1. AIM OF THE EXPERIMENT
Teaching how to polish and to etch samples before microscopic analyses.
2. THEORETICAL INFORMATION
2.1. Sample Selection
In metallographic analysis, sampling is very important since the sample to be
investigated have to represent the chemical and physical properties of the whole
material, which makes the metallographic analyses valuable. In sampling, even there
are no strict rules, some principles are highly recommended to be obeyed. However,
in some case the one making sampling shall use his own knowledge and ability to
accomplish his responsibility. For example, during sampling of the hammered or
rolled material, it is important to examine both its longitudinal and transversal
sections. This suggestion is also valid when non-metallic inclusions would be studied
metallographically.
The analyses on the transversal sections of the hammered or rolled material reveal
that there is a differentiation in the microstructure of the sample from the surface to
the center of the material.
For the carburized or decarburized materials, it is also recommended to make analyses
on their transversal sections.
Added to this, in some cases depending on the quality of the investigation sampling
from the corner, thin and thick regions of the same material have to be done.
Choosing an appropriate cutter tool (such as saws, chisels, lathes, cutting stone,
hammer crusher and oxyacetylene) to make sampling is very crucial. Since every
cutting tool would destroy the material microstructure up to a limit, it is important to
decide which tool would give less damage to the working material. For instance, the
use of oxyacetylene as a cutting tool changes the metal phase to liquid, then solidify
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to form a solid phase. Thus not only the surface properties but the microstructure of
the metal sample (ex:grains) will also change because of the high temperature.
Moreover, the use of the saws, cutting stone and chisels induce high temperature into
the metal, hence with cold working process they can both have a negative effect on
material and change its structure.
In case of working with cutting tools other than the oxyacetylene, it is possible to
control the heating effect by making the cutting action very slowly and cooling the
material with water. Plastic deformation problem could be resolved by grinding the
surface gradually.
Oxyacetylene is only used when big sized sample would be taken from the bulk
material. Never use this tool in the final sampling method.
In principle, it is highly suggested to chose a cutting tool which results less plastic
deformation and less increase in temperature within the material. Because this
sampling method is only made to understand the properties of the bulk material,
therefore having a sample representing the bulk material properties is very important.
If the sample will be polished manually, then the sample must be mounted, which
limits the sample size to 25 mm in diameter, and 20 mm in longitude.
In metallography laboratories, mostly water circulated polishing discs are utilized.
Chose of the appropriate sized cut-off machines and polishing disc must be done by
the operator. These polishing discs are made of a mixture of Al2O3, SiC and
diamonds with a binder; afterward a pressure is applied to gather them in a disc. For
particular machines, polishing disc made of diamond is preferred. This kind of
machines are used to prepare samples to be investigated in electron microscope since
the deformation resulted from the cutting process must be minimum. Figure 2.1
reveals an example of the machine used in metallographic analyses laboratories.
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Figure 2.1. A typical cutting tool equipment used in laboratories
2.2. Mounting of the sample
The mounting operation accomplishes three important functions (1) it protects the
specimen edge and maintains the integrity of a materials surface features (2) fills
voids in porous materials and (3) improves handling of irregular shaped samples,
especially for automated specimen preparation.
Each sample is mounted by placing it in a mold--usually cylindrical in shape--and
filling the mold with a suitable material that embeds the sample and holds it firmly
during the grinding and polishing operations that ensue.
Two kinds of mounting procedure may be used:
A)
hot mounting under a compression
B)
Cold mounting
In hot mounting, the mold is filled with a synthetic powdered material that liquefies
and embeds the sample when the mold is heated and subjected to a moderate pressure.
Special devices, known as mounting presses, are needed for this operation (Figure
2.2.). Generally hydraulic press with 1000 kg/cm2 are used. The mounting material is
usually opaque, but it is often considerably harder, when set, than the cold-mounting
resins. A particular care must be taken on the material selection. The temperature
needed to melt the mounting powder (160oC approximately) will not be so high as to
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alter the structure of the metal sample.
Figure 2.2 A typical molding machine used in laboratories
The temperature and the pressure applied in hot molding may change the
microstructure of some metals, thus the use of cold mounting is preferred in that case.
In cold mounting, synthetic resins (such as epoxy, polyester, and acrylic) containing
resin and hardening chemicals at the same time are widely used. Once the
components are added, they are mixed and poured into the mold containing the
sample (Figure 2.3). The mixture will be cool after an exothermic polymerization
reaction. Different than the hot molding the mold can have any size, therefore it is
possible to analyze many small samples in one time.
