Metallographic Test Report

Metallography is the science and art of preparing a metal surface for analysis by grinding, polishing, and etching to reveal microstructual constituents. After preparation, the sample can easily be analyzed using optical or electron microscopy. A skilled technician is able to identify alloys and predict material properties, as well as processing conditions by metallography alone.

Metallographic and materialographic sample preparation seeks to find the true structure of the sample. Mechanical preparation is the most common method of preparing the samples for examination. Abrasive particles are used in successively finer steps to remove material from the sample surface until the needed result is archived. A large number of preparation machines for grinding and polishing are available, meeting different demands on preparation quality, capacity, and reproducibility.

A systematic preparation method is easiest way to achieve the true structure. Sample preparation must therefore pursue rules which are suitable for most materials. Different material with similar properties (hardness and ductility) will respond alike and thus require the same consumables during preparation.

Main aim of this examination to conduct failure analysis. Failure analysis consists of investigations to find out how and why something failed. Understanding the actual reason for failures is absolutely required to avoid recurrence and prevent failure in similar equipment. The analysis should also help with the understanding and improvement of design, materials selection, fabrication techniques, and inspection methods.



Contents:

I.Introduction
Objective of Metalography

II.Examination Procedure

a) Specimen Preparation
1)Grinding
2)Polishing
3)Etching

b) Examination Methods:
1) Microscopical Examination
2) Macroscopic Examination

III.Theory
1)Microscopical Examination
2)Macroscopic Examination

IV.Experimental Apparatus

V.Analysis – Results

1)Microscopical Examination
Specimen No 1
Specimen No 2
Specimen No 3
Specimen No 4

2)Macroscopic Examination

I .Sulphur Printing
II. Flow Lines
III. Welded Sections
VI.Discussion
VII.Conclusion
VIII.Referances

II. Introduction
Objective of Metalography

Metallography is the imaging of topographical or microstructural features on prepared surfaces of materials. The properties and performance of materials are controlled by the structures studied by metallography.

In this technique, planar surfaces are prepared to obtain a polished finish. Chemical or other etching methods are often used to delineate macrostructure and microstructure features,which provide information on phase distribution, grain size, solidification structure, and thermo-mechanical processing history.

Metallography is also used for characterizing the macroscopic and microscopic configuration at planar sections of welds, brazes, and fabricated components. In failure analysis, morphology or corrosion of cracks can be characteristic of failure mode. The prepared samples are examined by the unaided eye, light microscopy, and/or electron microscopy.

III. Examination Procedure

a)Specimen Preparation
A systematic preparation method is easiest way to achieve the true structure. Sample preparation must therefore pursue rules which are suitable for most materials. Different materials with similar properties (hardness and ductility) will respond alike and thus require the same consumables during preparation.

Grinding The purpose of the grinding step is to remove damage from cutting, planarize the specimen(s), and to remove material approaching the area of interest.

The most common metallographic abrasive used is Silicon Carbide – SiC. It is an ideal abrasive for grinding because of its hardness and sharp edges. For metallographic preparation, SiC abrasives are used in coated abrasive grinding papers ranging from very coarse 60 grit to very fine 1200 grit sizes. Some of the application procedures are given below.

• Soft non-ferrous metals – Initial grinding is recommended with 320 grit SiC abrasive paper followed by 400, 600, 800 and 1200 grit SiC paper. Because these materials are relatively soft they do not easily break down the SiC paper. Thus initial grinding with 320 grit is generally sufficient for minimizing initial deformation and yet maintain adequate removal rates. For extremely soft materials such as tin, lead and zinc it is also recommended that the abrasive paper be lightly coated with a paraffin wax. The wax reduces the tendency of the SiC abrasive to embed into the soft specimen.

• Soft ferrous metals – are relatvely easy to grind with the depth of deformation being a major consideration. 240 grit SiC abrasives provide a good initial start with subsequent use of 320, 400, 600, 800 and 1200 grit SiC.

