MICROSTRUCTURE EXAMINATION OF STEEL TITLE
Microstructure examination of steel OBJECTIVE •
To observe the constituents and structure of metals and their alloys by means of an optical microscope.
INTRODUCTION
Microscopic examination with digital imaging Microstructure analysis is conducted by microscopic examination, a process that studies the structure of materials under magnification. The properties of a material determine how it will perform under a given application and these properties are dependent on the material’s structure. Industrial processes or treatments such as casting, welding and heat treating are often applied to metals metals to prepare them for particular particular applications applications and to improve improve their characterist characteristics ics and properties. A microscopic examination may be conducted to evaluate the effects of a process on material using optical microscopy at low magnification or scanning electron microscopy (!M" under high magnification. There may be residual effects of these processes and treatments, inclusion or contaminants that can be explained by microstructure analysis and microscopic examination. In many cases, the investigation centers on the correlation between the resulting microstructure and the material properties. #or example, exposure of carbon and alloy steels to elevated temperatures during heat treatment can cause a loss or gain of carbon near the surfaces of the parts if the atmosphere in the furnace is not properly controlled. $ecarburi%ation causes the surface to be soft and wea& with little wear resistance, while unwanted carburi%ation can cause the surface to become too brittle. Also, if austenitic stainless steel does not see sufficient temperature for enough time or does not receive a sufficiently rapid 'uench during heat treating, the carbon in the alloy will form chromium carbides on the grain boundaries which will ma&e the material brittle and susceptible to inter granular corrosion. A sensiti%ation test will reveal this problem. n the other hand, scanning electron microscopy is used to determine abnormalities such as inclusions, segregation, and surface layers, as well as fracture features. )hen used in combination with energy disper dispersiv sivee *+ray *+ray spectr spectrosc oscopy opy (!$", (!$", the micros microstru tructu cture re analys analysis is can identi identify fy inclus inclusion ion type type and corrodents on the fracture face.
Figure 1 (Callister Jr W.D., 2010) 1
Microscopy (Optical)
)hen a polished flat sample reveals traces of its microstructure, it is normal to capture the image using macrophotography. More sophisticated microstructure examination involves higher powered instruments optical microscopy, electron microscopy, *+ray diffraction and so on, some involving preparation of the material sample (cutting, microtomy, polishing, etching, vapor+deposition etc.". The methods are &nown collectively as metallography as applied to metals and alloys, and can be used in modified form for any other material, such as ceramics, glasses, composites, and polymers. Two &inds of optical microscope are generally used to examine flat, polished and etched specimens a reflection microscope and an inverted microscope. -ecording the image is achieved using a digital camera wor&ing through the eyepiece.
TEOR! ( scitation, [Online], Available: htt:!!scitation.ai.org!content!ai!"ournal "
The theory exhibits all four attributes of normal grain growth uniformity, scaling, stability, and lognormality. A prime new feature of the theory is the division of the grains into topological classes (/ planar, 0/ spatial", each with a lognormal distribution of grain si%es. 1rowth is found to be controlled by the rate of loss of grains from the lowest topological class. 2omplete solutions are found for the grain growth &inetics of each class, as well as the transfer rates between classes. The latter result is used to explain how the median diameter of those classes in which grains are shrin&ing still manages to increase in the manner re'uired to &eep their number a constant fraction of the total population. A parabolic growth law is found for the median grain si%e of the whole population as well as the median grain si%e in each topological class. The growth constant for each class is found to increase approximately as the cu be of the planar topological parameter or the s'uare of the spatial topological parameter. The -hines‐2raig structural gradient is shown to be independent of time and hence a basic constant of normal grain growth. tability is due to a maximum in the grain boundary velocity with increasing grain si%e.
AVERA"E "RAIN INTERCE#T (A"I) METOD
The average grain intercept (A1I" method is a techni'ue used to 'uantify the grain + or crystal + si%e for a given material by drawing a set of randomly positioned line segments on the micrograph, counting the number of times each line segment intersects a grain boundary, and finding the ratio of intercepts to line length. Thus, the A1I is calculated as A1I 3 (number of intercepts"4(line length". A sample with small crystals will have a high A1I value compared to a sample with large crystals.
MATERIALS AND A##ARATUS • •
ptical microscope Metal specimen 2
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Torch light A cleaned cloth Abrasive papers
#ROCEDURE •
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#irst the specimen surface was grounded and polished to a smooth and mirror li&e finish. Then it was &ept in the optical microscope and the torch or a bulb was provided to supply the necessary enlightment. The structure of the specimen was captured through the optical microscope with na&ed eyes. Then in order to determine grain si%e a random length was chosen and five lines with the same
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length which were parallel to each other was drawn in the scanned paper of microstructures. Afterwards the grains covered by each of the drawn lines was counted separately. Thereafter the number of grains intersected by each line was added and the total was obtained. The average grain was calculated after dividing the total of the grains from the number of lines
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drawn. #inally the length of a line in 5m was divided by the average grains.
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OBSERVATIONS "rai$ Si%& D&t&r'i$atio$
6ength of a line 7 89 mm 6ength in 5m 7 (:9 4 :" ; 8 .:, >, = Total of grains 7 =.: ? 0 ? >.: ? > ? = 3 /@ Average grains 7 /@ 4 : 3 >. 1rain si%e 7 89 4 >. 3 8.@>: 5m • • • •
ome rusted areas were seen in brown colour which were randomly oriented.. The surface loo&ed yellowish due to the light which was used to enlighten the area. Barallel lines were visible due to the scratches uttered while polishing the specimen. The grain boundaries were not visible as an optical microscope was used.
CALCULATIONS •
In order to determine the ATM grain si%e number (n" it is necessary to employ the following e'uation. log < 3 ( n+" log
( #$uation 1" 3
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To determine the grains per s'uare inch regarding a specific magnification in the microscope can be obtained as follows.
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Total grains 3 Average grains 3
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1rain si%e
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3
( #$uation 2% total of grains intersected from each grain Total number of grains
RESULTS • • •
Total grains Average grains 1rain si%e
3 =.: ? 0 ?>.: ?> ?= 3 /@ 3 /@ 4: 3>. 3 89 4 8. 3 8.@>: 5m
DISCUSSION 4
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CONCLUSION 7
REFERENCES
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Callister Jr W.D., R.D.G. (2010) 'Materials Sciece a! "#ieeri# ' Wile$. Experiment 3, %&lie, aila*le+ tt-+///.cs.e!tare"320Metall#ra-$.t . Materials Testing, %&lie, aila*le+ tt-+///.la*testi#.csericesaterials8 testi#etallr#ical8testi#icrsc-ic8e9aiati:stas.;<42#0s.!-= .
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