2

THE EFFECT OF PRE-WELD HEATING ON SOME

MECHANICAL PROPERTIES OF WELDED DUCTILE

IRON USING PURE NICKEL ELECTRODE

BY

OLUWASEGUN RICHARD, AJAYI

(MSE/2009/005)

A THESIS SUBMITTED TO

DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING,

FACULTY OF TECHNOLOGY, OBAFEMI AWOLOWO UNIVERSITY,

ILE-IFE, OSUN STATE

IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE AWARD OF

BACHELOR OF SCIENCE (B.Sc.) DEGREE IN MATERIALS SCIENCE AND

ENGINEERING.

MAY, 2014

3

CHAPTER ONE

INTRODUCTION

1.1 Background Information

Cast iron has its earliest origins in China between 700 and 800 B.C. Until this period

ancient furnaces could not reach sufficiently high temperatures. The use of this newly

discovered form of iron varied from simple tools to a complex chain suspension bridge

erected approximately 56 A.D. Cast iron was not produced in mass quantity until fourteenth century A.D.

The next significant development in cast iron was the first use of coke in 1730 by an

English founder named Darby. Coke could be used more efficiently than coal, thus

lowering the cost and time necessary to yield a final product.

This discovery, with stronger and more efficient cast iron alloys being produced, led to

larger scale production of cast irons with varying properties. Due to this revolution, better casts were available for more versatile roles, such as James Watt's first steam

engine, constructed in 1794. In 1810, Swedish chemist Bergelius, and German physicist

Stromeyer discovered that by adding Silicon to the furnace, along with scrap and pig

iron, consistently stronger cast iron can be produced.

In 1885 Turner added ferrosilicon to white iron to produce stronger gray iron castings.

This little knowledge about cast iron greatly restricted its use both domestically and in

industrial-based engineering applications.

4

In the later 20th century the major use of cast irons consisted of pipes, thermal

containment units, and certain machine or building entities which were necessary to

absorb continuous vibrations.

According to Sharma (2010), Cast iron is an iron-carbon cast alloy containing other

elements, and is made by re-melting pig iron, scrap, and other additions. Cast iron is

made when pig iron is re-melted in small cupola furnaces (similar to the blast furnace

in design and operation) and poured into molds to make castings.

In order to distinctly differentiate cast iron from steel and cast steel, cast Iron is

generally defined as an alloy of Iron with carbon content in the range (2.0-6.67%) and

usually with more than 0.1 % Silicon which ensures the solidification of the final

phase with a eutectic transformation1.

It is therefore obvious that, with such high carbon content, cast iron is very brittle and

has low ductility. Hence, cast iron cannot, or is practically difficult to be cold-worked.

However, cast iron flows readily when fluid; it is easily cast into intricate shapes that

can be machined after cooling and aging. It is the cheapest of the cast materials. Cast

iron without the addition of alloying elements is weak in tension and shear, strong in

compression and has low resistance to impact, deformation and wear resistance. Pierre

(2000) records that with proper alloying, the corrosion resistance of cast irons can

equal or exceed that of stainless steels and nickel-base alloys.

Therefore, with those inherent properties, cast irons have become engineering

materials with a wide range of applications, and are used in pipes, machines and

automotive industry parts, such as cylinder heads (declining usage), cylinder blocks,

and gearbox cases (declining usage). It is resistant to destruction and weakening by

oxidation (rust).

________________________ 1Eutectic Transformation is an invariant transformation, where liquid iron (at about 1,130oC and 4.3% C)

transforms to Ledeburite (i.e austenite at about 2.0%C, and cementite at about 6.67%C)

5

Further development and improvement were made in the area of cast irons which led

to the discovery of the different classes of cast irons with different microstructural

morphologies thus giving distinctively superb engineering properties and hence wide

range of engineering applications.

These classes include white cast iron, gray cast iron, malleable cast iron,

ductile/spheroidal cast iron, and compacted cast iron.

White cast iron, as seen in figure 1.0, has large amount of carbide phases in the form

of flakes or spheroids, surrounded by a matrix of either Pearlite or Martensite which is

the result of metastable solidification. White cast iron has a white crystalline fracture

surface because fracture occurs along the iron carbide plates with considerable

strength and insignificant ductility.

Fig. 1.0; Microstructure of White Cast Iron

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Gray cast iron, as seen in figure 1.1, has graphite flakes surrounded by a matrix of

either Pearlite or -Ferrite. Exhibits gray fracture surface due to fracture occurring

along Graphite plates which is the product of a stable solidification with considerable

strength and insignificant ductility.

Gray cast iron is by far the oldest and most common form of cast iron. As a result, it is

assumed by many to be the only form of cast iron, hence, the terms "cast iron" and

"gray iron" are used interchangeably. Gray cast iron is named because its fracture

surface has a gray appearance (due to the high volume fraction of graphite flakes). It

contains carbon in the form of flake graphite in a matrix which consists of ferrite,

pearlite or a mixture of the two.

Fig. 1.1; Microstructure of Gray Cast Iron

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Malleable cast iron, as seen in figure 1.2, is cast as white iron and then malleabilized

or heat treated so as to impart ductility. Malleable cast iron consists of tempered

Graphite in an -Ferrite or Pearlite matrix.

Fig. 1.2; Microstructure of Malleable Iron

Ductile iron, as seen in figure 1.3, also called ductile cast iron or nodular cast iron is a

type of cast iron which was invented in 1943 by Keith Millis (OSOWE, 2008). While

most varieties of cast iron are brittle, ductile iron is a much more ductile material due

to its nodular graphite inclusions. Ductile (Nodular) cast iron is an alloy of iron,

carbon and silicon that has been melted and poured into a mould to form a shape. It

has graphite nodules surrounded by a matrix of either -Ferrite, Bainite, or Austenite.

The ductility is a result of the carbon forming spheres of graphite in the ferrite/pearlite

matrix, rather than the flakes found in gray cast iron. The transformation from a flake

to a sphere is achieved by treating the molten iron with magnesium prior to pouring.

Ductile iron is characterized by having all of its graphite occurs in microscopic

spheroids. Although this graphite constitutes about 10% by volume of ductile iron, its

compact spherical shape minimizes the effect on mechanical properties.

8

The shape of the graphite is established when the metal solidifies, and it cannot be

changed in any way except by re-melting the metal. It has phenomenal strength and

impact resistance compared to Gray Iron, along with many other advantages, created a

rapid increase in the demand for ductile iron.

Fig, 1.3; Microstructure of Ductile Iron

The common grades of ductile iron differ primarily in the matrix structure that

contains the spherical graphite. These differences are the result of differences in

composition, differences in the cooling rate of the casting after it is cast, or as a result

of heat treatment.

The matrix structure and hardness also can be changed by heat treatment. The high

ductility grades are usually annealed so that the matrix structure is entirely carbon-free

ferrite. The intermediate grades are often used in the as-cast condition without heat

treatment and have a matrix structure of ferrite and pearlite. The ferrite occurs as rings

around the graphite spheroids. Because of this, it is called bulls-eye ferrite.

9

The high strength grades are usually given a normalizing heat treatment to make the

matrix all pearlite, or they are quenched and tempered to form a matrix of tempered

martensite. However, ductile iron can be moderately alloyed to have an entirely

pearlitic matrix as-cast.

