KWAME NKRUMAH UNIVERSITY OF SCIENCE AND

TECHNOLOGY, KUMASI

COLLEGE OF ENGINEERING

DEPARTMENT OF MATERIALS ENGINEERING

TOPIC:

APPRAISAL OF LOCALLY MANUFACTURED U-BOLT.

A THESIS SUBMITTED TO THE MATERIALS ENGINEERING

DEPARTMENT, KWAME NKRUMAH UNIVERSITY OF

SCIENCE AND TECHNOLOGY, KUMASI IN PARTIAL

FULFILMENT OF THE REQUIREMENT FOR THE BSC DEGREE

IN METALLURGICAL ENGINEERING.

BY

ANSA-ASARE KWAKU DUAH

AGBORKEY, KOFI TEYE JUDE

BOSOKAH, EMENYO

SUPERVISOR

PROF. S. KWOFIE

May, 2018

ii

DECLARATION

We hereby declare that this is our own work towards the requirements necessary for an award

of a BSc. Degree and to the best of our knowledge, it contains no material previously

published by another person, nor material which has been accepted for any degree, except

where due acknowledgement has been made in text.

ANSA-ASARE, KWAKU DUAH …………………………. ………………………….

(2202014) Signature Date

AGBORKEY, JUDE KOFI TEYE …………………………. ………………………….

(2201214) Signature Date

BOSOKAH, EMENYOH …………………………. ………………………….

(2203214) Signature Date

PROF. S. KWOFIE ………………………… ………………………….

(Supervisor) Signature Date

iii

ACKNOWLEDGEMENT

We thank the Almighty God, whom we serve for the grace and strength given us to finish this

project. We would like to thank our supervisor Professor Samuel Kwofie for his patience,

guidance and inputs, without which the project would not have been a success.

Our gratitude goes out to Mr. Yeboah and Mr. Appiah of Suame Magazine for their

contributions in helping us get access to the manufacturing process and obtaining U-bolts.

We thank Mr. Solomon Nettey of Tema Steel Company Ltd for his assistance during our test.

We are highly indebted to our families for their endless support, inspiration and

encouragement and also our friends for their support.

iv

ABSTRACT The U-bolt is a bolt, bent in the shape of the letter „U‟ with screw threads at the ends. U-bolts

are made of metal (steel) and are used to clamp down the leaf springs attached to the axle of a

vehicle. It plays an important part in the suspension systems of heavy duty vehicles and

trucks. The aim of this project was to appraise the locally manufacture U-bolts in Suame,

Kumasi. This project started with a visit to Suame to study the local production process of the

U-bolt and obtain samples for laboratory tests. Two shops were visited and a sample taken

from each shop and an additional foreign U-bolt. The tests conducted include hardness test,

chemical composition test, and metallography to determine the hardness, chemical

composition and microstructure of the U-bolts. After the analysis of the results obtained, it

was realized that the local U-bolts had inconsistent properties although when compared to the

Society of Automotive Engineers standard SAE-J429, they met the chemical requirement of

some Grades of U-bolts material. It was concluded that the inconsistency in the local U-bolt

properties stem from its production process. In a bid to improve on the properties of the local

U-bolt, recommendations on the application of a mechanized process and an additional

Carburizing procedure were made.

v

TABLE OF CONTENT DECLARATION ............................................................................................................................. i

ACKNOWLEDGEMENT ............................................................................................................. iii

ABSTRACT ................................................................................................................................... iv

LIST OF FIGURES ...................................................................................................................... vii

LIST OF TABLES ....................................................................................................................... viii

CHAPTER ONE ............................................................................................................................. 1

1.0 INTRODUCTION ................................................................................................................. 1

1.1 BACKGROUND ................................................................................................................... 1

1.2 PROBLEM STATEMENT ................................................................................................... 3

1.3 JUSTIFICATION .................................................................................................................. 3

1.4 AIM AND OBJECTIVES ..................................................................................................... 3

1.5 SCOPE OF WORK ............................................................................................................... 4

CHAPTER TWO ............................................................................................................................ 5

2.0 LITERATURE REVIEW ...................................................................................................... 5

2.1 INTRODUCTION TO U-BOLTS ........................................................................................ 5

2.2 FUNCTIONS OF THE U-BOLT .......................................................................................... 5

2.3 U-BOLT MATERIAL AND MECHANICAL REQUIREMENT........................................ 7

2.4 HEAT TREATMENT ........................................................................................................... 9

2.5.1 Hardening ..................................................................................................................... 10

2.5.2 Annealing...................................................................................................................... 10

2.5.3 Normalizing .................................................................................................................. 10

2.5.4 Tempering ..................................................................................................................... 11

2.6 METAL FORMING PROCESSES ..................................................................................... 11

2.6.1 Rolling .......................................................................................................................... 11

2.6.2 Extrusion ....................................................................................................................... 12

2.6.3 Forging.......................................................................................................................... 12

2.6.4 Drawing ........................................................................................................................ 13

2.7 TESTING OF U-BOLTS .................................................................................................... 13

2.7.1 Tensile Test................................................................................................................... 13

2.7.2 Hardness Test ............................................................................................................... 14

2.7.3 Metallography ............................................................................................................... 14

2.7.4 Chemical Analysis ........................................................................................................ 15

2.8 U-BOLT PRODUCTION PROCESS ................................................................................. 15

2.9 U-bolt Failure ...................................................................................................................... 18

vi

CHAPTER THREE ...................................................................................................................... 20

3.0 METHODOLOGY .............................................................................................................. 20

3.1 Visit to Suame ..................................................................................................................... 20

3.2 Hardness Test ...................................................................................................................... 24

3.3 Chemical Analysis............................................................................................................... 25

3.4 Metallography ..................................................................................................................... 25

CHAPTER FOUR ......................................................................................................................... 27

4.0 RESULTS AND DISCUSSION ......................................................................................... 27

4.1 Hardness Test Results ...................................................................................................... 27

4.2 Chemical Test results....................................................................................................... 30

4.3 Metallography Results ..................................................................................................... 31

CHAPTER FIVE .......................................................................................................................... 35

5.0 CONCLUSION AND RECOMMENDATION .................................................................. 35

5.1 CONCLUSION ................................................................................................................... 35

5.2 RECOMMENDATION ...................................................................................................... 35

REFERENCES ............................................................................................................................. 36

vii

LIST OF FIGURES Figure 1.1: Image showing the U-bolt together with the axis, plate and leaf springs.