• Hard ferrous metals – require more aggressive abrasives to achieve adequate material removal. Thus coarse SiC abrasives (120 or 180 grit) are recommended for stock removal requirements. Once planarity and the area of interest are obtained a standard 240, 320, 400 and 600 grit series is recommended.

• Super alloys – are generally of moderate hardness but have extremely stable elevated temperature characteristics and corrosion resistance. The procedures for preparing super alloys is very similar to that for most non-ferrous metals.

• Ceramics – are extremely hard, corrosion resistant and brittle materials. They fracture producing both surface and subsurface damage. Proper grinding minimizes both of these forms of damage. This requires the application of a semi-fixed abrasive which are held rigidly for grinding but can be dislodged under high stress in order to minimize subsurface damage. The abrasive size is also important because very coarse abrasives will remove material quickly but can seriously damage the specimen. For ceramics, consideration of the damage produced at each preparation step is critical to minimizing the overall preparation sequence. • Composites – are perhaps the most difficult specimens to prepare because of the wide range of properties for the materials used. For example, a metal matrix composite (MMC) such as silicon carbide ceramic particles in an aluminum metal matrix is a difficult specimen to prepare. This composite contains extremely hard/brittle ceramic particles dispersed in a relatively soft/ductile metal matrix. As a rule of thumb, initial grinding should focus on metal planarization and grinding to the area of interest. The secondary grinding steps require focusing on the ceramic particles and typically requires the use of diamond abrasives.

2) Polishing Polishing is the most important step in preparing a specimen for microstructural analysis. It is the step which is required to completely eliminate previous damage. Ideally the amount of damage produced during cutting and grinding was minimized through proper blade and abrasive grinding so that polishing can be minimized. To remove deformation from fine grinding and obtain a surface that is highly reflective, the specimens must be polished before they can be examined under the microscope. Polishing is a complex activity in which factors such as quality and suitability for the cloth, abrasive, polishing pressure, polishing speed and duration need to be taken into account. The quality of the surface obtained after the final polishing depends on all these factors and the finish of the surface on completion of each of the previous stages.

Polishing Cloths

There are three types of polishing clothes; Woven, Non-Woven and Flocked.
- Woven cloths offer ‘hard surface’ polishing properties and guarantee flat pre-polishing, without deterioration of the edges. - Non-woven cloths, are used on very hard materials for high precision surface finishing such as glass, quartz, sapphire and semi-conductors.
- The Flocked cloths, guarantee a super-polished finish. The polishing duration must be as short as possible, to avoid inclusions from being extracted.

Diamond products
Diamond, due to its exceptional hardness and cutting capacity, has become the first choice abrasive in metallographic polishing. Diamonds for metallographic grinding and polishing are available in two different crystalline shapes: Polycrystalline (P) and monocrystalline (M). Polycrystalline diamonds provide vast numbers of small cutting edges. In the metallographic preparation process these edges result in high material removal, while producing only a shallow scratch depth.
Monocrystalline diamonds are more block-shaped and provide few cutting edges. These diamonds give high material removal with a more variable scratch pattern. For high requirements, the (P)-type diamonds are chosen. The (M) type diamonds are best suited for all-purpose polishing. Diamond products are usually available in three forms; diamond paste, diamond suspension and diamond spray.
Polycrystalline diamond as compared to monocrystalline diamond provides better surface finishes and higher removal rates for metallographic specimen preparation. The features and advantages of polycrystalline diamond include the following:
• Higher cutting rates
• Very uniform surface finish
• More uniform particle size distribution
• Higher removal rates (self sharpening abrasives)
• Harder/tougher particles
• Blocky shaped
• Hexagonal microcrystallites (equally hard in all directions)
• Extremely rough surface (more cutting points)
• Surface area 300% greater than monocrystalline diamond
• No abrasion-resistant directionality

3) Etching
The purpose of etching is two-fold. Grinding and polishing operations produce a highly deformed, thin layer on the surface which is removed chemically during etching. Secondly, the etchant attacks the surface with preference for those sites with the highest energy, leading to surface relief which allows different crystal orientations, grain boundaries, precipitates, phases and defects to be distinguished in reflected light microscopy. There are many tried and tested etchants available but there are mandatory safety issues associated with the preparation and use of all of these. Consequently, you are limited to using just three preprepared solutions (Table). Please adhere to the general safety regulations provided to you at the beginning of the academic year.