Chemical analysis of this ductile iron has shown that it contains the following

constituent elements;

TABLE 1.0: Chemical Composition of Ductile Cast Iron

Element Composition (%) Element Composition (%)

Carbon 3.00-4.00 Copper 0.01-0.02

Silicon 2.00-2.90 Vanadium 0.00-0.00

Maganese 0.20-0.50 Titanium 0.00-0.00

Phosphorous 0.01-0.04 Aluminium 0.00-0.07

Sulphur 0.02-0.03 Boron 0.00-0.03

Chromium 0.00-0.00 Tin 0.00-0.00

Molybdenum 0.00-0.00 Cobalt 0.00-0.00

Nickel 0.00-0.00 Iron 90.00-93.00

Ductile iron has been utilized in a wide variety of mechanical applications such as

friction wedges (railway tracks), gear components and agricultural use due to its low

cost, high tensile strength, fatigue resistance and wear resistance.

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Heat treatment may be defined as an operation or series of operations involving heating and

cooling of metals / alloys in their solid state with the sole aim of impacting desirable

properties (Umoru, 2012).

Heat treatment is an endeavor to obtain the maximum efficiency of the material under the

demanding conditions of service.

The operation of heating metal/materials to some pre determined temperature before

engaging in actual welding is called preheating [3]

. The details and the modes may be

different in various situations but in general the purpose is to influence the cooling

behavior after welding so that shrinkage stresses will be lower (relative to welding

without preheating) and cooling rate will be milder thus making adequate preheating

to prevent martensite formation in the heat affected zone and thereby provide

improved toughness and ductility.

When a ductile iron is welded, it is heated up and the heated portion has a micro

structure that is different from that of the base metal and this is called the Heat

Affected Zone [HAZ] (J. E. RAMIREZ et. al 2005)

1.2 Statement of the Problem

Since the invention of ductile iron, the welding of ductile iron has been studied and

many papers have been published (Voigt et al., 2003). Ductile iron is a material which

presents unique welding problems because of its strongly heterogeneous

microstructure consisting of spheroidal graphite in a matrix of alloyed ferrite.

11

Poor welding of ductile cast iron is due primarily to the formation of high carbon

content martensite and massive iron carbide in the heat affected zone. Martensite

formed in the heat affected zone is due to less than desirable preheat procedures.

The poor welding of ductile iron has been hitherto created a lot of problems with the

use of the welded ductile iron in service owing to disparity in the mechanical

properties of the base metal from the properties of the Heat Affected Zone.

The difference in the mechanical properties such as ductility, toughness and tensile

strength is as a result due to high brittleness and hardness of the heat affected zone

(HAZ) and the cold cracking susceptibility of welds. Due to the welding problem

associated with ductile iron in ships, bridges, pressure vessels, industrial machinery,

automobile, rolling stock, fabrication industries and many other fields, there exists a

high interest to establish a welding procedure for ductile cast iron as this material has

high mechanical properties as well as low cost.

1.3 Brief Review of Past Works

According to Tadashi Kasuya et al. (2004), while searching the methods for predicting

maximum hardness of Heat Affected Zone and selecting necessary Preheat

temperature for Steel Welding concluded that the hard microstructure of the HAZ is

responsible for the property deterioration of weld and cold cracking susceptibility.

It has been established that the behavior, say mechanical, of cast iron, and its

subsequent applications in engineering service areas, depend greatly on the

morphology of its microstructure. From the research conducted by Radzikowska

(1980), he explained that the matrix of gray, nodular, compacted and malleable cast

irons can be pearlitic, ferritic-pearlitic, or ferritic which thus influences the mechanical

properties of cast irons.

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Earlier research were reported on the welding of spheroidal graphite ductile iron

comparing the mechanical properties obtained from making use of pure Ni electrode

and a cheap Ni-Fe electrodes, M. Pascual et. al (2009) at a preheating temperature of

350o

C. It was revealed that the welding of the spheroidal ductile cast iron welded by

means of arc welding techniques using nickel based electrode gave improved ductility

and low hardness as relative to the welded ductile iron making use of cheap Ni Fe

electrode. It was found that nickel can dissolve the graphite present in ductile iron

(parent material) germinating as spherulites with a lower average size, fragile carbides

and martensitic structures are not formed thus facilitating uniform compositions in the

weldment towards providing a high ductility and low fragility.

According to PRADESHI Ram et. al 2012, most of the welding of cast iron is repair

welding. Carbon pickup and resulting cracks are the main concerns when welding CI.

The casting process is never perfect, especially when dealing with large components.

Instead of scrapping defective castings, they can often be repaired by welding.

Naturally, the very high carbon concentration of typical CIs causes difficulties by

introducing brittle martensite in the heat-affected zone of weld. It is therefore

necessary to preheat to a temperature of 450 C, followed by slow cooling after

welding, to avoid cracking. The effect of preheat temperature on the microstructure

obtained in the heat-affected zone HAZ and the carbide zone in the weld metal

adjacent to HAZ has been studied in welds for the as ductile cast irons.

Studies have also been done on reducing the effect of non uniform heating and cooling

in weld metal and in base metal which generates a harder Heat Affected Zone (HAZ),

cold crack susceptibility and residual stress in weldment through a method of

preheating thus slowing the heating and cooling rate of the base metal and weld heat

affected zone (BIPIN KUMAR et al. 2010).

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1.4 Aims and Objectives of the Research

The aim of this research work is to study the effect of pre-weld heating on some

mechanical properties of welded ductile iron by preheating at 410o

C prior to welding

with electric arc welding using Ni filler electrode.

According to the study of the previous work reviews that effective preheat treatment

operation is the primary means by which acceptable heat affected zone properties,

minimum potential for hydrogen induced cracking and minimum residual stresses are

created thus improving the ductility and toughness of the ductile cast iron.

Rajnovic et al. (2004), depending on the applied thermal conditions, it is observed that

an attractive combination of both microstructural and mechanical properties can be

obtained in ductile iron. With this in mind, the aim of this research work is therefore to

focus on the production of a uniform microstructure in the weld and the base metal of

the ductile iron with superb mechanical properties using preheat treatment operation at

410o C prior to welding using the pure Ni filler metal electrode.

However, the objectives are:

To machine the ductile iron to the required mechanical testing sizes.

To perform metallographic operation.

To perform some mechanical testing on the machined samples of the ductile

iron.

To isothermally heat treat (i.e. Preheat) the ductile iron samples.

To perform welding operation on the ductile iron samples.

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At the end of the research work, the following objectives are expected to be achieved:

a. Machining of the ductile iron rods into the specific shapes and sizes of

the mechanical testing sample using the lathe machine, the milling machine

and the electric cutting machine.

b. Metallographic operation of the as-cast, as-welded and the preheated

samples of the ductile iron to reveal its microstructural morphology and to

perform mechanical testing on the machined samples.

c. Pre-weld heating operation at 410oC of the spheroidal ductile iron prior

to welding, performing metallographic operation of the preheated-welded

spheroidal ductile iron to reveal its microstructural morphology and to

determine some of its mechanical properties.

d. Evaluation of all the results on the mechanical properties of the samples

of spheroidal ductile iron at the different stages of the research and to ascertain

whether pre-weld heating improves the mechanical properties of welded

ductile iron using nickel Ni electrode.

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1.5 Scope of the Research

This research work seeks to cover the extent of the effect of preheat treatment

operation at temperatures of 410 on the mechanical properties of welded ductile

cast iron making use of pure Ni electrode and therefore characterizing the

microstructure as well as the mechanical properties of the parent (base) ductile iron

and the preheated welded ductile iron with the sole aim of property comparison. To

this end, no consideration will be given to the use of other heat-treatment procedures

and other filler metal electrodes.

