Metallography
Metallography is the study of materials microstructure. Analysis of a materials microstructure
helps us determining if the material has been processed correctly and is therefore a critical step for
determining product reliability and for determining why a material failed. The basic steps for
proper metallographic specimen preparation include:
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Sectioning and Cutting
Mounting
Grinding
Polishing
Final Polishing
Etching
Microscopic analysis
Sectioning:
We will be using metacut machine. Metacut is an instrument to cut the metallic sheets without deforming
its microstructure. In this report we will discuss basic introduction of the metacut. Procedure to use the
metacut. Principles to use the metacut. In our laboratory we used metacut with Silicon Carbide disk.
It is a very good abrasive material with the hardness of 300 – 350 HB. We can also use alumina
and tungsten carbide disk instead of silicon carbide disk. The only condition is that the material
should be abrasive.
Procedure:
Using metacut is simple and easy. There is no hard and fast rule to use it. Following are the basic steps
to use metacut.
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Mark the reference point if the sample is irregular.
From that point mark proper dimensions according to your need.
Clamp the sample in metacut appropriately such that the point to cut is in the center of the two
holders.
Recheck that disk cut at proper place by getting down the disk without turning on it.
Figure 1:Operating buttons
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Then close the hood so that spark does not reaches the operator.
Then turn on the pump first and then press the start button.
Now down the lever very slowly that the sample cut down gradually and uniformly.
When the sample is completely cut take the lever back to its place.
Now stop the disk and power off the pump and at last open the hood.
At the end file the edges to remove sharp ends.
Precautions:
Safety precautions are very important for any experiment. It is for our betterment. We have to follow
them properly.
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Wear lab coat so that clothes do not get any stain.
Do not apply instant force on the lever otherwise the disk will break.
Do not open the hood until the disk and pump stopped.
For mega applications keep dimensions properly.
Metacut is easy to use as compare to disk cutter or any other hand cutting tool. It also gives precision.
Less material is waste in metacut. There are less chances of accidents in metacut because the disk and
sample are placed in closed chamber. Heat treatment of the material is not occur which also leads to
remain the microstructure remain same.
Hot Mounting:
Hot mounting is also called compression mounting. It is the process of compressing and
heating bakelite powder (phenolic resin) around a piece of metal in order to form a solid disk
that can be used for easy handling of specimen. It is usually used for small, sharp and fragile
specimen which are otherwise difficult to handle. The details regarding procedural
specifications are mentioned later on in the report.
Figure 2:HOT MOUNTING PRESS
Procedure:
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Bring the stage parallel to the top surface.
Place your specimen on the cleaned stage with the required surface facing downwards.
Release pressure slowly and add Bakelite powder.
Figure 3: BAKELITE POWDER
Repeat step 3 until the entire specimen is covered with powder.
Release all the pressure, such that the stage goes all the way down to the bottom of the
pressure vessel.
Place the cap on top and let it slide in. (Do not forcefully close the cap).
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Using the lever apply pressure, allowing the bottom movable mould to move up, such
that the top and bottom moulds hold the specimen firmly.
After the temperature reaches 60ºC, apply pressure using the lever and maintain the
pressure to 25MPa (4000 Psi).
When the temperature reaches 180ºC (curing temperature), the timer will automatically
start the countdown of 5 minutes (curing time).
The temperature will start to drop because of water which is used as a cooling agent.
When the temperature drops to 40ºC, stop the machine.
After a few min release the cap and take out your specimen.
Grind the specimen for a perfect finish.
The handling and grinding of the mounted specimen was much easier and precise than the
unmounted and sharp specimen.
Grinding:
Grinding is basically the first step to remove material. Like any other cutting process,
grinding removes material in the form of chips. For proper grinding of a material, its
abrasive grains must wear and self-sharpen at a consistent rate. Grinding is basically done
to produce a surface of sample that is without scratches and highly reflective.
Procedure:
The experimental procedure of grinding of mild steel mounted sample is explained below:
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Take a mounted (both hot and cold) sample of mild steel.
