After melting, the metal
was poured into a ladle with two pockets at the bottom. In one pocket ferro- silicon 75
(base) and ferro-silicon- magnesium were placed while the other pocket was kept empty.
The alloys were covered with 1/8 inches thick plate to delay the reaction and to avoid
vaporization of alloying elements.
3.1.7 Inoculant
Ferro-silicon inoculant was used for the production of ductile iron samples
alloyed with copper, nickel and a combination of copper and nickel.
The ductile iron samples were inoculated for melts alloyed with lanthanum by
traditional ladle inoculation method. The inoculant was added to the metal as it was
transferred from the furnace to the pouring ladle. The turbulence quickly dissolved the
inoculant. For the first three melts ferro-silicon was used as an inoculant. It proved to be
unsuccessful in removing the carbides in the castings. Later, for the remaining four melts,
it was replaced with super-seed.
lxiii
3.1.8 Chemical Analysis
Standard coin samples were chilled cast for chemical analysis for every melt.
The samples were taken from the middle of casting. The chemical analysis of samples of
ductile iron is given in the table 3.10.
Table 3.10 Chemical Analysis of Ductile Iron Alloyed with Lanthanum
Elements Melt Number
MELT 1 MELT 2 MELT 3 MELT 4
C 3.71 3.45 3.40 3.41
Si 2.45 2.75 2.63 2.68
Mn 0.111 0.105 0.107 0.106
P 0.016 0.022 0.021 0.022
S 0.008 0.006 0.008 0.008
Cr 0.027 0.028 0.026 0.028
Mo 0.002 0.001 0.001 0.001
Ni 0.029 0.028 0.029 0.029
Al 0.019 0.018 0.017 0.016
Cu 0.016 0.015 0.016 0.015
Mg 0.064 0.071 0.058 0.060
Sn 0.001 0.001 0.001 0.001
Ti 0.005 0.005 0.005 0.006
V 0.008 0.008 0.007 0.008
La 0.000 0.006 0.020 0.030
Fe Bal. Bal. Bal. Bal.
lxiv
3.1.9 Filtration of Ductile Iron
The ductile iron produced at the University of Birmingham was filtered with a
Sedex ceramic foam filter having 10 pores per inch to get slag free samples for the study.
A ceramic filter was used for every melt to produce high quality ductile iron. It was
placed at the bottom of the sprue, as shown in Figure 3.1. The dross (slag) is relatively
high in ductile iron. Oxides are the principal constituents of slag/dross in cast iron and
come from furnace refractories, ladle lining, moulds, and the oxidation of dissolved
magnesium and silicon during the melting and pouring. The use of ceramic filter means
that the running system can be used for its primary purpose of metal delivery to the cavity
of the casting, whilst cleanliness is controlled by filter, to give inclusion-free casting and
to improve yield [37].
3.2 MICROSTRUCTURE
Two samples from each melt were taken, one from the middle and the other
from the bottom. These samples were sectioned from the test bars. Later these were
mounted in thermoplastic and marked for identification. Conventional metallographic
preparation techniques were used. The microstructural study was carried out using Leica
optical microscope and Olympus microscope. The nodule count and nodularity of the
samples was carried out using an image analyzer installed at the university of
Birmingham, UK.
3.3 SALTS USED
The salts used for austempering were purchased from the local market. Different
companies are selling their salts with their own fabricated names.
lxv
3.4 EQUIPMENTS USED
3.4.1 Melting Furnaces
Different furnaces were used for the melting of metal during the present work.
These are listed below.
1. Gas Fired Furnace:
The gas fired furnace of capacity 60 kg was used to produce the ductile iron for
the initial heats. It was fitted with a blower. Natural gas was fed to the furnace for the
melting of the metal.
2. Induction Furnaces:
Two furnaces were used for the melting of iron. The Inductotherm medium
frequency induction furnace of capacity 28 kg was used. It was installed at the University
of Birmingham, UK. The second furnace was a high frequency induction furnace of
capacity 100 kg installed at a commercial foundry, Lahore (Pakistan).
3.4.2 Heat Treatment Furnaces
Different types of furnaces were used for the heat treatment of tensile samples.
The majority of samples were heat treated at the University of Engineering and
Technology, Lahore (Pakistan) and some of the samples were heat treated by ADI
Treatment, UK. The details of furnaces are given below.
Muffle Furnace
The austenitizing heat treatment of the samples were carried out in a muffle
furnace installed at Materials Research Laboratory, Research Centre University of
Engineering and Technology, Lahore ( Pakistan ). The samples were austenitized in
lxvi
Carbolite Furnace, Type GPC 13/36 with 9000 watts and its maximum temperature limit
was 1300 oC .
Vertical tube furnace
The tensile samples were austempered in salt bath placed in a vertical furnace
fitted at Research Centre of University of Engineering & Technology, Lahore (Pakistan).
The samples were austempered in salt bath using Carbolite Furnace Type VCF 12/10
with 3000 watts and its maximum temperature limit was 1200oC.
Quench Austempered Furnace
The heat treatment of some of the tensile samples was carried out by ADI
Treatment Ltd., UK as the heat treatment facilities were not available at the time of
experimentation at the University Birmingham, UK. The ADI Treatment Ltd., were kind
enough to carry out the heat treatment at their premises. The organization is ISO 9000:
2000 certified. It is equipped with most modern furnaces using controlled atmosphere
belt. ADI Treatment Ltd. had installed the world‟s largest sealed quench austempering
furnace facility which provides controlled atmosphere heat treatment. The patented
design incorporated a controlled atmosphere bath with recirculating roof fans, radiant
tubes, intermediate purge transfer chamber, and vestibule austempering quench tank.
3.4.3 Microscopes Used
For microstructural study the following microscopes and image analyzer were
used.
Optical Microscopes
Leica optical microscope was used for microstructural study which was installed
at the University of Birmingham, UK. It was fitted with a camera. The second
lxvii
microscope which was used was Olympus Inverted Metallurgical Microscope PME-3-
312B installed at the Research Centre of the University of Engineering and Technology,
Lahore. The specimens for the microscope were prepared by using conventional method.
For etching the samples 4 % nital was used.
Scanning Electron Microscope
Hitachi S-3000H scanning electron microscope was used for the microstructure
study. The microscope is fitted at Research Centre of the University of Engineering &
Technology, Lahore, (Pakistan)
Image Analyzer
To find out the nodularity and nodule count, the image analyzer was used fitted
at Interdisciplinary of Research Centre, University of Birmingham, UK
3.4.4 Tensile Testing Machines
The tensile samples alloyed with lanthanum were tested using Instron Universal
Testing Machine Model 1195, capacity 100 kn, installed in the laboratory of Pakistan
Quality & Standards Control Authority, Lahore. The machine uses interchangeable load
cell to detect the load on the sample under test. The load is measured by an electrical
sensing device which produces signals corresponding to load variations.