1.6 Justification for the Research

Hitherto, the poor welding of ductile cast iron has created a lot of problems in the use

of the ductile iron in service. The welding of ductile cast iron refers to the maximum

hardness of the heat affected zone (HAZ) and the cold cracking susceptibility of welds

which renders the heat affected brittle and more susceptible to fracture.

Therefore, if this research work records success, then the poor welding problems

associated with the use of ductile cast iron in service would be drastically reduced and

the use of the ductile cast iron will receive wider engineering applications with cost

effectiveness, with improved properties such as high strength, high toughness, high

wear resistance and sound damping. Hence, the tentacles of its application in the

industrial sector will undoubtedly be extended.

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However, in a grey cast iron, if the cooling rate through the eutectoid temperature is

sufficiently slow, then a completely ferritic matrix is obtained with the excess carbon

being deposited on the already existing graphite. White cast irons are hard and brittle

and they cannot be easily machined whereas grey cast irons are softer with a

microstructure of graphite in transformed-austenite and cementite matrix. The graphite

flakes have a low density and hence compensate for the freezing contraction, thus

giving good casting free from porosity (Schelling and Eash 1957).

The flakes of graphite have a good damping characteristics and good machinability

because the graphite acts as a chip breaker and lubricates the cutting tools. In

applications involving wear, the graphite is beneficial because it helps to retain

lubricants. However, the flakes of graphites also are stress concentrators, leading to

poor toughness. The recommended applied tensile stress is therefore only a quarter of

its actual ultimate tensile strength.

Sulphur in cast irons is known to favour the formation of graphite flakes. The graphite

can be induced to precipitate in a spheroidal shape by removing the sulphur from the

melt using a small quantity of calcium carbide.

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This is followed by a minute addition of magnesium or cerium which restricts the

preferred growth directions and hence leads to isotropic growth resulting in spheroids

of graphites. The calcium treatment is necessary before the addition of magnesium

since magnesium also has an affinity for both sulphur and oxygen, whereas, its

spheroidizing ability depends on its presence in the liquid iron. The magnesium is

frequently added as an alloy with iron and silicon (Fe-Si-Mg) rather than as pure

magnesium.

However, magnesium tends to encourage the precipitation of cementite, so silicon is

also added (In the form of ferro-silicon) to ensure the precipitation of carbon as

graphite. The ferro-silicon is known as an inoculant. Spheroidal graphite cast iron has

excellent toughness and its used widely, for example in crankshafts. The latest

breakthrough in cast irons is where the matrix of spheroidal graphite cast iron is not

pearlite, but bainite.

18

However, in grey cast irons, if the cooling rate through the eutectoid temperature is

sufficiently slow, then a completely ferritic matrix is obtained with the excess carbon being

deposited on the already existing graphite. White cast irons are hard and brittle and they

cannot easily be machined whereas grey cast irons are softer with a microstructure of

graphite in transformed-austenite and cementite matrix. The graphite flakes have a low

density and hence compensate for the freezing contraction, thus giving good castings free

from porosity (Schelling and Eash, 1957).

The flakes of graphite have good damping characteristics and good machinability as the

graphite acts as a chip-breaker and lubricates the cutting tools. In applications involving

wear, the graphite is beneficial because it helps retain lubricants. However, the flakes of

graphite also are stress concentrators, leading to poor toughness. The recommended applied

tensile stress is therefore only a quarter of its actual ultimate tensile strength.

Sulphur in cast irons is know to favour the formation of graphite flakes. The graphite can be

induced to precipitate in a spheroidal shape by removing the sulphur from the melt using a

small quantity of calcium carbide.

However, magnesium tends to encourage the precipitation of cementite, so silicon is also

added in the form of ferro-silicon to ensure the precipitation of carbon as graphite. The ferro-

silicon is known as an inoculants. Spheroidal graphite cast iron has excellent toughness and

is used widely, for example in crankshafts. The latest breakthrough in cast irons is where the

matrix of spheroidal graphite cast iron is not pearlite but bainite.

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2.2 Ductile Cast Iron

Ductile cast iron is an alloy of iron, carbon and silicon that has been melted and

poured into a mould to form a shape. The ductility is a result of the carbon forming

spheres of graphite in the ferrite/pearlite matrix, rather than the flakes found in grey

cast iron. This transformation from a flake to a sphere is achieved by treating the

molten iron with magnesium prior to pouring.

Ductile cast iron has become a popular cast metal material which is widely applied in

modern industrial production, because of its low cost and desirable properties such as

good castability, convenient machining property, better wear resistance, etc (Xin Tong

et al.,2009).

Spheroidal graphite cast iron (SGCI) is a FeC alloy structural material. Due to its

attractive properties, such as high castability, excellent wear resistance and relatively

low cost as compared with alloy steels of equivalent mechanical properties, SGCI is

widely used in automotive components, like crankshafts and bearing journals.

A. Roula and G.A. Kosnikov (2008) investigated the manganese distribution and effect

on graphite shape in advanced cast irons. The manganese contribution to a change of the

graphite shape (in nodular graphite cast irons) has never been revealed. They made

obvious the negative action of this element on the nodularization of graphite.

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2.3 The Ductile Iron Family

Ductile iron is not exclusively a single material, but a family of materials offering a

wide range of properties obtained through microstructure control. The common

features that all ductile irons share is the roughly spherical shape of the graphite

nodules. These nodules act as crack-arresters and make ductile iron ductile. This

feature is essential to the quality and consistency of ductile iron, and is measured and

controlled with a high degree of assurance by competent ductile iron foundries. With a

high percentage of graphite nodules present in the structure, mechanical properties are

determined by the ductile iron matrix.

The importance of matrix in controlling mechanical properties is emphasized by the

use of matrix name to designate the following types of ductile iron.

2.3.1 Ferritic Ductile Iron

Graphite spheroids in a matrix of ferrite provides an iron with good ductility, good

impact resistance and with a tensile and yield strength equivalent to a low carbon steel.

Ferritic ductile iron can be produced as-cast but may be given an annealing heat

treatment to assure maximum ductility and low temperature toughness.

2.3.2 Ferritic Pearlitic Ductile Iron

These are the most common grade of ductile iron and are normally produced in the

as-cast condition. The graphite spheroids are in a matrix containing both ferrites and

pearlite. Properties are intermediate between ferritic and pearlitic grades, with good

machinability and low production cost.

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2.3.3 Pearlitic Ductile Iron

Graphite spheroids in a matrix of pearlite results in an iron with high strength, good

wear resistance and moderate ductility and impact resistance. Machinability is also

superior to steels of comparable physical properties.

The preceding three types of ductile irons are the most common and are usually used

in the as-cast condition, but ductile iron can also be alloyed and/or heat treated to

provide the following grades for a wide variety of additional applications.

2.3.4 Martensitic Ductile Iron

Using sufficient alloy additions to prevent pearlite formation and a quench-and-temper

heat treatment produces this type of ductile iron. The resultant tempered martensite

matrix develops very high strength and wear resistance but with lower levels of

ductility and toughness.

2.3.5 Bainitic Ductile Iron

This grade of ductile iron can be obtained through alloying and/or heat treatment to

produce a hard, wear resistance material.