(Ventura,2009.) .......................................................................................................................... 1

Figure 1.2: Types of U-bolts (U-bolt types, n.d) ....................................................................... 2

Figure 2.1: Outlines showing the functions of U-bolt ............................................................... 6

Figure 2.2: Flow diagram illustrating hot forming process for U-bolt production .................. 16

Figure 2.3: Hot bending of U-bolt (Bending-Portland, n.d) .................................................... 17

Figure 2.4: Flow diagram representing cold forming process for U-bolt production .............. 17

Figure 2.5: cold bending of U-bolt (the importance of the U-bolt…, n.d) .............................. 18

Figure 3.1: Flow diagram representing production process at Shop A.................................... 20

Figure 3.2: Flow diagram representing production process at Shop B .................................... 20

Figure 3.3: Measuring and cutting of rod to required length ................................................... 21

Figure 3.4: Cutting of threads using a center lathe machine .................................................... 22

Figure 3.5: Bending of heated steel rod ................................................................................... 23

Figure 3.6: Heating of U-bolt threads ...................................................................................... 23

Figure 3.7: Painting of finished U-bolt .................................................................................... 24

Figure 3.8 Image representing hardness test points along the U-bolt ...................................... 25

Figure 4.1: Graph of hardness (HB) values of Points 1 to 8 for Samples A, B and F ............. 28

Figure 4.3: Microstructure image of Sample A (a) top (b) neck ............................................. 32

Figure 4.4: Microstructure Image of Sample B (a) top (b) neck ............................................. 32

Figure 4.5: Microstructure Image of sample F (a) top (b) neck ............................................... 33

Figure 4.6 Microstructure of steel rod samples ....................................................................... 33

viii

LIST OF TABLES Table 2.1 Chemical composition and Treatment (SAE-J429, 2011) ......................................... 8

Table 2.2 Mechanical requirements (SAE-J429, 2011) ............................................................. 9

Table. 4.1 Results from Hardness Test along the U-bolt ......................................................... 27

Table 4.2 Chemical Compositions of U-Bolt and steel rod samples ....................................... 31

1

CHAPTER ONE

1.0 INTRODUCTION

1.1 BACKGROUND

The U-bolt is a bolt, bent in the shape of the letter „U‟ with screw threads at the ends. U-bolts

are made of metal, and are used to clamp down the leaf springs attached to the axle of the

vehicle. The U-bolt‟s purpose is to hold down the leaf spring unto the axis, hence forming a

three component solid set: the axis, the leaf springs, and the supporting plate as depicted in

Figure 1.1. These three combine to form part of a vehicle‟s suspension system, forming a

complex union with the U-bolt.

Figure 1.1: Image showing the U-bolt together with the axis, plate and leaf springs.

(Ventura,2009.)

The U-bolt is responsible for holding down the differential (which houses the axis to which

your wheels are bolted. The tyres are attached to the wheels). The U-bolt also secures the leaf

spring and restricts it from rocking and unwanted spring flex. The suspension system of the

vehicle exists in order to absorb shocks that are encountered when the vehicle is in motion.

The U-bolt, which forms part of the suspension system, supports all the tensile stresses

2

generated by the suspension during operation. They work under action and reaction forces,

always in the opposite direction than that of the leaf springs.

U-bolts come in different shapes and sizes depending on the suspension system design and

the shape of the mating parts of a vehicle. The U-bolt used in a long haul lumber truck would

be different than that of a van. They are mainly categorized based on three basic types of

bends; round bend, semi-round bend and square bend. Figure 1.2 shows the basic bend types

of U-bolts and a custom forged top variation.

Figure 1.2: Types of U-bolts (U-bolt types, n.d)

In Ghana, U-bolts are produced from steel rods by blacksmiths. There are large markets for

locally made U-bolts in places such as Abossey Okai- Greater Accra, Kokompe- Takoradi

and Suame Magazine-Kumasi.

In Kumasi, most of these metal working activities started as traditional family businesses

which involved blacksmithing, goldsmithing and making of brass artifacts. The need for

cheap and readily available spare parts, brought about the production of some products used

to service and repair vehicles and machines using these processes. This is particularly

common in Suame Magazine in Kumasi, where there is a huge market for these products.

3

1.2 PROBLEM STATEMENT

Quality of these locally produced U-Bolts cannot be defined by these craftsmen as there is no

proper quality assessment in their production process. Due to the lack of formal training of

these craftsmen, they employ no mechanized process, quality control, scientific input and no

code of conduct which might culminate into inconsistent product quality and varying product

design. The main focus of these artisans is the shape, the factors that go into the production of

the U-Bolt such as material type and composition, microstructure, forging temperature and

amount of force applied are not relevant to these local artisans.

1.3 JUSTIFICATION

The U-bolt plays a critical role in the leaf spring suspension assembly, and as such required

to perform its functions properly as designed by the engineers. The U-bolt requires certain

properties in order to execute its functions as designed, and these properties can be achieved

through its production processes hence compromises cannot be made during its production.

A lot of drivers and vehicle owners depend on the local craftsmen for their U-bolts because

they are cheap compared to those imported from other countries. The blacksmiths therefore

have an advantage and dominate the local market with locally manufactured U-Bolts.

However, as to whether the properties of these locally manufactured U-bolts meet the

required standards is yet to be known.

1.4 AIM AND OBJECTIVES

This project aims at appraising U-bolts produced locally in Ghana.

The specific objectives include:

Determine the chemical composition of the locally manufactured U-bolt

Ascertain the hardness of the locally produced U-bolt by performing a hardness test

4

Determine the phases present in the locally produced U-bolt using microstructural

analysis

Determine the tensile strength of the locally produced U-bolt.

Perform all the tests mentioned above, on a foreign U-bolt and compare the results.

Develop an improved process route for U-bolt production that involves further

processing to achieve satisfactory U-bolt product.