The second tensile testing machine was Universal Tensile Testing Machine
Shimadzu UH-F-500 KNA that was used for the testing of samples alloyed with copper,
nickel and a combination of copper and nickel. This machine was installed at Civil
Engineering Department of University of Engineering and Technology, Lahore.
lxviii
Chapter - 4
RESULTS AND DISCUSSION
Different variables have been studied during the present research. The first
variable was the effect of austempering time on ductile iron. The second variable was the
effect of austenitizing temperature on the ductile iron. The third major variable was the
effect of alloying additions on the ductile iron. The alloying elements selected for this
purpose were copper, nickel, a combination of copper and nickel and lanthanum.
4.1 EFFECT OF AUSTEMPERING TIME ON DUCTILE IRON
To find out the effect of time on austempering time on tensile strength of ductile
iron temperature, the tensile samples were austenitized at 900oC for one hour and then
austempered at 270oC and 370
oC for three different length of time i.e. half an hour, one
hour and one and a half hours. The results are shown in Table 4.1.
It can be seen from the table 4.1 that the average tensile strength was 968.9
N/mm2, when the samples were austenitized at 900
oC and austempered for ½ hour at
270oC but it became 1360.9 N/mm
2 when the time was increased from half an hour to
one hour. When the austempering time was further increased to 1 ½ hour, it was revealed
that the tensile strength was decreased to 1312.3 N/mm2
at the same austempering
temperature i.e. 270oC. However, the tensile strength of the samples which were
austentized at 900oC and austempered for ½ hour at a temperature of 370
oC was 811.8
N/mm2. This value increased to 925.2 N/mm
2 when the sample was autenitized at 900
oC
lxix
and the austempered at 370 o
C for 1 hour. The austempering time was increased to 1 ½
hour. The tensile strength again decreased to 817.5 N/mm2.
Table 4.1: Effect of Time on the Tensile Strength of Ductile Iron
No. Austenitizing
Temp. oC
Austempering
Temp. oC
Austempering
Time
(Hours)
Elog. % UTS
N/mm2
1
2
3
900
900
900
270
270
270
½
1
1 1/2
1.2
1.3
1.2
968.9
1360.9
1312.3
4
5
6
900
900
900
370
370
370
½
1
1 1/2
2.5
2.7
2.6
811.8
925.2
817.5
5 Without any treatment 4.0% 696.4
Figure 4.1 Effect of time on tensile strength of ductile iron
austenitized at 900oC and austempered at 270
oC.
lxx
Figure 4.2 Effect of time on tensile strength of ductile iron
austenitized at 900oC and austempered at 370
oC.
Figures 4.1 and 4.2 show that there was a gradual increase of tensile strengths
when the samples were autenitized at 900oC and austempered at 270
oC and 370
oC.
Tensile strength went on increasing up to one hour austempering time in both the cases.
The tensile strength decreased when austempering time was further increased.
The effect of time on austempering was studied by changing the time duration of
austempering while other parameters i.e. austenitizing temperature was fixed at 900 o
C
and austempering temperatures were fixed at 270 oC and 370
oC. The austempering time
was changed from half an hour to one and a half hour. The tensile strength went on
increasing up to one hour but decreased at 1½ hour. It was observed that the optimum
time for austempering was one hour. The second stage started after the austempering time
was longer than one hour. In this stage, high carbon austenite decomposed to ferrite and
carbide.
lxxi
It was also pointed out by Y. Lin [38] that after completing the first stage, the
microstructure of matrix in austempered ductile iron (ADI) contains acicular ferrite and
carbon rich austenite, giving ADI the outstanding mechanical properties of high strength
and good ductility. If the isothermal holding time is long enough to permit the reaction to
reach the second stage, the carbon rich austenite will decompose into ferrite and carbide.
Therefore, the austempering holding time should be controlled at the completion of the
first stage and the reaction of second stage should be avoided [38-40].
The two temperatures selected for austempering were 270 oC and 370
oC. The
tensile strength was 1360.9 N/mm2 when the samples were austenetized at 900
oC and
austempered at 270 o
C for one hour but it decreased to 925.2 N/mm2 when these were
austempered at 370 o
C for one hour. The main reason of this change in value was the
formation of upper and lower bainite.
Whenever the isothermal transformation of ductile cast iron takes place, a two-
stage transformation is involved. In the first stage, austenite decomposes into ferrite and
high carbon austenite. In the second stage, this high carbon austenite decomposes into
ferrite and carbide. The formation of carbides is detrimental to mechanical properties, so
it should be avoided. The second stage also embrittles the material which affects the
mechanical properties.
The second stage occurs due to long austempering time. Therefore the time
should be optimum. This is the reason for keeping austempering time as the first variable
to be studied in this study.
lxxii
4.2 EFFECT OF AUSTENITIZING TEMPERATURE ON
DUCTILE IRON
The samples prepared at the University Birmingham, UK, were subjected to
different austenitizing temperatures to find out the best austenitizing temperature. The
temperature ranged from 850 o
C to 925 o
C for a fixed austempering time i.e. one hour .
The samples were then austempered at 270 o
C and 370 o
C for one hour which was found
to be the optimum austempering time according to the findings of the effect of time on
austempering. The results are shown in table 4.2.
Table 4.2 Effect of Austenitizing Temperature on Tensile Strength of Ductile Iron
Sample No Austenitizing
Temperatures C
Austempering
Temperatures C
Tensile Strength
N/mm2
1 850 270 1142.47
2 900 270 1313.32
3 925 270 1205.95
4 850 370 991.19
5 900 370 1117.84
6 925 370 1010.46
The samples were austenitized for one hour at 850 o
C and austempered at 270 o
C
the tensile strength was 1142.47 N/mm2. When the austenitizing temperature was
increased to 900 oC the tensile strength increased to 1313.32 N/mm
2 and on further
increasing the austenitizing temperature to 925oC, the tensile strength decreased to
1205.95 N/mm 2.
lxxiii
Afterwards, austenitizing temperatures were kept the same i.e. 850 oC, 900
oC
and 950 o
C but the austempering temperature was increased to the upper limit
temperature i.e. 370 oC to find out its effect on the tensile strength. The tensile strength
showed the same pattern as in the previous experiment.
When the samples were austempered at temperature 850 o
C and austempered at
370oC their tensile strength was 991.19 N/mm
2; when the austenitizing temperature was
increased to 900 oC, the tensile strength increased to 1117.84 N/mm
2, but it decreased to
1010.46 N/mm2
by austenitizing at 925oC.
Figure 4.3 Effect of austenitizing temperature on the tensile strength of ductile iron
austempered at 270oC.
lxxiv
lxxv
Figure 4.4 Effect of austenitizing temperature on the tensile strength of ductile iron
austempered at 370oC.