2.3.6 Austenitic Ductile Iron

By proper alloying, this grade of ductile iron can be obtained which offers good

corrosion resistance, good magnetic properties, and good strength and dimensional

stability at elevated temperatures.

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2.3.7 Austempered Ductile Iron (ADI)

ADI, the most recent addition to the ductile iron family, is a sub-group of ductile irons

produced by given conventional ductile iron a special austempering heat treatment.

Nearly twice as strong as pearlitic ductile iron, ADI still retains high elongation and

toughness. This combination provides a material with superior wear resistance and

fatigue strength.

In order to use Ductile Iron with confidence, the design engineer must have access to

engineering data describing the following: Mechanical properties: elastic behavior,

strength, ductility, hardness, fracture toughness and fatigue properties. Physical

properties: thermal expansion, thermal conductivity, heat capacity and density.

Magnetic and electrical properties are also of interest in many applications. A

relationship exists between the mechanical and physical properties of conventional

Ductile Irons, with respect to the microstructure and indicates how composition and

other production parameters affect properties through their influence on

microstructure.

2.4 Physical Properties Of Ductile Iron

The behavior of conventional ductile iron is determined mainly by its physical and

mechanical properties such as density, thermal expansion and conductivity, specific

heat, thermal resistivity, electrical resistivity, magnetic properties, wear resistance and

corrosion resistance.

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2.4.1 Density

The generally accepted value for the room temperature desity of ductile iron is 7.1

g/cm3.

Density is affected primarily by the percentage of graphitized carbon ( fuller,

1977), densities varying from 6.8 g/cm3

to 7.4 g/cm3

for high carbon ferritic and low

carbon pearlitic irons respectively. Density of typical cast steel 7.8 g/cm3 is almost

10% higher than that of ductile iron.

The replacement of a steel casting or forging with a lighter ductile iron improves the

strength of the component: weight ratio, reducing energy savings and lifetime costs,

especially in reciprocating components such as automotive crankshafts.

2.4.2 Thermal Expansion

The coefficient of linear thermal expansion of ductile iron depends primarily on the

microstructure, although it is influenced to a minor extent by temperature and graphite

structure. In unalloyed ductile iron, composition has only a slight influence on thermal

expansion, but alloyed austenite ductile irons can exhibit significantly different

expansion behavior.

2.4.3 Thermal Conductivity

The thermal and electrical conductivities of Gray and Ductile Irons are influenced

strongly by graphite morphology (Fuller, 1977). The conductivity is higher in Gray

Iron because of the semi-continuous nature of the graphite flakes. Because of the

influence of flake graphite on the conductivity, the volume fraction of graphite plays

an important role in Gray Iron, but not in Ductile Iron. In addition to graphite shape,

microstructure, composition and temperature also influence thermal conductivity.

Ferritic Ductile Irons have a higher thermal conductivity than pearlitic grades while

quenched and tempered irons have values between those of ferritic and pearlitic irons.

24

2.4.4 Specific Heat

Specific heat is the amount of energy required to increase the temperature of a unit

mass of a body by unit temperature. Generally specific heat increases with

temperature, reaching a maximum whenever a phase transformation occurs.

2.4.5 Electrical Resistivity

Ductile Irons, with discontinuous spherical graphite, have lower electrical resistivity

than Gray Irons which have semi-continuous flake graphite. Resistivity is primarily

affected by the addition of silicon and nickel both of which increase resistivity.

2.4.6 Magnetic Properties

The magnetic properties of Ductile Irons are determined mainly by their

microstructures. The spheroidal shape of the graphite particles in Ductile Irons gives

them higher induction and higher permeability than Gray Irons with a similar matrix.

Ferritic Ductile Irons are magnetically softer than paerlitic grades because they have

higher permeability and lower hysteresis loss. For maximum permeability and

minimum hysteresis loss, ferritic low phosphorus irons should be used.

2.4.7 Wear Resistance

Mechanical wear may be defined as surface deterioration and/or material loss caused

by stresses which arise from contact between the surfaces of two bodies. Wear is

primarily mechanical in nature but chemical reactions may also be involved.

25

Wear is a complex phenomenon and may involve one or more of the following

mechanisms:

Abrasive wear caused by the removal of material from one body due to contact

with a harder body.

Abhesive or frictional wear caused by the relative sliding contact of two

bodies.

Fretting or fatigue wear resulting from cyclic stresses caused by the relative

motion of two contacting bodies

2.5 Mechanical Properties of Ductile Iron

These are the properties which determine the performance of Ductile Iron in service

and serve as a good indication of the strength or weakness of the material.

2.5.1 Tensile Properties

The tensile properties of conventional Ductile Iron, especially the yield and tensile

strengths and elongation, have traditionally been the most widely quoted and applied

determinants of mechanical behavior. Most of the world-wide specifications for

Ductile Iron describe properties of the different grades of Ductile Iron primarily by

their respective yield and tensile strengths and elongation. Hardness values, usually

offered as additional information and impact properties, specified only for certain

ferritic grades, complete most specifications. Although not specified, the modulus of

elasticity and proportional limit are also vital design criteria.

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2.5.1.1 Evaluation of Tensile Properties

Some of the tensile properties that have been evaluated for Ductile Iron are tensile and

yield strengths, modulus of elasticity, poisson ratio, Elongation and Proportionality

limit.

(a) Tensile Strength

The tensile strength or ultimate tensile strength (UTS) is the maximum load in tension

which a material will withstand prior to fracture. It is calculated by dividing the

maximum load applied during the tensile test by the original cross sectional area of the

sample. Tensile strengths for conventional Ductile Irons generally range from 414

MPa for ferritic grades to over 1380 MPa for martensitic grades.

(b) Yield Strength

The yield strength or proof stress is the stress at which a material begins to exhibit

significant plastic deformation. The sharp transition from elastic to plastic behavior

exhibited by annealed and normalized steels gives a simple and unambiguous

definition of yield strength. For Ductile Iron, the offset method is used in which the

yield strength is measured at a specified deviation from the linear relationship between

stress and strain.

This deviation, usually 0.2% is included in the definition of yield strength or proof

stress in international specifications and is often incorporated in the yield strength

terminology,e.g.0.2% yield strength. Yield strengths for ductile iron typically range

from 275MPa for ferritic grades to over 620MPa for martensitic grades.

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(c) Modulus of Elasticity

Past researches have shown that, at low tensile stresses, there is a linear or

proportional between stress and strain. This relationship is known as Hookess Law

and the slope of the straight line is called the Modulus of Elasticity or Youngs

Modulus. Usually the initial stress-strain behavior of Ductile Iron lies between those of

mild steel and gray Iron.

Annealed or normalized mild steels exhibit elastic behavior until the yield point, where

plastic deformation occurs suddenly and without any initial increase in flow stress.

Ductile Iron exhibits a proportional or elastic stress-strain relationship similar to that

of steel but which is limited by the gradual onset of plastic deformation. The Modulus

of elasticity of Ductile Iron varies from 162 170 GPa.

(d) Poissons Ratio

Poissons Ratio is the ratio of lateral elastic strain to longitudinal elastic strain

produced during a tensile test. A commonly accepted value is 0.275.

(e) Elongation

Elongation is defined as the permanent increase in length, expressed as a percentage of

a specified gauge length marked in a tensile test bar which is produced when the bar is

tested to failure. Elongation is used widely as the primary indication of tensile ductility

and is included in many Ductile Iron specifications. Although shown as the uniform

elongation in Figure 2.2, elongation also includes the localized deformation that occurs

prior to fracture.

