mats 402 materials lab i - faculty of engineering and...

MATS 402 Materials Lab (I) Lab Manual

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German University in Cairo Faculty of Engineering and Materials Science Materials Engineering Department

\Faculties\Engineering Design\MATS\Materials Lab I

Laboratory Manual

MATS 402

Materials Lab I

2013


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Content

I. Laboratory Policies for students .2

II. Laboratory Safety Rules .5

III. Laboratory weight ...7

IV. Laboratory report 8

V. Experimental

Group 1: Macro/Microstructure

1. (A) Metallography .9

2. (B) X-ray ...20

Group 2: Heat treatment

3. (E) CCT diagram ..52

4. (J) Phase diagram .60

5. (F) Jominy end quench test ...71

6. (G) Carburizing 78

Group 3: Mechanical test

7. (H) Hardness..80

8. (D) Tensile 89

9. (C) Impact..97

10. (I) Strain gauge.101


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I. Laboratory Policies for Students

Introduction

The information presented herein is intended for all students working within the

equipment-related laboratories of the Materials Engineering Department. The policies

outline the laboratory access, working, machines handling and safety practices to be

followed to ensure the health and safety of all students, as well as to avoid any machine

machines damage.

Responsibilities

The Material's engineering equipment-related laboratory policies are written to make you

aware of your surroundings so that you will be less likely to be injured as you work.

Remember that you are responsible for:

Your own health and safety.

The health and safety of those around you.

The security and the safe use of equipment and facilities that you have been authorized to use.

Understanding and complying with all laboratory policies.

General Laboratory Policies

In order to manage risks, it is necessary to limit access to equipment, laboratories, and

certain storage facilities. The following general policies apply to ALL equipment-related

laboratories within department. Policy pertaining to laboratories identified and posted as

"machinery laboratory" is also to be followed in addition to the general polices outlined

below.

Access, Equipment Use, Safety and Rules

A faculty or staff member must be present in the lab in order for you to operate any foundry equipments.

You are allowed to access to the laboratory during the time of your scheduled laboratory and not at any time during open lab hours

assigned for other groups.

If you miss to attend the laboratory with your scheduled group and you want to make up the experiment with another group which you are not

normally scheduled, you MUST have accepted reason for the absence

and you MUST get the approval from the lab Coordinator. However,

as per the GUC policy announced to the students If a student

attends other than his/ her scheduled classes, the attendance,

assignments and quizzes will not be counted.


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You are not permitted to enter the lab if you are 15 minutes late than your schedule time.

You should read your experiments before coming to the lab, you will be pre-tested and graded before starting the experiment (10% from the

total lab weight)

Your behaviour during the lab time (the work with Materials, Machine and Computer) will be recognized and will be graded (20% from the

total lab weight). Keep the work area clean and tidy. When you have

finished for the day, make sure all tools, equipment, and supplies are

returned to their proper storage (including electronic components back

to drawers), and the equipment is shut down.

You should not attempt to operate equipment or apparatus unless you are specifically authorized to use that equipment, or you must ask one

of the lab supervisors. The cost of any damage will be directly

charged to you.

Do not attempt to modify or repair any equipment or apparatus unless you inform one of the lab supervisors.

You should locate posted information regarding emergency contact information and identify the location of fire extinguishers and eye

washes (if appropriate) within the laboratory.

You should review and understand all additional posted access, safety warnings, and safety policies for the laboratory.

All injuries that occur in the laboratory must be reported immediately to Police and Safety Services and one of the lab supervisors.

If you create a hazard you must control it. It is important to notify and involve a faculty member or technician where the hazard is located.

Consumption of food and drink is prohibited in those laboratories where such restrictions are posted.

Suitable clothing and footwear as determined by the Materials Eng. Department must be

worn in the laboratory. Additionally, please read carefully the following laboratory

safety Rules


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II. Laboratory Safety rules

IInn ccaassee ooff aa ffiirree::

Notify your lab supervisor. If the fire cannot be contained, sound the alarm. Exit

using the procedure below (when the fire alarm sounds).

IIff tthhee ffiirree aallaarrmm ssoouunnddss::

Exit out the main door of the laboratory to the end of the hall. Move a safe distance

away from the building.

IInn ccaassee ooff aa ssppiillll::

Notify one of the supervisors immediately.

If skin or eyes are affected, move immediately to the eye-wash station and flush with

water.

IInn ccaassee ooff bbuurrnn::


Move immediately to the sink and flush the affected area with water.

IInn ccaassee ooff aann iinnjjuurryy::


IInn ccaassee ooff mmeecchhaanniiccaall mmaallffuunnccttiioonn::


EEmmeerrggeennccyy nnuummbbeerrss::

Fire:

Ambulance:

Public Safety:

Health Protection Office:


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SSaaffeettyy RRuulleess::

You must wear impact resistant safety glasses with permanent plastic fixtures on the

side. You also must have shoes that are non-porous in nature. During laboratory

periods when experiments are being conducted, you must also wear

pants/jeans/slacks. Depending on other experiments, further precautions may be

needed. If this is the case, your teaching assistant will give you guidance.

ABSOLUTELY NO EATING, DRINKING, OR SMOKING IN THE MATERIALS

SCIENCE LAB. Please do not discard food or beverage containers in the lab waste

cans. They must be discarded before entering the lab.

Violation of the safety rules will result in you being asked to leave the lab. If you are

asked to leave, you will take a zero for the lab report.

Feedback is always welcome for suggestions for improvement of the lab and/or

about safety concerns.

MMaatteerriiaallss EEnnggiinneeeerriinngg DDeeppaarrttmmeenntt

Prof. Dr. rer. nat. Ahmed Abd El-Aziz

Aziz


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III. Laboratory weight

Quiz (theoretical) 20%

Class work (Practical) 40%

Report (theoretical) 40%

Total 100%

Practical (experiment and report)

Received at: ___________________________

Report: - 5 points

ID : ________ ________ ________

Pre-test : _____/ 20% _____/ _____/

Behaviour (Class work,

Preparation, Measurements,

Cleaning) : _____/ 40% _____/ _____/

Report (Introduction,

Procedure, Results, Discussion,

Figures, Conclusion) : _____/ 40% _____/ _____/

Total : / 100% / /

____________ Signature

Very important: Please submit your report within one week from the experiment time.


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IV. Laboratory report

What should be in the report

1. Write precisely your report, introduction, experimental procedure, results and

discussion. Do not include more than very brief necessary details of the

experimental procedure.

3. As a result, explain and analyze the diagrams of the figures what you will draw

or get from the experiment.

4. As discussion describe and discuss your obtained results

5. Write some sentences as a summery about your results.


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V. Experimental

{Group 1}: Macro/Microstructure

Exp. (A): Metallography 1. Introduction

Metallography can be defined as the visual study of the constitution and structure of

materials. Metallographic examinations can be broadly classified into two types namely,

macroscopic examinations and microscopic examinations. Macroscopic examinations

refer to the observations carried out at a magnification of x10 of less. Microscopic

examinations, on the other hand, refer to the examination of the structure at a

magnification greater than x10. Microscopic examinations, depending on the nature of

information to be extracted, can be accomplished using an Optical Microscope (up to

x2000) or Scanning Electron Microscope (up to x 50000) or a Transmission Electron

Microscope (up to x500000). For most of the routine purposes in optical microscope is

used to obtain first hand information on the geometric arrangement of the grains and

phases in a material. In order to retain the information visualized using the microscope,

microstructural details are often recorded on a 35 mm film or a Polaroid film or by

digital camera.

The study of microstructaral details is important due to its correlation with the ensuing

mechanical properties of the material. As an example, if material A exhibits a more

homogeneous and refined microstructure than material B , it may very well be anticipated

that material A will exhibit better room temperature properties when compared to

material B.

In order to metallographically examine a specimen, it is essential to learn about the

various steps that are required to prepare it. The following section briefly describes the

various steps involved in the metallographic preparation of the samples.

Ferrite

Ferrite -perrlite


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Eutectoid

Hypo-Eutectoid

Fig. 1: Examples of grain structures of carbon steel and cast iron.

The basic operation outlining the metallographic preparation

of the specimens is as follows: 1. 2.1 Selection of the Size of the Specimen: The selection of the size of the specimen is dependent on the nature of material and the

information to be gathered. Normally, the linear dimensions may vary from 5 mm to 30

mm while the thickness is kept lower than the linear dimensions.