Figure 4.3 and 4.4 show that there is a gradual increase of tensile strength upto
900oC (austenitizing temperature) i.e. 1313.3 and 1117.8 N/sq-mm when the samples
were austempered at 270oC and 370
oC respectively. When austenitizing temperature was
increased to 925oC and austempered at 270
oC and 370
oC the values decreased to 1205.9
and 1010.4 N/sq-mm respectively.
It was observed that the higher austenitizing temperatures are not good for
austempering. J R Keough [16] also pointed out that higher austenitizing temperature
made transformation problematic during austempering and reduced mechanical properties
after austempering.
Similarly P. Shanmugan [41] found that lower austenitizing temperatures are
better for fatigue strength. Susal K. et al. [42] also investigated the influence of
lxxvi
austenitizing temperature on ductile iron. They found that yield strength went on
increasing up to 898 oC and the strength decreased at 927
oC. They found that the yield
strength was 1228.7 MPa when the samples were austenitized at 871 oC and austempered
at 302 oC. When the temperature was increased to 898
oC, the yield strength increased to
1246.6 MPa .But when the austentizing temperature was further increased to 927 oC the
yield strength decreased to1195.5 MPa under the same condition. The findings of the
present study are supported by the experiments conducted by P.Shanmugan and
Susal K.et al.
4.3 EFFECT OF ALLOYING ELEMENTS ON DUCTILE IRON
Alloying additions are used for different purposes but mainly to increase
mechanical properties especially to increase hardenability. Several alloying elements are
used. Molybdenum is effective in increasing the hardenability [43]. There are some
disadvantages in the use of larger molybdenum addition; these are relatively high cost
and effect of this element also reduces ductility [44-46]. Manganese is a relatively cheap
element but it tends to segregate in cast irons during solidification and additions
exceeding 0.3 wt.% to austempered nodular irons reduce ductility as a result of
embrittlement at cell boundaries [47]. However, Molybdenum has relatively small
carbide- stabilizing effect and causes a large increase in hardenability, so it has been
studied by several researchers [48-50].
In order to make austempered ductile iron with the required strength and
ductility, alloying elements can be added to conventional ductile iron. These elements
must play roles to avoid pearlite formation as well as stabilize austenite during
austempering treatment. In that way ductile irons can produce a supersaturated austenite
lxxvii
and therefore ausferrite phase can be achieved [51]. Molybdenum plays a significant role
in increasing the hardenability of ductile iron [52]. A combination of nickel, copper and
molybdenum was typically added to ductile iron [53-56].
4.3.1 Effect of Copper on the Tensile Strength of Ductile Iron
To find out effect of copper, four heats were made with 0.0 wt. %, 0.5 wt %, 1.0
wt. % and 1.5 wt. % by wt. in a commercial foundry. The tensile samples were machined
from the castings. The samples were austenitized in a Carbolite muffle furnace at a
temperature of 900 C for one hour and austempered at 270 oC and 370
oC for one hour.
Then the tensile test was performed. The results are tabulated in table 4.3.
Table 4.3 Effect of Copper on Tensile Strength of Ductile Iron
Copper UTS N/mm
2
0.0 wt %
UTS N/mm2
0.5 wt %
UTS N/mm2
1.0 wt %
UTS N/mm2
1.5 wt %
Without heat-
treatment 495.3 517.6 581.7 705.7
Austempered
at 270 oC
938.8 988.8 1096.1 1222.4
Austempered
at 370 oC
698.0 816.7 828.3 911.6
The tensile strength of ductile iron samples was 495.3 N/mm2 without any
addition of copper to the heat. When copper addition of 0.5 wt % was made in the ductile
iron, the tensile strength increased to 517.6 N/mm2. With the copper addition of 1.0 wt %,
the tensile strength increased to 581.7 N/mm2. When the copper addition was further
lxxviii
increased to 1.5 wt %, the tensile strength again increased to 705.7 N/mm2. Figure 4.5
shows the gradual increase of tensile strength with the increase of copper content without
any heat treatment.
The samples were then austenitized at 900 oC for one hour and austempered at
270 oC. The tensile strength of ductile iron samples was 938.8 N/mm
2 without any
addition of copper to the melt. With the copper addition of 0.5 wt % in the ductile iron
the tensile strength increased to 988.8 N/mm2. When the copper addition was increased to
1.0 wt %, the tensile strength was increased to 1096.1 N/mm2. When the copper addition
was further increased to 1.5 wt %, the tensile strength was also increased to 1222.4
N/mm2.
Now the austempering temperature was increased. After austenitizing at 900 o
C
for one hour, the samples were transferred quickly to salt bath maintained at 370 oC for a
time period of one hour. The tensile strength of ductile iron samples was 698.0 N/mm2
without any addition of copper. With the copper addition of 0.5 wt % in the ductile iron
the tensile strength increased to 816.7 N/mm2. The copper addition was further increased
to 1.0 wt %, the tensile strength also increased to 828.3 N/mm2. When the copper
addition was increased to 1.5 wt %, the tensile strength increased to 911.6 N/mm2.
(table 4.3). The graphical representation of increase of tensile strength when the samples
were austempered at 370oC is shown in figure 4.6.
lxxix
Fig. 4.5 Effect of copper on tensile strength without any heat treatment
Figure 4.6 shows a similar increase of tensile strength with the increase of
copper when the samples were austempered at 270 oC for one hour.
Fig. 4.6 Effect of copper on tensile strength when austenitized at 900 oC and
austempered at 270 oC.
lxxx
Figure 4.7 shows the same pattern of increase of tensile strength when the
samples were austenitized at 900 oC for one hour and austempered at 370
oC for one
hour.
Fig. 4.7 Effect of copper on tensile strength when austenitized at 900 oC and
austempered at 370 oC for one hour.
The present results are similar to the research conducted by Yoon-Jun Kim et al,
[51]. In their study, the samples were alloyed with copper and molybdenum and
austenitized at 910 oC for 90 minutes and subsequently austempered in salt bath. They
found that copper and molybdenum addition played an effective role in the formation of
ausferrite structure as well as an increment of mechanical properties such as tensile
strength and hardeability.
Another study by A.A. Cushway [43] revealed that copper addition up to 1.5 wt.
percent increased the hardenability of nodular iron. He further found that the addition of
copper above 1.5 percent resulted in no further increase in hardenability. In the present
study the tensile strength also went on increasing up to 1.5 wt. percent of copper addition.
lxxxi
Guerin et al [57] made alloying addition of copper, tin and a combination of
manganese and copper to ductile iron. They found that the use of manganese (% Mn >
0.4 %) or tin (% Sn > 0.07 %) caused the formation of embritlling intercellular phases.