2% Nital Ferric Chloride Sodium Hydroxide Steel Stainless steel Aluminium & alloys
- Copper & alloys Zinc & alloys
- – Magnesium & alloys
A polished sample is etched using a cotton tip dipped in the etchant. Etching should always be done in stages, beginning with light attack, an examination in the microscope and further etching only if required. If you overetch a sample on the first go then the polishing procedure will have to be repeated.

b) Examination Methods:

A)Microscopical Examination

This is performed on samples either cut to size or mounted in a resin mold. The samples are polished to a fine finish, normally one micron diamond paste, and usually etched in an appropriate chemical solution prior to examination on a metallurgical microscope. This is performed on samples either cut to size or mounted in a resin mold. The samples are polished to a fine finish, normally one micron diamond paste, and usually etched in an appropriate chemical solution prior to examination on a metallurgical microscope. In order to obtain reproducible results,with good contrast in th image, the specimen surface is polished and subsequently etched with reagents before microscopic examination.Grain boundaries ares often anodic to the bulk metal in the interior of the grain and so are etched away preferentially and delineated.Staining is produced by the deposition of solid etch product on the speciemen surface.

B)Macroscopical Examination

Macroetching is the procedure in which a specimen is etched and evaluated macro structurally at low magnifications. It is a frequently used technique for evaluating steel products such as billets, bars, blooms, and forgings. There are several procedures for rating a steel specimen by a graded series of photographs showing the incidence of certain conditions and is applicable to carbon and low alloy steels. A number of different etching reagents may be used depending upon the type of examination to be made. Steels react differently to etching reagents because of variations in chemical composition, method of manufacturing, heat treatment and many other variables.

Macro-Examinations are also performed on a polished and etched cross-section of a welded material. During the examination, a number of features can be determined including weld run sequence, important for weld procedure qualifications tests. As well as this, any defects on the sample will be assessed for compliance with relevant specifications. Slag, porosity, lack of weld penetration, lack of sidewall fusion and poor weld profile are among the features observed in such examinations. It is normal to look for such defects either by standard visual examination or at magnifications of up to 50X. It is also routine to photograph the section to provide a permanent record. This is known as a photomacrograph.

IV. Theory

A)Microscopical Examination
Microstructural examination may provide various information about list below
1) The grain size of specimens
2) The amount of interfacial area per unit volume
3) The dimensions of constituent phases
4) The amount and distribution of phases
For grain size measurement the grains along a line,circle, or within a known area are counted. In lineer intercept method, the grains intercepted by a theoretical line on the specimen surface are counted. The average grain diameter is given by: d=C/nL.M
d= The average grain diameter
C= Some constant greater than 1 ( typically, a value of C=1,5 is adequate)
nL= per unit length of a test line
M= Corrected for the magnification
Magnifications up to 1000x can be obtained with a resolution of 2 um.
B)Macroscopical Examination
The reaction of the sulphuric acid with the sulphide regions of the steel produces hydrogen sulphide gas, which reacts with the silver bromide in the paper emulsion, forming a characteristic Brown to gray-black deposit of silver sulphide. These reactions may be expressed as follows:
FeS + H2SO4 ~ FeSO4 + H2S
Or MnS + H2SO4 ~ MnSO4 + H2S
H2S + 2 AgBr ~ Ag2S + 2 HBr

V. Experimental Apparatus
In Microscopy examination is done with the prepared metal specimen, employing magnifications with the optical microscope of from 100x to as high as 2000x
In Macroscopy the examination of the structural characteristics of metal is done by unaided eye or the aid of a low-power misroscope ,usually under 10x
VI. Anaysis – Results
1) Microscopical Examination
Specimen No 1 :
Ductile Cast Iron By adding a small amount (0.05wt %) of magnesıum to the molten metal of the gray iron composition , spherodial graphite precipitates rather than flakes are produced. Ductility is increased by a factor of 20 , and strength is doubled