The objectives mentioned above are necessary because the chemical composition, hardness,

microstructure and strength are material properties that are indicative of the U-bolt‟s quality

and in-service durability.

1.5 SCOPE OF WORK

This project does not intend to produce a U-Bolt. The extent of the work that will be done

will cover identification of local and foreign U-Bolt, their quality assessment using laboratory

tests and the development of a new production process to improve upon the properties of the

U-bolts.

Samples will be selected from the manufactured products and run through a series of tests in

the laboratory. Comparisons of the mechanical properties of the local U-bolt and the foreign

U-bolt shall be made and the existing local production process shall be reviewed. The

materials laboratory and any other laboratory with the requisite equipment will be used for

carrying out these tests. The results from these tests will be analyzed to identify process

inefficiencies. A new production process will be suggested to improve the properties of the

locally produced U-bolts.

5

CHAPTER TWO

2.0 LITERATURE REVIEW

2.1 INTRODUCTION TO U-BOLTS

A U-bolt is a bolt in the shape of the letter U with screw threads on both ends. They are

versatile fasteners used to secure pipes, conduits, cables and machinery, or as an anchor in

foundations and roofs. They are single pieces cold or hot formed fasteners manufactured from

wire and round stock. U-bolts are used mainly in the automotive industry for the suspension

assembly of trucks, Sport Utility Vehicles (SUV), load carrying vehicles and other heavy

vehicles. Suspension systems are created with springs that absorb part of the shock when a

truck hits a bump, allowing the tires and axle to move independently and softening the impact

to the rest of the truck. These type of suspensions use a spring which is composed of thin

strips of metal called leaves arranged on top of each other and bolted together in the center

creating a reinforced bow-like item called the leaf spring.

The primary purpose of the U-bolt in the suspension assembly is to provide the force required

to clamp the leaf spring and related components firmly together [Kong et al. (2014)]. The U-

bolts are of three basic types with a forged top variation, depending on the suspension design

and the shape of the mating parts. They are characterized by the shape of their bends as

depicted in Figure 1.2

2.2 FUNCTIONS OF THE U-BOLT

The U-bolt provides the clamping force required to clamp the leaf spring, top plate, axle

seat, axle and bottom plate firmly together as depicted by Figure 2.1(a) and are critical to

the overall stability of the spring operation. They work to perform the following

functions;

6

1. Prevents flexing of the leaves of the spring which could cause dangerous stresses. The

center hole in each leaf acts as a stress concentration point which will lead to rapid

leaf breakage if spring flexing is not totally eliminated.

2. Prevents the shearing of the center bolt by reducing the horizontal forces acting on it.

This is achieved by the firm clamping of the spring to the axle seat.

3. Proper clamping of the spring by the U-bolts provides the desired spring stiffness and

contributes to maintaining the vehicle‟s ride height and handling characteristics as

originally specified for the vehicle.

(a) (b)

Figure 2.1: Outlines showing the functions of U-bolt

(a) Diagram showing cross-section of a suspension system (Axle, Suspension, Leaf Spring U-bolts -

Stengel Bros. Inc., 2016)

(b) U-bolt locking leaf springs into place (Byron Lamb, 2015)

7

2.3 U-BOLT MATERIAL AND MECHANICAL REQUIREMENT

The material commonly used for the manufacturing of U-bolts are steels. Steels are alloys of

iron and carbon with a carbon weight percent between 0.02% and 2.11%, it often contains

other alloying elements such as manganese, chromium, nickel and/or molybdenum but it is

the carbon content that turns iron to steel (Groover, 2010). The type of steel used depends

generally on the load bearing capacity of the unit. Irrespective of the load capacity, the torque

requirement during assembly may demand a higher grade material.

The Society of Automotive Engineers standard SAE-J429 have the recommended grades of

material for U-bolt manufacture. SAE-J429 covers the mechanical and material requirements

for inch series fasteners used in the automotive industries in sizes up to 1

inches. Table 2.1,

shows the chemical compositions and treatment according to the SAE-J429 standard.

8

Table 2.1 Chemical composition and Treatment (SAE-J429, 2011)

SAE-

J429

Grade

Material Carbon % Phosphorus %

max

Sulfur %

Max

treatment

2 Low or medium

carbon steel

0.15 – 0.55 0.025 0.025 No

treatment

4 Medium carbon

steel

0.28 – 0.55 0.025 0.13 Cold drawn

5 Medium carbon

steel

0.28 – 0.55 0.025 0.025 Quenched &

tempered

Carbon steel

with additives

(e.g. Bo or Cr or

Mn)

0.15 – 0.40 0.025 0.025 Quench &

tempered

8 Medium carbon

steel

0.25 – 0.55 0.025 0.025 Quenched &

tempered

Alloy steel with

additives (e.g.

Bo or Cr or Mn)

0.20 – 0.55 0.025 0.025 max Quenched &

tempered

Due to the variations in chemical composition and treatment methods, the yield strength and

tensile strength varies for the grades. Table 2.2 indicates the grades of U-bolts and their

mechanical requirements;

9

Table 2.2 Mechanical requirements (SAE-J429, 2011)

SAE-J429

Grades

Nominal size,

inches

Proof load,

Psi

Tensile strength,

Psi

Yield

strength,

Psi

2 ¼ through ¾ 55,000 74,000 57,000

Over ¾ through

1-1/2

33,000 60,000 36,000

4 ¼ through 1 –

1/2

65,000 115,000 100,000

5 ¼ through 1 85,000 120,000 92,000

Over 1 through

1-1/2

74,000 105,000 81,000

8 ¼ through 1- 1/2 120,000 150,000 130,000

2.4 HEAT TREATMENT

Heat treatment involves various heating and cooling procedures performed to effect

microstructural changes in a material which in turn affects it mechanical properties (Groover,

2010). It uses phase transformation during heating and cooling to change microstructure in a

solid state. It can be done at various times during the production of a part; can be done before

shaping to soften the material for easy working, can be done during metal working (forming)

so the material can be subjected to further deformation, and it can be done at the end of the

production process to achieve the final strength and hardness required of the product. Basic

heat treatment processes include; hardening, annealing, normalizing and tempering. These

processes result in different properties in a material but are all carried out in three basic steps;

heating, soaking and cooling (Groover, 2010).