28

Figure 2.2; Typical stress-strain curve for Ductile Iron.

Source: www.ductile.org/didata/section3/figures

29

iii. Manganese; The presence of manganese leads to pearlite and carbide

formation. This causes an increase in hardness and difficulty in machinability of

Ductile Iron.

iv. Nickel: It is preferred ferrite strengthener for ferrite Ductile Irons requiring

maximum low temperature toughness.

v. Phosphorus: It is present as an impurity element in Ductile Iron and has a

strong embrittling effect at levels as low as 0.02 percent.

vi. Silicon: A reduction in silicon level reduces both the yield and tensile strengths

of the ferritic iron but it enhances toughness at low temperature conditions.

(g) Environment

The performance of any material in service is controlled mainly by the prevailing

environmental conditions. Like some steels, the ambient temperature tensile properties

of certain grades of Ductile iron can be reduced significantly by prolonged exposure to

certain environments. Past researches have shown that tensile strength and elongation

followed similar trends, but the loss of strength and ductility begins at lower hardness

levels of about 175 BHN and then increases slowly.

30

2.5.2 Hardness

Hardness may be defined as a measure of a materials resistance to localized plastic

deformation. Hardness usually implies a resistance to deformation and for metals; it is

the measure of their resistance to permanent or plastic deformation.

There are three general types of hardness measurements depending on the manner in

which the test is conducted and these are:

Scratch Hardness

Indentation Hardness

Rebound or Dynamic Hardness

Only indentation hardness is of major engineering interest for metals. Hardness tests

are performed more frequently than any other mechanical test for several reasons:

They are simple and inexpensiveordinarily no special specimen need be

prepared, and the testing apparatus is relatively inexpensive.

The test is nondestructivethe specimen is neither fractured nor excessively

deformed; a small indentation is the only deformation.

Other mechanical properties often may be estimated from hardness data, such

as tensile strength.

The hardness of Ductile Iron is usually and best measured by the Brinell Hardness test,

wherein a 10mm diameter hardened steel or tungsten carbide ball is pressed into a flat

surface of the workpiece. Hardness is expressed as a Brinell Indentation Diameter

(BID) or a Brinell Hardness Number (BHN). Hardness may also be described as

BHN/3000 to indicate that the force applied to the ball is 3000kg which is the normal

value for ferrous materials.

31

The size of the Brinell indentation and its related volume of plastic deformation is

large relative to the scale of the microstructure and as a result, an average hardness is

obtained which exhibits good reproducibility for similar microstructures.

2.5.2.1 Brinell Hardness Test

In Brinell tests, as in Rockwell measurements, a hard, spherical indenter is forced into

he surface of the metal to be tested. The diameter of the hardened steel (or tungsten

carbide) indenter is 10.00 mm (0.394 in.). Standard loads range between 500 and 3000

kg in 500-kg increments; during a test, the load is maintained constant for a specified

time (between 10 and 30 s). Harder materials require greater applied loads. The Brinell

hardness number, HB, is a function of both the magnitude of the load and the diameter

of the resulting indentation.

This diameter is measured with a special low-power microscope, utilizing a scale that

is etched on the eyepiece. The measured diameter is then converted to the appropriate

HB number using a chart; only one scale is employed with this technique.

Maximum specimen thickness as well as indentation position (relative to specimen

edges) and minimum indentation spacing requirements are the same as for Rockwell

tests. In addition, a well-defined indentation is required; this necessitates a smooth flat

surface in which the indentation is made.

32

2.5.2.2 Rockwell Hardness Test

The Rockwell tests constitute the most common method used to measure hardness

because they are so simple to perform and require no special skills. Several different

scales may be utilized from possible combinations of various indenters and different

loads, which permit the testing of virtually all metal alloys (as well as some polymers).

Indenters include spherical and hardened steel balls having diameters of and in. (1.588,

3.175, 6.350, and 12.70 mm), and a conical diamond (Brale) indenter, which is used

for the hardest materials.

With this system, a hardness number is determined by the difference in depth of

penetration resulting from the application of an initial minor load followed by a larger

major load; utilization of a minor load enhances test accuracy. On the basis of the

magnitude of both major and minor loads, there are two types of tests: Rockwell and

superficial Rockwell. For Rockwell, the minor load is 10 kg, whereas major loads are

60, 100, and 150 kg. When specifying Rockwell and superficial hardnesses, both

hardness number and scale symbol must be indicated. The scale is designated by the

symbol HR.

2.5.3 Impact Fracture

Prior to the advent of fracture mechanics as a scientific discipline, impact testing

techniques were established so as to ascertain the fracture characteristics of materials.

It was realized that the results of laboratory tensile tests could not be extrapolated to

predict fracture behavior; for example, under some circumstances normally ductile

metals fracture abruptly and with very little plastic deformation.

33

Impact test conditions were chosen to represent those most severe relative to the

potential for fracturenamely, (1) deformation at a relatively low temperature, (2) a

high strain rate (i.e., rate of deformation), and (3) a triaxial stress state (which may be

introduced by the presence of a notch).

2.5.3.1 Impact Testing Techniques

Qualitatively, the fracture behavior of materials may be determined using Charpy and

Izod impact testing techniques. On the basis of the temperature dependence of

measured impact energy (or appearance of the fracture surface), it is possible to

ascertain whether or not a material experiences a ductile-to-brittle transition and the

temperature range over which such a transition occurs. Two ASTM4

standardized tests,

the Charpy and Izod, were designed and are still used to measure the impact energy,

sometimes also termed notch toughness. The Charpy V-notch (CVN) technique is

most commonly used in the United States. For both Charpy and Izod, the specimen is

in the shape of a bar of square cross section, into which a V-notch is machined.

As shown in figure 2.3, the load is applied as an impact blow from a weighted

pendulum hammer that is released from a cocked position at a fixed height h. Upon

release, a knife edge mounted on the pendulum strikes and fractures the specimen at

the notch, which acts as a point of stress concentration for this high-velocity impact

blow.

4. ASTM Standard E 23, Standard Test Methods for Notched Bar Impact Testing of MetallicMaterials.)

34

Figure 2.3; Impact testing Machine

35

2.5.3.2 Ductile-To-Brittle Transition

One of the primary functions of Charpy and Izod tests is to determine whether or

not a material experiences a ductile-to-brittle transition with decreasing temperature

and, if so, the range of temperatures over which it occurs. The ductile-to-brittle

transition is related to the temperature dependence of the measured impact energy

absorption.

It is possible to ascertain whether or not a material experiences a ductile-to-brittle

transition and the temperature range over which such a transition occurs. Low-strength

steel alloys typify this behavior, and, for structural applications, should be used at

temperatures in excess of the transition range. Furthermore, low-strength FCC metals,

most HCP metals, and high-strength materials do not experience this ductile-to-brittle

transition.

36

2.5.4 Preheat Treatment Operation

The heat treatment of spheroidal graphite irons can considerably alter the

microstructure of the matrix, with little or no effect on the size and shape of the

graphite achieved during casting. The matrix microstructures resulting from heat

treatment can vary from ferrite-pearlite to tempered martensite.

Preheating involves raising the temperature of the base metal in the region to be

welded to a predetermined temperature prior to carrying out the welding process.