2.2 Mounting the Specimen:

The primary purpose of mounting specimens is for convenience in handling specimens

of difficult shapes or sizes during the subsequent steps of preparation and examination.

A secondary purpose is to protect and preserve extreme edges or surfaces defects during

preparation. Specimens also may require mounting to accommodate various types of

automatic devices used in laboratories or to facilitate placement on the microscope stage.

An added benefit of mounting is the ease with which a mounted specimen can be

identified by name, alloy number, or laboratory code number for storage by scribing the

surface of the mount without damage to the specimen.

Hyper-Eutectoid

Martensite

Grey iron

Nodular (ductile) iron


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Mount Size and Shape

As the size of the specimen increases, so does the difficulty of keeping the specimen

surface area flat during grinding and polishing. A saving in the time required for the

preparation of one large metallographic specimen may be realized by sectioning the

specimen into two or more smaller specimens. A specimen having an area of

approximately 1/4 sq in. is perhaps the most suitable; the maximum area should be

limited to about 4 sq in. if possible. Thickness of the mount should be sufficient to

enable the operator to hold the mount firmly during grinding and polishing and thereby

to pervent a rocking motion and to maintain a flat surface. Circular mounts are

commonly 1 to 2 in. in diameter and are the most easily handled. The length-to-width

ratio of rectangular mounts should be limited to approximately 2 to 1 to facilitate

handling (See Figure 1).

Figure (2) A mounted specimen (shows typical dimensions)

Mounting Methods

The method of mounting should in no way be injurious the microstructure of the

specimen. Mechanical deformation and the heat are the most likely sources of injurious

effects. The mounting medium and the specimen should be compatible with respect to

hardness and abrasion resistance. A great difference in hardness or abrasion resistance

between mounting media and specimen promotes differential polishing characteristics,

relief, and poor edge preservation. The mounting medium should be chemically resistant

to the polishing and etching solutions required for the development of the microstructure

of the specimen.

Mounting Methods:

Clamp Mounting

Compression Mounting

Cold Mounting

Conductive Mounting

Compression mounting

Compression mounting, the most common mounting method, involves molding around

the specimen by heat and pressure such molding materials as bakelite diallyl phthalate

resins, and acrylic resins. Bakelite and diallylic resins are thermosetting, and acrlyic

resins are thermoplastic. Both thermosetting and thermoplastic materials require heat and

pressure during the molding cycle, but after curing, mounts made of thermosetting

materials may be ejected from the mold at maximum temperature. Thermoplastic


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materials remain molten at the maximum molding temperature and must cool under

pressure before ejection.

Mounting presses equipped with molding tools and heater, are necessary for

compression mounting. Readily available molding tools for mounts having diameters of

25, 30, 40, 50 mm. It consists of a hollow cylinder of hardened steel, a base plug, and a

plunger (See Figure 3).

A specimen to be mounted is placed on the base plug, which is inserted in one end of the

cylinder. The cylinder is filled with appropriate amount of molding material in powder

form, and the plunger is inserted into open end of the cylinder. A cylindrical heater is

placed around the mold assembly, which has been positioned between the platens of the

mounting press. After the prescribed pressure has been exerted and maintained on the

plunger to compress the molding material until it and the mold assembly have been

heated to the proper temperature (see Table 1, the finished mount may be ejected from

the mould by forcing the plunger entirely through the mold cylinder.

Not all materials or specimens can be mounted in thermosetting or thermoplastic

mounting mediums. The heating cycle may cause changes in the microstructure, or the

pressure may cause delicate specimens to collapse or deform. The size of selected

specimen may be to large to be accepted by the available mold sizes. These difficulties

are usually overcome by cold mounting.

Fig(3) Struer LabosPress-1 machine


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Table (1) embedding order of Struers LaboPress-1 machine.

2.3 Grinding

Grinding is a most important operation in specimen preparation. During grinding the

operator has the opportunity of minimizing mechanical surface damage that must be

removed by subsequent polishing operations. Even if sectioning is done in a careless

manner, resulting is severe surface damage; the damage can be eliminated by prolonged

grinding. However, prolonged polishing will do little toward eliminating severe surface

damage introduced by grinding.

The grinding procedure involves several stages, using a finer paper (higher number) each

time. Each grinding stage removes the scratches from the previous coarser paper.

Between each grade the specimen is washed thoroughly with water to prevent

contamination from coarser grit present on the specimen surface.

Rough Grinding: Rough grinding is carried out on the emery belt surface in order to

round off the corners, if necessary and to remove deep scratches from the surface.

Fine Grinding:

Fine grinding involves rubbing of the specimen surface against the silicon carbide

powders bonded onto specially prepared papers. There are various grit sizes of silicon

carbide papers and. the ones normally used are 400 grit, 600 grit and 1000 grit papers.

These papers are normally mounted on a flat surface. Grinding involves holding the

specimens face downwards on the abrasive paper followed by rubbing in forward and

backward directions until the surface is covered with an even pattern of fine scratches.

The process is repeated with successively finer grade papers (increase in grit number).

With each change of paper, the specimen should be turned through 90 to facilitate the

observation of the disappearance of the previous scratch marks. In addition, at every


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new stage the specimen and equipment should be washed of grit and dirt from the

preceding grinding.

All grinding should be done wet, provided water has no adverse effects on any

constituents of the microstructure. Wet grinding minimizes loading of the abrasive with

metal removed from the specimen being prepared. Water flushes away most of the

surface removal products before they become embedded between adjacent abrasive

particles. Thus the sharp edges of the abrasive particle remain exposed to the surface of

the specimen throughout the operation.

Another advantage of the wet grinding is the cooling effect of the water. Considerable

frictional heat can develop at the surface of a specimen during grinding and can cause

alterations of the true microstructure - for example, tempering of martensite in steel -

that cannot be removed during polishing. Wet grinding provides effective control of

overheating.

2.4 Polishing:

Polishing discs are covered with soft cloth impregnated with abrasive diamond particles

and an oily lubricant. Particles of two different grades are used : a coarser polish -

typically with diamond particles 6 microns in diameter which should remove the

scratches produced from the finest grinding stage, and a finer polish typically with

diamond particles 1 micron in diameter, to produce a smooth surface. Before using a finer

polishing wheel the specimen should be washed thoroughly with warm water followed by

alcohol to prevent contamination of the disc.

Rough Polishing:

This stage involves the polishing of the specimen surface on a rotating wheel using

alumina or diamond abrasive with a particle size of about 3 microns. Polishing aids

include diamond particle suspension or alumina powder suspension. In the polishing

stage, the specimen is moved around the wheel in the direction opposite to the wheel

itself.

Fine Polishing:

This stage involves the removal of very fine scratches and the thin distorted layer

remaining from the rough polishing stages. Fine polishing is usually carried on a

polishing wheel using fine alumina particles with an average size of less than 1 micron

(normally 0.5 micron size is used). Fine polishing, if properly carried out, yields a

scratch free surface ready for etching.


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Fig(3) Struers LaboPol 5 Sample Polishing, Grinding device

2.5 Etching:

Etching is carried out on the properly dried specimen obtained from fine polishing step.

Etching involves chemically treating the specimen surface using a mild acidic or alkaline

solution. The etching differentially attacks various microstructural features as a result of

their different chemical affinities. This differential attack leads to a non-similar

reflection of light into the objective lens leading to the generation of contrast between

the various microstructural features.

For each type of material, there is appropriate etching solution, See Table (2)

After etching is successfully carried out, the specimen can be taken to the optical

microscope for microstructural examination.

Etching Solution Nital or Picral Ferric Chloride Cds

Material Cast Iron Stainless steel Cast iron, Steel

- Copper & alloys

Ferritic & Martensitic

Stainless steels

Table (2) Etching solutions

2.6 Cleanliness

Cleanliness is an important requirement for successful sample preparation. Specimens

must be cleaned after each step, all grains from one grinding and polishing step must be


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completely removed from the specimen to avoid contamination, which would reduce the

efficiency of the next preparation step.

2.7 Sample Storage

When polished and etched specimens are to be stored for long periods of time, they must

be protected from atmospheric corrosion. Desiccators and vacuum desiccators are the

most common means of specimen storage, althrough plastic coatings and cellophane tape

are sometimes used.