10

Heating-This is the first of the three basic steps in heat treatment. It involves raising the

temperature of the material to a suitable one, where the material forms a solid solution (the

various elements in the material fuse and cannot be identified individually).

Soaking - This step involves holding the material at the suitable temperature until the entire

part is heated throughout. The more mass the part has the longer it must be soaked.

Cooling - In this step the temperature of the part is reduced. The rate at which the

temperature is reduced determines the microstructure of the end product.

2.5.1 Hardening

This treatment process is done by heating the part to the suitable temperature (usually 30˚C -

50˚C above the upper critical temperature of the type of steel), held for some time at that

temperature depending on the thickness of the part and rapidly cooled in a quenching medium

such as oil, water or brine. This process increases the hardness and strength of metals but also

increases brittleness. A form of the hardening treatment is the case hardening, which aims at

producing parts which require a wear-resistant surface and a tough core. The most common

of the case hardening treatment process are Carburizing and Nitriding (Groover, 2010).

2.5.2 Annealing

This is done by heating the metal to a suitable temperature, soaking for a period of time

relative to the metal thickness and cooling to room temperature. The rate of cooling in this

process is quite slow hence require cooling in the furnace by switching of the furnace and

allowing the part and the furnace to cool together. Metals are annealed to relieve internal

stresses, soften them, refine grain structure and increase ductility (Callister, 2007).

2.5.3 Normalizing

Normalizing is achieved by heating the metal to the suitable temperature and soaking it for a

period relative to its thickness and then cooling in still air. Ferrous metals are normalized to

11

relieve internal stresses produced by machining, forging or welding. Normalized steels are

much tougher than other steels in any other condition (Callister, 2007).

2.5.4 Tempering

Tempering involves the reheating of a hardened metal to a suitable temperature and allowing

it to cool. This is done to relieve internal stresses and reduce brittleness of metal parts as the

rapid rate of cooling during the hardening process set up severe internal stresses (Callister,

2007).

2.6 METAL FORMING PROCESSES

Metal forming processes are processes of shaping metal parts and objects through mechanical

deformation. Forming operates on the material science principle of plastic deformation,

where the physical shape of a material is permanently deformed. Deformation processes are

achieved mechanically by the use of tools such as dies, punches, rolls etc. depending on the

type of process and can be carried out in either a cold, warm or hot state. The amount of

deformation imposed on a work piece depends on the amount of force applied. Most of the

forces applied in the processes are mainly compressive forces and for a successful

deformation, the material to be deformed must possess high ductility and a low yield strength.

Materials can be altered to suit required deformation properties by heating to a required

temperature. Examples of forming processes include rolling, extrusion, forging, drawing etc.

(Groover, 2010).

2.6.1 Rolling

A forming process in which the thickness of a metal piece (slab or plate) is reduced as it is

forced through two rotating rolls exerting compressive forces. Rolling is classified according

to the temperature of the metal being rolled. Rolling is termed “Hot rolling” if the

temperature of the metal being rolled is above its recrystallization temperature and “cold

12

rolling” if it is below the recrystallization temperature. Each type of rolling has its effects on

the grain structure (Groover, 2010).

2.6.2 Extrusion

A forming process in which a metal piece is made to take the shape of a die opening as its

new cross sectional area by forcing it through the die opening. The product form will have a

uniform cross sectional area over the entire length produced. Extrusion can be carried out

cold, warm or hot; each type having it unique effect on the grain structure which will in turn

affect the mechanical properties of the material (Groover, 2010).

2.6.3 Forging

A forming process that involves the use of compressive forces, either by impact or gradual

pressure to cause deformation or change in geometry of the work piece into desired shape.

Forging operations are achieved using two basic equipment namely forging hammer and or

the forging press. Most forging operations are carried out hot or warm due to the large

deformation required by the process and the need to reduce yield strength and increase

ductility of the work piece. This process is classified into three main categories, these are;

open-die forging, impression-die forging and flashless forging.

1. Open-die forging – the work is compressed between two dies, thus allowing the metal to

flow without constraint in a lateral direction relative to the die surface.

2. Impression-die forging – the die surfaces contain a shape or impression that is imparted

to the work during compression, thus constraining metal flow to a significant degree.

3. Flashless forging – the work is completely constrained within the die and no excess flash

is produced.

13

2.6.4 Drawing

A forming process whereby the cross sectional area of a round material (billet) is reduced, by

pulling it through a die opening of a relatively smaller cross section. This operation is usually

carried out cold. It defers from extrusion based on how the forces are applied and also the

amount of deformation (Groover, 2010).

2.7 TESTING OF U-BOLTS

Testing of U-bolts or metals in general, is a process or procedure used to check the properties

and integrity of the metal product. These include destructive and Non-destructive tests.

Destructive tests are those which end up destroying the material or product being tested. It is

often used to determine the mechanical properties such as strength, toughness and hardness of

a material. Much of mechanical design and verification calculations are based on the

parameters (results) obtained from such tests. Destructive tests include tests as; tensile test,

hardness test, metallography etc.

In determining the strength of a U-bolt, a standard tensile test is reliable since a huge

proportion of U-bolts are subject to tensile service load. The general hardness of a U-bolt is

usually not a consideration in the service life as there is often no wear involved. Yet a

hardness test may be required for the threads depending on the process through which they

were made, as additional hardness of the threads provides the necessary torque values during

assembly. Tensile strength is the primary determining factor of grade, material and diameter

selection of a U-bolt. Other additional tests may be conducted on the U-bolt to gain more

information.

2.7.1 Tensile Test

This is a physical experimental evaluation performed on materials to determine their

suitability for specific engineering or construction applications to ensure quality. It involves

14

applying a force on opposite ends of the specimen and pulling outwardly until the metal

breaks to determine the strain, stress, yield deformation and other properties unique to the

specimen. The application of the force is achieved using a tensile test machine. (Kong et al,

2014)

2.7.2 Hardness Test

Hardness test are mechanical tests which are carried out on a material to determine its

resistance to localized plastic deformation (Zamanzadeh et al, 2014). Hardness is determined

by measuring the permanent depth of indentation on a material caused by a fixed load. This

implies, the smaller the indentation, the harder the material. Examples of hardness tests

include the Rockwell and Brinell hardness test.