The best mechanical properties were obtained with 1.48 wt. per cent of copper. They
further found that manganese and tin were less effective than copper to harden and
strengthen ductile iron.
P.W. Sheton and A. A. Bonner [58] reported that when copper was added in
quantities exceeding the limits of solid solubility in ferrous alloys (0.7 wt. %) it
significantly improved its strength and toughness. These results are in agreement with the
present study.
4.3.2 Effect of Nickel on Tensile Strength of Ductile Iron
The effect of nickel on ductile iron was studied by preparing different heats with
1.0 wt.%, 2.0 wt.% and 3.0 wt. % nickel addition. The tensile samples were made from
the castings made by the Y block pattern. Then these tensile samples were subjected to
tensile test with and without heat treatment. The test results are tabulated in the table 4.4.
Table 4.4 Effect of Nickel on Tensile Strength of Ductile Iron
Nickel UTS N/ mm
2
0.0 wt. %
UTS N/ mm2
1.0 wt. %
UTS N/ mm2
2.0 wt. %
UTS N/ mm2
3.0 wt. %
Without heat-
treatment 495.3 552.4 575.3 628.2
Austempered
at 270 C 938.8 970.5 979.7 1082.5
Austempered
at 370 C 698.0 721.18 732.8 917.7
lxxxii
The present results showed that there is a gradual increase in the tensile strength
of ductile iron with the increase of nickel content without heat treatment. The tensile
strength was 495.3 N/mm2 without any addition of nickel. By adding of 1.0 wt.% of
nickel, the tensile strength increased to 552.4 N/mm2 (table 4.4). When 2.0 wt % nickel
was added in the ductile iron, the tensile strength increased to 575.3 N/ mm2. Further
increasing of nickel content to 3.0 wt %, the tensile strength also increased to 628.2
N/mm2.
The ductile iron samples were then heat treated by austenitizing the samples at
900oC for one hour and austempered at 270
oC for one hour. The tensile strength of
ductile iron samples was 938.8 N/ mm2 without any addition of nickel to the melt. With
the nickel addition of 1.0 wt. % in the ductile iron, the tensile strength increased to
970.5 N/ mm2.
When the quantity of nickel was increased to 2.0 wt % in the ductile iron, the
tensile strength again increased to 979.7 N/ mm2. When the nickel addition was further
increased to 3.0 wt. %, the tensile strength again increased to 1082.5 N/mm2 (table 4.4).
Now the salt bath temperature was increased to 370 o
C for austempering. After
austenitizing at 900 oC for one hour, the samples were austempered for one hour. The
tensile strength of ductile iron samples was 698.0 N/ mm2 without any addition of nickel.
With the nickel addition of 1.0 wt % in the ductile iron the tensile strength increased to
721.1 N/ mm2. When the 2.0 wt % nickel was added to the ductile iron, the tensile
strength also increased to 732.8 N/ mm2. When the nickel addition was further increased
lxxxiii
to 3.0 wt. %, the tensile strength again showed the similar tendency and it increased to
917.7 N/ mm2 (table 4.4).
The effect of nickel was studied by making melts with nil to 3 wt. % nickel. The
addition of varying quantities of nickel to the ductile iron showed a positive effect of
mechanical properties of ductile iron by increasing its tensile strength in a proportionate
manner. The graphs shown in figures 4.8, 4.9 and 4.10 revealed the gradual increase of
tensile strength with the increase of nickel content with and without any heat treatment.
Figure 4.8 shows that with the increase of nickel quantity the tensile strength
increased without any heat treatment.
Fig. 4.8 Effect of nickel on tensile strength without any heat treatment
When the samples are austenitized at 900 oC and austempered at 270
oC, the
increase in tensile strength can be seen in figure 4.9.
lxxxiv
Fig. 4.9 Effect of nickel on tensile strength when austenitized at 900 oC and
austempered at 270 oC
When the tensile samples were austenitized at 900 oC and austempered at 370
oC,
the tensile strength again increased. The slope in figure 4.10 is not as steep as in case of
austempering at 270 oC in figure 4.9.
Fig. 4.10 Effect of nickel on tensile strength when austenitized at 900 oC and
austempered at 370 oC
lxxxv
Cheng-Hsun Hsu et al [59] studied the mechanical properties of cobalt and
nickel alloyed ductile irons. They found that the highest strength was achieved with the
addition of 4.0 % nickel. They found that the tensile strength of unalloyed ductile iron
was 463 MPa but when the ductile iron was alloyed with 4. 0 wt % nickel the tensile
strength increased to 1025 MPa.
In the present study, the highest strength of ductile iron produced in a
commercial foundry using local raw materials without any heat treatment was 495.3
N/mm2 and with 3.0 wt % nickel, it increased to 628.2 N/ mm
2.There is tendency of
increasing tensile strength in all the samples. The author could make nickel addition up to
3.0 wt. % only due to financial constraints.
Both copper and nickel are austenite stabilizers, so they widen the austenite zone
of phase diagram. As both copper and nickel are austenite stabilizers. Both copper and
nickel move the nose of isothermal diagram to right, and make the transformation even
easier as the cooling rate is lower. Both copper and nickel increase the hardenability;
however the information of these two elements on the mechanical properties is limited.
4.3.3 Effect of a combination of Copper and Nickel on
Ductile Iron
In this study, ductile iron was alloyed in different combinations of copper and
nickel.
Three melts of ductile iron were made as given below
1. 0.5wt% copper and 1.0wt% nickel
lxxxvi
2. 1.0wt% copper and 2.0wt% nickel
3. 1.5wt% copper and 3.0wt% nickel.
The tensile strength was compared with unalloyed ductile iron. The test results
of tensile strength of unalloyed ductile iron and alloyed ductile iron with copper and
nickel together without heat treatment and with heat treatment are shown the table 4.5
Table 4.5 Effect of Copper and Nickel together on Tensile Strength of Ductile Iron
Copper
Nickel
UTS N/ mm2
0.0 wt %
0.0 wt %
UTS N/ mm2
0.5 wt %
1.0 wt %
UTS N/ mm2
1.0 wt %
2.0 wt %
UTS N/ mm2
1.5 wt %
3.0 wt %
Without heat-
treatment 495.3 544.0 552.7 576.8
Austempered
at 270 oC
938.8 1034.3 1055.3 1164.1
Austempered
at 370 oC
698.0 715.7 738.7 787.3
The tensile strength of ductile iron samples without any addition of copper and
nickel was 495.3 N/mm2 (without any heat treatment). When copper addition of 0.5 wt %
along with 1.0 wt % nickel was made in the ductile iron the tensile strength increased to
544.4 N/mm2. With the copper addition of 1.0 wt% and nickel 2.0 wt % in the ductile
iron, the tensile strength increased to 552.7 N/mm2. When the copper addition was further
increased to 1.5 wt % in combination of 3.0 wt %nickel, the tensile strength again
increased to 576.8 N/mm2, (table 4.5).
lxxxvii
The samples were then austenitized at 900 oC for one hour and austempered at
270 oC. The tensile strength of ductile iron samples was 938.8 N/mm
2 without any
addition of copper and nickel to the melt. With the copper addition of 0.5 wt % in
combination of 1.0 wt % nickel in the ductile iron the tensile strength increased to 1034.3
N/mm2, (table 4.5).