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SCOPE

In accordance with the subject matter covered in the present manual, the scope of this

laboratory exercise will be twofold:

1. To obtain experience in the metallographic preparation of metallic specimens, and 2. To observe the various microstructures in Steel sample

Equipment:

Struers LaboPol 5 sample Polishing device.

Zeiss microscope with digital Camera.

PROCEDURE

Steel samples have codes .

1. Mount the specimen using Struers LaboPress-1 device using the table (1) to find the correct temperature, pressure, time and number of spoons of molding

granules.

2. Grind the specimen using the coarser grade MD120 disc laid on the grinding disc. Hold the specimen face downwards on the MD 120. Turn on Struers LaboPol 5

for 6 min with 300 rpm. Lubricant is water.

3. Wash the specimen and repeat step 1 using the finer grade MD Allergo disc and the belonging abrasive liquid (9-15m). Turn on Struers LaboPol 5 for 5 min with

150 rpm.

4. Wash the specimen when only fine scratch marks are obtained. 5. Polish the specimen using a cloth-covered rotating MD Mol disc with diamond

liquid ( 3m) and lubricant RED. as the polishing agent until a flat and scratch-

free mirror -like finish is obtained ( 10min, 150 rpm) (see Table 3).

Speed Time(Min) Grain size

(m)

Lubricant Grinding/Pol

Disc

300 6 Water MD120

150 5 15-9 Green MD Allergo

150 10 3 Red MD Mol

150 3 1 Red MDNap

Table (3) Grinding and polishing routine

6. Wash first with water and immediately with alcohol and dry. 7. Have a look at the surface by using the microscope.


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8. Etch the surface of the specimen with 3% alcoholic Nitric (3 parts of concentrated nitric acid and 97 parts of ethyl alcohol by volume) for a few seconds till you see

any change at the sample.

9. Rinsing is most frequently used and consists of holding specimen under a stream of running water and wiping the surface with a soft brush or cotton swab.

10. After cleaning, specimens may be dried rapidly by rinsing in alcohol, benzene, or other low-boiling-point liquids, then placed und a hot-air drier for sufficient time

to vaporize liquids remaining in cracks and pores.

11. The specimen is now ready for observation (compare with examples of Fig. 1). 12. Observe the specimen under an optical microscope (See the procedure in the next

page).

13. Ask the experiment supervisor to have a look and if he is satisfied that your preparation has produced clearly observable microstructures then proceed to step I

1. If not, repolish and re-etch the specimen until the microstructures are

observable.

14. Sketch the general microstructural arrangement of the various distinguishable zones stating the magnification used.

15. Your sketches are to be of high quality. Label all important features neatly.

Guide for 12. Microscope Procedure

Fig(4) Zeiss Microscope Imager MAT

1. Turn the microscope on. 2. Push in the light path selector. 3. Place the specimen on the stage plate. 4. Move the 5x objective lens into place to focus on the specimen.


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5. Adjust the brightness. 6. Adjust the coarse and fine adjustment knob until object is focused. 7. Observe object by using the x-y-table. 8. Change the magnification step by step till 500x or 1000x and compare it with the

photos presented in the manual.

REFERENCES

1. W.D. Callister, Jr.,in "Material Science and Engineering, An Introduction," (John Wiley And Sons (SEA) Pte Ltd, Singapore, 1994).

2. R.E. Reed - Hill and R. Abbaschian, in "Physical Metallurgy Principles,"(PWS- Kent Publishing Co., Boston, USA, 1992).

3. Metals Handbook, ASM Desk Edition, Eds: H.E. Boyer and T.L. Gall, ASM, Metals Park, OH, USA, Vol. 2, 1985.

4. Metals Handbook: Metallography and Microstructure, Vol. 9, 9th Edition, ASM, Metals Park, OH, USA, 1985.

5. M.N.A. Hawlader, Metallography Laboratory Manual, 1984.

6. D.S. Clark and W.R. Varne, in "Physical Metallurgy for Engineers", (Van Nostrand, 1962).

7. G.L. Kehl, in "The Principles of Metallographic Laboratory Practice", (McGraw-Hill, 1949).


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{Group 1}: Macro/Microstructure

Exp. (B): X-ray

Bragg Reflection

Determining the Lattice constants of Monocrystals

Objects of the experiment

Investigating and comparing Bragg reflection at an LiF and an NaCl monocrystal.

Determining the lattice constant a0 of NaCl and LiF.

Introduction

X-rays

To generate x-rays we need a source of electrons, a means of accelerating the electrons to high speeds, and a target for the accelerated electron to interact with.

X-rays are produced when the free electrons cause energy to be released as they interact with the atomic particles in the target.

Nature of X Rays

X rays are electromagnetic radiation ranging in wavelength from about 100 to 0.01 .

The shorter the wavelength of the X ray, the greater is its energy and its penetrating

power. Longer wavelengths, near the ultraviolet-ray band of the electromagnetic

spectrum, are known as soft X rays Spectrum. The shorter wavelengths, closer to and

overlapping the gamma-ray range are called hard X rays Radioactivity. A mixture of

many different wavelengths is known as white X rays, as opposed to monochromatic

X rays, which represent only a single wavelength. Both light and X rays are produced by

transitions of electrons that orbit atoms, light by the transitions of outer electrons and X

rays by the transitions of inner electrons. In the case of bremsstrahlung radiation, X rays

are produced by the retardation or deflection of free electrons passing through a strong

electrical field.

X rays are produced whenever high-velocity electrons strike a material object. Much of

the energy of the electrons is lost in heat; the remainder produces X rays by causing

changes in the target's atoms as a result of the impact. The X rays emitted can have no

more energy than the kinetic energy of the electrons that produce them Energy.

Moreover,

the emitted radiation is not monochromatic but is composed of a wide range of wavelengths with a sharp, lower wavelength limit corresponding to the maximum

energy of the bombarding electrons. This continuous spectrum is referred to by

the German name bremsstrahlung, which means braking, or slowing down,

radiation, and is independent of the nature of the target.


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In addition to the continuous spectrum there are lines, known as the characteristic X rays, which represent wavelengths that depend only on the structure of the

target atoms. In other words, a fast-moving electron striking the target can do two

things:

It can excite X rays of any energy up to its own energy;

Or it can excite X rays of particular energies, dependent on the nature of the target atom.

David R. Lide CRC Handbook of Chemistry and Physics 75th edition, 10-227, CRC

Press. ISBN 0-8493-0475-X.

X-ray K-series spectral line wavelengths (nm)

for some common target materials

Target

K1 K2 K1 K2

Fe 0.17566 0.17442 0.193604 0.193998

Ni 0.15001 0.14886 0.165791 0.166175

Cu 0.139222 0.138109 0.154056 0.154439

Zr 0.070173 0.068993 0.078593 0.079015

Mo 0.063229 0.062099 0.070930 0.071359


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Principles

Bragg Law1

We study crystal structure through the diffraction of photons, neutrons, and electrons.

When the wavelength of the radiation is comparable or smaller than the lattice constant,

we may find diffracted beams in directions quite different from the incident direction.

W. L. Bragg presented a simple explanation of the diffracted beams from a crystal.

Suppose that the incident waves are reflected from parallel planes of atoms in the crystal,

with each plane reflecting only a very small fraction of the radiation, like a slightly

silvered mirror.

In mirror like reflection the angle of incidence is equal to the angle of reflection. The

diffracted beams are found when the reflections from parallel planes of atoms interfere

constructively as in Fig. 1

We treat elastic scattering, in which the energy of the x-ray is not changed on reflection.

Consider parallel lattice planes spaced d apart. The radiation is incident in the plane of

the paper. The path difference for rays reflected from adjacent planes is 2d sin

planes occurs when the path difference is an integral number n of wavelengths , so that

n 2 d sin

This is Bragg law. Bragg reflection can occur only for wavelength 2d. This is why

parallel planes add up in phase to give a strong reflected beam. If each plane were

perfectly reflecting, only the first plane of a parallel set would see the radiation, and any

wavelength would be reflected. But each plane reflects 10-3

to 10-5

of the incident

radiation, so that 103

to 105 planes may contribute to the formation of the Bragg-reflected

beam in a perfect crystal.