2.7.3 Metallography

Metallography is the study of the composition and microstructure of metals and alloys using

various techniques. It is used to reveal the microstructure of metals, which is affected by

alloy composition and processing conditions; including cold working, heat treatment and

welding. This is achieved by using equipment such as optical microscopes, scanning electron

microscopes (SEM), Transmission electron microscopes (TEM) etc. which helps in

magnifying the microstructure. Analysis of a material‟s microstructure aids in determining if

the material has been processed correctly and is therefore a critical step for determining

product reliability and/ or for determining why a material failed. In preparing a specimen for

metallography to determine its microstructure, a rigid step by step process must be followed;

in sequence, the steps include sectioning, mounting, coarse grinding, fine grinding, polishing,

etching and microscopic examination. The preparation procedure must be carefully followed

in order to reveal accurate microstructures.

15

2.7.4 Chemical Analysis

These are tests conducted on metal samples in order to determine the elemental composition

and their amounts in the sample. There are various techniques for chemical analysis tests,

some of these techniques are; Spark – Optical Emission Spectrometer(spark-OES),

Inductively Coupled Plasma – Optical Emission Spectrometer(ICP-OES), X-ray

fluorescence(XRF), ICP Atomic Emission Spectroscopy, Mass spectroscopy, etc.

2.8 U-BOLT PRODUCTION PROCESS

Raw Material

U-bolts are manufactured using alloy steel rods and bars as the raw material. These bars are

initially acquired by processing steel billets. A billet is a long steel bar of a square cross

section with dimension 40mm on a side or larger. (Groover, 2010)

The original material that is processed to produce a steel rod is Iron ore. The principal

ore used in the production of iron and steel is Hematite (Fe2O3). Other iron ores include

Magnetite (Fe3O4), Siderite (FeCO3) and Limonite (Fe2O3-1.5H2O). Iron ores are made up of

50% to around 70% iron, depending on grade (hematite is almost 70%). In addition, scrap

iron and steel are widely used today as raw materials in iron and steelmaking.

Other additives used in the steel production process include coke and limestone. Coke is a

high carbon fuel produced by heating bituminous coal in a limited oxygen atmosphere for

several hours, followed by water spraying in special quenching towers. Limestone

is a rock containing high proportions of calcium carbonate (CaCO3). They help in the

reduction of iron from the ores.

U-Bolt Processing

U-bolts are manufactured in two ways; either by cold forming or by hot forming, each

possessing unique qualities as a result of their production processes. These production

processes are illustrated by Figures 2.2 and 2.4.

16

Figure 2.2: Flow diagram illustrating hot forming process for U-bolt production

In the hot forming U-bolt production process as illustrated in Figure 2.2, steel rods of

required diameter are first cut up into appropriate lengths using a hydraulic cutting machine

and the tips chamfered using a lathe machine. This procedure is followed by the rolling of

threads on the ends of the bar using a Hydraulic Thread rolling machine; where a cylindrical

part is rolled between two dies in the machine. This operation is commonly done by cold

working. The center portion of the rod is marked to serve as a visual indicator of the

longitudinal center of the rod. The rods are then heated at the centers after having them

marked. The steel rod is heated to increase ductility for significant deformation. A bending

machine then bends the rod into a „U‟ as depicted by Figure 2.3, after which it is quenched

and tempered (temperature of about 425˚C).

Cutting of bar Chamfering

the tip Making of

thread

Heating Bending of

bolt Quench

Temper Coating

17

Figure 2.3: Hot bending of U-bolt (Bending-Portland, n.d)

Figure 2.4: Flow diagram representing cold forming process for U-bolt production

The U-bolts that are made by cold bending follow a process route illustrated in Figure 2.4,

they do not undergo heating, quenching and tempering as that made from hot bending

process. After rolling of the threads the steel rod is inserted into a Hydraulic rod bending

machine as shown in Figure 2.5, which has a mandrel designating a mark that shows the

center plane of the rod on the machine. A gripping die holds the rod in place as the machine

bends the rod by applying a load through a punch that comes into contact with the rod at the

middle. The rod is forced into the die as it forms a U-bend. The type of U-bolt formed is

dependent on the shape of the punch and the die.

Cutting of bar

Chamfering the tip

Making of thread

Cold bending

Coating

18

Figure 2.5: cold bending of U-bolt (the importance of the U-bolt…, n.d)

Finally after bending, the U-bolt is coated by Hot-Dip Galvanizing. This is the metallurgical

bonding of Zinc unto steel to prevent corrosion. The part is first dipped into caustic soda and

rinsed in sulphuric acid to etch the steel for maximum zinc penetration into the surface. The

bolts are dipped into molten zinc at 449o C depending on the size for three to five minutes.

The bolts are removed from the zinc and rapidly spun around to remove the excess zinc.

Subsequently the bolts are cooled in water tanks and packaged.

2.9 U-bolt Failure

No matter how perfect a product is after manufacture, it will always come to a point where

the part will no longer be able to perform its function. For certain components, this point is

not reached and it fails. The product does not achieve its expected lifespan. Several factors

account for this phenomenon.

With regards to the U-bolt, corrosion and fatigue are two factors that accelerate failure.

19

Corrosion

Corrosion is defined as the destructive and unintentional attack of a metal; it is

electrochemical and ordinarily begins at the surface (Callister, 2007). Corrosion is as a result

of the interaction of the U-bolt‟s surface with the environment. The U-bolt has a high iron

composition, which is chemically unstable in the environment. Iron naturally exists in nature

in combination with other elements. Whenever refined iron comes into contact with the

external environment, there is a tendency for it to return to its original ore state where it is

thermodynamically stable. Corrosion compromises the U-bolt as the dimensions, composition

and structural integrity is compromised.

Fatigue

Fatigue is a form of failure that occurs in structures subjected to dynamic and fluctuation

stresses (Callister, 2007). Fatigue on the other hand causes the U-bolt to fail mechanically.

