c6 metallography

8/8/2019 C6 METALLOGRAPHY

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METALLOGRAPHYMETALLOGRAPHY

Metallography iso the microscopic examination of the microstructure (grain

structure) of metals

the most important tool in metallurgy, provides invaluableinformation on the processing history and properties ofmetals

used as a quality control tool, in failure analysis and for alloydevelopment

It may involve techniques and tools such as visualinspection, low-powered magnification, optical microscopes,electron microscopes and X-ray crystallography.

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MACROSCOPIC vs MICROSCOPICMACROSCOPIC vs MICROSCOPIC

ANALYSISANALYSIS

To reveal the general structure oflarge areas of a specimen

Equipment: binocular microscope

Magnification: 10x

MACROGRAPH: a sketch of theetched surface of the specimenmade from a macroscopic

examination

PHOTOMACROGRAPH: aphotograph of the etched surfacemade from a macroscopicexamination

To gain a representative view ofmicrostructure

Equipment: metallurgical

microscope Magnification: 25x to 50x

100x to 1000x

METALLOGRAPH: ametallurgical microscopeequipped to photograph

microstructures and producephotomicrographs.

PHOTOMICROGRAPHS: aphotographs of microstructure

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MACROSCOPICMACROSCOPIC MICROSCOPICMICROSCOPIC


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PREPARATIONPROCESSOFPREPARATIONPROCESSOF

METALLOGRAPHICTESTPIECEMETALLOGRAPHICTESTPIECE

Metallographic tests: the most appropriate testsfor determining the constitution of steels

Metallographic tests include these following:

taking samples/sectioning

smoothing

polishing

etching

cleaning-drying

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A good representative of the sample.

Sectioned, ground and polished so as to minimize

disturbed or flowed surface metal caused bymechanical deformation, and thus to allow the true

microstructure to be revealed by etching.

Free from polishing scratches and pits and liquid

staining.

Polished so that inclusions are preserved intact.

Flat enough to permit examination at high

magnification.

A well-prepared metallographic specimen is:


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TAKINGSAMPLETAKINGSAMPLE

Samples may be taken for the following purposes:

to obtain a sample representative of a material. In thecase of laminated materials, a sample must be taken of

the sample in the non-laminated section, and thenanother, in the direction of the lamination.

to observe defects suspected to exist in a particulararea.

to study the causes of breakage in a part, in which case,

the sample must be taken from the area affected by thebreakage. An additional, healthy sample should also betaken from a different area of the same part, in orderto study the differences.

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Orientation ofSpecimenOrientation ofSpecimen--Square/Rectangular BarSquare/Rectangular Bar

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DIRECTIONOF WORKDIRECTIONOF WORK

TRANSVERSESECTION

LONGITUDINALSECTION

MICROSTRUCTURE

MICROS

TRUCTU

RE

Figure 1: Longitudinal & transverse orientations of specimensfrom a worked metal exhibit different microstructures.


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Orientation ofSpecimenOrientation ofSpecimen--Round BarRound Bar

DIRECTIONOF WORKDIRECTIONOF WORK

TRANSVERSESECTION

RADIAL

LONGITUDINAL

SECTION

MICROSTRUCTURE

MICROSTRUCTURE

TANGENTIAL

LONGITUDINALSECTION

Figure 2: Longitudinal & transverse orientations of specimens

from a worked metal exhibit different microstructures.


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Sectioning/ Specimen SelectionSectioning/ Specimen Selection

Sectioning is defined as the removal of a partial sized,representative specimen from a larger sample.

This is the main step that has to be taken when

preparing a specimen for a physical or microscopicanalysis.

When performing this operation the use of incorrectpreparation techniques may lead to falsemicrostructure.

The damage to a specimen during sectioning dependsupon the material being sectioned, the nature of thecutting device used, the cutting speed rate, and theamount and type of coolant used.

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The sample should be cut by means of a saw or abrasive disc.

In both cases, but especially so in the latter, refrigerationmust be intense, in order to keep the presence of heat from

modifying the original constitution of the part.One good rule of thumb for making certain that thetemperature is never excessive is to operate in such a waythat during cutting or later smoothing or polishing, the partnever becomes too hot to touch.

As regards the size of the sample, generally, the mostadequate would be from 20-25mm wide by 50mm long, and10-12mm thick

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Sectioning/

Specimen

Selection

Sectioning/

Specimen

Selection


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Mounting

After the metal is cut into a smallpiece, it is placed in the mountingmachine.