When the copper addition was increased to 1.0 wt % with 2.0 wt % nickel in the
ductile iron, the tensile strength increased slightly to 1055.3 N/mm2. The copper addition
was further increased to 1.5 wt % in combination of 3.0 wt %, the tensile strength showed
a similar pattern and it increased to 1164.1 N/mm2.
When the samples were austenitized at 900 oC for one hour and the
austempering temperature was increased to 370 oC, the tensile strength of ductile iron
samples was 698.0 N/mm2 without any addition of copper and nickel. With the copper
addition of 0.5wt % and 1.0 wt % nickel in the ductile iron the tensile strength increased
to 715.7 N/mm2. When the copper addition was further increased to 1.0 wt % along with
2.0 wt % nickel in the ductile iron, the tensile strength also increased to 738.7 N/mm2.
When the copper addition was further increased to 1.5 wt % in combination of 3.0 wt
nickel, the tensile strength again increased to 787.3 N/mm2 (table 4.5).
The samples with different combinations of copper and nickel were austenitized
at 900 oC and austempered at 270
oC and 370
oC. The results showed the same pattern as
with copper and nickel addition separately but there was not a significant increase in the
tensile strength in combination of copper and nickel.
lxxxviii
Fig. 4.11 Effect of copper and nickel without heat treatment.
Figure 4.12 Effect of copper and nickel together on tensile strength of ductile iron
when austenitized at 900 oC and austempered at 270
oC.
lxxxix
Fig. 4.13 Effect of copper and nickel on tensile strength when austenitized at
900oC and austempered at 370
oC
The above figures 4.11-4.13 show the gradual increase in tensile strength with
the increase of a combined effect of copper and nickel. To achieve a good hardenabilty
and tensile strength it is advisable to use comparatively cheap alloying addition i.e.
copper rather using an expensive nickel alloy. Both copper and nickel can be used to
increase the hardenability of ductile iron. More information on the effect of addition of
copper or nickel is limited [43].
4.3.4 Effect of Lanthanum on Ductile Iron
Lanthanum and other rare earth metals (REM) have been utilized in molten
metal processing in a number of ways. For ductile iron production, rare earth metals have
been used to modify cast iron eutectic structures. In addition to use REM to neutralize
xc
subversive elements (e.g. bismuth) in ductile iron production, the use of REM to
deoxidize and desulphurize ferrous melts is well recognized [2]. A sizable amount of
literature has been produced regarding the practical application of rare earth metals in
industrial situations [60-65]. The ability of rare earth especially cerium and lanthanum to
chemically combine with bismuth to form intermetallic compounds has been well
established [66-69]. The neutralization of trace elements in the ductile cast iron has been
reported by the addition of 0.0007- 0.0015 wt. % rare earth metal through the nodulizing
alloy [67]. The interaction of rare earths with subversive elements e.g. with bismuth may
be understood by consulting appropriate phase diagrams [appendix 1 & appendix 2]. The
diagrams show that both cerium and lanthanum form high melting point compounds with
bismuth [2].
4.3.4.1 Effect of Lanthanum on Nodule Count and Nodularity of Ductile Iron
To find out the effect of lanthanum on nodule count and nodularity, four melts
of ductile iron were made with the addition of 0.006%, 0,020% and 0.030 wt % of
lanthanum and without addition of lanthanum. Two samples from each melt were taken;
one from the middle and the other from the bottom. These samples were sectioned from
the test bars. Later these were mounted in thermoplastic and marked for identification.
Conventional metallographic preparation was used. The microstructure study was carried
out using Leica Optical Microscope. The nodule count and nodularity of the samples
were carried out using the image analyzer installed at University of Birmingham, UK.
The microstructures showing the nodule count with and without different lanthanum
contents are given in figure 4.14. The effect of lanthanum on nodule count with varying
composition ranged from nil to 0.03 wt. % on ductile iron is given in table 4.6.
xci
Table 4.6 Effect of Lanthanum on Nodule count and Nodularity on Ductile Iron
The nodule count of ductile iron was 377 without any lanthanum with 81%
nodularity. It increased to 444 with 0.006% lanthanum. The nodularity was 83%. When
the content of lanthanum was increased to 0.020 %, the nodule count increased slightly to
448 and nodularity was 81 %. When the content of lanthanum was further increased to
0.030 %, the nodule count also increased to 467. The nodularity was almost the same
i.e.82%.
The microstructures of as-cast ductile iron etched with 4% natal with and
without different lanthanum contents are shown in figure 4.15.
Melt No. Lanthanum, % Nodule Counts Nodularity, %
MELT 1 0.000 377 81
MELT 2 0.006 444 83
MELT 3 0.020 448 81
MELT4 0.030 467 82
xcii
Figure 4.14 Effect of lanthanum on the nodule count of ductile iron
100µm 100µm
100µm 100µm
xciii
(a) (b)
(c) (d)
Figure 4.15 Micrographs of ductile iron with (a) 0.00 wt.%, (b) 0.006 wt.%,
(c) 0.02 wt.% and (d)0.03 wt. % lanthanum etched 4 % nital
100µm 100µm
100µm 100µm
xciv
The nodule count went on increasing with the increase of lanthanum. A
significant increase in the nodule count was noticed with very small amount of lanthanum
i.e. 0.006 wt % of lanthanum. Afterwards there was a slight increase of nodule count with
a further increase of lanthanum up to 0.03 wt %. So the maximum benefit of the alloy can
be achieved with a very small amount of lanthanum content.
The nodularity was almost the same i.e. 82%. Figure 4.16 shows a gradual
increase in the nodule count with the increase of lanthanum and figure 4.17 shows that
there is no significant change in nodularity with the increase of lanthanum. The graph is
almost a straight line as shown in fig 4.17.
Figure 4.16 Effect of lanthanum on nodule count of ductile iron
0
50
100
150
200
250
300
350
400
450
500
0 0.006 0.02 0.03
Percentage of Lanthanum
No
du
le C
ou
nt
xcv
Figure 4.17 Effect of lanthanum on nodularity of ductile iron
The results of this study are similar to the results of Horie et al [24 ] who found
that the nodule count increased when the La: S ratio was between 2.5 and 6.0. In the
current study, the La: S ratios are 1.0, 2.5 and 3.7. It was observed that the nodule count
increased even when the ratio of La: S was less than 2.5 i.e. 1.0. More research is needed
to ascertain the effect of other lone rare earth elements e.g. cerium to find out the effect
on nodule count and nodularity of ductile iron. Lanthanum when added along with
cerium produced higher nodule counts than cerium alone [70]. Lalich [71] and Wallace
[72] reported that nodule counts increased as the La: Ce ratio increased toward 2.0.