During in-service operation, the part is subjected to loads of a cyclic nature. This alternating

load nature causes defects that exist within the microstructure to propagate. Subsequently

fracture occurs at the point of maximum stress concentration.

20

CHAPTER THREE

3.0 METHODOLOGY

3.1 Visit to Suame

Suame Magazine was visited with the aim to identify shops which specialized in the

production of U-bolts and also study the processes through which the U-bolts are made

locally. Two shops were selected to be studied. Observation and recording of the production

processes were done at the selected blacksmith shops. Figures 3.1 and 3.2 illustrates the

production process of the U-bolts at Shop A and Shop B respectively.

Figure 3.1: Flow diagram representing production process at Shop A

Figure 3.2: Flow diagram representing production process at Shop B

measuring and cutting of steel

rods

machining of steel rods at

machine shop

heating and bending of

machined rods

heating of threaded U-bolt

section

finishing

(painting the U-bolt)

measuring and cutting of steel rods

machinining of steel rods at machine shop

heating and bending of machined rods

finishing

(painting of U-bolt)

21

Process Description

Measuring and Cutting: at this stage; rods of preferred diameters are selected, measured

using a tape measure as shown in Figure 3.3 and cut using a hacksaw to lengths based on the

size of U-bolt being produced. These cut out rods are then sent to the machine shop for

shaping.

Figure 3.3: Measuring and cutting of rod to required length

Machine Shop: using a center lathe machine, the fins on the rods are machined off to obtain

a smooth cylindrical bar and required diameter followed by chamfering of the tips of the rods

and then the cutting of threads as shown in Figure 3.4. The threads are cut to different lengths

depending on the size of the U-bolt being produced.

22

Figure 3.4: Cutting of threads using a center lathe machine

Heating and Bending: the center of the rods are marked and then placed in a hearth. The

middle part of the rods are heated to red hot (temperature in the furnace of about 800-880˚C),

taken out and placed in three metal studs arranged on a flat plate in the form of a triangle. The

rod is inserted within the studs with the marked part placed just by the first stud and bent

against it using manual force as shown in Figure 3.5. The bent rod is then placed on an anvil

and forged by hammering to complete the bend to the required shape of a “U”. The bent rods

are left in an open area to cool.

23

Figure 3.5: Bending of heated steel rod

Heating of threads: the threads of the U-bolts are heated in the hearth as shown in Figure

3.6, followed by quenching in water. They are heated to red hot and quenched in water a few

minutes after being brought out of the hearth.

Figure 3.6: Heating of U-bolt threads

Finishing: this is the final stage of the U-bolt production in Suame. The U-bolt is coated with

red paint as shown in Figure 3.7 and the threaded part optionally dipped in dirty engine oil.

24

Figure 3.7: Painting of finished U-bolt

3.2 Hardness Test

The surface of the U-bolts were prepared by using the grinding machine to remove the

finishing paint and even the surface of the U-bolts. Markings were made on one side of the

U-bolt along the surface; from the top to the leg of the U-bolt as depicted by Figure 3.8 at an

interval of 2 inches, with the exception of the bend (between points 2 and 3) where the

interval was reduced to 1 inch. A portable Brinell hardness tester was used to measure the

hardness at each marked point. The hardness of each point was measured three times and an

average was taken to represent the hardness of that point.

25

Figure 3.8 Image representing hardness test points along the U-bolt

Intervals between the points are same (2 inches apart). The ‘point 1’ represents the center of the rod that

was bent to form the U-bolt

3.3 Chemical Analysis

The samples for the chemical analysis were taken from the three U-bolts at point 1 in Figure

3.9 and a section of the unprocessed rods from which the U-bolts are made. The surfaces of

the samples were prepared by grinding using a grinding machine and subsequent polishing

using a polishing wheel to obtain an even and smooth surface for chemical testing with an

Atomic Emission Spectrometer in the chemical laboratory of Tema Steel Company Limited.

3.4 Metallography

The samples were taken from points 2 and 4 in figure 3.8 for all the U-bolts with additional

samples from the unprocessed rods from which the U-bolts are made. The surfaces of the

samples were ground, polished and etched before being analyzed under the microscope. Each

sample was ground on six different grades of abrasive paper, beginning with the coarsest

which is the P100 grade through P280, P400, P600, P800 to the finest P1000 grade to remove

all forms of scratches to obtain a smooth surface. The samples after going through the

grinding stage were polished, using the polishing wheel with a polishing cloth. The polishing

was done using an aluminum oxide suspension as abrasive. After obtaining a mirror like

26

surface, the samples were etched. The etching was done by dipping the prepared surfaces in

Nital for about five seconds, followed by rinsing in water and then alcohol. The sample

surfaces were examined under an optical microscope and images of the microstructure were

taken.

27

CHAPTER FOUR

4.0 RESULTS AND DISCUSSION

Samples A and B are samples from the products selected from shops A and B respectively.

Samples F are samples taken from the foreign U-bolts.

4.1 Hardness Test Results

The hardness values taken along the U-bolt points in Figure 3.8 are presented in Table 4.1.

The hardness points are at an interval of 2 inches with the exception of the bend (between

Points 2 and 3 in figure 3.9) which were reduced to 1 inch.

Table. 4.1 Results from Hardness Test along the U-bolt

Points Along U-bolt, Pt Sample A, HB Sample B, HB Sample F, HB

POINT 1 131 127 152

POINT 2 136 158 152

POINT 3 89 158 141

POINT 4 134 123 147

POINT 5 133 160 123

POINT 6 227 229 162

POINT 7 222 233 187

POINT 8 193 176 218

28

Figure 4.1: Graph of hardness (HB) values of Points 1 to 8 for Samples A, B and F

The unevenness of the Graph in Figure 4.1 is as a result of the non-uniform hardness of the

U-bolt. Hardness of U-bolt samples A and B although not equal shows a relatively unsteady

change with steep slopes due to large differences in hardness along the U-bolt.