1 See Charles Kittel: Introduction to Solid State Physics

dsin

d


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Braggs law of reflection describes the diffraction of plane waves at a monocrystal as the

selective reflection of the waves at a set of lattice planes within the crystal. Due to the

periodicity of the crystal, the lattice planes of a set have a fixed spacing d. An incident

wave with the wavelength is reflected with maximum intensity when the Bragg

condition

n = 2 d sin (I)

n: diffraction order

: wavelength

d: spacing of lattice planes

is fulfilled

Fig. 2 Three-dimensional representation of the structure of NaCl

d: Spacing of lattice planes in [100]-direction

a0: lattice constant

The angle shows the direction of the incident and reflected wave with respect to the set

of lattice planes and is often referred to as the glancing angle.

In a cubic crystal with NaCl structure, the lattice planes run parallel to the surfaces of the

crystals unit cells in the simplest case. Their spacing d corresponds to one half the lattice

constant: d = a0/2

(II)

This lets us use (I) as an equation for determining the lattice constant a0:

n = a0 sin (III)

In other words, to determine a0 we need to measure the glancing angle q for a known

wavelength l and diffraction order n. This method is more precise when the glancing

angles are also measured in higher diffraction orders. In this experiment, the


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molybdenum x-rays are used as radiation of a known wavelength. Table 1 shows its

Table 1: Wavelengths of the characteristic x-ray radiation of Molybdenum.

Line /pm

k 71.08

K 63.09

A Geiger-Mller counter tube is used to detect the x-rays; this instrument and the crystal

are both pivoted with respect to the incident x-ray beam in 2 coupling the counter tube

is turned by twice the angle of the crystal (cf. Fig. 3). The zero point = 0 is

characterized by the fact that the lattice planes and the axis of the counter tube are

parallel to the incident x-ray beam. As the lattice planes are seldom precisely parallel to

the surface of the crystal, the zero point of each crystal must be calibrated individually.

Fig. 3 Schematic diagram of diffraction of x-rays at a monocrystal and 2q coupling

between counter-tube angle and scattering angle (glancing angle) 1 collimator, 2

monocrystal, 3 counter tube

Setup

Setup in Bragg configuration:

Fig. 4 shows some important details of the experiment setup.

Fig. 4 Experiment setup in Bragg configuration


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Carrying out the experiment2

You will carry out the experiment for NaCl crystal and you will find data for LiF crystal

in next page to calculate its lattice parameter.

Notes:

NaCl and LiF crystals are hygroscopic and extremely fragile. ; avoid mechanical stresses

on the crystals; handle the crystals by the short faces only. If the counting rate is too low,

you can reduce the distance s2 between the target and the sensor somewhat. However, the

distance should not be too small, as otherwise the angular resolution of the goniometer is

no longer sufficient to separate the characteristic K and K lines.

a) Bragg reflection at an NaCl monocrystal: Fig. 5 Front panel of the X ray Apparatus.

Recording the diffraction spectrum:

2 For more details see attached PDF file [ the manual of the machine]


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Get yourself familiar with the X ray apparatus [see the attached the manual of the

machine and ask your instructor]

Using b3 [Parameter selecting key] adjust the High Voltage to 35 kV and the current

filament to 1 mA.

Press the COUPLED key to activate 2q coupling of target and sensor and set the lower

limit of the target angle to 4 and the upper limit to 24.

Start the software X-ray Apparatus or clear any existing measurement data using the

button or the F4 key.

Start measurement and data transfer to the PC by pressing the SCAN key.

When you have finished measuring, save the measurement series under an appropriate

name by pressing the button or the F2 key.


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Fig. 6 Diffraction spectrum of x-rays in Bragg reflection to the third diffraction order at

an LiF monocrystal with logarithmic display of counting rate R.

Parameters of x-ray tube: U = 35 kV, I = 1 mA

Evaluation

In each diagram3, click the right mouse button to access the evaluation functions of the

software X-ray Apparatus and select the command Calculate Peak Center to evaluate

the diffraction spectra.

Using the left mouse button, mark the full width of each peak and write down the

center values in a table as the glancing angle.

For each glancing angle , calculate the values sin and plot these value pairs in a

diagram. In each case, the results lie along a straight line through the origin; in

accordance with (III), its slope corresponds to the lattice constant a0.

Questions

Which of these is not involved in the diffraction of X-rays through a crystal?

a) Electron transitions b) Crystallographic planes c) Nuclear interactions

Constructive interference

2 What is the largest wavelength of radiation that will be diffracted by a lattice plane of the interplanar spacing d?

a. 0.5d b. d c. 2d d. No limit

3 A crystal has a primitive lattice with a spacing between (100) planes is

0.420 nm. What will the value of the Bragg angle () be for the 100 reflection of X-rays of wavelength 0.154 nm?

a. 5.3 b. 10.6 c. 21.2 d. 42.4

4 How could X-ray diffraction be used to determine the phase diagram of an alloy?

To what voltage would you have to go in order to see the characteristic spectral lines

K and K for tungsten? Would both lines appear simultaneously or would one appear

5 and then the other only after the voltage had been further raised?

3 For LiF crystal you will get the data from the instructor.


52

{Group 2}: Heat Treatment

Exp. (E): CCT Diagram by using Dilatometer

1. General Description of a Dilatometer

Dilatometers serve the measurement of a thermal change in length. This change can be a

reversible change or a sum of reversible and irreversible

Fig. 1 General view of Dilatometer DIL801

Change in length,

Phase transformation,

Mass transfer,

Crystallization,

Change in modification and sintering.

Principally, samples (solids, liquids, powders, bulk materials, foils, and fibers) lying in a

sample holder are linearly heated as the case may be cooled. The sample temperature is

recorded by a thermocouple (up to 1550 oC).

The change of length is transmitted by means of a push rod from furnace on linear

variable differential transducer (LVDT).

Measurements can also be carried out under vacuum or inert gas.

The record change in length is strictly a measure a difference. The single push rod

Dilatometer DIL 801 measure the difference between the sample and the sample holder

(See Fig 2).


53

Fig(2) the measuring head for Dilatometer

1.1 Dilatometer system components

2. Definitions

2.1 Linear Thermal Expansion: The change in length of a material resulting from a temperature change. Linear thermal

expansion is symbolically represented by L/L0, where L is the observed change in

length (L = L1 - LO), and LO and L1 are the lengths of the specimen at reference temperature T0 and test temperatures T1. Linear thermal expansion is dimensionless, it is often expressed as a percentage, or in parts per million (such as mm/m) units.

2.2 Mean Coefficient of Linear Thermal Expansion: The linear thermal expansion per change in temperature. The mean coefficient of linear

thermal expansion, a, is defined as:

= 1/L0 [(L1 - L0) / (T1 - T0)] = [1/L0 (L/T)]

(It is customary to designate the coefficient of thermal expansion with the greek letter

alpha (). For the mean coefficient, a bar is placed over it, and is referred to as alpha-

bar. In industry, frequently the whole process is referred to as "CTE testing".)

The value of the mean coefficient must be accompanied by the values of the two

temperatures.


54

2.3 Instantaneous Coefficient of Linear Thermal Expansion: The slope of the linear thermal expansion curve at temperature T. Instantaneous

coefficient of linear thermal expansion represented by:

T = (1/L0) L/dT

The value of the instantaneous coefficient must be accompanied by the temperature at

which it is determined.

There are two main types of transformation diagram that are helpful in selecting the

optimum steel and processing route to achieve a given set of properties. These are

time-temperature transformation (TTT) and continuous cooling transformation (CCT)

diagrams.

CCT diagrams are generally more appropriate for engineering applications as

components are cooled (air cooled, furnace cooled, quenched etc.) from a processing

temperature as this is more economic than transferring to a separate furnace for an

isothermal treatment.

2.4Time-temperature transformation (TTT) diagrams

T (Time) T(Temperature) T(Transformation) diagram is a plot of temperature versus the logarithm of time for a steel alloy of definite composition.

It is used to determine when transformations begin and end for an isothermal (constant temperature) heat treatment of a previously austenitized alloy.

In other words a sample is austenitised and then cooled rapidly to a lower temperature and held at that temperature whilst the rate of transformation is

measured, for example by dilatometry. Obviously a large number of experiments

is required to build up a complete TTT diagram.