Adjust the grinding paper on the wheel starting from 200.
The water pipe is switched on which will work as lubricant to avoid heating of
sample.
Turn on the machine and start grinding by placing the sample on the rotating
wheel with paper adjusted on it.
Speed of the wheel is adjusted according to requirement.
According to requirement, paper is changed in sequence (220-400-600-10001500-2000)
After grinding of the mounted samples, we produced finely grinded samples having
smooth surface without scratches and deformation. Along with that, the samples
produced are plane and highly reflective. To eliminate these problems, careful handling
of sample is necessary. Along with that patience is required as it is a lengthy process in
field of metallurgy.
We will do polishing and etching to clearly watch the microstructure of the material. Polishing is
done to obtain mirror like surface and etching is done to visible the grain boundaries under
microscope.
Polishing and Etching
Polishing:
Polishing is the process of creating a smooth and shiny surface by rubbing it or using a chemical
action, leaving a surface with a significant specular reflection. In some materials (such as metals,
glasses, black or transparent stones), polishing is also able to reduce defuse reflection to minimal
values.
In polishing machine there is a wheel which rotates and polish the surface of a material. Polishing
wheels come in a wide variety of types to fulfil a wide range of needs. The most common
materials used for polishing wheels are wood, leather, canvas, cotton cloth, plastic, felt, paper,
sheepskin, impregnated rubber, canvas composition, and wool; leather and canvas are the most
common.
Etching:
Etching Is a Chemical or Electrolytic Process Used after Metallographic Grinding and Polishing
Procedures.
In our lab we use 98% ethanol and 2% nitric acid.
Etching Enhances the Contrast on Surfaces in Order to Visualize the Microstructure or
Macrostructure. Etching in metallography exerts a controlled influence on the surface profile or
optical properties at grain boundaries, phases, or grain surfaces, thus enabling microscopic
inspection and additional use of optical filters in the microscope. Preparation before the etching
process must be adapted to the investigation target.
Procedure:
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First of all we take the grinded sample which was grinded at 2000 amre paper.
Now we use polishing machine to polish the surface of the sample in which we use woven
cloth.
 While doing polishing we also add alumina powder solution drop by drop into the woven
cloth so that it can polish the surface well.
 After polishing we wash our sample carefully and also dry it before etching.
 Now we drop etching agent (which are 98% ethanol and 2% nitric acid) on the sample to
make grain boundaries visible.
 After adding etching agent for 3 seconds, we immediately rinse the specimen. It is
also necessary to dry the sample after etching.
 Now our sample is ready to examine microstructure.
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After polishing we examine that there is no scratch on specimen but also no grain
boundaries are visible.
So, in order to make grain boundaries visible we did etching. Now our sample is
ready to examine microstructure.
Metallurgical Microscopy
Metallurgical microscopy allows small features of a sample to be analyzed in detail. The laboratory
routinely uses its range of microscopes for initial examination and characterization to plan the next
steps for work or to confirm customer’s observations prior to proceeding to more detailed investigations
by other laboratory techniques such as SEM/EDX or reverse engineering.
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Figure 4: Parts of Metallurgical Microscope
Procedure:
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First of all, place the sample (which is grinded well, polished and then etched) on the
stage of the metallurgical microscope.
Select the required lens that can magnified the sample i.e. 50X,100X,300X etc.
Now look through the eyepiece and adjust the coarse adjustment knob so that the
image become clear.
Now move the fine adjustment knob to make the microstructure clearer.
Place the camera to the body tube of the metallurgical microscope.
Open the software from the computer and attach the camera to the computer.
Adjust the magnification.
Now take a picture of the object by clicking on the snap button in the software.
Now save the image of the object.
REFERENCES:
https://www.kemet.co.uk/products/metallography/microscopes/metallurgical-microscopes
http://www.substech.com/dokuwiki/doku.php?id=metallurgical_microscope
Struers. (n.d.).
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