Preparation of Specimens Report

7/30/2019 Preparation of Specimens Report

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1.Introduction:Metallography, this word mean the study of the structure and constitution and components of

metals and alloys, using microscopy and magnifications.

Using microscopy gives the experimenter a lot of critical information like size and shape of the

grains, presence of micro defects (such as segregation, hair cracks, and nonmetallic inclusions),

and distribution of secondary phases.

And the metallographic examination used in quality control to predict and explain the mechanical

properties.

Grains, known as crystallite region separated regularly. Boundary grain is an interface between

two grains, or crystallites. And tend to decrease the electrical and thermal conductivity of the

material. See Fig (1) grains and boundary grain.

At first place the experimenter should prepare a specimen to

determine the grains. Using some machine to do a processes on the

specimen, at the beginning sectioning (cutting), and passing through

mounting, course grinding, fine grinding, and polishing, etching and

finally microscopic examination. See Fig (2)

Fig (1): Grains & Boundary Grain

FIG (2): Preparation Steps


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2.Objective: To prepare the specimens surfaces to be examined by the microscope. To learn and to gain experience in the preparation of metallographic specimens. To be able to prepare my future specimen. To become familiar with machines using to prepare the specimen. To make sense with relation between number of grains and mechanical properties.

3.Equipment:The experiment has been done step by step in sequence using a lot of machines listed below:

1. Minitom: Small, automatic precision cut-off machine for sectioning all material graphicand ceramic specimens. This machine cuts the specimen slowly to produce fine surfacewithout heavy inclusions. See Fig(3)

2. Metason: Ultrasonic Cleaning for effective ultrasonic cleaning of metallographic,ceramic, and mineralogical specimens. See Fig (4)

Principle of work:The ultrasound which is transferred to the liquid causes the propagation of pressure waves

in the tank. These pressure waves involve a continuous change of the pressure in the liquidso that, as a result of sub-pressure, innumerable microscopic evacuated bubbles are formed

which will soon expand and implode (capitation).These implosions give the cleaning liquid a high speed locally so that dirt and loose

particles are removed from the specimens which are immersed into the liquid.

3. LaboPress-3: Semi-automatic mounting press for hot mounting of all materials. SeeFig(5)

FIG (5): LaboPress-3 FIG (4): Metason FIG (3): Minitom


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4. LaboPol (grinding and polishing machine): Machines for grinding, lapping and polishingfor 200 or 230 mm discs. See Fig(6)

5. LaboForce: Specimen movers for semi-automatic preparation of materialographicspecimens on LaboPol grinding and polishing machines. See Fig (7)

6. LectroPol-5: Automatic, microprocessor controlled electrolytic polishing and etching ofmetallographic specimens. See Fig (8)

MD-System Consumables:

a. MD-DiscTM is the supporting disc for all preparation surfaces. See FIG (9)b. MD-PrimoTM is a SiC embedded disc designed for plane grinding materials in the HV 40-

150 hardness range. See FIG (10)

FIG (6): LaboPol

FIG (10): MD-Primo FIG (9): MD-Disc

Fig (7): LaboForce

Fig (8): LectroPol-5


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c. MD-AllegroTM Fine grinding of materials harder than HV150 is carried out in one step onMD-Allegro using DP-Suspension or DP-Spray. See Fig (11)

d. MD-LargoTM Fine grinding of soft materials in the HV 40-150 hardness range is carriedout in one step on MD-Largo using DP-Suspension or DP-Spray. See Fig (12)

And there another type of MD-System Consumables, like MD-ClothsTM

, MD-Pian and MD-Fuga.

4.Procedure:1. At first choose the material to test, then apply the sectioning process using Minitom

machine which gives a specimen with smooth surface by slowly sectioning.

a. Adjust the position of the cut by turning the micrometer screw.b. Push weights of the specimen holder arm back.c. Adjust the cutting pressure with the front weight.d. Set the adjustable knob, CUTTING SPEED, to the lowest speed.e. Press the button, START.f. Adjust the speed of the cut off wheel on the adjustable knob, CUTTING speed, (for

127 mm (5'') cut-off wheels the speed should be not exceed 300 rpm).

2. Clean the specimen from smooth inclusions, by using ultrasonic cleaning machineMetason (cleaning machine)

g. Start cleaning, set the timer for the amount of time you wish the specimen to becleaned.

