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IMAGE ANALYSIS & QUANTITATIVE METALLOGRAPHY

A. N. Sinha

Scientist

National Metallurgical Laboratory

Jamshedpur - 831 007

INTRODUCTION

The metallographic examination constitutes simply a planar

section view of a three dimensional structure. It is not enough to

recognize this fact; one must also understand how shape in a three

dimensional construction can degenerate into traces in random planar

section. In fact, one must be able by mental visual skill to recreate from

slices of hard boiled egg, the oblate ellipsoid when they came. For

example, an inclusion, or porosity can appear to be different when

observed in different planer section and the assessment of volume will

be qualitative and only quantitative to the extent of visual judgement.

Actual quantitative evaluation of inclusion and porosity, their shape and

size distribution are very important for predicting the mechanical

properties of the metals and alloys. Similarly, the grain size is another

important factor in the hardenability of steels, ductility of brass and in

the ductile-brittle transition of alloys. The amount of ferrite in stainless

steels is a factor in their foregeability. The average flake size of graphite

is a control parameter in the strength of gray cast iron. These are only a

few instances where numerical limits to metallographic parameters are

practical factors in quality or production control. Since we can not

obtain large number of specimens to get correct three dimensional

picture, quantitative analysis is an appropriate alternative. Two factors

must be assumed in all such quantitative studies. The planar section or

sections be representative of the whole. In the matter of arriving at

average values for dimensions of distributed particles one must have a

knowledge of the actual or approximate shape of the particles. Few

particles are actually perfectly spherical. but this is a mathematical

convenience.

Kinds of Measurement

The quantities one may wish to determine can be derived from

one or more of several kinds of measurements based upon real analysis,

point counting, and lineal analysis.

Areal analysis involves the measurement with a planimeter (or by

other means) of the area of a microconstituent intercepted by a planar

cross section. Point counting involves the superposition of an

appropriate line grid such as transparent graph paper into a

microstructural feature, counting the number per unit area of grid

intersections which fall on that type of feature. Lineal analysis involves

the estimate of the proportion per unit length of a random line

superimposed on a micrograph occupied by the specific type of

microstructural feature. Lineal analysis involves the estimate of the


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proportion per unit length of a random line superimposed on a

micrograph occupied by the specific type of microstructural feature. In

another version of this, a count is made of the number of intersections

per unit length which a random line makes with a specific type of

distributed particle.

A basic rule in quantitative observations made on planar section is

that all three ratios are equal and that in turn they are equal to the

volume fraction.

Va a Na La

V

= A

=

N =

L

Earlier it used to be made manually and quantitative

measurements were very time consuming and difficult. Thanks to

computer technology, a large variety of Automatic Image Analysers are

available and now we can measure grain size, particle size, distribution,

aspect ration of odd shaped flakes of cast iron, retained austenite in

ferrite, and ferrite in stainless steel with ease. However, the basic

principle remains the same.

Measurement of Grain size

Most of the basic difficulties in quantitative metallographv are

contained in the problem of assignment of numerical magnitudes to the

grains of a polycrystalline aggregate. All the grains may not be of same

size and so on. Therefore, an average statistical concept is taken into

consideration assuming all grains to be spherical.

One of the basic measurements in grain size determination is the

number of grains counted per unit area of planar field of observation.

From analysis of the distributions of sections which will be encountered

in space occupied by truncated octahedra, the ASTM system gives the

following relationship :

n 2N-1

where n = number of grains per square inch.

at 100 x magnification

& N= ASTM grain size number

Standard charts are available and by comparing the

microstructures at 100 x magnification one can determine the ASTM

grain size of the microstructures. Image Analysers are now available for

this with suitable programming.

Measurement of Particle Sizes

The volume fraction of a dispersed phase can be determined by

simple lineal, areal, or point count analysis and this determination is

independent of the shape, ideal or otherwise, which the particles

posses. This is not true, however. for the problem of estimating the

number of particles per unit volume and the dimensions of particles. In

both the cases, the shape of the particle must be known. In practice


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one must be able to decide whether the distributed phase approximates

a sphere, a disc, a rod, an ellipsoid, or some such ideal geometric

shape. Relationships have been derived for particle density and

dimensions using parameters measurable on a polished section. Each

relationship pertains to a specific geometric shape and assumes a

uniform size distribution. The problem of nonuniform size distribution

must be considered separately. However, all these rigorous

mathematical derivations are beyond the scope of present course.

Quantitative metallography equipments can do these without any human

factor, except to use them with skill. A histogram of particle size and

volume can obtained easily which gives the idea of distribution.

Normally inclusions are determined by this method. However, for day to

day work, ASTM charts are available which are designated A, B, C & D

for Sulphides, Alumina, Silicates and Oxides types of inclusions. They

are thick and thin. All these charts are at 100 x magnifications. By

comparison with these charts, inclusions of the microstructures can be

identified and quantified. The volume % of these inclusions can be

measured with the help of Image Analyser.

Automatic Image Analyser

Now-a-days a large number of versatile programmed Automatic

Image Analysers are available for quantitative metallography. These can

be operated manually also. The image analyser is interfaced with an

optical microscope which produces microstructures and is transmitted

to TV screen with the help of TV camera. It works on the basic

principle of differentiating between the greyness of phases or particles,

which can be initiated electronically as per choice. Once initiated.

these phases or particles can be measured both manually as well as by

automatic computer programming. The image analysers have got three

options, namely area, count and intercept functions. Sometimes we may

like to edit the picture on TV screen. Therefore, image editors are also

available with image analyser. Besides optical microscope. the TV

camera can be shifted for analysing the photographs also. No

metallography laboratory can be complete without image analyser.

However. for routine type of industrial work ASTM charts for

ASTM grain size. inclusions charts for Alumina. Sulphides, Silicates and

Oxides are sufficient, as the image analysers are very costly and are

needed for advanced quantitative metallography only. Here, one thing

must be mentioned that special etching is needed to produce different

greyness to the required phases. without which image analyzer is

useless. Probably, colour metallography laboratory is a must.

REFERENCES

1.

R.T. Howard and M.Cohen. Trans. AIME 172, 413-426 (1947).

2.

C.S,Smith and L.Guttman, Trans. AIME, 197, 81-87 (1953).

3.

J.E.Hillard and J.W.Cahn. Trans, AIME, 221, 344-352. 1961.

4.

R.L.Fullman, Trans, AIME, 197, 447-452 (1953).

5.

William Rostoker and James R.Dvovak, Interprtation of

Metallographic Structures, Academic Press, New York, 1965.

6. Metals Handbook - ASM

J-3


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