2.5 Continuous cooling transformation (CCT) diagrams

It is a plot of temperature versus the logarithm of time for a steel alloy of definite composition. It measures the extent of transformation as a function of time for a

continuously decreasing temperature. For example a sample is austenitised and

then cooled at a predetermined rate and the degree of transformation to another

phase is measured. Obviously a large number of controlled cooling experiments

are needed to build up a complete CCT diagram.


55

3. Scope

The dilatometric technique is used to build the Continuous Cooling Transformation

(CCT) diagram for 42CrMo4 Steel

4. Material and Experimental Procedures

1-The material used in this work is a commercial 42CrMo4steel with the chemical

composition given in Table 1.

C Si Mn Cr Mo

0.41 0.30 0.70 1.10 0.20

2- To construct the CCT diagrams, dilatometric tests are carried out using 8 mm

diameter, 50 mm long samples (check the length and dimension using vernier caliper).

3- The sample is placed into a holder, usually called the dilatometer tube. The sample,

when it expands, pushes the tube and the push-rod in opposite directions. This movement

is sensed by a transducer. The tube and the transducer are fixed to the same reference

surface with the moving member of the transducer coupled to the push-rod.

Mounting and demounting of the sample will be done by the assistant only!!!

4- Temperature Control and Measurement:

Usually a thermocouple (RtRh10Pt-Pt) is used. It is imperative to measure the

temperature of the sample region (somewhat away from the sample (why??)) precisely,

and to control the furnace to provide uniform sample temperature (see Fig. 3).

Fig. 3 Structure of measuring system

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1516-14392001000300002#tab01#tab01


56

5- Temperature program:

The sample is heated continuously to 850 oC with heating rate 100K/min, annealed at 850

oC 10 min, and then cooled at different cooling rates (why?)

Do not start the experiment without the OK of the assistant! The assistant must be present for starting.

6- Evaluation of data:

2. Use the evaluation software installed on the control computer.

Make a plot of the length change (y-axis) versus temperature (x-axis). Determine the phase transition temperature (see fig. 4).

Determine a in the rang from 820 o

C -790 oC.

Make a plot of the length change (y-axis) versus time (x-axis). Determine the time at which the phase transformations begine

Make a plot of the temperature (y-axis) versus the time (x-axis linear or log scale).

Mark the phase transition temperature, and time which is determined

before (see fig. 4).Calculate the personal error

Insert the results into the respective CCT diagram! Compare experimental data with theoretical data

References:

1. William D. Callister, Introduction of Material science, Ch.10. 2. http://www.matter.org.uk/steelmatter/metallurgy.


57

Appendix (CCT diagram)

Fig. 4 Example of such a measurement evaluation


58


59

Fig 5 example of CCT diagram


60


Exp. (J): Tin-lead phase diagrams

NOTE: Read the Safety and Procedure sections completely

before starting the lab. Lead is a poison, wear gloves or use

forceps when handling, and do not breathe fumes.

N.B. Report any problems or breakage. You will not be penalized for problems, even if you helped to create them. We want only to fix problems, not to fix blame.

1. Objectives

To make 4 different alloys of lead and tin (5, 20, 60 and 90% by weight tin).

To melt samples of 100% lead and 100% tin.

To determine a rough phase diagram of the Pb-Sn alloy system.

To observe and record the microstructure of each alloy.

To report on the relative mechanical properties of these alloys.

2. Materials and equipment

Ring stand with ring and clamp, wire triangle, two-jaw clamp to hold thermocouple.

K-type thermocouple

Propane torch with flint lighter/lighter

Crucible, Crucible tongs

Dish, aluminum, weighing

Spatula, stainless, large

Spatula, stainless, small

Digital balance

Forceps, metal, coarse

Tin, bulk stock cut up


61

Lead, chunks of cut up pencil lead

Plastic beaker

Metallurgical microscope

Stereo microscope

Paper towel

3. Safety

1. Read the Material Safety Data Sheets (MSDS) provided for lead and tin, before

starting work.

2. Note position of nearest fire extinguisher.

3. Note position of nearest telephone. It has emergency numbers marked on it.

4. Turn on fume hood fan and light switches (switches on either side of fume hood glass).

Check that Magnehelic gauge indicates acceptable flow rate/pressure differential (see

sign on fume hood for acceptable reading).

5. All team members must wear goggles or face shields at all times.

6. Keep fume hood glass about half way down and keep the glass between your face and

the experiment.

7. Remember that crucibles and samples are hot. Carefully use tongs to handle crucibles.

When pouring molten material, grip edge of crucible firmly and carefully with point of

tongs. Trying to cradle crucible does not work.

8. Apply only enough heat to the crucible to liquefy the sample. DO NOT continue to

heat indefinitely after melting is complete; you may be able to turn down torch and still

maintain melt.

9. Melt should not become red hot. If it does, it is probably covered in excess dross. In

any event, do not plunge thermocouple into it until it has taken on the liquid silver

appearance of mercury. Otherwise the thermocouple may break.

10. Turn off torch firmly but gently -- gas control is by way of a delicate needle valve

which is easily damaged by overtightening.

Sn-Pb

http://www.doitpoms.ac.uk/tlplib/phase-diagrams/images/crucible.jpg


62

11. LIGHT AND USE TORCH ONLY WHEN ACTUALLY HEATING SAMPLES

DO NOT LEAVE IT ON OTHERWISE THIS IS WASTEFUL AND DANGEROUS

4. Procedure

N.B. Start your work with the high % tin alloys. These melt at lower temperatures and

will give you a feel for temperature, melt time, use of thermocouple, etc.

N.B. Please read each numbered section of this procedure completely before starting to

perform the operations called for by that section.

N.B. You should not have to adjust the position of the crucible just slide whole torch

assembly to one side for lighting or to remove it as a heat source.

1. Use an aluminum weighing dish and weigh out the correct proportion of lead and tin.

Record the weights of your metals. Use larger pieces of material to get close to the

desired weights and then use progressively smaller pieces to reach the actual final weight.

Your sample mixtures need only be within ~ +/- 1% of the stated sample proportions, but

be sure to record exactly what the final figures are.

Use the coarse metal tweezers and/or spatulas to handle the tin and lead.

A total weight of about 50 grams for each sample should suffice --this will provide the

needed depth-of-melt of approximately 5-10 mm. The 90% tin mixture will require about

40-45 grams of tin, and the 5% tin mixture will require about 45-50 grams of lead.

Be sure that the balance is at zero with nothing on either pan. Your sample goes on the

right pan of the balance.

2. Mix the weighed samples in a small crucible. Place the crucible in the ring stand

triangle.

3. Make sure the thermocouple is raised well away from the propane torch so that the

torch flame will not affect it. Move the torch to a convenient spot for lighting and

adjustment of the controls. Light it with the flint striker/lighter and adjust the flame so

that its flared blue cones extend about 1. -1.5 cm beyond the end of the torch. Adjust the

position of the torch so that the end of the highest flame is just about 3- 5 cm. from the

bottom of the crucible. It does not have to be pointing directly up at the crucible; pointing

at the bottom of the crucible from below and to one side is fine. DO NOT TURN UP

THE FLAME OR MOVE THE TORCH CLOSER IN ORDER TO TRY TO SPEED

THINGS UP. Ensure that the flame heats the crucible evenly.

4. While waiting for the melting to occur, fill the plastic beaker at least 3/4 full of cold

water.


63

5. Melt should take about 5-10 minutes, depending on mix and flame intensity. Your

sample will melt a little faster if you have small pieces on the bottom to melt first -- once

there is a liquid present, heat transfer to the other pieces will be a bit quicker.

As material melts, stir the melt carefully with the stainless steel spatula. Do not splash

melt out of crucible or hit thermocouple, which should be clamped with the tip well out

of the crucible at this point.

Use the small spatula to remove any excessive dross or scum (the granular junk floating

on the surface), and place this in a weighing dish to cool, then in the designated waste

container. Lab staff will dispose of this waste correctly later. Most dross will be dark

coloured and may even look like lead that refuses to melt. If you see a lot of coloured

dross (yellow, white or red), turn heat down slightly.

Failed samples may also be placed in this waste container.

6. When material has melted, is reasonably free of dross and has a silvery appearance,

carefully turn propane torch flame down, but not off, and gently lower the thermocouple

into the melt, while watching the temperature reading to ensure that thermocouple does

not go off scale (remove it if it does and cool sample). Clamp thermocouple gently but

firmly so that it is suspended in the middle of the crucible and does not touch the crucible

bottom. About 2-3 mm off bottom of crucible is good.