Before mounting, it is important to make rough grinding to clean the small burrs
formed after the cutting process.
After the mounting process if an electropolishing will be done, mounting the sample
in a conductive mold is highly recommended. Thus, generally an addition of copper
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or iron into the thermoset or thermoplasticmaterial is done to make the mold
conductive.
Figure 2.3. Cold mounting process
If the brittle edges of a sample would be analyzed, the edges have to be fed up. So,
plating the sample with copper, nickel or iron up to 3-5 μm will make the edges
stronger; hence metallographic investigation of the edges becomes possible.
If numerous samples are studied in one time, coding the sample after the mounting
will prevent to have any problem in future.
2.3. Grinding and Polishing
Once the sample has been cut off and mounted, the resin block must be ground flat to
make a microscopic investigation. The standard procedure at this stage is to make
grinding with abrasive discs, progressively finer. The aim is to cancel the resultant
effects of the previous grinding step and have a smooth sample surface.
The grain size of the abrasives is given as micron or mesh. The sandpaper is noted
with its number. Mostly, the sandpapers are made of SiC. Table 2.1 demonstrates that
by increasing the sandpaper number, the grain size is decreased.
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Table 2.1.The sand paper properties
Sand paper number
Long name
Short name
Grain
Size
(micron)
80
4
-
210-177
150
3
-
105-88
180
2
-
88-74
240
1
-
553-45
320
0
1/0
37-31
400
00
2/0
31-27
600
000
3/0
22-18
As it was previously discussed once the sample is cut off by a saw, there will be
resulting traces on the cross sectional surface of the sample. Figure 2.4 schematically
shows the resulting microstructure of a sample after being cut off by a saw.
The polishing of a sample will be done progressively in 4 steps:
1.
Rough grinding
2.
Fine grinding
3.
Rough polishing
4.
Fine polishing
The aim of the rough grinding is to remove any traces left form the sampling process.
First the curs and the traces from the cutting process will be cleaned up then grinding
with 0, 150, 240 and 320 numbered sand papers are used for grinding. In rough
grinding process, it is crucial to prevent heating of the sample, hence rough grinding
with water is widely used. The presence of liquid (water in that case) not only cools
the sample but also supplies homogenous media for grinding to get a well prepared
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sample.
Figure 2.4. the cross sectional view of the samples after being cut off by a saw. The
affected regions are named as A,B, and C. D shows the original microstructure of the
sample
For a fine grinding, 400, 600, 800, 1000 and 1200 numbered papers are used. In
mechanical polishing, the sample must be held rigidly, so that it does not rock or
move out of a single grinding plane. Starting with the coarsest grit paper, the sample
is moved backwards and forwards over the paper until a uniform ground finish is
obtained. It is then carefully washed under running water, examined, and returned to
the next grade of paper, rotating the sample through 900 before grinding
recommences. This process is repeated for the subsequent grinding papers, rotating
the specimen through 90o on each paper. It is very important completely to eliminate
the scratches from the previous grinding stage otherwise they will not be removed in
polishing (Figure 2.5).
It is important to prevent any inclination formation on the sample surface, resulted
from the difference in the forces applied on the sample, during the grinding process.
An inclined surface will cause difficulty when the sample is analyzed under the
optical microscope.
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Figure 2.5. The recommended grinding directions are AA and BB.
Rough and fine grinding are done by using polishing discs. Their diameters are in the
range of 20-25cm. They are made of brass or bronze (Figure 2.6). Their rotation
speeds change from 150 to 350 rpm. High speed is preferred during the rough
polishing and low speed is chosen in fine polishing.
(a) (b) (c)
Figure 2.6–Polishing wheels with discs a) polishing wheels unit b) polishing wheel,
c) automatic polishing wheels
The discs are recovered by canvas, hard broadcloth, flannel, nylon, poplin or velvet
depending on the polishing step and the sample properties. The main property of the
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polishing tissue is to have a homogenously ticking textured. The polishing discs may
also be utilized in grinding process; in this case they are covered by sandpapers.
The abrasives utilized during polishing are made of
alumina (15- 0,3 μm) or
alumina (0,05 μm); and both have to be applied on the surface with distilled
water, in a suspension. During the polishing of the sample on the discs, the alumina
solution is applied on the polishing tissue at intervals. Here in it is important to notice
the moisture level of the polishing tissue. The minimum moisture content is removed
immediately (in 1-5 seconds) once the disc contacts with air.If the moisture level is
less than this, there could have some staining on the sample surface. On the other
hand, excessive moisture will prolong the polishing time and generates pinholes on
the sample surface.
