c4 metallography

8/10/2019 c4 Metallography

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SHARMIWATI


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METALLOGRAPHY

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 MICROSCOPIC

ANALYSIS

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: metallurgicalmicroscope

Magnification: 25x to 50x

100x to 1000x

METALLOGRAPH:ametallurgical microscopeequipped to photographmicrostructures and producephotomicrographs.

PHOTOMICROGRAPHS:aphotographs of microstructure

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MACROSCOPIC MICROSCOPIC


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PREPARATION PROCESS OF

METALLOGRAPHIC TEST PIECE

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 by

mechanical 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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TAKING SAMPLE

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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TAKING SAMPLE

Orientation of Specimen-Square/Rectangular Bar

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DIRECTION OF WORK

TRANSVERSESECTION

LONGITUDINALSECTION

MICROSTRUCTURE

MICROSTRUCTURE

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


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TAKING SAMPLE

Orientation of Specimen-Round Bar

DIRECTION OF WORK

TRANSVERSESECTION

RADIALLONGITUDINAL

SECTION

MICROSTRUCTURE

MICROSTRUCTURE

TANGENTIAL

LONGITUDINALSECTION

Figure 2: Longitudinal & transverse orientations of specimens

from a worked metal exhibit different microstructures.


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TAKING SAMPLE

Sectioning/ 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 whenpreparing a specimen for a physical or microscopicanalysis.

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

microstructure.

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The damage to a specimen during sectioning depends upon thematerial being sectioned, the nature of the cutting device used,the cutting speed rate, and the amount and type of coolant usedThe 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 frommodifying 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 way that

during cutting or later smoothing or polishing, the part neverbecomes too hot to touch.

As regards the size of the sample, generally, the most adequatewould be from 20-25mm wide by 50mm long, and 10-12mm thick

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TAKING SAMPLE

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 by

means of a disc sander.

The pressure of the sample on the sander must beslight 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 removeoxides

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POLISHING

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 DISC POLISHER.

The surface must be free from pits(small, sharp

depressions) & 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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Rough polishing is usually done with the laps rotating at 500 to600 rpm.

Cloths with a medium or high nap are ordinarily used on slowrotating laps (less than 300 rpm) for intermediate and finalpolishing.

Felt or billiard cloths (100% virgin wool), used with 0.3 micronaluminum oxide or other comparable abrasive, are excellent forintermediate polishing of soft metals (most nonferrous alloys andlow carbon steels) and final polishing of hard materials (such ashardened 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 inreflectivity (10% or greater) can be viewed without etching.

This is true of microstructural features with strong colordifferences 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

must be employed..SHARMIWATI

ETCHING


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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 theappropriate reagent.

It is impossible to lay down general rules for the time ofetching. 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 beinsufficiently etched, the etching process can usually be

repeated without further preparation of the surface. A specimen that is over-etched can only be corrected by

repolishing and then re-etching for a shorter time.


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Common Etchants

There are a number of more common etching solutions.Several of them are listed in the following table.

The following table lists the most commonly used etchants

Etchant Composition Conc. Conditions Comments

ASTM No. 30

AmmoniaHydrogen Peroxide(3%)DI Water

62.5 ml125 ml62.5 ml

Mix Ammonia andwater before addingperoxide. Must beused fresh. Swab 5-45 seconds

For etching copper, copperalloys and copper-silveralloys.

Adler Etchant

Copper ammoniumchloride

Hydrochloric acidFerric chloride,hydratedDI Water

9 grams

150 ml45 grams75 ml

Immersion is

recommended forseveral seconds

For etching 300 series

stainless steel and othersuperalloys

CarpentersStainlessSteel Etch

FeCl3CuCl2Hydrochloric acidNitric acidEthanol

8.5 grams2.4 grams122 ml6 ml122 ml

Immersion etchingat 20 degreesCelsius

For etching duplex and300 series stainless steels.

Kalling's No. 2CuCl2Hydrochloric acidEthanol

5 grams100 ml100 ml

Immersion orswabbing etch at 20degrees Celsius

For etching duplex and400 series stainless steelsand Ni-Cu alloys andsuperalloys.

Kellers Etch

Distilled waterNitric acidHydrochloric acidHydrofluoric acid

190 ml5 ml3 ml2 ml

10-30 secondimmersion. Use onlyfresh etchant

Excellent for aluminumand titanium alloys.
http://www.metallographic.com/Technical/Etching.htm


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Klemm'sReagent

Sodium thiosulfatesolutionPotassium

metabisulfite

250 mlSaturated5 grams

Etch for a fewseconds to minutes

For etching alpha-betabrass, bonze, tin, cast ironphosphides, ferrite,martensite, retained

austenite, zinc and steeltemper embrittlement.

Krolls

Reagent

Distilled waterNitric acidHydrofluoric acid

92 ml6 ml2 ml

Swab specimen upto 20 seconds

Excellent for titanium andalloys.

NitalEthanolNitric acid

100 ml1-10 ml

Immersion up to afew minutes.

Most common etchant forFe, carbon and alloyssteels and cast iron -Immerse sample up from

seconds to minutes; Mn-Fe, MnNi, Mn-Cu, Mn-Coalloys.

Marble'sReagent

CuSO4Hydrochloric acidWater

10 grams50 ml50 ml

Immerse or swabfor5-60 seconds.

For etching Ni, Ni-Cu andNi-Fe alloys andsuperalloys. Add a fewdrops of H2SO4 toincrease activity.


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Murakami'sK3Fe(CN)6KOHWater

10 grams10 grams100 ml

Pre-mix KOH andwater before addingK3Fe(CN)6

Cr and alloys (use freshand immerse); iron andsteels reveals carbides; Mo

and alloys uses fresh andimmerse; Ni-Cu alloys foralpha phases use at 75Celcius; W and alloys usefresh and immerse; WC-Coand complex sinteredcarbides.

PicralEthanolPicric acid

100 ml2-4 grams

Seconds to minutesDo not let etchantcrystallize or dry explosive

Recommended formicrostructures containing

ferrite, carbide, pearlite,martensite and bainite.Also useful for magneticalloys, cast iron, high alloystainless steels andmagnesium.

VilellasReagent

Picric AcidHydrochloric acid

Ethanol

1 gram5 ml

100 ml

Seconds to minutes

Good for ferrite-carbidestructures (temperedmartensite) in iron and

steel


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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 thespecimen 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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