3. Take the specimen to hot mounting using LaboPress-3 to add surrounding by an organicpolymeric powder which melts under the influence of heat _about 200 _ and highpressure applied by a piston, to give a high quality mold with intimate contact between the

sample and the polymer.

FIG (12): MD-LargoFIG (11): MD-Allegro


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Using table (1&2) and the equation below, the experimenter know the amount of polymer which

should use, time heating and cooling processes and the forces and temperature of the last two. And

this is important to avoid something wrong as shown in Fig (10)

Min no. of Spoons =.

And the factor 1.2 is multiplied to eliminate variation in the resin densities and the measuring

during mounting, height of 1 Spoon (15 ml) of Resin after Mounting in an Empty Cylinder.

To apply this equation we have to know that cylinder Dia. 30 mm and sample height in Polyfast

process 11.3 mm (0.44 in.) and in Multifast process 8.9 mm (0.55in.)

We use the Polyfast process, then height of sample 11.3 mm,

Then, Min number of spoons =..

= 0.904 1

Cylinder dia: 30 mm

Resin: mm inch

PolyFast 11.3 0.44

MultiFast (black) 8.9 0.35

Cylinder dia. Resin heating Cooling

30 mm

Type Quantity Time Temp. Force Time Rate

[spoons] [ml] [min] [oC] [kN] [min]

PolyFast 1.5 23 4-6 180 20 2-3 HIGH

MultiFast

(block)2 30 4-6 180 20 2-3 HIGH

Fig (10): Errors

Table (2): Mounting Process

Table (1) Resin Height


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a. Placing the specimen1- Press the hold RAM UP to raise the lower ram to its upper limit.2- Apply "Mould Release Agent" to the surface of the lower arm.3- Place the specimen on the ram. The specimen must be clean, dry, and free fromgrease. 5the distance between the specimen and the cylinder wall must be minimum

3 mm to avoid cracks in the resin.

b. Pouring resin over the specimen1- Press and hold RAM DOWN to the lower the ram to its lower limit.2- Fill a suitable amount of resin into the cylinder by means of the enclosed funnel.

c. Installing the top closure.1-Remove the resin from the upper part of the mounting cylinder.2-Clean cylindrical surface of the upper ram. Cured resin can easily be removed

without damage to the surface of the ram using the scraper.

3-Apply "Mould Release Agent" to all accessible surfaces of the upper ram.4-Place the top closure with upper ram on the mounting cylinder.5-Press the closure straight down, turning it counterclockwise until you hear a stick.6-Press the top closure down and turn it clockwise until its lower limit.7-Turn the top closure quarter of a turn back.

b) Starting the mounting process1- Set the force.2- Set the heating time (5 minutes).3- Set the heating temperature (180/150oC).4- Set the cooling time (2.5 minutes).5- Press start, and the process will run automatically.

c) Stopping the mounting processThe machine automatically stops and relieves the pressure when the cooling time has

elapsed.

d) removing the top closure1- Turn the top closure counter clockwise until released from the thread.2- Press Ram up to raise the lower ram to its upper limit.3- Place the top closure in the top closure holder.

4. The fourth step is to clean the specimen surface from impurity, scratches, flaws, stains,inclusions and other imperfections which make the surface of specimen more difficult to

test, be free from all traces of disturbed metal, and reveal no evidence of chipping due to

brittle intermetallic compounds and phases and to retain non-metallic inclusions. And to

achieve last objective, the specimen must pass through grinding and polishing using

LaboPol machine.


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Firstly insert the suit grinding disc into machine, then put the specimen on grinding disc

and adjust the force applied on the specimen. As shown in Table (3)

In grinding and polishing process, there is three types of grinding disc and used according to the

surface and other articles. In this experiment the MD-Largo and MD-Primo used for grindingprocess. And MD-Dac and MD-Nap used for polishing process, and there is a disc to make the

final surface smooth. And see table (4) for grinding and table (5) polishing processes.

IndicationForce

(N)

0 0

1 2.5

2 5

3 7.5

4 10

5 12.5

6 15

7 17.5

8 20

Surface MD-Primo 220 MD-Largo

Abrasive SiC DP-Suspension

Lubricant Water Green/Blue

rpm 300 150

Force(N) 120 180

Time (min) Until Plane 5

Table (3): Applied Force

Table (4): Grinding processes


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5. Electrolytes(Etching): LectroPol-5Electrolytes:

1- Electrolyte E5-I, E5-II and E5-III: for electrolytic polishing of steel with disturbingimpurities, low carbon steel, cast iron and copper alloys.