7. Record liquid temperature. Turn off the propane torch carefully to avoid spilling the

crucible. Be sure valve is fully and firmly off, but dont overtighten. Begin the rest of the

temperature measurements and observations. Cooling time will be about 1-2 minutes; if

cooling happens too quickly, try turning the torch flame down but not off. It is suggested

that temperature readings and observations of the melt be made about every 5 seconds.

MAKE YOUR DATA CHART UP AHEAD OF TIME, LEAVING ROOM FOR PLOTS

OF TEMPERATURE/TIME MEASUREMENTS, AND SOME ROOM FOR

COMMENTS ON THE VISCOSITY OF THE MELT. Stir gently with the small spatula

during cooling. Describe viscosity in terms of butter, margarine, putty, plasticine, glue,

syrup, etc., whatever comes to mind.

8. Remelt sample and record temperatures and observations, both on the way up and

during cooling again. Remelt time may differ from the original melt time, because you

are now melting the alloy, not its components.

9. Remelt sample one last time (recording data on the way up) and then lift the

thermocouple well out of crucible. Grasping the edge of the crucible firmly with the

tongs, VERY CAREFULLY pour half of the melt into the water in the plastic beaker and

the other half into one of the aluminum weighing dishes have this sitting on the square

of wire screening to speed the cooling process and to prevent damage to the counter top.

10. When the samples have cooled, identify them (i.e. by marking them) to avoid

confusion later.


64

11. Repeat procedure for other tin/lead proportions and for the 100% lead and 100% tin

samples.

12. Using the microscopes, observe any crystal structure on the surface of the alloy

specimens.

13. Carefully scrape crucible clean with spatula when finished and deposit scrapings in

the designated waste container mentioned above. The crucible wont look like new, but it

should not have a thick film of metal in it.

14. Turn off light and fan in fume hood. MAKE SURE TORCH IS TURNED OFF.

15. Plot temp/time observations and comment (4 data sets plotted on one graph for each

sample).

16. Save all samples and return to Gary in an envelope with your group name on it.


65

Appendix (phase diagram)

Cooling curves

Cooling curves for pure substances

Suppose you have some pure molten lead and allow it to cool down until it has all

solidified, plotting the temperature of the lead against time as you go. You would end up

with a typical cooling curve for a pure substance.

Throughout the whole experiment, heat is being lost to the surroundings - and yet the

temperature doesn't fall at all while the lead is freezing. This is because the freezing

process liberates heat at exactly the same rate that it is being lost to the surroundings.

Energy is released when new bonds form - in this case, the strong metallic bonds in the

solid lead.

If you repeated this process for pure liquid tin, the shape of the graph would be exactly

the same, except that the freezing point would now be at 232C. (The graph for this is

further down the page.)

Cooling curves for tin-lead mixtures

A sample curve

If you add some tin to the lead, the shape of the cooling curve changes. The next graph

shows what happens if you cool a liquid mixture containing about 67% lead and 33% tin

by mass.


66

There are lots of things to look at:

Notice that nothing happens at all at the normal freezing point of the lead. Adding

the tin to it lowers its freezing point.

Freezing starts for this mixture at about 250C. You would start to get some solid

lead formed - but no tin. At that point the rate of cooling slows down - the curve

gets less steep.

However, the graph doesn't go horizontal yet. Although energy is being given off

as the lead turns to a solid, there isn't anything similar happening to the tin. That

means that there isn't enough energy released to keep the temperature constant.

The temperature does stop falling at 183C. Now both tin and lead are freezing.

Once everything has solidified, the temperature continues to fall.

Changing the proportions of tin and lead

If you had less tin in the mixture, the overall shape of the curve stays much the same, but

the point at which the lead first starts to freeze changes.

The less tin there is, the smaller the drop in the freezing point of the lead.

For a mixture containing only 20% of tin, the freezing point of the lead is about 275C.

That's where the graph would suddenly become less steep.

BUT . . . you will still get the graph going horizontal (showing the freezing of both the tin

and lead) at exactly the same temperature: 183C.

As you increase the proportion of tin, the first signs of solid lead appear at lower and

lower temperatures, but the final freezing of the whole mixture still happens at 183C.


67

That continues until you have added enough tin that the mixture contains 62% tin and

38% lead. At that point, the graph changes.

This particular mixture of lead and tin has a cooling curve which looks exactly like that

of a pure substance rather than a mixture. There is just the single horizontal part of the

graph where everything is freezing.

However, it is still a mixture. If you use a microscope to look at the solid formed after

freezing, you can see the individual crystals of tin and lead.

This particular mixture is known as a eutectic mixture. The word "eutectic" comes from

Greek and means "easily melted".

The eutectic mixture has the lowest melting point (which is, of course, the same as the

freezing point) of any mixture of lead and tin. The temperature at which the eutectic

mixture freezes or melts is known as the eutectic temperature.

What happens if there is more than 62% of tin in the mixture?

You can trace it through in exactly the same way, by imagining starting with pure tin and

then adding lead to it.

The cooling curve for pure liquid tin looks like this:


68

It's just like the pure lead cooling curve except that tin's freezing point is lower.

If you add small amounts of lead to the tin, so that you have perhaps 80% tin and 20%

lead, you will get a curve like this:

Notice the lowered freezing point of the tin. Notice also the final freezing of the whole

mixture again takes place at 183C.

As you increase the amount of lead (or decrease the amount of tin - same thing!) until

there is 62% of tin and 38% of lead, you will again get the eutectic mixture with the curve

we've already looked at.


69

The phase diagram

Constructing the phase diagram

You start from data obtained from the cooling curves. You draw a graph of the

temperature at which freezing first starts against the proportion of tin and lead in the

mixture. The only unusual thing is that you draw the temperature scale at each end of the

diagram instead of only at the left-hand side.

Notice that at the left-hand side and right-hand sides of the curves you have the freezing

points (melting points) of the pure lead and tin.


70

Note: The two lines meeting at the eutectic point have been simplified slightly so that they are drawn as

straight lines rather than slight curves. It doesn't affect the argument in any way. I haven't been able to find

the actual data to plot them accurately, so the simplification is to avoid giving the impression that I actually

know exactly what the curves look like!

To finish off the phase diagram, all you have to do is draw a single horizontal line across

at the eutectic temperature. Then you label each area of the diagram with what you would

find under the various different conditions.


71


Exp. (F): Jominy End Quench Test

Objectives

Student will learn about:

How to harden the steel alloy by carrying out the Jominy End Quench Test.

The hardness

Effect of alloying element on the microstructure

Phase transformation of steel

1. Introduction The Jominy end-quench test is the standard method for measuring the

hardenability of steels. This describes the ability of the steel to be hardened in depth by

quenching. Knowledge about the hardenability of steels is necessary to select the

appropriate combination of alloy steel and heat treatment to minimize thermal stresses

and distortion in manufacturing components of different sizes. Hardenability depends on

the chemical composition of the steel and also be can affected by prior processing

conditions, such as the austenitizing temperature. It is not only necessary to understand

the basic information provided from the test, but also to understand how the information

obtained from the Jominy test can be used to understand the effects of alloying in steels

and the steel microstructure.

Hardening of steels can be understood by considering that on cooling from high

temperature, the austenite phase of the steel can transform to either martensite (Fig. 1a) or

a mixture of ferrite and pearlite (Fig. 1b). The ferrite/pearlite reaction involves diffusion,

which takes time. However, the martensite transformation does not involve diffusion and

essentially is instantaneous. These two reactions are competitive, and martensite is

obtained if the cooling rate is fast enough to avoid the slower formation of ferrite and

pearlite. In alloyed steels, the ferrite/ pearlite reaction is further slowed down, which

allows martensite to be obtained using slower cooling rates. Transformation to another

possible phase (bainite) can be understood in a similar way.

Steels having high hardenability are required to make large high-strength

components, such as large extruder screws for injection molding of polymers, pistons for

rock breakers, mine-shaft supports, aircraft undercarriages, as well as for small, high-

precision components, such as die-casting molds, drills and presses for stamping coins.