Polishing is carried out by holding the specimen against the rotating polishing cloth.
To minimize directional polishing effects, and prevent the formation of comet like
defects on the sample surface the sample should be rotated in a direction opposite to
that of the wheel rotation.
The compression exerted onto the sample is advantageous during rough polishing, but
has to minimize during fine polishing. It is difficult to specify how much pressure
must be used: too little pressure retards the rate of polishing and may result in some
pitting of the surface; too much pressure may distort the surface. The correct
polishing pressure varies with different metals and can only be learned through
practice.
As a result of a successive polishing process, the surface of the sample has a mirror
like surface appearance. But it would be better the surface properties are checked by
observing via an optical microscope (100x magnification).
If the sample is observed via an optical microscope before the etching process, its
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surface has to be cleaned with an alcohol and dried in air. If any stain of water left on
the surface, this will destroy the homogenous etching effect.
In general, diamond is not used in polishing. However for the samples having two
phases (hard inclusion in soft matrix) it can be used after the rough polishing process.
This will decrease the process duration and enhances the precision of the analysis.
The usual range of diamond powder sizes are: s15 micron to one-quarter micron (15
to 0.25 μm). The diamond powders are usually supplied as tubes of paste. The
diamond paste is mostly kept in a syringe. A low viscosity metallographic oil (carbon
tetrachloride) is used as polishing liquid with diamond paste to distribute diamond
powders homogenously on the sample surface.
2.4. Etching
When the polished sample is investigated under the optical microscope, it is rarely
happened to see the structure of the sample. Only the non-metallic inclusions,
porosities and cracks may be easily seen from the surface. Thus, it is beneficial to
investigate the sample structure under the microscope after the polishing step.
However the polished sample surface reflects the light equally, it would be too
difficult to detect any differentiation on the sample surface; hence a generation of a
contrast is highly required in that case.
Thus the crystalline structure (grains and grain boundaries) of the polished surface is
revealed by etching with a proper etchant. This process is named as “chemical
etching” or “etching” in brief. It is based on the rate difference in chemical attack
depending upon chemical composition, energy content, and grain orientation of the
sample. Since the grain boundaries are attacked at a greater rate than the proper grain
due to higher energy content of the grain boundaries. In addition, the presence of
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chemically different phases results in variations in the rate of chemical attack. These
changes in the rate of chemical attack produce deviations both in angle and depth of
certain portions of the surface. Thus the light is reflected in varying amounts
depending on the angle and depressions of the portion of the surfaces resulting in light
and dark regions. In this manner the crystalline microstructure of the specimens are
revealed.
2.4.1. Etching of a sample having multi-phases structure
The etching mechanism of the multiphase structure has electrochemical properties.
Once the sample is reacted with the reactive, a potential difference between the
different phases is aroused. The phase having higher electropositive potential behaves
like a positive electrode and starts to be dissolved during etching. The other phase
behaves like a negative electrode (cathode) will stand out without any change.
The concentration of the etching solution and the process duration have to be
optimized to prevent over etching effect of the sample.
During etching the dissolution of the positively induced phase, will result in a crater
formation. During the microscopic analysis, the light can not reflect this region, thus a
darkness is detected. On the other hand, for the negatively induced phase a reflection
of light occurs and bright appearance is detected (Figure 2.7).
Figure 2.7. Etching of a sample having multi-phases structure
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2.4.2. The etching of pure metals and one phase structured materials
The etching of homogenous one phase structured material is different than that of the
multiphase structured samples. In this case,the etching mechanism is based on an
electropositive process since the potential difference between the matrix material and
inclusions or the bulk material and the grain boundaries are so small that it is highly
difficult to detect any difference among them. Thus, it is aimed to notice the
differentiation in the grain depending on their dissolution rate. The grains having the
same orientation have the same brightness when the sample is observed under the
optical microscope. This process is called as “oriented grain brightness”. This
difference may be detected particularly in long etching process duration.
In case of grain boundaries, since they exhibit high energy compared to the bulk
material, they are preferentially dissolved when the sample contacts with the reactive
resulting in the formation of cracks among the valleys (Figure 2.8).
Figure 2.8– In grain boundaries a) the valley formation after the etching process b)
view of the grain boundaries under the optical microscope investigation.