2- Mix E5-I, E5-II and E5-III just before use. For etching surfaces for material graphicpreparations

3- Electrolyte D2: for electro polishing of copper and copper alloys.In this experiment the External Etching method was used, and as an Electrolyte, by the stepslisted below:

1- Connect the plug for the unit for the external etching to the back of the control unit.2- Fill the bowl with the required electrolyte.3- Seize the specimen with the pair of pliers.4- Dip the specimen face down into the electrolyte.5- As soon as the specimen is immersed in the electrolyte, countdown of the pre-set time

starts.

6- When the time has expired the voltage is switched off and the etching process is stopped.Note: Avoid contact between the etching bowl and specimen or the pair of pliers. This will

result in a short circuit and the specimen not being etched correctly.

Surface MD-Dac MD-Nap or MD-Chem

Abrasive DP-Suspension OP-S or OP-U

Grit / Grain size 3m -

Lubricant Green/Blue -

rpm 150 150

Force(N) 190 60

Time (min) 5

Table (5): Polishing Process


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6.Results:After these processes, the specimen is prepared to test in microscope and get a picture which showthe grains. And use a picture to determine the ASTM of copper alloy. See Fig (11)

Sample

material

Etchant Composition Remarks

Carbon

steel

(Usually 2%)

(nitric acid)

HNO3 1-5 ml

Ethyl alcohol 100ml

Immersion. Time ranges

from a few seconds to a minute

Carbon

steel

Picral (picric

acid)

Picric acid 4g

Ethyl alcohol 100ml

Immersion. Time ranges

from a few seconds to

Over a minute.

Aluminum Hydrofluoricacid

HF (concen) 0.5ml

H2O 99.5mlSwab for 15 sec.

Table (6): Electrolytes (Etching).

Fig (11): Grains of Copper Alloy


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Why should the specimen be roughly washed after each stage during either grinding orpolishing?

As soon as the process finishes the sample must be washed and cleaned to avoid any further attack

of the surface

Why is fine grinding performed wet?Because it has higher capacity per unit mill volume, it makes possible the use of wet screening or

classification for close product control, it eliminates the dust problem, and it makes possible the

use of simple handling and transport methods such as pumps, and pips

What is the purpose of etching metallographic samples?Etching metallographic samples uses to determine the ASTM which used to indicate the ductility

of the substance, and other mechanical properties. The purpose of etching is tw0-fold. Grinding

and polishing operation produce a highly deformed, thin layer on the surface which is removed

chemically during etching.

What are the advantages of electrolytic polishing? High speed. Can be reproducible. No mechanical deformation. Can produce excellent surfaces for examination.

7.Determination the ASTM:

Metals, except in a few instances, are crystalline in nature and, except for single crystals; they

contain internal boundaries known as grain boundaries. When a new grain is nucleated during

processing (as in solidification or annealing after cold working), the atoms within each growing

grain are lined up in a specific pattern that depends upon the crystal structure of the metal or alloy.

With growth, each grain will eventually impinge on others and form an interface where the atomic

orientations are different.

The number of grains per square inch at 100x magnification is:

n100 = nm * [M / 100]2

Where: n100 is the number of grains per square inch at 100x magnification.

nm is the number of grains per square inch at the magnification M of the photomicrograph.


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The ASTM grain size number N is calculated from the following equation:

= lnln 2 + 1And this a example to calculate the grain size,

From Fig (12),

M = 200,

A =

.

. = 9.92 .nm = 36 grain /9.92 inch

2 = 3.63 grain / in2

n100 = nm * [M / 100]2

n100 = 3.63*(200/100) ^2 = 14.52 grain / in

N = [ln (n) / ln (2)] + 1

N = [ln (14.52) / ln (2)] + 1 = 4.86 grain / in.2

Fig (12): Approximate Grains


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8.Comments & Recommendations:The experiment was useful and very long, and gain experience in the preparation of metallographicspecimens.

9.References: http://en.wikipedia.org/wiki/Grain_boundary http://en.wikipedia.org/wiki/Metallography http://www.struers.com/default.asp?doc_id=262 http://www.eng-tips.com/viewthread.cfm?qid=141206

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Preparation of Specimens Report

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