Tanaka et al. [73] found that high carbon-reacted austenite increased with the increasing
nodule count and with increasing nodule count, the tensile strength and fatigue properties
improved.
xcvi
4.3.4.2 Effect of Heat treatment with Lanthanum on Tensile Strength
The effect of heat treatment on ductile iron with lanthanum was studied. For this
purpose the test bars castings with different contents of lanthanum were machined as per
figure 4.18 The dimensions of tensile samples are given in table 4.7
Figure 4.18 Schematic diagram of tensile test sample
Table 4.7 Dimensions of the Tensile Specimen (mm)
Diameter
(d)
Gauge
length (Lo)
Minimum
parallel
length (Lc)
Minimum
transition
Radius (r)
Grip
Diameter
(D)
Total
Length (Lt)
6.75 34 37 26 12 100
After machining, the samples with and without lanthanum were austenitized in
Carbolite muffle furnace at optimum austempering temperature which was found from
previous experiments i.e. 900 oC for one hour and the austempered at 270
oC and 370
oC
xcvii
in a salt bath for one hour. After the completion of heat treatment the samples were
subjected to tensile test. The results of the tests are shown in table 4.8.
Table 4.8 Effect of Lanthanum on the Tensile Strength of Ductile Iron
The results showed that the tensile strength went on increasing with the increase
of lanthanum. It was 537.21 N/mm2 without lanthanum and without any heat treatment.
With the addition of a small amount of lanthanum i.e. 0.006 wt.% the tensile strength
increased to 562.7 N/mm2. The lanthanum content was increased to 0.02 wt.% the tensile
strength also increased to 594.85 N/mm2. When the quantity of lanthanum was further
increased i.e. 0.03 wt.% the tensile strength again increased to 597.3 N/mm2. These
results showed that the optimum lanthanum content is 0.03 wt.% to get the maximum
tensile strength.
The samples of the same composition were heat treated by austenitizing at
900oC for one hour and austempering at 270
oC for one hour. The tensile strength was
1359.19 N/mm2 without lanthanum. It also went on increasing with the increase of
S. No
Austenitized
at 900 oC and
austempered
at
Without
La(A)
Tensile
strength
N/mm2
0.006 % La(B)
Tensile
strength
N/mm2
0.02 % (C)
Tensile
strength
N/mm2
0.03% La (D)
Tensile
strength
N/mm2
1 370 901.82 974.68 978.23 992.80
2 270 1359.19 1454.69 1542.13 1547.14
3 Without
Treatment 537.21 562.70 594.85 597.31
xcviii
lanthanum. It was 1454.69 N/mm2 with a very small amount of lanthanum i.e. 0.006
wt.%. It increased to 1542.14 N/mm2 with the addition of 0.02 wt.% lanthanum and it
also slightly increased to 1547.14 N/mm2 with 0.03 wt.% lanthanum.
By changing the austempering temperature to 370 o
C keeping other parameters
constant, the tensile strength was found to be 901.82 N/mm2 without lanthanum. A
similar pattern was observed with the addition of 0.006wt. % , 0.02 wt.% and 0.03 wt.%
lanthanum, the tensile strength increased to 974.68 N/mm2, 978.23 N/mm
2 and
992.80
N/mm2
respectively.
The effect of heat treatment on ductile iron was significant. The tensile strength
was doubled with the application of austempering treatment. S. Salman et al [74] studied
various austempering temperatures on fatigue properties. He found that when ductile iron
specimens were applied to various austempering temperatures and austenitized at 900oC,
the hardness decreased at higher austempering temperature. In their study, the hardness
was 45 HRC when the specimens were austenitized at 900oC and austempered at 235
oC.
The austempering temperature was increased to 300oC the hardness decreased to
40 HRC. The temperature was further increased to 370oC, the hardness again decreased
to 28 HRC. The hardness value decreased with the increasing of austempering
temperature due to coarse bainitic structure (upper bainite) compared to lower bainite
structure at lower temperature.
The effect of austempering temperature is mostly carried out on hardness and
fatigue strength of ductile iron. There is very little detailed research on the effect of
xcix
austempering temperature on the tensile strength of ductile iron, so this study would be
useful for researchers working on this particular area.
4.3.4.3 Effect of Heat treatment on Microstructure of Ductile Iron
To find out the effect of heat treatment on the ductile iron the samples of ductile
iron with lanthanum content of 0.006 wt %, 0.02 wt % and 0.03 wt % and without
lanthanum were austenetized at 900oC for one hour and austempered at 270
oC and 370
oC
for one hour.
The micrographs taken from samples austempered at 270oC are shown in figure
4.20. These micrographs clearly show the matrix of acicular bainite and distribution of
graphite nodules. Figure 4.19 shows the micrographs of ductile iron austenitized at 900oC
and austempered at 370oC for one hour.
c
Figure 4.19 Micrographs of ductile iron austenitized at 900oC and austempered at
370oC for one hour with (a) 0.0 % La (b) 0.006 % La (c) 0.02 % La
(d) 0.03 % La.
25 µm
25 µm 25 µm
25 µm
(a) (b)
(c) (d)
ci
Figure 4.20 Micrographs of ductile iron austenetized at 900oC and austempered
at 270oC for one hour with (a) 0.0 % La (b) 0.006 % La (c) 0.02 % La
(d) 0.03 % La.
25 µm
25 µm 25 µm
25 µm
(a) (b)
(c) (d)
cii
Different temperature can be selected to get the desired structure to suit the
properties of ductile iron. If more hardness and tensile strength is required then the lower
austempering temperature in the range of 250oC should be chosen but if the lower
hardness and lower tensile strength is required then the upper austempering temperature
in the range of 370oC should be chosen to get coarse bainite structure.