Preheating may be applied to help prevent cold-cracking, reduce hardness in the heat-

affected zone, reduce residual stresses, and reduce distortion.

The operation of heating metal to some pre determined temperature before engaging in

actual welding is called preheating. The details and the modes may be different in

various situations but in general the purpose is to influence the cooling behavior after

welding so that shrinkage stresses will be lower (relative to welding without

preheating) and cooling rate will be milder. When a ductile iron is welded, it is heated;

the heated portion has a micro structure that is different from that of the base metal

and this is called the Heat Affected Zone [HAZ] (J. E. RAMIREZ et al., 2005).

Preheat

treatment operation in welding refers to the heating of a part in a furnace prior to

welding operation to ensure a slow enough cooling rate so that the part's distortion and

HAZ is minimal.

Pre-heating prepares metal to make it more receptive to welding. The importance of

preheating increases with the thickness of the base metal because of the rapid self

quench capability, and with the rigidity of the welded structure because of the derived

constraints. In general the higher the preheat temperature and the lower the heat input,

the conditions are more favorable for limiting martensite formation and its hardness,

37

hopefully contributing to higher quality welds. If the entire part is already at an

elevated temperature before welding, after welding the heat affected zone will have no

place to release the 'extra' heat because the area adjacent to the HAZ will be at

relatively the same temperature. Thus a slow cooling rate of the HAZ, prevents a

brittle, martensitic weld from forming.

2.5.5 Welding

Welding, in engineering, any process in which two or more pieces of metal are joined

together by the application of heat, pressure, or a combination of both. Most of the

processes may be grouped into two main categories: pressure welding, in which the

weld is achieved by pressure; and heat welding, in which the weld is achieved by heat.

Heat welding is the most common welding process used today.

Welding is used in ships, bridges, pressure vessels, industrial machinery, automobile,

rolling stock and many other fields. Problems associated with welding are common

issues in these fields. Welding of steel refers to the maximum hardness of the heat

affected zone (HAZ) and the cold cracking susceptibility of welds. When steel is

welded non uniform heating and cooling in weld metal and in base metal generates

harder Heat Affected Zone (HAZ), cold crack susceptibility and residual stress in

weldment. The best way to minimize above difficulties is to slow the heating and

cooling rate of the base metal and weld heat affected zone. However there are many

methods for reducing the effects of above problems and one of them is preheating

and/or post heating.

38

Due to their high carbon content, all cast irons have a common factor affecting their

welding. During the welding of cast irons, the iron immediately adjacent to the weld

metal is heated to its fusion or melting point. After welding, the entire heat-affected

zone cools very rapidly. During this heating and cooling, some of the graphitic carbon

dissolves and diffuses into the iron, and, as a result, carbides tend to form at the edge

of the fusion zone, and high-carbon martensite and bainite tend to form in the

remainder of the heat-affected zone. The formation of these hard, brittle

microconstituents increases the susceptibility to cracking.

The lower surface-to-volume ratio of the nodular graphite in Ductile Iron as compared

to that for the flake graphite in Gray Iron results in less carbon dissolution and the

formation of fewer carbides and less high-carbon martensite. Welding involves the

fusion of both a filler metal (welding consumable) and the base metal adjacent to the

weld zone. the high carbon content of Ductile Iron can lead to the formation of

carbides in the fusion zone (FZ) and martensite in both the FZ and heat affected zone

(HAZ) adjacent to the FZ unless correct procedures are followed. however, with the

use of appropriate materials and procedures, Ductile Iron castings can be successfully

joined to other Ductile Iron castings and to steel by fusion welding.

During welding, rapid heating and cooling take place which produce severe thermal

cycle near weld line region. Thermal cycle cause non uniform heating and cooling in

the material, thus generating harder heat affected zone, residual stress and cold

cracking susceptibility in the weld metal and base metal. Detrimental residual stresses

commonly result from differential heating and cooling.

39

Due to contraction of metal along the length of the weld is partially prevented by the

large adjacent body of cold metal. Hence residual tensile stresses are set up along the

weld. The properties of welds often cause more problem than the base metal

properties, and in many cases they govern the overall performance of the structure.

These all are a problem in the process of production. To get rid of these problems heat

treatment before welding (Preheating) is employed. Effective preheat are the primary

means by which acceptable heat affected zone properties and minimum potential for

hydrogen induced cracking are created.

2.5.6.1 ARC WELDING

Several methods have been employed successfully to arc-weld Ductile Iron to itself

and other materials with acceptable properties in both the weld and base metal. the

properties of shielded metal arc welded Ductile Irons were greatly improved by the

introduction over 40 years ago of the high-Ni and Ni-Fe electrodes. these electrodes

produce high-nickel fusion zones that are relatively soft and machinable but have

adequate tensile strength, ductility and fatigue strength. The short arc metal inert gas

(MIG) welding process, by virtue of its controlled, low heat input, reduced harmful

structureal changes in the base metal HAZ. combining the benefits of Ni-base filler

wire with the short-arc MIG process has resulted in welds with tensile properties that

are equivalent to the base Ductile Iron and fatigue strengths that are 65% and 75%

respectively of the fatigue limits of unwelded pearlitic and ferritic Ductile Irons.

40

2.5.6.2 Welding Of Ductile Iron

Welding of the cast iron is commonly characterized by calculation of the carbon

equivalent (CE). By CE no-crack temperature for the cast iron is determined. No-crack

temperature is the preheating temperature, above which the cooling rates will be

lowered enough that the material will not cause formation of any cracks due to

welding. CE is calculated using the weight percents of the elements in the chemical

composition according to Equation 1;

The no-crack temperature is correlated with the CE by the equation 1. The lower the

no-crack temperature is the more weldable the cast iron is.

Here the preheating is determined by CE and main structure of the cast iron to be

welded. Additionally, the thickness of the material will be another factor determining

the cooling rate after welding. Increased thickness causes an increase in cooling rates

thus welding of the thicker pieces will be lower requiring increased preheating

temperatures.

2.5.6.3 Welding Of Cast Iron

Ab Pascual et al. have studied welding nodular cast iron with oxyacetylene (OAW)

and shielded metal arc welding (SMAW) using 98.2% Ni and Fe-Cr-Ni alloy filler

materials respectively. They have concluded that welding ductile cast iron with or

without preheat is possible but preheating almost always increases weld quality and

ductility. OAW results very poor weld metal properties whereas SMAW yields an

amount of ductility in the weld metal.

41

Furthermore, using Ni electrodes is another factor increasing the ductility which

hinders the carbide formation. El-Banna has studied welding ductile cast iron in as-

cast and fully ferritized states using SMAW process with ENiFe-CI filler material. He

has worked on different preheating temperatures and again concluded that ductile cast

iron can successfully be welded with or without preheating using Ni based electrodes

but in order to achieve certain mechanical properties a preheating temperature of 200-

300C is required. Additionally he stated that Rm values required from the base

materials can only be met in ferritized components. In as-welded specimens

ledeburitic carbide structures and local melting around the graphite nodules are

observed. With application of preheating various pearlite and martensite ratios instead

of carbide were formed.

Again in a study carried out by El-Banna et al. restoration properties of pearlitic cast

iron using SMAW with various filler materials as Ni, Fe-Ni alloy, Ni-Cu alloy,

stainless and ferritic steel is studied. Also subcritical annealing at 677C is applied.