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If the etching time exceeds a limit, the preferential dissolution of the sample ends up
and the dissolution of the whole sample starts. This case is named as “excessive
etching”. The microscopic view of the over-etched sample is only dark. To be able to
analyze this sample, it is recommended to make fine polishing and etching again.
2.4.3. Reactives used in etching
Generally the reactives used in metallographic etching are formed when the solvents
(one or more) made of water, alcohol, glycerin and glycol mixed with different
organic/anorganic acids, various alcalines and other complex compounds.
The reactivity and the effect of the reactives depend on
a)
Hydrogen ion concentration b) Hydroxyl ion concentration c)the blackening
effect of one or more components.
Each compounds needs an appropriate reactive. The reactives used in steel and
hammered iron are given in Table 2.2. For each application, the reactive must be
prepared freshly and used immediately.
The etching process is very easy but needs a particular attention. The main important
point of this process is to chose the appropriate reactive. In order to achieve the
desired effect, the application rules of the reactive where the concentration of the
reactive, the process duration and conditions have to be obeyed.
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Table 2.2 –The etching reactive used in steel and hammered iron
The reactive composition
1-10 ml HNO3 90-99 ml methanol or
ethanol
4 gr picric acid 100 ml ethanol
1-4 gr K3Fe(CN)6 10 gr KOH 100 ml
water
The application
It is known as Nital.This is the mostly used solution
for the etching of iron, alloyed - pure stell and cast
iron. It demonstrates the ferrite grain boundaries and
phases within the sample structure.
It is known as Pikral.It is recommended to be used for
samples having ferrites and cementite phass. This
reactive can not demonstrate ferrite’s grain
boundaries.
It has to be used freshly. The sample is etched when
the sample temperature arrives to its boiling degree.
The Cementite phases are seen as black and the
perlite phases are noticed as brown.
100 ml ethanol
The reactive of Vilella. It is used for samples having
ferrite and carbide structures.
5 ml HCl 1 gr picric acid
It gives information about the austenite grain size and
notices the phases present in the stainless steel.
30 ml water 25 ml ethanol 40 ml HCl
5 gr CuCl2
The reactive of Fry. It is used to demonstrate the
defects in the steel resulted from the cold deformation
process.
15 ml HCl 20 ml water 1 gr
CuCl2 5 gr picric acid 135 ml ethanol
10 gr oxalic acid 100 ml water
The reactive of Noren. It is used to show the cracks
formed in the region affected by the welding process.
This is an electrolytic etching reactive used for steel.
The distance between the cathode and the anode
material is kept as 25 mm, the applied voltage is set to
6V. The carbides become remarkable in 15-30
seconds, the grain boundaries become outstanding in
45-60 seconds.
2.4.4. Electrolytic etching
The electrolytic etching of a sample is done in an electrolysis cell. This process is
particularly used for the electrolytic polishing of stainless steel. The solution used in
electrolytic polishing may be acidic/basic or salt contained. The samples will be
anode in most cases. D.C. current is applied generally on the specimen. The etching
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process is controlled by varying the applied potential value and process duration.
2.4.5. Macro-etching
This process is utilized to reflect the heterogeneity exist in the metals and alloys.
Added to this,
- Different structural defects left from the solidification or mechanical process
(differentiation in grain size, traces left from the deformation process, columnar or
dendritic structures,…),
-Heterogeneities resulted from the carburization or decarburization processes,
segration and carbid-ferrite band formation,
- Physical defects (porosities, crackes,…) may be noticed by the macro etching
process.
Moreover, to observe the welding, heat treatment and quenching effects (the depth of
the affected region, the presence of cracks), on the sample microstructure we can also
utilize macro-etching process
In steel industry, this process is widely used for quality control. The structure of the
ingot, segregation and inclusion presence are detected by this process.
In hammered alloys, by following the traces left from the hammering, an optimization
in hammering process is also achieved.
In order to make macro-etching, a grinding up to 400 numbered sandpaper is done.
Mostly, there is no need to polish the sample surface before the macro-etching.
2.5. Metallurgical (Optic) Microscope
The microstructure of the metals and alloys is investigated by metallurgical
microscopes.An optic microscope has the maximum 2000x magnification and 1000Å
separation efficiency (lateral dissolution). For higher magnification analyses, electron
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microscopy has to be used, which is not in the scope of this study. Unlike biological
microscopes, metallurgical microscopes must use reflected light. Figure 2.9 presents a
simplified ray diagram of the illuminating and imaging system of a metallurgical
microscope.