Specimen No 2 :

Gray Cast Iron It has a grey fracture surface with finely faceted structure.A significant silicon content ( 2 to 3 wt %) promotes graphite (C) precipitation rather than cememntite (Fe3C) . The sharp, pointed graphide flakes contribute to the characteristic brittleness in gray iron

Specimen No 3 :
White Cast Iron It has a characteristic white ,crystalline fracture surface: Large amounts of Fe3C are formed during casting, giving a hard , brittle material.

Specimen No 4 :
Low Carbon Stainless Steel That steel has two constituents, which are ferrite and pearlite. The light coloured region of the microstructure is the ferrite. The grain boundaries between the ferrite grains can be seen quite clearly. The dark regions are the pearlite. Decreasing the size of the grains and decreasing the amount of pearlite improves the strength, ductility and the toughness of the steel.

2) Macroscopical Examination

I) Sulphur Printing
Impurities is desiarable that their amounts should be minimal and homogeneously distributed within the product. These impurities degrade the mechanical properties of the steel, especially sulphur content in steel makes it brittle.
Sulphur may exist chemically in steel in one of two forms, either as manganese sulphide or as iron sulphide. Sulphur printing detects and permanently records the distribution of sulphur in steel.

The examination of properly prepared sulphur print will disclose quite clearly, because of the presence of darkly colored areas of silver sulphide, the precise location of sulphur inclusions on the prepared surface of the metal.

II) Flow Lines
Flow lines as revealed by macroetching in forgings are a natural consequence of applied mechanical working. If the flow pattern shows highly selective etching characteristics, it is likely that the material may be defective and may contain an excessive amount of inclusions and segregated areas.

The orientation of this pattern with respect to the plane of the prepared surface indicates the direction of metal flow during deformation. The flow lines are made visible because the elongated inclusions of impurities, such as oxides, sulphides and other elongated heterogeneous areas are selectively attacked by the etching reagent.

III) Welded Sections

In ferritic welds, the specimen of interest is prepared in a manner described for metallographic specimens and finally alternately polished and etched in saturated picral to remove disturbed metal. The prepared surface is then etched for 10 to 20 sec. in 5% nital, after which the surface is thoroughly washed and lightly rubbed on a metallographic polishing cloth until the columnar grains in the weld metal show distinctly. This procedure is repeated several times to lessen the light reflectivity characteristics of the surface and to produce some relief of the
macrostructure. The specimen of interest is then etched by immersion for about 2 min. in saturated picral, followed by thorough washing in cold running water and swabbing with a tuft of cotton to remove the loosely adhering reaction products formed on the surface. VII. Discussion
1)
2)In this sitiation sulphur distributed homogeneously so that this material is not brittle as much as the material that has segratgated points.Being gather together for sulphur inclusions maket he part more brittle.
3)Lathing makes the flow lines appear.The reason of appearing flow lines to cut and take out the particle.When casting occuring there is not any big flow lines appear because casting doesn’t cut the lines by the way it is done as particle is liquid.
4)It effect tensile and pressing stress of material and make pressing stres 10 multıple than tensile stres.
5)

6) . The different light reflections of welded material and welding consolidative shows the structure of whole section such as dispersion of welding material or air holes, that minimize the quality of welding. It can be interpreted from the image whether the welding done efficiently, faulty or incompletely. So, the stability of the welding section can be estimated. VIII. Conclusion

In both macroscopical examination and the microscopical examination the fundemental is to study the characteristic or constitution of a metal in relation to its physical and mechanical properities.In macroscopy this procedure is more sketchy and more memorable so that grinding , polishing and etching the three crucial preperation types should be learnt exactly.In microscopy , microscope used and structures check and decided which belong to sources .It is also studied that sulphur inclusions as segregated points and lathing(destroy flow lines) make particle brittle.
IX. Referances
1) General prosedure taken from MATERIAL TESTING Laboratory Manual – 2006