Effect of heat input, preheating and filler materials was examined. When using the

ferritic filler material, preheating at 300C becomes the best option for narrowing the

melt region and HAZ with discontinuous carbide and bainite. It is seen that PWHT has

reduced the maximum hardness values slightly and finally multipass welding lowers

the width of melt region and microhardness of HAZ. Using filler materials with Ni

content can overcome carbide formation however; with ferritic filler a continuous

carbide network is observed around the fusion line and HAZ yielded a martensitic

structure. Pouranvari carried out a study on welding cast iron using SMAW with Ni

based electrodes.

42

He also applied PWHT to the welded pieces. Due to possibility of increasing amount

and continuity of carbides preheating is not used and formation of cracks was not

reported. Material was fully annealed and a nearly uniform hardness profile is

achieved. Again nickel based filler is used to prevent ledeburitic carbide formation in

the structure of the weld piece but due to dilution very high carbon contents are come

across which cannot be compensated with Ni. This excess amount precipitated as

graphite in fusion zone. In PMZ ledeburitic and martensitic structure formation occurs,

constructing a hard and brittle network among fusion line. Voigt et al. have studied

general HAZ structures of ductile cast irons. SMAW with ENi-CI filler material used

with about 300C of preheating. Sub-critical annealing and full annealing is applied to

the specimens. In as weld specimens carbides are formed surrounding the graphite

nodules and in intercellular regions between nodules. It is concluded that this

formation cannot be effectively prevented in PMZ.

2.5.6.4 HAZ Heat affected zone

Cast iron is generally considered as a difficult material to be welded. This is basically

due to two reasons: inherent brittleness of the cast iron and the effect of weld thermal

cycle on the metallurgical structure of the cast iron. Typically, four distinct regions are

formed when cast iron is welded, as follows:

Fusion zone (FZ) which is melted during welding process and is resolidified

upon cooling.

Partially melted zone (PMZ) which is the area immediately outside the FZ

where liquation can occur during welding.

43

Heat affected zone (HAZ) which is not melted but undergoes microstructural

changes.

Base metal (BM) which its structure remains unaffected during weld thermal

cycle.

The possibility of performing a welding process without building up a thermal

gradient in the parent metal is almost negligible. The temperature and the speed of the

welding process is very influential in deciding the spread of heat into the parent metal.

The thermal gradient will get compressed by the high power welding at high speed

(Houldcroft and John, 1988).

44

3.2.2 Metallographic preparation of samples

The metallographic preparation of samples was performed by creating a flat surface on

the samples of the As-cast, As-welded and preheated respectively, grinded on the

Beulah strip grinding machine and then polished on a polishing machine. The

photomicrographs of the microstructure were also taken to get information on the

graphite morphology and matrix features, obtained through a computer-based image

analyzing system.

3.2.3 Mechanical property testing

The ductile iron rods were carefully machined to the required sizes and shapes of the

tensile, impact and hardness specimen using the Lathe machine as seen in figure 3.2

and 3.2.1. The tensile specimen is such that the tensile-testing machine can grip it

easily on both ends while the impact specimen is machined with a notch of 2mm at the

mid-point of the specimen.

3.2.3.1 Tensile Strength Testing

A computerized tensometer was used to carry out the tensile strength test on the

samples and the graphs plotted instantaneously.

Tensile tests was performed on the machined samples with circular cross section. Test

pieces were screwed into or gripped in jaws of the automated tensile testing machine

and stretched by moving the grips apart at a constant rate while measuring the load and

the grip separation. This data is plotted as load vs extension and then converted to

engineering stress (load/original area) vs engineering strain (fractional change in

length over the test section assuming the deformation is uniform).

45

A milling machine was used in creating a groove along the longitudinal axis of the test

sample followed by metal arc welding of the grooved portion of the ductile iron

samples.

3.2.3.2 Impact Strength Testing

Impact strength is measured by allowing a pendulum to strike a grooved machined test

piece and measuring the energy absorbed in the break (AS1544). The Izod test is at

ambient temperature while the temperature controlled Charpy test (AS1544.2) uses

typically 10x10mm, rectangular cross section samples cut at specified orientations to

the material axes. The absorbed energy decreases at lower temperatures.

46

3.2.3.3 Hardness Testing

Hardness is not an intrinsic property of a material. During the Hardness Test, a flat

surface was made on the specimen and then grinding and polishing was done on the

surface. The compression attachment was assembled in the machine, one compression

die was inserted, and a Brinell ball bolster, the mercury on the hardness-testing

machine was set to zero, the polished surface was held against the Brinell ball and the

load was applied with the quick-acting handle.

3.2.4 Pre-weld heating of samples

A pre-weld heating of the Spheroidal ductile iron was performed on the third group of

fifteen samples in an electric furnace to a temperature of 410o

C before engaging in

actual welding so as to influence the cooling behavior after welding and lowering the

shrinkage stresses that are relative to welding without preheating and thus makes the

cooling rate milder.

3.2.5 Welding of the samples

Welding involves the fusion of both a filler metal (welding consumable) and the base

metal adjacent to the weld zone. The high carbon content of Ductile Iron can lead to

the formation of carbides in the fusion zone (FZ) and martensite in both the FZ and

heat affected zone (HAZ) adjacent to the FZ unless correct procedures. A welding

current of 140 A, with a root gap of 1.5 mm was used in order to obtain a good weld

penetration using an electrode containing 97.6 % Ni.

47

3.3 Equipment/facility

Equipment used for this research are as follows:

1 Lathe Machine

2 Milling Machine

3 Electric Arc Welding

4 Muffle Furnaces

5 Grinding Machine

6 Polishing Machine

7 Metallurgical Microscope

8 Fatigue Testing Machine

9 Impact Testing Machine

10 Tensile Testing Machine

11 Vernier Calliper

48

3.4 Availability Of The Equipment

Some of the major equipments used for this research work are as listed in Table 2.1,

with their respective places of availability indicated.

Table 2.1: Equipments to Be Used and Their Places of Availability

S/N EQUIPMENT PLACE AVAILABLE

1 Lathe Machine Mech. Engineering Workshop, O.A.U.,

Ile-ife

2 Milling Machine FIIRO, Lagos.

3 Electric Arc Welding FIIRO, Lagos.

4 Muffle Furnaces M.S.E Laboratory, O.A.U., Ile-ife

5 Grinding Machine M.S.E Laboratory, O.A.U., Ile-ife

6 Polishing Machine M.S.E Laboratory, O.A.U., Ile-ife

7 Metallurgical Microscope M.S.E Laboratory, O.A.U., Ile-ife

8 Fatigue Testing Machine M.S.E Laboratory, O.A.U., Ile-ife

9 Impact Testing Machine M.S.E Laboratory, O.A.U., Ile-ife

10 Tensile Testing Machine CERD, O.A.U, Ile-fe

11 Vernier Calliper M.S.E Laboratory, O.A.U., Ile-ife

49

CHAPTER FOUR

RESULTS AND DISCUSSION

4.1 Experimental Results

The results of the Tensile strength, Impact strength and Hardness of the ductile iron

samples are as presented on Table 4.1, Table 4.2 and Table 4.3 respectively. The stress-

strain curve for the as-cast, as-weld and pre-weld heated samples are presented on Fig

4.1, Fig 4.2 and Fig 4.3 respectively. The microstructure of the as-cast, as-weld and

pre-weld heated samples are presented on Fig 4.4, Fig 4.5 and Fig 4.6 respectively.