In the as-cast ductile iron samples the nodules were almost spherical (nodularity
> 80%). The matrix consisted of mostly pearlite. The austempered ductile iron
microstructures are shown in figure 4.19 and 4.20. The microstuctural study of the
samples austempered at 270oC for one hour consisted of bainitic ferrite, carbide and
retained austenite. The isothermal treatment at 270oC produced a fine acicular structure
consisting of lower bainite, matensite and some retained austenite. Richard B. et al.[ 75 ]
did a similar study. They found that at 370oC austempering temperature coarse bainitic
ferrite needles were produced. When austempering below 300oC, the transformation
products were much finer and resembled steels with multiple lath formations containing
alternate platelets of ferrite and austenite.
ciii
Figure 4.21 SEM photograph of ductile iron austenitized at 900oC
and austempered at 370oC
Figure 4.22 SEM photograph of ductile iron austenitized at 900oC
and austempered at 370oC
civ
Figure 4.23 SEM photograph of ductile iron austenitized at 900oC
and austempered at 270oC
The samples which were austenitized at 900 C for one hour and austempered at
370 C for one hour were also examined using scanning electron microscope. A typical
upper bainite structure was observed as shown in figures 4.21 and 4.22. Similarly
samples which were a ustenitized at 900 C for one hour and austempered at 270 C for
one hour lower bainite was revealed as shown in figure 4.23. The observed structures are
consistent with previous work of K. Aslantas et al. [76] and C. K. Lin et al. [77].
When the austempering temperature was increased to 370 C, the morphology of
bainite also changed from acicular to plate-like. The amount of retained austenite also
cv
increased at higher temperature. At lower austempering temperature the strength is
higher.
The high strength obtained during isothermal heat treatment is due to the
transformation of austenite to bainitic ferrite. The presence of pearlite in the structure
may have a detrimental effect on the mechanical properties of ductile iron. This is why
the austenitized samples are quickly transferred to an austempering bath. When the
section size increases, the cooling rate should be necessary to avoid pearlite formation.
During quenching it cannot be attained in an unalloyed ductile irons. Hence it is
necessary to make alloying additions to the iron, which will move the heat treatment
curve to the right and this permits fully transformed structure to be obtained at lower
cooling rate [43].
cvi
Chapter - 5
CONCLUSIONS
1. An increase of austempering time upto one hour resulted in an increase in
tensile strength; however, it decreased when the time was increased.
2. The maximum tensile strength achieved was 1313.3 N/mm2 and 1117.8 N/mm
2
by austempering at 900oC and austempered at 270
oC and 370
oC, respectively.
3. Austempering at 270oC produced higher tensile strength in comparison with
austempering at 370oC which resulted in lower tensile strength in all samples.
4. With the application of austempering process, the tensile strength was doubled.
The tensile strength without any heat treatment was 694.4 N/mm2
and when the
samples were subjected to austempering heat treatment at 270 oC for one hour;
it increased to a value 1360.9 N/mm2.
5. With the increase of copper content, the tensile strength of ductile iron went on
increasing up to 1.5 wt % (705 N/mm2).
6. Nickel addition also increased the tensile strength of ductile iron. The highest
tensile strength was obtained with 3.0 wt % nickel.
7. The tensile strength increased when ductile iron was alloyed with a combination
of nickel and copper but there was no significant increase in the tensile strength.
cvii
8. The nodule count increased with the increase of lanthanum content in ductile
iron thus improving its strength and ductility. The maximum nodule count was
achieved with 0.03 wt. % lanthanum.
9. There was almost no effect of lanthanum on nodularity of the ductile iron. Good
nodularity i.e. 81% to 83% was achieved with a good selection of charge and
careful melting techniques.
10. It was observed that the nodule count of ductile iron increased with the increase
of lanthanum when the ratio of lanthanum sulphur was as low as 1.0.
11. Tensile strength went on increasing with the increase of lanthanum addition
when it was subjected to austempering treatment. The maximum tensile strength
was achieved with the addition of 0.03 wt. % lanthanum.
cviii
FUTURE WORK
More research is needed to ascertain the effect of other lone rare earth elements
such as Cerium.
Further work is necessary to establish the effects of multiple alloying elements
on the properties of ductile iron.
cix
REFERENCES
[1] P. C. Liu, T.X. Li, C. L. Li and C. R. Loper; Jr. AFS Transactions, 97(1989)
11-16
[2] U.H. Udomon and C.R. Loper; Jr. AFS Transactions, 93(1985) 519-522
[3] R. K. Buhr; AFS Transactions, 79(1971) 247-252
[4] H.F. Linebarger; American Chemical Society, (1981) 21-39
[5] ASM Handbook Committee; Properties and selection, Iron and Steel, Metals
Handbook, American Society for Metals , Metals Park, Ohio, Ninth Edition
(1990)3-56
[6] R. Elliot; Cast Iron Technology: An Introduction of Cast Iron, First Edition,
Butterworth, (1988) 1-19
[7] S. I. Karsay; Ductile Iron I Production, Quebec Iron and Titanium Corporation,
Canada, (1985) 9,71,88,103-104,109,111and 182
[8] Ductile Iron society, ductile Iron Data for Design Engineers.
http://www.ductile.org/didata/ pdf/didata2.pdf , 9
[9] W. F. Smith; Principals of Materials Science and Engineering, Cast Irons,
McGraw-Hill, Inc. Third Edition, 551-560
[10] I. C. Hughes; BCIRA, International Centre for Cast Metals Technology, Great
Britain, 647-666
cx
[11] T.A. Burns; Foundryman‟s Handbook, FOSECO (F.S.) Limited, Pergamon Press,
Ninth Edition, 296-338
[12] I. Kay; Modern Castings, (2004) 46
[13] A. Javaid, J. Thomson, M. Sahoo and K.G. Davis, AFS Transactions, 107 (1999)
[14] SORELMETAL, Ductile Iron Advantage,
http://www.sorelmetal.com/en/ductile/main_advantage.htm
[15] J. R. Keough, P. E.; International Pig Iron Secretariat Completion, (1995)
[16] J. R. Keogh; Section IV, Ductile Iron Society, (1998) 1-25
[17] H. L. Morgan; The British Foundryman, (1987) 98-108
[18] R. W. Hein, C. R. Loper, J. P. C. Rosenthal; Ductile Iron, Principals of Metal
Castings, Second Edition, McGraw-Hill Book Company
[19] Sulphur in ductile iron. www.industrycomnity.com
[20] A. A. Pereira, L. Boehs and W. L. Guesser; Journal of Materials Processing
Technology, 179(2006) 165-171
[21] M. H., AFS Transactions, 60(1952) 439-452
[22] J. F. Wallace and J. C. Sawyer; AFS Transactions, 76(1968) 386-404.
[23] T. K. McCluhan; AFS Transactions, 75(1967) 372-375
cxi
[24] H.Horie, S. Hiratsuka, T. Kowata, Y. Twamochi, Y. Shobuzawa and M.
Nakamura; Cast Metals, 3 (1990) 73-81
[25] D.M. Stefanescu, R.J. Warrick, L. R. Jenkins, G. Chen and F. Martinez; AFS
Transactions, 93(1985) 835-848.