From Figure 4.1, the first 4 points (i.e. Points 1, 2, 3 and 4) represent a flattened portion of

the U-bolt along the bend; Sample F presents a fairly crooked line with a gentle decreasing

gradient because of less deviation of the points from each other which is as a result of the

mechanized forging process employed to flatten that side of the U-bolt. The microstructure of

Sample F taken at Points 2 and 4 (see figure 3.8) shows an elongated grain in Figures 4.5 (a)

and (b) which resulted from mechanized forging process.

Sample B on the other hand at the same points (i.e. Points 1, 2, 3 and 4) presents a line with

an increase in gradient from point 1 to point 2, a straight line from point 2 to point 3 and then

a decrease in gradient from point 3 to 4. Sample A also presents a line with a small change in

gradient from point 1 to point 2 with a steep drop from point 2 to point 3 and another steep

rise from point 3 to point 4. These changes in slopes of the lines between each two points is

as a result of large differences in hardness within the region which is attributed to the

inconsistent force applied during the manual forging process of using a hammer and an anvil.

0

50

100

150

200

250

0 1 2 3 4 5 6 7 8 9

Sample A

Sample B

Sample F

29

From the microstructures of Samples A and B which are represented by Figures 4.3 and 4.4

respectively, there are no signs of large deformations of the grains as compared to that of

Sample F in Figure 4.5., implying the forces applied in forging was not large enough.

From points 5 to 6, there is an increase in hardness across the three samples. This is because

this section lies within the heat affected zone which was not forged, but because of the

temperature gradient there was an alteration in grain size from the large grains in Figures 4.3

and 4.4 to the small grain size of the parent material in Figure 4.6. Materials with relatively

smaller grain sizes are harder than those with large grain sizes. Points 6 and 7 represent the

hardness of the steel rods out of which the U-bolts were made.

Since hardness was not consistent from point 1-8 for any of the U-bolts, an average hardness

was calculated for each sample and the value used to calculate the tensile strength.

Average hardness for Sample A =

= 158.125

HB

Average hardness for Sample B =

= 170.5

HB

Average hardness for Sample F =

= 160.25

HB

Tensile strength can be calculated using the formula;

TENSILE STRENGTH (MPa) = Brinell hardness (HB) × 3.45 [Callister, D. (2007)].

Sample A tensile strength = 158.125 × 3.45 = 545.53 MPa

30

Sample B tensile strength = 170.5 × 3.45 = 588.225 MPa

Sample F tensile strength = 160.25 × 3.45 = 552.86 MPa

The tensile values calculated presents Sample B as the U-bolt with highest tensile strength

followed by Sample F and A in that order

4.2 Chemical Test results

From Table 4.2, samples A and B have been identified as Plain low carbon steels and sample

F as medium carbon steel. Low carbon steel have less than 0.3% C, medium carbon steels are

within 0.30% - 0.60%C and high carbon steel above 0.60%C. A look at the chemical

composition of Sample A (U-bolt product) and Sample Rod A (steel rod) suggests a slight

increase in Carbon content from 0.154 %C to 0.173 %C which is as a result of the heating for

about 5minutes in a local hearth fueled by palm kernel and charcoal. Considering the carbon

content of each sample from Table 4.2, sample F has the highest with 0.316% C, followed by

sample B with 0.226% C and sample A being the least with 0.173% C. Samples B and F are

harder than A because of their relatively high carbon content. Hardness and strength in steels

increases with increasing carbon content.

Apart from carbon, other elements in the composition has no major effect on the properties of

the U-bolts, based on their amounts. Copper, Nickel, Chromium, Molybdenum and Tin alter

the properties of the U-bolts positively on a minute scale, when existent in larger proportions.

Manganese, Sulphur, Silicon and Boron are also beneficial to the U-bolt, if their individual

amounts are sufficient, they would improve hardenability, machinability and strength

respectively. Increase in Phosphorus content will negatively affect the U-bolts, by causing

embrittlement and reduce toughness.

31

Table 3.2 Chemical Compositions of U-Bolt and steel rod samples

Element

content

Sample A

(%)

Sample B

(%)

Sample F

(%)

Sample Rod

A (%)

Sample Rod

B (%)

Iron 98.633 98.683 98.638 98.632 98.725

Carbon 0.173 0.226 0.316 0.154 0.185

Manganese 0.983 0.519 0.370 0.997 0.355

Phosphorous 0.024 0.003 0.066 0.024 0.044

Sulfur 0.020 0.012 0.032 0.020 0.010

Silicon 0.043 0.133 0.053 0.045 0.221

Copper 0.004 0.130 0.129 0.004 0.001

Nickel 0.027 0.074 0.083 0.027 0.024

Chromium 0.025 0.115 0.182 0.025 0.365

Molybdenum 0.064 0.071 0.069 0.063 0.063

Titanium 0.002 0.002 0.002 0.002 0.002

Aluminium 0.010 0.010 0.032 0.010 0.010

Cobalt 0.000 0.006 0.004 0.001 0.001

Tin 0.002 0.010 0.007 0.002 0.002

Boron 0.001 0.001 0.002 0.002 0.001

4.3 Metallography Results

During the production process of the local U-bolts, the steel rods are bent to the shape of the

“U” within a temperature range of 800˚C to 900˚C (Austenitic zone) after which they are left

to cool in an open space. The rate of cooling in an open space is relatively slow giving a

microstructure made up of ferrite and pearlite only as shown in Figures 4.3 and 4.4. The

32

grains of Sample F appear elongated unlike that of Samples A and B, which is as a result of

the mechanized forging process to flatten the U-bolt along the bend. The microstructure of

Samples Rod A and Rod B in Figures 4.6 (a) and (b) have very small grain sizes as compared

to the microstructures of the U-bolts in Figures 4.3 and 4.4.

(a) (b)

Figure 4.3: Microstructure image of Sample A (a) top (b) neck

Sample A shows a microstructure of Ferrite and Pearlite only. The percentage of ferrite is 79.15% and

that of pearlite is 20.85%.

(a) (b)

Figure 4.4: Microstructure Image of Sample B (a) top (b) neck

Pearlite

Pearlite

Ferrite

Ferrite

33

Sample B also shows a microstructure of Ferrite and Pearlite only. The percentage of ferrite is 72.77%

and that of pearlite is 27.23%.