Steels having low hardenability may be used for smaller components, such as chisels and

shears, or for surface-hardened components, such as gears, where there is a desire to

maintain a ferrite/pearlite microstructure at the core to improve toughness. The Jominy

end-quench test is the standard method to measure the hardenability of steels [DIN EN

ISO 642].


72

High hardness occurs where high volume fractions of martensite develop. Lower

hardness indicates transformation to bainite or ferrite/pearlite microstructures.

Fig 1 Microstructures observed in the Jominy end-quench test of a 0.4wt% carbon steel:

(a) untempered martensite; (b) ferrite and pearlite. Pearlite, the darker constituent, is a

eutectoid mixture of ferrite and iron carbide.

Effects of alloying and microstructure The Jominy end-quench test measures the effects of microstructure, such as grain size,

and alloying on the hardenability of steels. The main alloying elements that affect

hardenability are carbon, a group of elements including Cr, Mn, Mo, Si and Ni, and

boron.

Carbon Carbon controls the hardness of the martensite; increasing carbon content increases the

hardness of steels up to about 0.6wt% carbon. However, at higher carbon levels, the


73

critical temperature for the formation of martensite is depressed to lower temperatures.

The transformation from austenite to martensite may then be incomplete when the steel is

quenched to room temperature, which leads to retained austenite.

Fig 2 Schematic of typical hardness profile in a Jominy specimen.

The hardenability is described by a hardness curve for the steel (Fig. 2), or more

commonly by reference to the hardness value at a particular distance from the quenched

end.

Carbon also increases the hardenability of steels by retarding the formation of pearlite

and ferrite. Slowing down this reaction encourages the formation of martensite at slower

cooling rates. However, the effect is too small to be commonly used for control of

hardenability. Furthermore, high-carbon steels are prone to distortion and cracking during

heat treatment and can be difficult to machine in the annealed condition before heat

treatment. It is more common to control hardenability using other elements and to use

carbon levels of less than 0.4wt%.

Other alloying elements Cr, Mo, Mn, Si, Ni and V retard the phase transformation from austenite to ferrite and

pearlite. The most commonly used elements are Cr, Mo and Mn. The retardation is due to

the need for redistribution of the alloying elements during the diffusional phase

transfromation from austenite to ferrite and pearlite. The solubility of the elements varies

between the different phases, and the interface between the new growing phase cannot

move without diffusion of the slowly moving elements. There are quite complex

interactions between the different elements, which also affect the temperatures of the

phase transformation and the resultant microstructure. Alloy steel compositions are,

therefore, sometimes described in terms of a carbon equivalent, which describes the

magnitude of the effect of all of the elements on hardenability. Steels of the same carbon

equivalent have similar hardenability.


74

Boron Boron is a very potent alloying element, typically requiring 0.002 to 0.003wt% to have an

equivalent effect as 0.5wt% Mo. The effect of boron is independent of the amount of

boron, provided a sufficient amount is added. The effect of boron is greatest at lower

carbon contents and it typically is used with lower carbon steels.

Boron has a very strong affinity for oxygen and nitrogen, with which it forms

compounds. Boron can, therefore, only affect the hardenability of steels if it is in

solution. This requires the addition of "gettering" elements such as aluminum and

titanium to react preferentially with the oxygen and nitrogen in the steel.

Grain size Increasing the austenite grain size increases the hardenability of steels. The nucleation of

ferrite and pearlite occurs at heterogeneous sites such as the austenite grain boundaries.

Increasing the austenite grain size therefore decreases the available nucleation sites,

which retards the rate of the ferrite/pearlite phase transformation (Fig. 6). This method of

increasing the hardenability is rarely used because substantial increases in hardenability

require large austenite grain size, obtained through high austenitizing temperatures. The

resultant microstructure is quite coarse, with reduced toughness and ductility. However,

the austenite grain size can be affected by other stages in the processing of steel, and,

therefore, the hardenability of a steel also depends on the previous stages used in its

production.

Vickers Hardness

The Vickers hardness test uses a square pyramidal diamond indentor. The recorded

hardness depends on the indentation load and the width of the square indentation made by

the diamond. The indentation load is typically between 10 and 30 kg. The hardness

number is usually denoted by HV20 for Hardness Vickers 20 kg, for example.


75

2. Experimental procedure

In this test the hardenability of a low-alloy steel is compared to that of a plain carbon

steel.

The experimental procedure is as follows:

3. position a Jominy specimen (See Fig.7) on the tray and push the tray into the furnace set at 950C using tongs and heat resisting gloves.

Fig. 7 Jominy Specimen

4. Adjust the water column height in the Jominy end quenching tank to 65 mm above the orifice with faceplate valve wide open. Close the faceplate without

changing the water column height adjustment so that when the faceplate is opened

later on the water column will rise immediately to 65mm.

5. After 30 minutes in the furnace transfer the Jominy specimen to the specimen holder of the Jominy end quenching tank. Then after the Jominy specimen is in

place, turn on the water and quench the bottom end of the specimen (See Fig. 2).

Transfer from furnace to quench should be rapid (in minor than 5 seconds). You

are advised to practice the motions in advance. The first student with heat

resisting gloves open the furnace and a second student with tongs or large pliers

transfers the Jominy specimen to water bath. while the other one operates the

quick opening valve. Care should be taken to position the specimen so that it

hangs straight down and a uniform umbrella pattern of water rebounds from the

specimen bottom (See Fig.8).


76

Fig.8 schematic diagram of appliance of quenching

6. Leave the specimen in place with the water flowing for at least 10 minutes. 7. Remove the specimen from the holder and cool it in water. 8. Grind a flat on the side of the specimen at least 0.4mm deep. use 120 Grit paper..

Grind gently with coolant water flowing on the surface of the grinding paper.

9. Mark a scale on the flat as follows (See Fig. 3). Divide the first 16 from the quenched end into 2 mm increments. Divide the next 65 mm into 5mm

increments.


77

10. Measure the Vickers hardness HV30. Start at the quenched and work towards the other side. Thereafter, record the hardness at each increment that you have

marked off. It is important to keep the indentations in the center of the flat, since

errors will arise if they are at the edges of the flat.

3. Analysis 11. Make a plot of the hardness (y-axis) versus the distance from the quenched end

(x-axis).

4. References 1. D. R. Askeland, The Science and Engineering of Materials, Alt. Ed., PWS

Engineering, 1984, pp. 288-303, 351-376.

2. L. H. Van Vlack, Elements of Materials Science and Engineering, 5th ed., 1985, pp. 402 - 415, 431-435,439-445, 455-463, 469-478.

3. G. L. Kehl, Principles of Metallographic Laboratory Practice, McGraw-Hill, 1949, pp. 303-310, 229-240.

4. J. Wulff, et. al., Structure and Properties of Materials, Vol. 1, pp. 184-197; Vol. 2, pp. 123-128.

5. G. Guy, Physical Metallurgy for Engineers, 1962, Addison-Wesley, 1962, pp. 122-124, 294-298, 301-311.


78


Exp. (G): Carburizing

Carburization is a technique used to harden the surface of steels by diffusing into the

crystal lattice. The carbon enters the interstitial spaces between the iron atoms. It

strengthens the metal by distorting the crystal lattice, thus making it difficult for

dislocations to move. Carburization is a surface technique because, even at high

temperatures, diffusion is a slow process. The source of carbon can either be a gas or

solid carbon.

1. Objectives

The practical aims to familiarize you with a solution of Fick's Second Law of diffusion

by studying the diffusion of carbon into iron.

You will gain experience of heat treatment in furnaces, preparation of metallographic

sections and etching, The microscopy will require interpretation of more complicated

microstructures.

2. Safety

Care is needed in handling hot materials; use proper tongs, wear suitable protection

(gloves and visor) and don't leave hot materials on the bench without a notice that they

are hot . Normal safety precautions are adequate for the etches used here, i.e. lab coat,

gloves and eye protection.

3. Procedure

You are provided with two small pieces of Mild steel, this has a low C content. One will be as blank (for comparison) and the other is required for carburizing at

950 C.

Heat the samples in the oven at 950 oC for one hour. At the end turn off the oven, and let the samples cool in air to room temperature. This process is caused

normalization, and can be done several days or weeks before the experiment;

however the sample should be at room temperature for at least 24 hours before

proceeding to the next step.

To carburize your sample:

Clean the surface of the specimen by polishing up to 9 m.