Table 4.1 Tensile Test Results

Samples UTS Yield Strength % Elongation Fracture Strength

As-Cast

As-Weld

Pre-weld

heated

446

735

411

330

514

315

6

4

14

410

697

375

50

Table 4.2 Impact Test Results

Samples As-Cast As-Weld Pre-weld heated

1

2

3

4

5

5.20

8.88

6.25

6.70

6.43

4.80

5.00

4.00

5.30

5.00

9.20

8.30

9.70

9.40

8.70

Mean 6.70 4.82 9.06

Impact

Strength

51

Table 4.3 Hardness Test Results

Samples As-Cast As-Weld Pre-weld heated

1

2

3

4

5

245

233

230

282

236

330

480

290

380

520

189

163

174

176

173

Mean 245 397 175

Hardness

(BHN)

52

Figure 4.1 Tensile Stress Vs Tensile Strain curve of the As-Cast samples of Ductile

Iron

0

50

100

150

200

250

300

350

400

450

500

0 0.005 0.01 0.015 0.02 0.025 0.03

Ten

sile

Str

ess

Tensile Strain

As-Cast Ductile Iron

53

Figure 4.2 Tensile Stress Vs Tensile Strain curve of the As-Weld samples of Ductile Iron

54

Figure 4.3 Tensile Stress Vs Strain curve of the Pre-weld heated samples of Ductile Iron

-50

0

50

100

150

200

250

300

350

400

450

500

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07

Ten

sile

Str

ess

Tensile Strain

Pre-weld heated-welded

55

(a)

(b)

Figure 4.4 (a) Photomicrograph of the as-cast ductile iron without using Nital etchant (b)

Photomicrograph of the as-cast ductile iron using Nital etchant.

56

(

(c) (d)

Figure 4.5 (a) and (b) Photomicrographs of the fusion zone and heat affected zone of the

asweld ductile iron without using Nital etchant (c) and (d) Photomicrographs of the fusion

zone and Heat Affected zone of the asweld ductile iron using Nital etchant.

57

(c) (d)

Figure 4.6 (a) and (b) Photomicrographs of the fusion zone and heat affected zone of the pre-

weld heated samples of ductile iron without using Nital etchant (c) and (d) Photomicrographs

of the fusion zone and Heat Affected zone of the pre-weld heated samples of ductile iron

using Nital etchant.

58

DISCUSSION OF RESULTS

Table 4.1 shows the tensile test result performed on the ductile iron samples and it is

observed that the tensile strength of the as-weld samples of ductile iron is very high

having 735 MPa with least elongation which showed reduced impact strength and

ductility compared to that of the as-cast samples having tensile strength having 446

MPa which showed good tensile strength, improved impact strength and ductility as

shown on the stress-strain graph in Figure 4.2 and Figure 4.1.

Also it can be seen on table 4.1 that the tensile strength of the pre-weld heated

samples of ductile iron has 411 MPa which is very close to that of the as-cast samples

of ductile iron having 446 MPa but with improved mechanical properties as it

resulted in increased elongation, hence improved ductility and impact strength as

shown on the stress-strain graph in Figure 4.3 and Figure 4.1 respectively.

Table 4.2 shows the impact test result performed on the ductile iron samples and it is

observed that the impact strength of the as-welded samples of ductile iron is low due

to its high tensile strength and low ductility while the impact strength of the preheated

samples of ductile iron is improved over the as-cast impact strength due to improved

ductility.

59

Table 4.3 shows the hardness test result performed on the ductile iron samples and it

is observed that the hardness value of the as-welded samples of ductile iron is highest

signifying brittleness while the hardness value the preheated samples of ductile iron is

least as compared to the as-cast impact strength.

The as-weld samples of ductile iron behaved very hard, signifying brittleness due to

the rapid cooling of the as-weld ductile iron after the welding operation suggesting

the presence of ledeburite carbides and martensitic phases within its matrix which are

results of metastable solidification.

The figure 4.4 (a) and (b) are the photomicrographs, at a magnification of X200, of

the microstructures of the as-cast ductile iron without using etching and the

microstructure of the as-cast ductile iron after etching which reveals bull-eye

spheroids of graphites in the matrix of ferrite. Bull-eye spheroids of graphites are

formed when ferrites form rings around the spheroidal graphites.

The photomicrograph at a magnification of X200 shown in figure 4.5 (a) and (b) are

the fusion zone and heat affected zone microstructures of the as-weld ductile iron

without etching which show spheroids of graphites and the figures 4.5 (c) and (d) are

the fusion zone and heat affected zone microstructures of the as-weld ductile iron

with etching with nital which reveals ledeburite carbides and martensite phases in the

matrix of ferrite.

60

The photomicrograph at a magnification of X200 shown in figure 4.6 (a) and (b) are

the fusion zone and heat affected zone microstructures of the pre-weld heated ductile

iron without etching which reveal no change in microstructure when heated to 410

as compared with the as-cast samples and the figures 4.6 (c) and (d) are the fusion

zone and heat affected zone microstructures of the as-weld ductile iron using nital

etchant which reveal reduced graphite sizes in the matrix of ferrite.

The influence of thermal treatment on welded ductile iron shows that the pre-weld

heated samples of the as-cast ductile iron has uniform distribution of phases with

dissolved graphite present in the matrix and low average size of fragile carbides and

martensite structures were not formed. The thermal treatment resulted in relieving

residual stress and diminishing the cooling rate.

Table 4.3 shows the hardness of the parent ductile iron is found to be 245BHN.The

photomicrograph of the as-weld ductile iron shows a typical microstructure of the

weld joint welded with a Ni electrode without preheating. From the figure it is found

that the HAZ is visually darker due to less amount of ferritic matrix structure

resulting from the dissolution of ferrite in nickel. The hardness of HAZ was found to

be 397 BHN (138 % higher than that of parent metal).

From the table it is found that the pre-weld heated ductile iron hardness value is less

with the weld bead containing ferritic structure with a high concentration of small

graphite nodules distributed uniformly in the matrix resulting due to the dissolution of

graphite in nickel. Though the bead had higher ductility due to the uniform

distribution of graphite nodules.

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Hardness obtained in the HAZ was 175 BHN. The ductility was increased twice and

the rest of values were very similar. The mode of failure observed from tensile

test was found to be ductile in the bead.

The typical microstructure of a pre-weld heated cast iron using a Ni electrode reveals

that there is no change in the microstructure with the preheating at 410 and that

graphite in form of smaller spherolytes grew in the bead region and got distributed

uniformly. This smaller spherolyte forms may be due to nickel metal, which absorbs

carbon dissolving it in its metallic matrix. It is found that the ductility is increased and

hardness diminished. It is also found that the yield limit is not affected significantly.

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CHAPTER FIVE

5.0 CONCLUSION

It can be concluded from the thesis result that the As-welded ductile iron results in the

formation of carbides and martensite phases in the matrix of ferrites which embrittles

the welded ductile iron thus making it hard.

Preheat treatment operation prior to welding prepares the Ductile Iron to be more

receptive to welding conditions, having no microstructural changes in the ferritic

matrix thus increasing the ductility of the welded piece through minimizing hard and

fragile microstructures and achieving a tensile strength close to that of the as-cast.

5.1 RECOMMENDATION

It is therefore recommended that effective preheat treatment operation should be

performed on ductile irons prior to welding using high purity Ni electrodes as it

showed better welding, enhanced ductility and better uniform distribution of graphite.