(a) (b)

Figure 4.5: Microstructure Image of sample F (a) top (b) neck

Only ferrite and pearlite exist in the microstructure of Sample F. The ferrite percentage is 61.93%

whilst that of pearlite is 38.07%.

(a) (b)

Figure 4.6 Microstructure of steel rod samples (a) Sample Rod A and (b) Sample Rod B

Pearlite

Ferrite

Pearlite

Ferrite

34

The phases present in the individual microstructures of the various U-bolts are Ferrite and

Pearlite. Ferrite is a solid solution Carbon in BCC iron, containing a maximum carbon

weight percentage of 0.04% at 695˚C. Pearlite is a phase consisting of alternating layers of

ferrite and cementite (a hard brittle intermediate compound with the formula Fe3C having

6.67% carbon and 93.3% iron by weight).

The dark portions represent pearlite and the light portions represent ferrite. The percentages

of the phases present were calculated using the formula; ( )

and Pearlite % =( ).

35

CHAPTER FIVE

5.0 CONCLUSION AND RECOMMENDATION

5.1 CONCLUSION

From the analysis of the results obtained, it can be concluded that the difference in properties

between the foreign U-bolt and that produced locally is minimal and in specific instances, the

local U-bolt possess higher mechanical properties than that of the foreign product. The

problem with the locally made U-bolt is the inconsistency in its properties. These

inconsistencies stem basically from the production process and can be eliminated if they are

manufactured with constant production parameters such as forging force and temperature.

5.2 RECOMMENDATION

Some of the differences in the foreign U-bolt and the local U-bolt can be eliminated by

making changes in the production process of the locally produced U-bolt. The application of

a mechanized forging process will help reduce if not eliminate inconsistent product properties

at the flattened part.

A carburizing process will be sufficient to improve the mechanical properties of the U-bolt,

as it will increase the carbon content which will cause a case hardening effect. The case

hardening effect will improve the hardness of the U-bolt and will give a relatively soft core

which will increase the toughness. Carburizing at about 1000˚C for approximately 5 hours

will be sufficient to give a surface carbon content of about 0.3wt% C for a case thickness of

1mm with an inner carbon content of 0.2wt% C.

Given the nature of the local production process, the solid carburizing (pack carburizing)

process will be appropriate.

36

REFERENCES

1. Groover M. P., (2010). Fundamentals of Modern Manufacturing Materials,

Processes, and Systems, Fourth Edition. New York, John Wiley & Sons Inc. (p

99,pp 395 – 426,pp 656 – 666)

2. Yu, H., Güvenç, L., & Özgüner, Ü. (2017). Heavy duty vehicle rollover detection

and active roll control, 3114(December).

https://doi.org/10.1080/00423110701477529

3. Zamanzadeh, M., Larkin, E., Incorporated, C., Chaudhari, C. A., Waghulde, K. B.,

Cited, R., … Hewitt, R. A. A. (2014). ScienceDirect U-shaped Bolts Fracture

Failure Analysis.

4. Zamanzadeh, M., & Larkin, E. (2015). Fatigue Failure Analysis Case Studies.

Journal of Failure Analysis and Prevention, 15(6), 803–809.

https://doi.org/10.1007/s11668-015-0044-3

5. Callister, D. William, (2007). Material Science and Engineering. Seventh Edition.

Hoboken, NJ, John Wiley & Sons Inc. (pp 312 -346, pp 622 – 651, pp 227 - 237)

6. SAE-J429 (2011), Mechanical and material requirements for externally threaded

fasteners. Warrendale, PA: SAE.

7. Francis Walter & John Bailey, (1967). Fundamentals of Engineering Metallurgy.

First Edition. London. Cassell.

8. Karunakar, M. Shetty, (2006). Development of a Leaf Spring Load Transducer.

Vancouver. University of British Columbia.

9. J.M. Ventura, D.B.V. Castro, C.O.F.T. Ruckert, O. Maluf, W.W.B. Bose Filho, &

D. Spinelli (2009). JOURNAL OF MATERIALS ENGINEERING AND

PERFORMANCE, v.18, n.7, p.903-911.

https://doi.org/10.1080/00423110701477529

37

10. [Bending-Portland][online image](n.d). Retrieved May 31,2018 from

http://www.portlandbolt.com/about/manufacturing/bending/

11. [The Importance of the U-bolt - Questions Answered In This Video][online

image](n.d). Retrieved May 31, 2018 from https://ssa.com.au/solutions/the-

importance-of-the-ubolt.

12. Gordon Hanson, Richard Hildman & Steve Santee (1994). Guidelines for U-bolt

design. Clamps incorporated. Retrieved from

http://www.clampsinc.com/resources.html.

13. U-Bolt Application and Design | Engineers Edge | www.engineersedge.com (n.d).

Retrieved from

https://www.engineersedge.com/hardware/ubolt_application_and_design_9899.ht

m

14. [u-bolt-types][online image](n.d). Retrieved from

https://www.engineersedge.com/imagefiles/hardware/u-bolt-types.jpg

15. [Axle, Suspension, Leaf Spring U-bolts - Stengel Bros. Inc.](2016)[Online

image]. Retrieved from http://www.stengelbros.com/u-bolts/

16. Byron Lamb (2015). [Online image]. Retrieved from

http://photobucket.com/gallery/user/Byron_Lamb/media/cGF0aDovQXByaWwg

MjAxNSBCcm9uY28gU3RhbmNlIDAxMF96cHNxcW15MGJhcC5qcGc=/?ref=

http://www.portlandbolt.com/about/manufacturing/bending/https://ssa.com.au/solutions/the-importance-of-the-ubolthttps://ssa.com.au/solutions/the-importance-of-the-ubolthttp://www.clampsinc.com/resources.htmlhttp://www.engineersedge.com/https://www.engineersedge.com/hardware/ubolt_application_and_design_9899.htmhttps://www.engineersedge.com/hardware/ubolt_application_and_design_9899.htmhttps://www.engineersedge.com/imagefiles/hardware/u-bolt-types.jpghttp://www.stengelbros.com/u-bolts/