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The samples is packed in a ceramic crucible in a mixture of powdered charcoal and sodium carbonate activator (10 % w/w). The carbonate releases carbon dioxide

that reacts with C to give carbon monoxide and form a carburizing gas. A lid is

needed to exclude air.

Place your crucible in the furnace for 2 hours at 1000C.

Remove from the oven and quench the sample (hot) in water.

Perform a Vickers hardness test, and examine the surface under the optical microscopy.

Compare the data before and after carburizing and describe the microstructure.

Note: after carburizing the sample need to be polished; in particular it is important that

the rusted (oxide) surface is removed to expose a bulk cross-section. After grinding and

polishing, the specimens need to be etched, in 2% Nital. Check that the etch time is

sufficient to show the structure clearly across the whole carbon profile.


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{Group 3}: Mechanical tests

Exp. (H): Hardness

Scope

To study the effect of heat treatment of steel (Jominy-end quench test) and the

case hardening (carburizing) on the surface of the metals.

Introduction

The hardness test measures the resistance of a material to an indentor or cutting

tool. The indentor is usually a ball, pyramid or cone made of a material much harder than

that being tested. In most of the standard tests, a load is applied slowly by pressing the

indentor perpendicularly onto the surface being tested for a given period of time. The

load and/or the size of the ball may be varied according to the hardness of the material.

An empirical hardness number may be calculated from the results of such tests by

knowledge of the load applied and cross-sectional area or depth of the resulting

impression using appropriate formula. These tests should never be taken near the edge of

a sample or any closer than about three diameters from an existing impression. Most

hardness tests produce plastic deformation in the material and all variables that effect

plastic deformation effect hardness. For materials which work-harden in a similar

fashion, there is good correlation between hardness and the ultimate tensile strength.

Concept

The basic concept utilized in this test is that a set force is applied to an indenter in

order to determine the resistance of the material to penetration. If the material is hard, a

relatively small or shallow indentation will result, whereas if the material is soft, a fairly

large or deep indentation will result.

Methods of hardness measurement

Brinell, Rockwell, Vickers, and Knoop are frequently used methods for

determining hardness. These tests are often classified in one of two ways: either by the


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extent of the test force applied or the measurement method used. A macro test refers

to a test where a load >1 kg is applied; similarly micro refers to a test where a load of

1 kg of force is applied. Additionally, some instruments are capable of conducting tests

with loads as light as 0.01 g and are commonly referred to as ultralight or

nanoindentation testers.

Rockwell and Brinell testers fall into the macro category, whereas Knoop testers

are used for microindentation tests. Vickers testers are employed for both macro and

microindentation tests (This technique is applied in the laboratory of GUC (C5)). The

measurement methods available include a visual observation of the indentation or a depth

measurement of the indentation. Rockwell and some nanoindentation testers are capable

of determining the depth of the indentation, whereas Brinell, Knoop, and Vickers testers

require an indentation diameter measurement.

Principal

The basis of static indentation tests is that an indenter is forced into the surface of

the material being tested for a set duration. When the force is applied to the test piece

through contact with the indenter, the test piece will yield. After the force is removed,

some plastic recovery in the direction opposite to the initial flow is expected, but over a

smaller volume. Because the plastic recovery is not complete, biaxial residual stresses

remain in planes parallel to the free surface after the force is removed. The hardness

value is calculated by the amount of permanent deformation or plastic flow of the

material observed relative to the test force applied. The deformation is quantified by the

area or the depth of the indentation. The numerical relationship is inversely proportional,

such that as the indent size or depth increases, the hardness value decreases.

Hardness values can be directly compared only if the same test is used, since the

geometry of the indenter and force applied influence the outcome of the test. For each

type of hardness test conducted, a different equation is used to convert the measured

dimension, depth or diameter, to a hardness value. The Brinell hardness value is

calculated by dividing the test force by the surface area of the indentation. The test


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parameters taken into account are the test force and ball diameter while the indentation

diameter is measured. For Rockwell tests, the hardness value is determined by the depth

of indentation made by a constant force impressed upon the indenter. The test parameters

taken into account are the test force (major and minor load) and the indenter geometry

(ball or diamond cone), while the depth of penetration between the application of the

minor load and major load is measured. Vickers hardness values are calculated in the

same manner as Brinell tests. The projected area, instead of the surface area, is used when

computing Knoop values. Table1 lists common applications for the tests.

The test parameters taken into account for Vickers and Knoop tests are identical

and include the test force and diamond indenter geometry while the indentation diameter

is measured. The illustrations in Figure1 demonstrate the indentations.


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Procedures

1-Adjust the Microscope illumination so that all edges

are sharp, but without a halo.

2- First of all set the vertical middle line parallel to the

indentation diagonal in the field of view. Whereby the

indentation should be near to the vertical middle line.

3-Use the lower adjustment knob to set the indentation so

that

-The lower tip extends past the lowest short

measurement line, and

-The upper tip touches one of the short measurement

lines.

4- Use the lateral adjustment knob to bring the next long measurement line to the lower

tip of the indentation. Whereby the upper edge of the measurement line and the tip of

the indentation must just touch one another.


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5-Count the whole measurement line separations

between the indentation tips to be measured.

Read off the scale value from (a) the table for the

objective lenses. Multiply scale value (a) by the

number of whole measurement line distances.

6- Now read off the numerical value from the scale

and read the scale constant (b) from the table for

the objective lenses. These values are also

multiplied by one another.

Hidden setting knob


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7- Add both measured values in step 5 and 6

8- The resulting length of the diagonal is then converted to HV (Vicker hardness) using

the following table


86


87


88


89

{Group 3}: Mechanical tests

Exp. (D): Tensile

Objectives

The objective of the experiment is to demonstrate the elastic and plastic properties of

metals. Students will learn about:

How to perform a tensile test on metals.

Measurements of stress and strain in tensile tests

Interpretation of stress strain curves

Derivation of material properties like Youngs modulus, yield stress/0.2% proof stress, tensile strength and other mechanical properties.

1. Introduction

The tensile test, also known as tension test, is one of the most important and most

fundamental tests in mechanical testing of materials. Tensile tests provide information on

the strength and ductility of materials under uniaxial tensile stresses for almost all types

of materials. Tensile tests are simple, relatively inexpensive, and fully standardized. The

result of the tensile test is the so-called stress-strain-curve. From this curve, several

material properties can be derived, like elastic (Youngs) modulus, yield stress, tensile

strength and strain at fracture. Other data, which might be obtained, are elastic limit or

proportional limit, reduction of area and other properties. This information may be useful

in comparisons of materials, alloy development, quality control, and design under certain

circumstances.

The direct, but simplified result of the test uses the engineering stress and engineering

strain. An advanced evaluation of the test yields also true stresses and true strains.

The test procedures are outlined in several standards: ASTM E-8, DIN EN 10002-1, ISO

6892, JIS Z2241.

3. Test Samples

For different materials different standardized sample shapes have been developed. A

tensile sample in general has a central part with a constant reduced cross section, and

ends with a larger cross section where the sample will be fixed in the machine. The

design of the specimen has to guarantee that the multi-axial stresses due to gripping and

fixing the sample do not disturb the uniaxial stress state in the central reduced section.


90

The elongation of the sample will be measured at the central part of the sample. There,

the so-called gauge length l0 shall be marked. The actual gauge length depends on the

cross section of the sample. The parallel part with the reduced cross section must be

larger than the gauge length, at least by the sample width or diameter. The transition

between the central parallel part and the ends of the specimen must be a smooth curve

with radius larger than a specified minimal radius.

In this experiment hot and cold rolled steel samples and a non-ferrous alloy (brass) shall

be compared. The flat specimen shape will be used (see figure and table below).

Figure 1: Flat tensile specimens according to the ASTM standard

2. Test Procedure

The tensile test makes use of a tensile machine, which consists of a very stiff load frame,

a movable cross-head, suitable devices to grip the sample, a load cell, motors and a

control unit. A tensile machine shall include also a device to measure the elongation of

the sample (extensometer).

The tensile test is performed by gripping the ends of the sample and elongating the

sample. The force, acting on the sample, is measured by a load cell continuously during

the experiment and may be plotted versus the elongation. I.e. we measure and record the

force, necessary to obtain the elongation of the sa

mats 402 materials lab i - faculty of engineering and...

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