[26] Austempered Ductile Iron, The Casting Development Centre,
www.thidk.co.uk/adi.htm
[27] Why ADI? http:www aditreatments.applications.php
[28] ADI Disadvantage. www.ctiparts.com/austempered.htm
[29] R. A. Harding, The Foundryman, (1993)193-208
[30] Z. Yicheng and Others; Proceedings of 46th
International Foundry Congress,
Madrid, 1979, Paper 7
[31] M. Johansson, A. Vesanen and Retting; Antriebstechnik, 15(1976) 593-600
[32] M. Johansson, Transaction of American Foundrymen’s Society, 85(1977)
117-122
[33] H. Mayer and B. B. Sulzer; ASME and Amax Inc. 2nd
International Conference
on ADI, University of Michigan, (1986) 99-115
[34] K.L. Hayrynen, K.R. Brandenberg and J.R. Keough; AFS Transactions (2002)
929-938
cxii
[35] R. Krish, Brandenberg and K. L. Hayrynen, 2002 World Conference on ADI,
cosponsored by Ductile Iron Society and the American Foundry Society, (2002)
[36] M. Ashraf and T.U. Din, unpublished Research
[37] H. Clark, The foundryman , 84(1991) 366-368
[38] B. Y. Lin, Scipta Metallurgica, 32(1995) 1363-1367
[39] M. Jonansson, AFS Trans., 85(1977) 117
[40] D.J. Moore, T. N.Rouns and K. B. Rudman; AFS Trans., 93(1985) 705
[41] P. Shanmucam , P. Prasad Rao, K. Rajendra Udupa and N. Venkataraman ,
Journal Materials Science, (2004) 4933-4940
[42] S. K. Putatunda and P. K. Gadicherla; Matreials Science and Engineering A,
268(1999) 15-31
[43] A.A. Cushway; The Effects of Copper and Nickel in the Production of
Austempered Nodular Irons, BCIRA Report, (1984)
[44] E. Dorazil and others; AFS International Cast Metals Journal, 7(1982) 52-62
[45] R.D. Schelleng; Transactions of the AmericanFoundrymen’s Society, 77(1969)
223-228
[46] K. Nilsson; Austempered and Quench-hardened Nodule Cast Iron-Production and
Mechanical Properties, BCIRA Report, (1979)
cxiii
[47] J.W. Boyes and N. Carter; BCIRA Journal, 14(1966) 175-183
[48] C.C. Reynolds, W.T. Whittington and H.F. Taylor; Transactions of the American
Foundrymen’s Society, 63 (1955) 116-122
[49] W. Scholz and M. Semchyshen; Modern Castings, 53(1968) 53-72
[50] J.E. Bevan and W. G. Scolz; Transactions of the American Foundrymen’s Society,
85 (1977) 271-276
[51] Y. J. Kim, H. Shin, H. Park and J. D. Lim; Materials Letter, (2007)
[52] Y.J. Park, P.A. Morton, Gagne and R. Goller; Trans. Am. Fish. Soc. , (1984) 92
[53] O. Eric, L. Sidjanin, Z. Miskovic, S.Zec and M.T. Jovanovic; Mter. Lett., (2004)
58
[54] P.P. Rao and S.K. Putatunda; Metall. Mater. Trans., A Phys. Metall. Mater.Sci.,
(1997) 28
[55] R.B. Gundlach and J.F. Janowak; Met. Prog., (1985) 12
[56] J.F. Janowak, R.B. Gundlach, G.T. Eldis and K. Rohrting; AFS Int, Cast Met. J.
(1982) 6
[57] L. Guerin, M. Gagne; Foundryman, 80(1987) 336-344
[58] P.W. Shelton and A. A. Bonner; Journal of Materials Processing Technology,
(2006)
cxiv
[59] C.H. Hsu, M. L. Chen and C. J. Hu; Materials Science and Engineering A,
444(2007) 339-346
[60] N.N. Yakushin et al.; Russian Casting Production, (1974) 74
[61] V. M. Popov et al.; Russian Casting Production, (1974) 420
[62] M. J. Lalich; Foundry M & T , (1978) 118
[63] K.A. Gschneider; Rare Earth Alloys” D. van Nostrand Co., Inc., Princeton, NJ ,
(1961)
[64] Y.M. Savitskiy and V. F. Terekhova; Problems of the Theory and Use of Rare
Earth Metals” Science Publication House, Moscow, (1964)
[65] Proceedings of Electric Furnace Conference, 31(1973) 153-181
[66] C. E. Bates and J. F. wallance, AFS Transactions, 75(1967) 815-838
[67] J. Sawayer and J.F. Wallance, AFS Transactions, 76(1968) 386-404
[68] C. E. Bates and J. F. Wallance; AFS Transactions, 74 (1966) 513-524
[69] R. Morrogh; BCIRA Journal, (1952) 292
[70] M. J. Gross, G. Wistchuff, H. Colthurst and D. R. Askeland; AFS Transactions,
104, (1996) 497-510
[71] M. J. Lalich; AFS Transactions, 82(1974) 441-448
cxv
[72] J. F. Wallace, P. Du, H. Q, Su, R.J. Warrick and L.R. Jenkins; AFS Transactions,
93 (1985) 813-834
[73] Y. Tanaka, H. Kage; Materials Transaction, JIM, 33(1992) 543-557
[74] S. Salman, F. Findik and P. Topuz; Materials & Design, (2006)
[75] R. B. Gundlach and J. F. Janowak, Metal Progress, (1985)19-26
[76] K. Aslantas, S. Tasgetiren and Y.Yalcin, Eng Fail Anal, 11(2004)935-941
[77] C. K. Lin and Y. L. Pai; Int J Fatigue, 21(1999) 45-54
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Appendix 1
The bismuth-cerium phase diagram [2]
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Appendix 2
The lanthanum-bismuth phase diagram [2]
cxviii
LIST OF PUBLICATIONS
[1] Production of Austempered ductile Iron and its Importance”. National Journal
Pakistan Engineer, 2006.
[2] Effect of Austempering Temperature on Tensile Strength of ductile Iron. National
Journal “The Pakistan engineer”, 2007.
[3] Effect of Lanthanum on Nodule count and Nodularity of ductile Iron. Journal of
Rare Earths. (HEC approved International Journal), 2007.
[4] Production of Carbide Free Thin ductile Iron Castings. Accepted for publication
in Journal of University of Science and Technology, Beijing, China. (HEC
approved International Journal).
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INTERNATIONAL CONFERENCES
The following papers have been accepted for presentation in the International
conferences. The presentation could not be made due to financial constraints.
[1] “Effect of Heat Treatment on Austempered ductile Iron” in International
conference on Advances in solidification Processes, June 7-10, 2005, Sweden,
Paper No. XIV : 8.
[2] “Effect of Heat Treatment on ductile Iron”, International conference by IOM,
Materials congress, 5-7 April, 2006, Paper No. C0602/175, London, UK.
[3] Effect of Lanthanum on Tensile Strength of ductile Iron”, by Euromat 2007,
Paper No. 895, Germany.