Light incident upon the specimen is reflected back from the specimen surface. Any
light that reflects back from specimen features which are approximately normal to the
optical axis (i.e. features that are perpendicular to the incident light beam) will enter
the objective, pass through the plane glass reflector, travel on to the eyepiece, and will
form the bright portion of the image one sees. Any light that is reflected back from
features inclined to the optical axis (i.e. features that are not perpendicular to the
incident light beam) will be scattered and will not enter the objective. Such features
will thereby appear dark in the image one sees. The final image of the specimen,
formed by the eyepiece(s) of the microscope, is thus bright for all features normal to
the optical axis and dark for inclined features. In this way, the various microstructural
features of a metallographic specimen (such as grain boundaries that have been etched
to produce grooves with inclined edges, precipitate particles, and non-metallic
inclusions) areall revealed in the image of the specimen. Figure 2.10 presents a
schematic diagram showing bright and dark image areas corresponding to reflection
from normal or inclined features on the specimen surface.
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Figure 2.9 Specimen image under bright-field illumination.
Figure 2.10Schematic representation of an etched sample having two phases.
3. EQUIPMENTS USED in EXPERIMENTS
- Different cut off machines and equipment used to get a sample to be analyzed from
the bulk material.
- Sample marking machine
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- Mounting machine and equipment (Bakelite machine, Bakelite powder, resin mold,
polyester resin)
- Grinding system, numerous sand papers (80 - 1200 numbers)
- Polishing system (polishing baize, alumina, distilled water, diamons paste, etc)
- Sample drying system
- Etching equipment (alcohol, nitric acid, picric acid, cotton etc)
4. EXPERIMENTAL PROCEDURE
1- A sample with 1cm length is cut off,
2- The burrs of the sample is cleaned up by abrasive stone,
3- Then the sample is mount in Bakelite mold.
4-With an abrasive stone the burrs of the Bakelite mold is removed off, then the
sample is marked with a machine.
5- Grinding the sample with sand papers 80, 150, 320, 400, 600, 800 and 1200
6- After the grinding process, the sample is rinsed with water.
7- Once the rough grinding step is finished, the sample is polished with the polishing
disc number 1. The suspension named as 1 is used during the experiment.
8- For fine polishing, the sample is polished by using the polishing disc named as 2.
The suspension called as 2 is used during the experiment.
9- Once the polishing step is end up, the sample is rinsed with distilled water.
10- The sample will be rinsed with alcohol and dried in the drying machine.
11- The sample will be observed by using an optical microscope with 100x
magnification to decide if the polishing is enough or not. (If it’s not enough then you
can continue to polish the sample)
12- Some amount of cotton is kept by using pincers. The cotton is immerged into
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Nital solution then applied on the sample surface. This process continues up to a limit
where the surface of the sample is not polished anymore (becomes opaque). (be
careful, not over-etched the sample!!!)
Nital = %1-5 Nitric acid (HNO3) + %95-99 Alcohol(CH3OH),
13- when the sample is etched, it has to be rinsed with water then with alcohol, finally
the sample is dried in the drying machine.
14-Under the optic microscope the sample’s structure will be investigated. If the
etching is not enough to detect the differentiation in the microstructure, then the last
two steps will apply once again.
5. HOMEWORKS
1- What were the tools you used to prepare bulk material for sampling? What
parameters helped you to chose those tools? How did you make sample selection?
2- What was the reactive you used during the etching process? What is the procedure
that you applied while making etching process (indicate the process duration
particularly)? Please indicate schematically how to get a view of an etched surface
when it was investigated by an optical microscope.
3- Draw schematically the view of the sample detected under the optical microscope
with 100x magnification, before and after the etching processes.
4- Please indicate precisely the problems that you faced while preparing the sample
for the experiments and making the experimental analyses. Moreover, explain in
detail the solution that you find to resolve all those problems.
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REFERENCES
1- GEÇKINLI, A.E., 1989. Metalografi, I.Chapter, I.T.U.,1391, Istanbul.
2- ASM, 1973. Metals Handbook, Vol.8, 8th Ed., Ohio.
3- GEÇKINLI, A.E., ÇGIMENOGGLU, H., YILDIRIM, S., BAYDOĞAN, M.,
2001. Çelik ve Dökme Demirlerin Metalografisi ve Mekanik Muayenesi,
TMMOB Metalurji Mühendisleri Odası, Istanbul.
4- KEHL G. L. The Principles of Metallographic Laboratory Practice.
5- RHINES F. N. Phase Diagrams in Metallurgy, Their Development and
Application.
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