With the aid of a thermosettingresin powder, the specimen ismounted by the heat and pressureapplied to the powder.

This causes the specimen to besurrounded by a plastic likematerial, in a cylindrical shape.

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The mounting operation accomplishesthree important functions:

it protects the specimen edge and maintains theintegrity of a materials surface features

fills voids in porous materials

improves handling of irregular shaped samples, especiallyfor automated specimen preparation.


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GRINDING/SMOOTHING

Initial smoothing to flatten the sample is done bymeans of a disc sander.

The pressure of the sample on the sander must be

slight in order to avoid distortion and excessivescratching of the metal.

This operation means going over the sample withdifferent grain sandpapers (400/600/800/1,000).

Grinding uses sandpaper of different grades to remove

oxides

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POLISHINGPOLISHING

Polishing is the final step in production a surface that isflat, scratch free, and mirror finish on the specimen forexamination of a metal's microstructure under amicroscope.

Before final polishing is started, the surface conditionshould be at least as good that obtained by grinding with a400-grit (25 microns) abrasive.

The specimen is polished using DISCPOLISHER.

The surface must be free from pits (small, sharpdepressions) & subsurface deformation effects that leadto artifacts when the specimen is etched.

PITS are caused by the polishing operation that removetiny nonmetallic particles from the metal surface.

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POLISHINGCLOTHS

The requirements of any good polishing cloth include the abilityto hold an abrasive, long life, absence of any foreign materialthat may cause scratches, and absence of any processing

chemicals that may react with the specimen. The cloths most frequently used are canvas, low-nap, cotton,

nylon, silk and Pelon. These cloths are stretched tight on thelaps and fastened securely, usually by a band-type clamp.

Some cloths are available with a contact adhesive on the back,

which greatly simplifies installation on the wheel. Afterinstallation, the cloths are charged with the appropriateabrasive (usually in sizes from 15 microns down to 1 microns) andcarrier.


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Rough polishing is usually done with the laps rotating at 500to 600 rpm.

Cloths with a medium or high nap are ordinarily used on

slow rotating laps (less than 300 rpm) for intermediate andfinal polishing.

Felt or billiard cloths (100% virgin wool), used with 0.3micron aluminum oxide or other comparable abrasive, areexcellent for intermediate polishing of soft metals (most

nonferrous alloys and low carbon steels) and final polishingof hard materials (such as hardened alloy steels)


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Although certain information may be obtained from as-polished specimens, the microstructure is usually visible onlyafter etching.

Only features which exhibit a significant difference in

reflectivity (10% or greater) can be viewed without etching. This is true of microstructural features with strong color

differences or with large differences in hardness causingrelief formation.

Cracks, pores, pits, and nonmetallic inclusions may be

observed in the as-polished condition. In most cases, a polished specimen will not exhibit its

microstructure because incident light is uniformly reflected.

Since small differences in reflectivity cannot be recognizedby the human eye, some means of producing image contrast

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ETCHINGETCHING


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Etching is usually performed by subjecting the polishedsurface to the chemical action of some appropriate reagent.

The specimen to be etched is treated by immersion in the

appropriate reagent. It is impossible to lay down general rules for the time of

etching. Usually the desired effect will be produced betweenten seconds and two minutes.

The specimen after etching should be washed in a stream of

running water and subsequently in alcohol or acetone. The surface should be dried untouched by holding in a current

of warm air, such as supplied the laboratory shop air supply.


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When selecting etching times, it is more desirable tounder-etch than to over-etch.

If a specimen, after a first attempt is found to be

insufficiently etched, the etching process can usually berepeated without further preparation of the surface.

A specimen that is over-etched can only be corrected byrepolishing and then re-etching for a shorter time.


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The Metallurgical Microscope

A horizontal beam of light is deflected by a plane glassreflector, upward and through a microscope objective ontothe surface of the specimen.

A certain amount of incident light will be reflected from the

specimen surface back through the objective lens system andthen through a second lens system, the microscope eyepiece.

The total visual magnification obtained by the combination ofa given eyepiece and objective is equal to the product of themagnifications of the two systems.

These magnifications are usually marked clearly on theappropriate parts.


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When examining a metallographic specimen, theobjective of lowest magnifying power should first beused.

Subsequently, greater detail of particular areas can beobtained by using progressively higher magnifications.

The different objectives are mounted on a rotatinghead, so that their focal planes are very nearly at thesame level.

Thus, after focusing at the lowest magnification, onlysmall adjustments should be necessary at highermagnifications.

c6 metallography

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